Staff directory Gustau Catalán Bernabé

Gustau Catalán Bernabé

ICREA Research Professor and Group Leader
gustau.catalan(ELIMINAR)@icn2.cat
Oxide Nanophysics

ORCID: 0000-0003-0214-4828

Publications

2024

Coexistence of ferroelectric and ferrielectric phases in ultrathin antiferroelectric PbZrO3 thin films
Ying Liu; Ranming Niu; Roger Uriach; David Pesquera; José Manuel Caicedo; José Santiso; Julie M. Cairney; Xiaozhou Liao; Jordi Arbiol; Gustau Catalán Microstructures; 4 (4) 2024. 10.20517/microstructures.2024.12.
Domain wall dynamics in tungsten trioxide: Evidence for polar domain walls
Eckstein, Jack T; Yokota, Hiroko; Domingo, Neus; Catalan, Gustau; Aktas, Oktay; Carpenter, Michael A; Salje, Ekhard K H Physical Review b; 110 (9): 094107. 2024. 10.1103/PhysRevB.110.094107. IF: 3.200
Flexophotovoltaic Effect and Above-Band-Gap Photovoltage Induced by Strain Gradients in Halide Perovskites
Wang, Zhiguo; Shu, Shengwen; Wei, Xiaoyong; Liang, Renhong; Ke, Shanming; Shu, Longlong; Catalan, Gustau Physical Review Letters; 132 (8): 086902. 2024. 10.1103/PhysRevLett.132.086902. IF: 8.100
Metal poles around the bend
Catalan, Gustau Nature Physics; 2024. 10.1038/s41567-024-02402-6. IF: 18.100
Non-Destructive Tomographic Nanoscale Imaging of Ferroelectric Domain Walls
He, Jiali; Zahn, Manuel; Ushakov, Ivan N; Richarz, Leonie; Ludacka, Ursula; Roede, Erik D; Yan, Zewu; Bourret, Edith; Kezsmarki, Istvan; Catalan, Gustau; Meier, Dennis Advanced Functional Materials; 2024. 10.1002/adfm.202314011. IF: 18.500
Photostrictive Actuators Based on Freestanding Ferroelectric Membranes
Saptam Ganguly; David Pesquera; Daniel Moreno Garcia; Usman Saeed; Nona Mirzamohammadi; José Santiso; Jessica Padilla; José Manuel Caicedo Roque; Claire Laulhé; Felisa Berenguer; Luis Guillermo Villanueva; Gustau Catalán Advanced Materials; 2024. 10.1002/adma.202310198. IF: 27.400
Strain-gradient-induced modulation of photovoltaic efficiency
Zhiguo Wang; Haozhang Zhong; Zhiyong Liu; Xiaotian Hu; Longlong Shu; Gustau Catalán Matter; : 101930 - 101930. 2024. 10.1016/j.matt.2024.11.024. IF: 18.400
Surface polarization profile of ferroelectric thin films probed by X-ray standing waves and photoelectron spectroscopy
Hoang, Le Phuong; Spasojevic, Irena; Lee, Tien-Lin; Pesquera, David; Rossnagel, Kai; Zegenhagen, Joerg; Catalan, Gustau; Vartanyants, Ivan A; Scherz, Andreas; Mercurio, Giuseppe Scientific Reports; 14 (1): 24250. 2024. 10.1038/s41598-024-72805-1. IF: 3.800
Switchable tribology of ferroelectrics
Cho, Seongwoo; Gaponenko, Iaroslav; Cordero-Edwards, Kumara; Barcelo-Mercader, Jordi; Arias, Irene; Kim, Daeho; Lichtensteiger, Celine; Yeom, Jiwon; Musy, Loic; Kim, Hyunji; Han, Seung Min; Catalan, Gustau; Paruch, Patrycja; Hong, Seungbum Nature Communications; 15 (1): 387. 2024. 10.1038/s41467-023-44346-0. IF: 14.700
Switching Dynamics and Improved Efficiency of Free-Standing Antiferroelectric Capacitors
Saeed, Umair; Pesquera, David; Liu, Ying; Fina, Ignasi; Ganguly, Saptam; Santiso, Jose; Padilla-Pantoja, Jessica; Roque, Jose Manuel Caicedo; Liao, Xiaozhou; Catalan, Gustau Advanced Electronic Materials; 2024. 10.1002/aelm.202400102. IF: 5.300

2023

Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO3
Liu, Y; Niu, RM; Majchrowski, A; Roleder, K; Cordero-Edwards, K; Cairney, JM; Arbiol, J; Catalan, G Physical Review Letters; 130 (21): 216801. 2023. 10.1103/physrevlett.130.216801. IF: 8.600
Tunable Molecular Electrodes for Bistable Polarization Screening
Spasojevic, I; Santiso, J; Caicedo, JM; Catalan, G; Domingo, N Small; 19 (30): e2207799. 2023. 10.1002/smll.202207799. IF: 13.300
Weak low-temperature polarity in a PbZrO3 single crystal
Roleder, K; Catalan, G; Glazer, AM; Baker, JS; Ko, JH; Naqvi, FH; Junaid, SB; Majchrowski, A; Trybula, Z; Zareba, J; Lazar, I; Kajewski, D; Koperski, J; Soszynski, A Physical Review b; 107 (14): L140102. 2023. 10.1103/physrevb.107.l140102. IF: 3.700

2022

Intrinsic flexoelectricity of van der Waals epitaxial thin films
Shu L., Wang Z., Liang R., Zhang Z., Shu S., Tang C., Li F., Zheng R.-K., Ke S., Catalan G. Physical Review B; 106 (2, 024108) 2022. 10.1103/PhysRevB.106.024108.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO3 and SrTiO3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films. © 2022 American Physical Society.

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The emancipation of flexoelectricity
Arias I., Catalan G., Sharma P. Journal of Applied Physics; 131 (2, 020401) 2022. 10.1063/5.0079319.

[No abstract available]

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2021

Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO3Thin Film Deposited in a Composition-Spread Geometry
Bugallo D., Langenberg E., Ferreiro-Vila E., Smith E.H., Stefani C., Batlle X., Catalan G., Domingo N., Schlom D.G., Rivadulla F. ACS Applied Materials and Interfaces; 13 (38): 45679 - 45685. 2021. 10.1021/acsami.1c08758. IF: 9.229

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices. © 2021 American Chemical Society.

