Staff directory Enric Canadell Casanova

Enric Canadell Casanova

Visiting Senior Researcher
Institut de Ciència de Materials de Barcelona (ICMAB)
Theory and Simulation



  • 2 × 2 charge density wave in single-layer TiTe2

    Guster B., Robles R., Pruneda M., Canadell E., Ordejón P. 2D Materials; 6 (1, 015027) 2019. 10.1088/2053-1583/aaf20b.

    A density functional theory study concerning the origin of the recently reported 2 × 2 charge density wave (CDW) instability in single-layer TiTe2 is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a 2 × 2 distortion. The study suggests that the magnitude of the semimetallic overlap between the valence band top at - and the conduction band bottom at M is a key factor controlling the tendency towards the distortion. It is also shown that tensile strain stabilizes a 2 × 2 CDW, and we suggest that this fact could be further used to induce the instability in doublelayers of TiTe2, which in the absence of strain remain undistorted in the experiment. The driving force for the CDW instability seems to be the same phonon mediated mechanism acting for singlelayer TiSe2, although in single-layer TiTe2 the driving force is smaller, and the semimetallic character is kept below the transition temperature. © 2018 IOP Publishing Ltd.

  • Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe 2

    Guster B., Rubio-Verdú C., Robles R., Zaldívar J., Dreher P., Pruneda M., Silva-Guillén J.Á., Choi D.-J., Pascual J.I., Ugeda M.M., Ordejón P., Canadell E. Nano Letters; 19 (5): 3027 - 3032. 2019. 10.1021/acs.nanolett.9b00268.

    Bulk and single-layer 2H-NbSe 2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2H-NbSe 2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2H-NbSe 2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2H-NbSe 2 may provide further understanding of its low temperature state and the competition between different instabilities. © 2019 American Chemical Society.

  • Polarization dependence of angle-resolved photoemission with submicron spatial resolution reveals emerging one-dimensionality of electrons in NbSe3

    Valbuena M.A., Chudzinski P., Pons S., Conejeros S., Alemany P., Canadell E., Berger H., Frantzeskakis E., Avila J., Asensio M.C., Giamarchi T., Grioni M. Physical Review B; 99 (7, 075118) 2019. 10.1103/PhysRevB.99.075118.

    In materials with nearly commensurate band filling the electron liquid may spontaneously separate into components with distinct properties, yielding complex intra-and interunit cell ordering patterns and a reduced dimensionality. Polarization-dependent angle-resolved photoemission data with submicron spatial resolution demonstrate such an electronic self-organization in NbSe3, a compound considered to be a paradigm of charge order. The new data indicate the emergence of a novel order, and reveal the one-dimensional (1D) physics hidden in a material which naively could be considered the most three dimensional of all columnar chalcogenides. The 1D physics is evidenced by a new selection rule-in two polarizations we observe two strikingly different dispersions each closely resembling apparently contradicting results of previous studies of this material. © 2019 American Physical Society.


  • First principles analysis of the CDW instability of single-layer 1T-TiSe2 and its evolution with charge carrier density

    Guster B., Canadell E., Pruneda M., Ordejón P. 2D Materials; 5 (2, 025024) 2018. 10.1088/2053-1583/aab568. IF: 7.042

    We present a density functional theory study of the electronic structure of single-layer TiSe2, and focus on the charge density wave (CDW) instability present on this 2D material. We explain the periodicity of the CDW from the phonon band structure of the undistorted crystal, which is unstable under one of the phonon modes at the M point. This can be understood in terms of a partial band gap opening at the Fermi level, which we describe on the basis of the symmetry of the involved crystal orbitals, leading to an energy gain upon the displacement of the atoms following the phonon mode in a 2 × 1 structure. Furthermore, the combination of the corresponding phonons for the three inequivalent M points of the Brillouin zone leads to the 2 × 2 distortion characteristic of the CDW state. This leads to a further opening of a full gap, which reduces the energy of the 2 × 2 structure compared to the 2 × 1 one of a single M point phonon, and makes the CDW structure the most stable one. We also analyze the effect of charge injection into the layer on the structural instability. We predict that the 2 × 2 structure only survives for a certain range of doping levels, both for electrons and for holes, as doping reduces the energy gain due to the gap opening. We predict the transition from the commensurate 2 × 2 distortion to an incommensurate one with increasing wavelength upon increasing the doping level, followed by the appearance of the undistorted 1 × 1 structure for larger carrier concentrations. © 2018 IOP Publishing Ltd.