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Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging
Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295

The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH

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Effect of Humidity on the Writing Speed and Domain Wall Dynamics of Ferroelectric Domains
Spasojevic I., Verdaguer A., Catalan G., Domingo N. Advanced Electronic Materials; 8 (6, 2100650) 2021. 10.1002/aelm.202100650. IF: 7.295

The switching dynamics of ferroelectric polarization under electric fields depends on the availability of screening charges in order to stabilize the switched polarization. In ferroelectrics, thin films with exposed surfaces investigated by piezoresponse force microscopy (PFM), the main source of external screening charges is the atmosphere and the water neck, and therefore relative humidity (RH) plays a major role. Here, it is shown how the dynamic writing of domains in BaTiO3 thin films changes by varying scanning speeds in the range of RH between 2.5% and 60%. The measurements reveal that the critical speed for domain writing, which is defined as the highest speed at which electrical writing of a continuous stripe domain is possible, increases non-monotonically with RH. Additionally, the width of line domains shows a power law dependence on the writing speed, with a growth rate coefficient decreasing with RH. The size of the written domains at a constant speed as well as the creep-factor μ describing the domain wall kinetics follow the behavior of water adsorption represented by the adsorption isotherm, indicating that the screening mechanism dominating the switching dynamics is the thickness and the structure of adsorbed water structure and its associated dielectric constant and ionic mobility. © 2021 The Authors. Advanced Electronic Materials published by Wiley-VCH GmbH.

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Mechanical reading of ferroelectric polarization
Stefani C., Langenberg E., Cordero-Edwards K., Schlom D.G., Catalan G., Domingo N. Journal of Applied Physics; 130 (7, 0059930) 2021. 10.1063/5.0059930. IF: 2.546

Flexoelectricity is a property of dielectric materials whereby they exhibit electric polarization induced by strain gradients; while this effect can be negligible at the macroscale, it can become dominant at the nanoscale, where strain gradients can turn out to be tremendous. Previous works have demonstrated that flexoelectricity coupled with piezoelectricity enables the mechanical writing of ferroelectric polarization. When considering ferroelectric materials with out-of-plane polarization, the coupling of piezoelectricity with flexoelectricity can insert a mechanical asymmetry to the system and enable the distinction of oppositely polarized domains, based on their nanomechanical response. Using atomic force microscopy and, more specifically, contact resonance techniques, the coupling of flexoelectricity to piezoelectricity can be exploited to mechanically read the sign of ferroelectric polarization in a non-destructive way. We have measured a variety of ferroelectric materials, from a single crystal to thin films, and domains that are polarized down always appear to be stiffer than oppositely polarized domains. In this article, we demonstrate experimentally that the phenomenon is size-dependent and strongly enhanced when the dimension of the material is reduced to nanoscale in thin films. Ultimately, we demonstrate how the sensitivity in mechanical reading of ferroelectric polarization can be improved by appropriately tuning the mechanical stiffness of the cantilevers. © 2021 Author(s).

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Metallic Diluted Dimerization in VO2 Tweeds
Sandiumenge F., Rodríguez L., Pruneda M., Magén C., Santiso J., Catalan G. Advanced Materials; 33 (9, 2004374) 2021. 10.1002/adma.202004374. IF: 30.849

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal–insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale. © 2021 Wiley-VCH GmbH

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Non-linear nanoscale piezoresponse of single ZnO nanowires affected by piezotronic effect
Lozano H., Catalán G., Esteve J., Domingo N., Murillo G. Nanotechnology; 32 (2, 025202) 2021. 10.1088/1361-6528/abb972. IF: 3.874

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal-semiconductor-metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor-metal junctions. By directly measuring the experimental I-V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d 33eff ∼ 8.6 pm V-1-12.3 pm V-1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d 33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d 33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics. © 2020 IOP Publishing Ltd.

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Origin of large negative electrocaloric effect in antiferroelectric PbZr O3
Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.

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Piezoelectricity in nominally centrosymmetric phases
Aktas O., Kangama M., Linyu G., Catalan G., Ding X., Zunger A., Salje E.K.H. Physical Review Research; 3 (4, 043221) 2021. 10.1103/PhysRevResearch.3.043221. IF: 0.000

Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the "symmetry-forbidden"piezoelectric effect ranges from ∼1 to 10-5 pm/V (∼0.5% to ∼2×10-5% of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon. © 2021 authors. Published by the American Physical Society.

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Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films
Rodríguez L., Sandiumenge F., Frontera C., Caicedo J.M., Padilla J., Catalán G., Santiso J. Acta Materialia; 220 (117336) 2021. 10.1016/j.actamat.2021.117336. IF: 8.203

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices. © 2021

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2020

Control of lateral composition distribution in graded films of soluble solid systems A1-xBx by partitioned dual-beam pulsed laser deposition
Sakai J., Roque J.M.C., Vales-Castro P., Padilla-Pantoja J., Sauthier G., Catalan G., Santiso J. Coatings; 10 (6, 540) 2020. 10.3390/COATINGS10060540. IF: 2.436

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries aswell as for high-throughputmaterials exploration.We herein propose amethod to prepare epitaxial lateral compositionally-graded films using a dual-beampulsed laser deposition (PLD)method with two targets separated by a partition. Tuning the ambient pressure and the partition-substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1-x)PbTiO3-xPbZrO3 and (1-x)LaMnO3-xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD. © 2020 by the authors.

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Investigation of The Cellular Response to Bone Fractures: Evidence for Flexoelectricity
Núñez-Toldrà R., Vasquez-Sancho F., Barroca N., Catalan G. Scientific Reports; 10 (1, 254) 2020. 10.1038/s41598-019-57121-3. IF: 3.998

The recent discovery of bone flexoelectricity (strain-gradient-induced electrical polarization) suggests that flexoelectricity could have physiological effects in bones, and specifically near bone fractures, where flexoelectricity is theoretically highest. Here, we report a cytological study of the interaction between crack stress and bone cells. We have cultured MC3T3-E1 mouse osteoblastic cells in biomimetic microcracked hydroxyapatite substrates, differentiated into osteocytes and applied a strain gradient to the samples. The results show a strong apoptotic cellular response, whereby mechanical stimulation causes those cells near the crack to die, as indicated by live-dead and caspase staining. In addition, analysis two weeks post-stimulation shows increased cell attachment and mineralization around microcracks and a higher expression of osteocalcin –an osteogenic protein known to be promoted by physical exercise. The results are consistent with flexoelectricity playing at least two different roles in bone remodelling: apoptotic trigger of the repair protocol, and electro-stimulant of the bone-building activity of osteoblasts. © 2020, The Author(s).

View abstract
James F. Scott (1942-2020)
Catalan G., Dawber M., Gregg M., Morrison F., Ramesh R., Zubko P. Nature materials; 19 (6): 580. 2020. 10.1038/s41563-020-0692-x. IF: 38.663

[No abstract available]

View abstract
Mechanical Softness of Ferroelectric 180° Domain Walls MECHANICAL SOFTNESS of FERROELECTRIC 180 DEGREE ... STEFANI CHRISTINA et al.
Stefani C., Ponet L., Shapovalov K., Chen P., Langenberg E., Schlom D.G., Artyukhin S., Stengel M., Domingo N., Catalan G. Physical Review X; 10 (4, 041001) 2020. 10.1103/PhysRevX.10.041001. IF: 12.577

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct-softer-compared to the domains. This effect is observed in different ferroelectric materials (LiNbO3, BaTiO3, and PbTiO3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Photoflexoelectric effect in halide perovskites
Shu L., Ke S., Fei L., Huang W., Wang Z., Gong J., Jiang X., Wang L., Li F., Lei S., Rao Z., Zhou Y., Zheng R.-K., Yao X., Wang Y., Stengel M., Catalan G. Nature Materials; 19 (6): 605 - 609. 2020. 10.1038/s41563-020-0659-y. IF: 38.663

Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials1–3, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

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Self-Pixelation Through Fracture in VO2 Thin Films
Laura Rodríguez, Elena del Corro, Michele Conroy, Kalani Moore, Felip Sandiumenge, Neus Domingo, José Santiso, Gustau Catalan Acs Applied Electronic Materials; 2 (5): 1433 - 1439. 2020. 10.1021/acsaelm.0c00199. IF: 0.000
Strain-Engineered Ferroelastic Structures in PbTiO3 Films and Their Control by Electric Fields
Langenberg E., Paik H., Smith E.H., Nair H.P., Hanke I., Ganschow S., Catalan G., Domingo N., Schlom D.G. ACS Applied Materials and Interfaces; 12 (18): 20691 - 20703. 2020. 10.1021/acsami.0c04381. IF: 8.758

We study the interplay between epitaxial strain, film thickness, and electric field in the creation, modification, and design of distinct ferroelastic structures in PbTiO3 thin films. Strain and thickness greatly affect the structures formed, providing a two-variable parameterization of the resulting self-assembly. Under applied electric fields, these strain-engineered ferroelastic structures are highly malleable, especially when a/c and a1/a2 superdomains coexist. To reconfigure the ferroelastic structures and achieve self-assembled nanoscale-ordered morphologies, pure ferroelectric switching of individual c-domains within the a/c superdomains is essential. The stability, however, of the electrically written ferroelastic structures is in most cases ephemeral; the speed of the relaxation process depends sensitively on strain and thickness. Only under low tensile strain - as is the case for PbTiO3 on GdScO3 - and below a critical thickness do the electrically created a/c superdomain structures become stable for days or longer, making them relevant for reconfigurable nanoscale electronics or nonvolatile electromechanical applications. Copyright © 2020 American Chemical Society.

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Temperature-independent giant dielectric response in transitional BaTiO3 thin films
Everhardt A.S., Denneulin T., Grünebohm A., Shao Y.-T., Ondrejkovic P., Zhou S., Domingo N., Catalan G., Hlinka J., Zuo J.-M., Matzen S., Noheda B. Applied Physics Reviews; 7 (1, 011402) 2020. 10.1063/1.5122954. IF: 17.054

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics. © 2020 Author(s).

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2019

Converse flexoelectricity yields large piezoresponse force microscopy signals in non-piezoelectric materials
Abdollahi A., Domingo N., Arias I., Catalan G. Nature Communications; 10 (1, 1266) 2019. 10.1038/s41467-019-09266-y. IF: 11.878

Converse flexoelectricity is a mechanical stress induced by an electric polarization gradient. It can appear in any material, irrespective of symmetry, whenever there is an inhomogeneous electric field distribution. This situation invariably happens in piezoresponse force microscopy (PFM), which is a technique whereby a voltage is delivered to the tip of an atomic force microscope in order to stimulate and probe piezoelectricity at the nanoscale. While PFM is the premier technique for studying ferroelectricity and piezoelectricity at the nanoscale, here we show, theoretically and experimentally, that large effective piezoelectric coefficients can be measured in non-piezoelectric dielectrics due to converse flexoelectricity. © 2019, The Author(s).

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Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures
Menéndez E., Sireus V., Quintana A., Fina I., Casals B., Cichelero R., Kataja M., Stengel M., Herranz G., Catalán G., Baró M.D., Suriñach S., Sort J. Physical Review Applied; 12 (1, 014041) 2019. 10.1103/PhysRevApplied.12.014041. IF: 4.532

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe75Al25 (at. %) thin film on top of a (110)-oriented Pb(Mg1/3Nb2/3)O3-32PbTiO3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe75Al25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures. © 2019 American Physical Society.

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Ferroelectric Domain Walls in PbTiO3 Are Effective Regulators of Heat Flow at Room Temperature
Langenberg E., Saha D., Holtz M.E., Wang J.-J., Bugallo D., Ferreiro-Vila E., Paik H., Hanke I., Ganschow S., Muller D.A., Chen L.-Q., Catalan G., Domingo N., Malen J., Schlom D.G., Rivadulla F. Nano Letters; 19 (11): 7901 - 7907. 2019. 10.1021/acs.nanolett.9b02991. IF: 12.279

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the roomerature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10-9 K m2 W-1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics. Copyright © 2019 American Chemical Society.

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Flexoelectric Fracture-Ratchet Effect in Ferroelectrics
Cordero-Edwards K., Kianirad H., Canalias C., Sort J., Catalan G. Physical Review Letters; 122 (13, 135502) 2019. 10.1103/PhysRevLett.122.135502. IF: 9.227

The propagation front of a crack generates large strain gradients and it is therefore a strong source of gradient-induced polarization (flexoelectricity). Herein, we demonstrate that, in piezoelectric materials, a consequence of flexoelectricity is that crack propagation is helped or hindered depending on whether it is parallel or antiparallel to the piezoelectric polar axis. The discovery of crack propagation asymmetry proves that fracture physics cannot be assumed to be symmetric in polar materials, and indicates that flexoelectricity should be incorporated in any realistic model. © 2019 American Physical Society.