  • Anisotropic features in the electronic structure of the two-dimensional transition metal trichalcogenide TiS3: Electron doping and plasmons

    Silva-Guillén J.A., Canadell E., Ordejón P., Guinea F., Roldán R. 2D Materials; 4 (2, 025085) 2017. 10.1088/2053-1583/aa6b92. IF: 6.937

    Analysis of the band structure of TiS3 single-layers suggests the possibility of changing their physical behaviour by injecting electron carriers. The anisotropy of the valence and conduction bands is explained in terms of their complex orbital composition. The nature of the Fermi surface and Lindhard response function for different doping concentrations is studied by means of firstprinciples DFT calculations. It is suggested that for electron doping levels x (number of electrons per unit cell) ~0.18-0.30e- the system could exhibit incommensurate charge or spin modulations which, however, would keep the metallic state whereas systems doped with smaller x would be 2D metals without any electronic instability. The effect of spin-orbit coupling in the band dispersion is analysed. The DFT effective masses are used to study the plasmon spectrum from an effective low energy model. We find that this material supports highly anisotropic plasmons, with opposite anisotropy for the electron and hole bands.


  • Electronic structure of 2H-NbSe2 single-layers in the CDW state

    Silva-Guillén J.Á., Ordejón P., Guinea F., Canadell E. 2D Materials; 3 (3, 035028) 2016. 10.1088/2053-1583/3/3/035028. IF: 9.611

    Adensity functional theory study of NbSe2"Qsingle-layers in the normal non-modulated and the 3"Q¡Ñ"Q3 CDWstates is reported.Weshow that, in the single layer, the CDW barely affects the Fermi surface of the system, thus ruling out a nesting mechanism as the driving force for the modulation. The CDW stabilizes levels lying around 1.35 eV below the Fermi level within the Se-based valence band but having a substantial Nb-VNb bonding character. The absence of interlayer interactions leads to the suppression of the pancake-like portion of the bulk Fermi surface in the single-layer.Weperform scanning tunneling microscopy simulations and find that the images noticeably change with the sign and magnitude of the voltage bias. The atomic corrugation of the Se sublayer induced by the modulation plays a primary role in leading to these images, but the electronic reorganization also has an important contribution. The analysis of the variation of these images with the bias voltage does not support a Fermi surface nesting mechanism for the CDW. It is also shown that underlying graphene layers (present in some of the recent experimental work) do not modify the conduction band, but do affect the shape of the valence band of NbSe2"Qsingle-layers. The relevance of these results in understanding recent physical measurements for NbSe2"Qsingle-layers is discussed. © 2016 IOP Publishing Ltd.

  • Nanotexturing to Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap

    Brotons-Gisbert M., Andres-Penares D., Suh J., Hidalgo F., Abargues R., Rodríguez-Cantó P.J., Segura A., Cros A., Tobias G., Canadell E., Ordejón P., Wu J., Martínez-Pastor J.P., Sánchez-Royo J.F. Nano Letters; 16 (5): 3221 - 3229. 2016. 10.1021/acs.nanolett.6b00689. IF: 13.779

    Manipulating properties of matter at the nanoscale is the essence of nanotechnology, which has enabled the realization of quantum dots, nanotubes, metamaterials, and two-dimensional materials with tailored electronic and optical properties. Two-dimensional semiconductors have revealed promising perspectives in nanotechnology. However, the tunability of their physical properties is challenging for semiconductors studied until now. Here we show the ability of morphological manipulation strategies, such as nanotexturing or, at the limit, important surface roughness, to enhance light absorption and the luminescent response of atomically thin indium selenide nanosheets. Besides, quantum-size confinement effects make this two-dimensional semiconductor to exhibit one of the largest band gap tunability ranges observed in a two-dimensional semiconductor: from infrared, in bulk material, to visible wavelengths, at the single layer. These results are relevant for the design of new optoelectronic devices, including heterostructures of two-dimensional materials with optimized band gap functionalities and in-plane heterojunctions with minimal junction defect density. © 2016 American Chemical Society.