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Giant bulk photovoltaic effect in solar cell architectures with ultra-wide bandgap Ga2O3 transparent conducting electrodes
Pérez-Tomás A., Chikoidze E., Dumont Y., Jennings M.R., Russell S.O., Vales-Castro P., Catalan G., Lira-Cantú M., Ton –That C., Teherani F.H., Sandana V.E., Bove P., Rogers D.J. Materials Today Energy; 14 (100350) 2019. 10.1016/j.mtener.2019.100350. IF: 0.000

The use of ultra-wide bandgap transparent conducting beta gallium oxide (β-Ga2O3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes - one of them - transparent) which is not constrained by the Shockley–Queisser limit for open-circuit voltage (Voc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides. © 2019 Elsevier Ltd

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Periodicity-Doubling Cascades: Direct Observation in Ferroelastic Materials
Everhardt A.S., Damerio S., Zorn J.A., Zhou S., Domingo N., Catalan G., Salje E.K.H., Chen L.-Q., Noheda B. Physical Review Letters; 123 (8, 087603) 2019. 10.1103/PhysRevLett.123.087603. IF: 9.227

Very sensitive responses to external forces are found near phase transitions. However, transition dynamics and preequilibrium phenomena are difficult to detect and control. We have observed that the equilibrium domain structure following a phase transition in ferroelectric and ferroelastic BaTiO3 is attained by halving of the domain periodicity multiple times. The process is reversible, with periodicity doubling as temperature is increased. This observation is reminiscent of the period-doubling cascades generally observed during bifurcation phenomena, and, thus, it conforms to the "spatial chaos" regime earlier proposed by Jensen and Bak [Phys. Scr. T 9, 64 (1985)PHSTER0281-184710.1088/0031-8949/1985/T9/009] for systems with competing spatial modulations. © 2019 American Physical Society.

View abstract
Surface polarization feels the heat
Gustau Catalan, Beatriz Noheda Nature; 575 (7784): 600 - 602. 2019. 10.1038/d41586-019-03494-4.

A crystal’s surface has been found to behave as a distinct material that has temperature-dependent electrical polarization — despite the rest of the crystal being non-polar.

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Vehicle Classification System Based on Ferroelectric Materials
Fina I., Martí X., Catalan G. 2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings; (9034960) 2019. 10.1109/ISAF43169.2019.9034960.

Vehicle classification taking into account the weight of the vehicle and its number of shafts can be relevant for adequate control of vehicles mobility. Piezoelectric materials can be used to infer both parameters in a reliable manner. Also piezoelectric sensors can be cost-effective. In the present work, we focus our attention in the characterization of a piezoelectric cable, where the piezoelectric active material is PVDF. We show that the piezoelectric cable can be used to classify vehicles and count their number of shafts, using a simply signal processing. © 2019 IEEE.

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2018

A Solar Transistor and Photoferroelectric Memory
Pérez-Tomás A., Lima A., Billon Q., Shirley I., Catalan G., Lira-Cantú M. Advanced Functional Materials; 28 (17, 1707099) 2018. 10.1002/adfm.201707099. IF: 13.325

This study presents a new self-powered electronic transistor concept “the solar transistor.” The transistor effect is enabled by the functional integration of a ferroelectric-oxide thin film and an organic bulk heterojunction. The organic heterojunction efficiently harvests photon energy and splits photogenerated excitons into free electron and holes, and the ferroelectric film acts as a switchable electron transport layer with tuneable conduction band offsets that depend on its polarization state. This results in the device photoconductivity modulation. All this (i.e., carrier extraction and poling) is achieved with only two sandwiched electrodes and therefore, with the role of the gating electrode being taken by light. The two-terminal solar-powered phototransistor (or solaristor) thus has the added advantages of a compact photodiode architecture in addition to the nonvolatile functionality of a ferroelectric memory that is written by voltage and nondestructively read by light. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectricity in antiferroelectrics
Vales-Castro P., Roleder K., Zhao L., Li J.-F., Kajewski D., Catalan G. Applied Physics Letters; 113 (13, 132903) 2018. 10.1063/1.5044724. IF: 3.495

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO3 and AgNbO3) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity. © 2018 Author(s).

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Flexoelectricity in Bones
Vasquez-Sancho F., Abdollahi A., Damjanovic D., Catalan G. Advanced Materials; 30 (9, 1705316) 2018. 10.1002/adma.201705316. IF: 21.950

Bones generate electricity under pressure, and this electromechanical behavior is thought to be essential for bone's self-repair and remodeling properties. The origin of this response is attributed to the piezoelectricity of collagen, which is the main structural protein of bones. In theory, however, any material can also generate voltages in response to strain gradients, thanks to the property known as flexoelectricity. In this work, the flexoelectricity of bone and pure bone mineral (hydroxyapatite) are measured and found to be of the same order of magnitude; the quantitative similarity suggests that hydroxyapatite flexoelectricity is the main source of bending-induced polarization in cortical bone. In addition, the measured flexoelectric coefficients are used to calculate the (flexo)electric fields generated by cracks in bone mineral. The results indicate that crack-generated flexoelectricity is theoretically large enough to induce osteocyte apoptosis and thus initiate the crack-healing process, suggesting a central role of flexoelectricity in bone repair and remodeling. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectricity in Bones (vol 30, 1705316, 2018)
Vasquez-Sancho, Fabian; Abdollahi, Amir; Damjanovic, Dragan; Catalan, Gustau; Advanced Materials; 30 (21): e1801413. 2018. 10.1002/adma.201801413. IF: 21.950
Piezoelectric Mimicry of Flexoelectricity
Abdollahi A., Vásquez-Sancho F., Catalan G. Physical Review Letters; 121 (20, 205502) 2018. 10.1103/PhysRevLett.121.205502. IF: 8.839

The origin of "giant" flexoelectricity, orders of magnitude larger than theoretically predicted, yet frequently observed, is under intense scrutiny. There is mounting evidence correlating giant flexoelectriclike effects with parasitic piezoelectricity, but it is not clear how piezoelectricity (polarization generated by strain) manages to imitate flexoelectricity (polarization generated by strain gradient) in typical beam-bending experiments, since in a bent beam the net strain is zero. In addition piezoelectricity changes sign under space inversion but giant flexoelectricity is insensitive to space inversion, seemingly contradicting a piezoelectric origin. Here we show that, if a piezoelectric material has its piezoelectric coefficient asymmetrically distributed across the sample, it will generate a nonzero bending-induced polarization impossible to distinguish from true flexoelectricity even by inverting the sample. The effective flexoelectric coefficient caused by piezoelectricity is functionally identical to, and often larger than, intrinsic flexoelectricity: our calculations show that, for standard perovskite ferroelectrics, even a tiny gradient of piezoelectricity (1% variation of piezoelectric coefficient across 1 mm) is sufficient to yield a giant effective flexoelectric coefficient of 1 μC/m, three orders of magnitude larger than the intrinsic expectation value. © 2018 American Physical Society.

View abstract

2017

Domain wall magnetoresistance in BiFeO3 thin films measured by scanning probe microscopy
Domingo N., Farokhipoor S., Santiso J., Noheda B., Catalan G. Journal of Physics Condensed Matter; 29 (33, 334003) 2017. 10.1088/1361-648X/aa7a24. IF: 2.678

We measure the magnetotransport properties of individual 71° domain walls in multiferroic BiFeO3 by means of conductive-atomic force microscopy (C-AFM) in the presence of magnetic fields up to one Tesla. The results suggest anisotropic magnetoresistance at room temperature, with the sign of the magnetoresistance depending on the relative orientation between the magnetic field and the domain wall plane. A consequence of this finding is that macroscopically averaged magnetoresistance measurements for domain wall bunches are likely to underestimate the magnetoresistance of each individual domain wall. © 2017 IOP Publishing Ltd.

View abstract
Ferroelectrics as Smart Mechanical Materials
Cordero-Edwards K., Domingo N., Abdollahi A., Sort J., Catalan G. Advanced Materials; 29 (37, 1702210) 2017. 10.1002/adma.201702210. IF: 19.791

The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact-resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO3) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
On the persistence of polar domains in ultrathin ferroelectric capacitors
Zubko P., Lu H., Bark C.-W., Martí X., Santiso J., Eom C.-B., Catalan G., Gruverman A. Journal of Physics Condensed Matter; 29 (28, 284001) 2017. 10.1088/1361-648X/aa73c3. IF: 2.678

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO3 films sandwiched between the most habitual perovskite electrodes, SrRuO3, on top of the most used perovskite substrate, SrTiO3. We use a combination of piezoresponse force microscopy, dielectric measurements and structural characterization to provide conclusive evidence for the ferroelectric nature of the relaxed polarization state in ultrathin BaTiO3 capacitors. We show that even the high screening efficiency of SrRuO3 electrodes is still insufficient to stabilize polarization in SrRuO3/BaTiO3/SrRuO3 heterostructures at room temperature. We identify the key role of domain wall motion in determining the macroscopic electrical properties of ultrathin capacitors and discuss their dielectric response in the light of the recent interest in negative capacitance behaviour. © 2017 IOP Publishing Ltd.

View abstract
The profile of researchers moving towards scientific entrepreneurship
Marti X., Fina I., Catalan G., Veà A. Supporting University Ventures in Nanotechnology, Biomaterials and Magnetic Sensing Applications: Pol; : 143 - 157. 2017. 10.1007/978-3-319-61237-9_7.

We review some of the skills that scientists who have become scientific entrepreneurs share. We advocate that it is possible to include technology transfer as a part of the scientific investigation not only as a collateral funding source but also as a way to reinforce the scientific advances and resulting communication. Traditionally, the technology-transfer process of scientific results has been mediated mainly by patents, even before the first prototype testing has been performed. Instead, we discuss the critical benefits of fast prototyping and develop strategies how to adequately monetize and establish the price of the products even at the prototype level. We also list several common failures of a technology-transfer project and some preventive actions to avoid them. © Springer International Publishing AG 2018. All rights reserved.

View abstract

2016

A flexoelectric microelectromechanical system on silicon
Bhaskar U.K., Banerjee N., Abdollahi A., Wang Z., Schlom D.G., Rijnders G., Catalan G. Nature Nanotechnology; 11 (3): 263 - 266. 2016. 10.1038/nnano.2015.260. IF: 35.267

Flexoelectricity allows a dielectric material to polarize in response to a mechanical bending moment and, conversely, to bend in response to an electric field. Compared with piezoelectricity, flexoelectricity is a weak effect of little practical significance in bulk materials. However, the roles can be reversed at the nanoscale. Here, we demonstrate that flexoelectricity is a viable route to lead-free microelectromechanical and nanoelectromechanical systems. Specifically, we have fabricated a silicon-compatible thin-film cantilever actuator with a single flexoelectrically active layer of strontium titanate with a figure of merit (curvature divided by electric field) of 3.33MV â '1, comparable to that of state-of-the-art piezoelectric bimorph cantilevers. © 2016 Macmillan Publishers Limited.

View abstract
Above-Bandgap Photovoltages in Antiferroelectrics
Pérez-Tomás A., Lira-Cantú M., Catalan G. Advanced Materials; 28 (43): 9644 - 9647. 2016. 10.1002/adma.201603176. IF: 18.960

The closed circuit photocurrent and open circuit photovoltage of antiferroelectric thin films were characterized both in their ground (antipolar) state and in their polarized state. A sharp transition happens from near zero to large photovoltages as the polarization is switched on, consistent with the activation of the bulk photovoltaic effect. The AFE layers have been grown by a solution processing method (sol?gel synthesis followed by spin coating deposition) onto fluorine-doped tin oxide (FTO), a transparent conducting oxide with low sheet resistance and a higher resilience to high-temperature processing than indium tin oxide and a standard for solar cells such as organometal trihalide perovskites. Light absorption confirmed that the PZO films are, as expected, wide-band gap semiconductors with a gap of 3.7.8 eV and thus highly absorbing in the near-ultraviolet range. On a virgin sample, there is no shortcircuit photocurrent, consistent with the antipolar nature of the ground state. As an external bias voltage is applied, the current remains negligible until suddenly, at the coercive voltage, a spike is observed, corresponding to the transient displacement current caused by the onset of polarization.