  • Quasiparticle spectra of 2H-NbSe2: Two-band superconductivity and the role of tunneling selectivity

    Noat Y., Silva-Guillén J.A., Cren T., Cherkez V., Brun C., Pons S., Debontridder F., Roditchev D., Sacks W., Cario L., Ordejón P., García A., Canadell E. Physical Review B - Condensed Matter and Materials Physics; 92 (13, 134510) 2015. 10.1103/PhysRevB.92.134510. IF: 3.736

    We have studied the superconducting state of 2H-NbSe2 by scanning tunneling spectroscopy along two different crystal orientations, the c and the a/b axes. Along the c axis a large gap is dominant in the spectra, while a smaller gap is measured along the a/b axis. We show that these spectra are accurately described by the McMillan model where the small gap is induced through the coupling to the band associated with the large gap. In order to assign the small and large gaps to specific parts of the 2H-NbSe2 Fermi surface, the electronic structure was studied using first-principles calculations. While we cannot exclude the possibility of intrinsic anisotropy of the gaps, we propose that the large gap opens in the Fermi surface cylinders located around the corner K points while the sheets located around Γ are associated with the small gap. An additional component of the Fermi surface, a selenium based pocket, plays an essential role in the tunneling process. The role of the charge density wave occurring in this material is also discussed. Finally, we are able to give a coherent description of the observed characteristics of the tunneling spectra of 2H-NbSe2 as well as the differences with 2H-NbS2 where no charge density wave state is present. Further experimental work, such as high-resolution ARPES, would be very useful to confirm our interpretation. The approach and modeling developed here could also be relevant for other compounds of the dichalcogenide family. © 2015 American Physical Society.

  • Tunneling and electronic structure of the two-gap superconductor MgB2

    Silva-Guillén J.A., Noat Y., Cren T., Sacks W., Canadell E., Ordejón P. Physical Review B - Condensed Matter and Materials Physics; 92 (6, 064514) 2015. 10.1103/PhysRevB.92.064514. IF: 3.736

    A combined experimental (superconductor-insulator-superconductor tunneling spectra) and theoretical (density functional theory) study of the two-gap superconductor MgB2 is reported. The calculations confirm that the small gap is associated with a π band mostly based on the boron pz orbitals leading to the three-dimensional band component of the Fermi surface. This channel almost completely dominates the tunneling images and spectra for c-axis-oriented samples and not the two-dimensional σ band. The origin of this effect is due to the faster decay of the electronic states associated with the boron px and py orbitals compared to those associated with the boron pz orbitals, together with the symmetry properties of the wave functions. The calculated tunneling channels and partial density of states for each band agree with the values deduced from precise fits of experimental tunneling spectra. The present approach provides a framework for the understanding of tunneling spectra and the nature of superconducting gaps of other multigap superconductors. © 2015 American Physical Society.


  • Density-wave instability in α- ( BEDT-TTF)2KHg(SCN) 4 studied by x-ray diffuse scattering and by first-principles calculations

    Foury-Leylekian P., Pouget J.-P., Lee Y.-J., Nieminen R.M., Ordejón P., Canadell E. Physical Review B - Condensed Matter and Materials Physics; 82 (13, 134116) 2010. 10.1103/PhysRevB.82.134116.

    α- (BEDT-TTF)2KHg(SCN)4 develops a density wave ground state below 8 K whose origin is still debated. Here we report a combined x-ray diffuse scattering and first-principles density functional theory study supporting the charge density wave (CDW) scenario. In particular, we observe a triply incommensurate anharmonic lattice modulation with intralayer wave vector components which coincide within experimental errors to the maximum of the calculated Lindhard response function. A detailed study of the structural aspects of the modulation shows that the CDW instability in α- ( BEDT-TTF) 2 KHg ( SCN) 4 is considerably more involved than those following a standard Peierls mechanism. We thus propose a microscopic mechanism where the CDW instability of the BEDT-TTF layer is triggered by the anion sublattice. Our mechanism also emphasizes the key role of the coupling of the BEDT-TTF and anion layers via the hydrogen bond network to set the global modulation. © 2010 The American Physical Society.