View abstract
Enhanced flexoelectric-like response in oxide semiconductors
Narvaez J., Vasquez-Sancho F., Catalan G. Nature; 538 (7624): 219 - 221. 2016. 10.1038/nature19761. IF: 38.138

Flexoelectricity is a property of all dielectric materials whereby they polarize in response to deformation gradients such as those produced by bending. Although it is generally thought of as a property of dielectric insulators, insulation is not a formal requirement: in principle, semiconductors can also redistribute their free charge in response to strain gradients. Here we show that bending a semiconductor not only generates a flexoelectric-like response, but that this response can in fact be much larger than in insulators. By doping single crystals of wide-bandgap oxides to increase their conductivity, their effective flexoelectric coefficient was increased by orders of magnitude. This large response can be explained by a barrier-layer mechanism that remains important even at the macroscale, where conventional (insulator) flexoelectricity otherwise tends to be small. Our results open up the possibility of using semiconductors as active ingredients in electromechanical transducer applications. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

View abstract
Ferroelectric Domain Structures in Low-Strain BaTiO3
Everhardt A.S., Matzen S., Domingo N., Catalan G., Noheda B. Advanced Electronic Materials; 2 (1, 1500214) 2016. 10.1002/aelm.201500214. IF: 0.000

Epitaxial strain in ferroelectric films offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO3 has been predicted to order in a variety of phases and domain configurations when grown under low strain on low mismatched substrates, but little experimental evidence of that region of the phase diagram exist. Here, epitaxial BaTiO3 thin films are grown on NdScO3 substrates under ≈0.1% strain. A monoclinic ca1/ca2 phase, with 90° periodic in-plane domain configuration and small additional out-of-plane component of polarization, is stabilized at room temperature and investigated using piezoelectric force microscopy and X-ray diffraction. Above 50 °C, this phase is transformed into an a/c phase with alternating in-plane and out-of-plane polarizations and forming zigzag domain walls between up-polarized and down-polarized superdomains. Both types of domain patterns are highly anisotropic, giving rise to very long domain walls. Above 130 °C, the paraelectric phase is observed. The occurrence of a phase transition close to room temperature, a low symmetry ca1/ca2 phase, and the formation of periodic domains make of this material a promising candidate for high piezoelectric response. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

View abstract
Flexoelectric MEMS: Towards an electromechanical strain diode
Bhaskar U.K., Banerjee N., Abdollahi A., Solanas E., Rijnders G., Catalan G. Nanoscale; 8 (3): 1293 - 1298. 2016. 10.1039/c5nr06514c. IF: 7.760

Piezoelectricity and flexoelectricity are two independent but not incompatible forms of electromechanical response exhibited by nanoscale ferroelectrics. Here, we show that flexoelectricity can either enhance or suppress the piezoelectric response of the cantilever depending on the ferroelectric polarity and lead to a diode-like asymmetric (two-state) electromechanical response. © 2016 The Royal Society of Chemistry.

View abstract

2015

Dielectric relaxation in YMnO3 single crystals
Adem U., Mufti N., Nugroho A.A., Catalan G., Noheda B., Palstra T.T.M. Journal of Alloys and Compounds; 638: 228 - 232. 2015. 10.1016/j.jallcom.2015.02.207. IF: 2.999

We have investigated the origin of the dielectric relaxation in YMnO3 single crystals. Two distinct dielectric relaxation features were observed at low (200≤T≤373K) and high (300≤T≤450K) temperatures. Analysis of our detailed frequency, electrode and thickness dependent dielectric measurements and ac conductivity data as well as the use of single crystals allow us to get a comprehensive picture of these relaxations. The low temperature relaxation is attributed to the Maxwell-Wagner type effects originating from the dipoles at the surface while the high temperature one is suggested to originate from hopping of charge carriers resulting from the second ionization of oxygen vacancies. © 2015 Elsevier B.V. All rights reserved.

View abstract
Ferroelectrics: Negative capacitance detected
Catalan G., Jimenez D., Gruverman A. Nature Materials; 14 (2): 137 - 139. 2015. 10.1038/nmat4195. IF: 36.503

[No abstract available]

View abstract
Fracture toughening and toughness asymmetry induced by flexoelectricity
Abdollahi A., Peco C., Millán D., Arroyo M., Catalan G., Arias I. Physical Review B - Condensed Matter and Materials Physics; 92 (9, 094101) 2015. 10.1103/PhysRevB.92.094101. IF: 3.736

Cracks generate the largest strain gradients that any material can withstand. Flexoelectricity (coupling between strain gradient and polarization) must therefore play an important role in fracture physics. Here we use a self-consistent continuum model to evidence two consequences of flexoelectricity in fracture: the resistance to fracture increases as structural size decreases, and it becomes asymmetric with respect to the sign of polarization. The latter phenomenon manifests itself in a range of intermediate sizes where piezo- and flexoelectricity compete. In BaTiO3 at room temperature, this range spans from 0.1 to 50 nm, a typical thickness range for epitaxial ferroelectric thin films. © 2015 American Physical Society.

View abstract
Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films
Domingo N., López-Mir L., Paradinas M., Holy V., Železný J., Yi D., Suresha S.J., Liu J., Rayan Serrao C., Ramesh R., Ocal C., Martí X., Catalan G. Nanoscale; 7 (8): 3453 - 3459. 2015. 10.1039/c4nr06954d. IF: 7.394

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices. This journal is © The Royal Society of Chemistry.

View abstract
Large Flexoelectric Anisotropy in Paraelectric Barium Titanate
Narvaez J., Saremi S., Hong J., Stengel M., Catalan G. Physical Review Letters; 115 (3, 037601) 2015. 10.1103/PhysRevLett.115.037601. IF: 7.512

The bending-induced polarization of barium titanate single crystals has been measured with an aim to elucidate the origin of the large difference between theoretically predicted and experimentally measured flexoelectricity in this material. The results indicate that part of the difference is due to polar regions (short-range order) that exist above TC and up to T∗≈200-225°C. Above T∗, however, the flexovoltage coefficient still shows an unexpectedly large anisotropy for a cubic material, with (001)-oriented crystals displaying 10 times more flexoelectricity than (111)-oriented crystals. Theoretical analysis shows that this anisotropy cannot be a bulk property, and we therefore interpret it as indirect evidence for the theoretically predicted but experimentally elusive contribution of surface piezoelectricity to macroscopic bending-induced polarization. © 2015 American Physical Society. © 2015 American Physical Society.

View abstract
Mechanical tuning of LaAlO3/SrTiO3 interface conductivity
Sharma P., Ryu S., Burton J.D., Paudel T.R., Bark C.W., Huang Z., Ariando, Tsymbal E.Y., Catalan G., Eom C.B., Gruverman A. Nano Letters; 15 (5): 3547 - 3551. 2015. 10.1021/acs.nanolett.5b01021. IF: 13.592

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage. © 2015 American Chemical Society.

View abstract
Nanomechanics of flexoelectric switching
Očenášek J., Lu H., Bark C.W., Eom C.B., Alcalá J., Catalan G., Gruverman A. Physical Review B - Condensed Matter and Materials Physics; 92 (3, 035417) 2015. 10.1103/PhysRevB.92.035417. IF: 3.736

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a "flexoelectric size effect" that mirrors the ferroelectric one. ©2015 American Physical Society.

View abstract
Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor
Pérez-Tomás A., Catalàn G., Fontserè A., Iglesias V., Chen H., Gammon P.M., Jennings M.R., Thomas M., Fisher C.A., Sharma Y.K., Placidi M., Chmielowska M., Chenot S., Porti M., Nafría M., Cordier Y. Nanotechnology; 26 (11, 115203) 2015. 10.1088/0957-4484/26/11/115203. IF: 3.821

The gallium nitride (GaN)-based buffer/barrier mode of growth and morphology, the transistor electrical response (25-310 C) and the nanoscale pattern of a homoepitaxial AlGaN/GaN high electron mobility transistor (HEMT) have been investigated at the micro and nanoscale. The low channel sheet resistance and the enhanced heat dissipation allow a highly conductive HEMT transistor (Ids>1 A mm-1) to be defined (0.5 A mm-1 at 300 C). The vertical breakdown voltage has been determined to be ∼850 V with the vertical drain-bulk (or gate-bulk) current following the hopping mechanism, with an activation energy of 350 meV. The conductive atomic force microscopy nanoscale current pattern does not unequivocally follow the molecular beam epitaxy AlGaN/GaN morphology but it suggests that the FS-GaN substrate presents a series of preferential conductive spots (conductive patches). Both the estimated patches density and the apparent random distribution appear to correlate with the edge-pit dislocations observed via cathodoluminescence. The sub-surface edge-pit dislocations originating in the FS-GaN substrate result in barrier height inhomogeneity within the HEMT Schottky gate producing a subthreshold current. © 2015 IOP Publishing Ltd.

View abstract
Persistence of ferroelectricity above the Curie temperature at the surface of Pb(Z n1/3 N b2/3) O3-12%PbTi O3
Domingo N., Bagués N., Santiso J., Catalan G. Physical Review B - Condensed Matter and Materials Physics; 91 (9, 094111) 2015. 10.1103/PhysRevB.91.094111. IF: 3.736

Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer ("skin") that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at Tc=200C, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of Nb5+ - a small ion with d0 orbital occupancy - stabilizes the ferroelectricity of the skin layer. © 2015 American Physical Society.

View abstract

2014

Origin of the enhanced flexoelectricity of relaxor ferroelectrics
Narvaez, J.; Catalan, G. Applied Physics Letters; 104 (16) 2014. 10.1063/1.4871686. IF: 3.515
Physics of Ferroic and Multiferroic Domain Walls
Catalan G. Springer Series in Materials Science; 198: 225 - 247. 2014. 10.1007/978-3-642-55375-2_9.

Ferroic materials are defined by having an order parameter that can be oriented in more than one direction. Within a ferroic material, then, there can be regions (domains) with different orientation of the order parameter: magnetic domains in ferromagnets, polar domains in ferroelectrics, twins in ferroelastics. Domain walls, or domain boundaries, are the separations between adjacent domains. In the last few years, domain walls have moved from being regarded as an inevitable by-product of the domains, to regions of interest in their own right, with unique electronic properties that may be used as the active ingredient in new electronic device paradigms, in what has been called "domain wall nanoelectronics". The present book chapter outlines the basic physics of domain walls from their thickness and internal structure to their properties and dynamics. We will draw the connection between the fundamental properties and their experimental observation. The last section will discuss current unresolved challenges in this exciting and emerging field. © Springer-Verlag Berlin Heidelberg 2014.

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2013

Flexoelectric effect in solids
Zubko, P.; Catalan, G.; Tagantsev, A.K. Annual Review of Materials Research; 43: 387 - 421. 2013. 10.1146/annurev-matsci-071312-121634. IF: 16.179
Local properties of the surface layer(s) of BiFeO3 single crystals
Domingo, N.; Narvaez, J.; Alexe, M.; Catalan, G. Journal of Applied Physics; 113 2013. 10.1063/1.4801974. IF: 2.210
Thickness scaling of ferroelastic domains in PbTiO3 films on DyScO3
Nesterov, O.; Matzen, S.; Magen, C.; Vlooswijk, A.H.G.; Catalan, G.; Noheda, B. Applied Physics Letters; 103 2013. 10.1063/1.4823536. IF: 3.794

2012

Domain wall nanoelectronics
Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J.F. Reviews of Modern Physics; 84: 119 - 156. 2012. 10.1103/RevModPhys.84.119.
Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg 1/3Nb 2/3)O 3: I. Strain analysis and a static order parameter
Carpenter, M.A.; Bryson, J.F.J.; Catalan, G.; Howard, C.J. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045901.
Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg 1/3Nb 2/3)O 3: II. Strainorder parameter coupling and dynamic softening mechanisms
Carpenter, M.A.; Bryson, J.F.J.; Catalan, G.; Zhang, S.J.; Donnelly, N.J. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045902.
Magnetotransport at domain walls in BiFeO 3
He, Q.; Yeh, C.-H.; Yang, J.-C.; Singh-Bhalla, G.; Liang, C.-W.; Chiu, P.-W.; Catalan, G.; Martin, L.W.; Chu, Y.-H.; Scott, J.F.; Ramesh, R. Physical Review Letters; 108 2012. 10.1103/PhysRevLett.108.067203.
Mechanical Writing of Ferroelectric Polarization
Lu, H. ; Bark, C.W.; Esque de los Ojos, D.; Alcala, J. ; Eom, C. B.; Catalan, G. ; Gruverman, A. SCIENCE; 336 (6077): 59 - 61. 2012. DOI: 10.1126/science.1218693.
Structural, spectroscopic, magnetic and electrical characterization of Ca-doped polycrystalline bismuth ferrite, Bi 1xCa xFeO 3x/2 (x0.1)
Sardar, K.; Hong, J.; Catalan, G.; Biswas, P.K.; Lees, M.R.; Walton, R.I.; Scott, J.F.; Redfern, S.A.T. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/4/045905.
Surface phase transitions in BiFeO 3 below room temperature
Jarrier, R.; Marti, X.; Herrero-Albillos, J.; Ferrer, P.; Haumont, R.; Gemeiner, P.; Geneste, G.; Berthet, P.; Schülli, T.; Cevc, P.; Blinc, R.; Wong, S.S.; Park, T.-J.; Alexe, M.; Carpenter, M.A.; Scott, J.F.; Catalan, G.; Dkhil, B. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.184104.

2011

Flexoelectric rotation of polarization in ferroelectric thin films.
Catalan, G. ; Lubk, A.; Vlooswijk, A. H. G.; Snoeck, E.; Magen, C. ; Janssens, A.; Rispens, G.; Rijnders, G.; Blank, D. H. A.; Noheda, B. Nature Materials; 10: 963. 2011. .
Skin layer of BiFeO3 single crystals
Martí, X.; Ferrer, P.; Herrero-Albillos, J.; Narvaez, J.; Holy, V.; Barrett, N.; Alexe, M.; Catalan, G. Physical Review Letters; 106 2011. 10.1103/PhysRevLett.106.236101.

2010

Comment on "Nanometer resolution piezoresponse force microscopy to study deep submicron ferroelectric and ferroelastic domains" [Appl. Phys. Lett. 94, 162903 (2009)]----NO ES ARTICULO
Vlooswijk, A.H.G.; Catalan, G.; Noheda, B. Applied Physics Letters; 97 2010. 10.1063/1.3467005.
Electric-field control of the metal-insulator transition in ultrathin NdNiO3 films
Scherwitzl, R.; Zubko, P.; Lezama, I.G.; Ono, S.; Morpurgo, A.F.; Catalan, G.; Triscone, J.-M. Advanced Materials; 22: 5517 - 5520. 2010. 10.1002/adma.201003241.
Landau theory of domain wall magnetoelectricity
Daraktchiev, M.; Catalan, G.; Scott, J.F. Physical Review B - Condensed Matter and Materials Physics; 81 2010. 10.1103/PhysRevB.81.224118.
Neutron diffraction study of the BiFeO3 spin cycloid at low temperature
Herrero-Albillos, J.; Catalan, G.; Rodriguez-Velamazan, J.A.; Viret, M.; Colson, D.; Scott, J.F. Journal of Physics Condensed Matter; 22: 256001 - 256005. 2010. 10.1088/0953-8984/22/25/256001.
The β-to-γ transition in BiFeO 3: A powder neutron diffraction study
Arnold, D.C.; Knight, K.S.; Catalan, G.; Redfern, S.A.T.; Scott, J.F.; Lightfoot, P.; Morrison, F.D. Advanced Functional Materials; 20: 2116 - 2123. 2010. 10.1002/adfm.201000118.
The flexoelectricity of barium and strontium titanates from first principles
Hong, J.; Catalan, G.; Scott, J.F.; Artacho, E. Journal of Physics Condensed Matter; 22 2010. 10.1088/0953-8984/22/11/112201.

2009

Domains in Ferroelectric Nanodots
A. Schilling; D. Byrne; G. Catalan; K. G. Webber; Y. A. Genenko; G. S. Wu; J. F. Scott; J. M. Gregg Nano Letters; 9: 3359. 2009. .
Effect of chemical substitution on the Néel temperature of multiferroic Bi1¿xCaxFeO3
G. Catalan; K. Sardar; N. S. Church; J. F. Scott; R. J. Harrison; S. A. T. Redfern Physical Review B; 79: 212415. 2009. 10.1103/PhysRevB.79.212415.
Effect of wall thickness on the ferroelastic domain size of BaTiO3
G. Catalan; I. Lukyanchuk; A. Schilling; J. M. Gregg; J. F. Scott Journal of Materials Science; 44: 5307. 2009. 10.1007/s10853-009-3554-0.
Elastic and electrical anomalies at low-temperature phase transitions in BiFeO3
S. A. T. Redfern; Can Wang; J. W. Hong; G. Catalan; J. F. Scott Journal of Physics Condensed Matter; 20: 452205. 2009. 10.1088/0953-8984/20/45/452205.
Epitaxial TbMnO3 thin films on SrTiO3 substrates: a structural study
C. J. M. Daumont; D. Mannix; S. Venkatesan; G. Catalan; D. Rubi; B. J. Kooi; J. Th. M. De Hosson; B. Noheda Journal of Physics Condensed Matter; 21: 182001. 2009. 10.1088/0953-8984/21/18/182001.
Origin of ferroelastic domains in free-standing single-crystal ferroelectric films
I. A. Luk¿yanchuk; A. Schilling; J.M. Gregg; G. Catalan; J.F. Scott Physical Review B; 79: 144111. 2009. 10.1103/PhysRevB.79.144111.
Physics and applications of BiFeO3
G. Catalan; J. F. Scott Advanced Materials; 21: 2463 - 2485. 2009. 10.1002/adma.200802849.

2008

Conduction at domain walls in oxide multiferroics
J. Seidel; L. W. Martin; Q. He; Q. Zhan; Y.-H. Chu; A. Rother; M. E. Hawkridge; P. Maksymovych; P. Yu; M. Gajek; N. Balke; S. V. Kalinin; S. Gemming; H. Lichte; F. Wang; G. Catalan; J. F. Scott; N. A. Spaldin; J. Orenstein; R. Ramesh Nature Materials; 8: 229 - 234. 2008. 10.1038/nmat2373.
Conformal oxide coating of carbon nanotubes
S. Kawasaki; G. Catalan; H. J. Fan; M. M. Saad; J. M. Gregg; M. A. Correa-Duarte; J. Rybczynski; F. D. Morrison; T. Tatsuta; O. Tsuji; and J. F. Scott Applied Physics Letters; 92 (5): 53109. 2008. 10.1063/1.2841710.
Fractal walls and domain size scaling in thin films of multiferroic BiFeO3
G. Catalan; H. Béa; S. Fusil; M. Bibes; P. Paruch; A. Barthélémy; and J. F. Scott Physical Review Letters; 100: 27602. 2008. 10.1103/PhysRevLett.100.027602.
Landau Theory of Multiferroic Domain Walls
M. Daraktchiev; G. Catalan; J. F. Scott Ferroelectrics; 375: 122 - 131. 2008. 10.1080/00150190802437969.
Progress in perovskite nickelate research
G. Catalan Phase Transitions; 81: 729. 2008. 10.1080/01411590801992463.
ß phase and ¿-ß metal-insulator transition in multiferroic BiFeO3
R. Palai; R. S. Katiyar; H. Schmid; P. Tissot; S. J. Clark; J. Robertson; S. A. Redfern; G. Catalan; J. F. Scott Physical Review B - Condensed Matter and Materials Physics; 77: 14110. 2008. 10.1103/PhysRevB.77.014110.
Solution-process coating of vertical ZnO nanowires with ferroelectrics
S. Kawasaki; H. J. Fan; G. Catalan; F. D. Morrison; T. Tatsuta; O. Tsuji; J. F Scott Nanotechnology; 19: 375302. 2008. 10.1088/0957-4484/19/37/375302.