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It has been recently shown [I. Errea, B. Rousseau, and A. Bergara, Phys. Rev. Lett. 106, 165501 (2011)] that the phonons of the high-pressure simple cubic phase of calcium are stabilized by strong quantum anharmonic effects. This was obtained by a fully ab initio implementation of the self-consistent harmonic approximation including explicitly anharmonic coefficients up to fourth order. The renormalized anharmonic phonons make possible to estimate the superconducting transition temperature in this system, and a sharp agreement with experiments is found. In this work, this analysis is extended in order to study the effect of anharmonicity in the isotope effect. According to our calculations, despite the huge anharmonicity in the system, the isotope coefficient is predicted to be 0.45, close to the 0.5 value expected for a harmonic BCSsuperconductor. ; I.E. is grateful to the Department of Education, Universities and Research of the Basque Government for financial support. The authors acknowledge financial support from UPV/EHU (Grant No. IT-366-07) and the Spanish Ministry of Science and Innovation (Grant No. FIS2010-19609-C02-00). ; Peer reviewed

4 páginas, 3 figuras.-- PACS numbers: 63.20.kg, 63.20.dk, 63.20.Ry, 74.25.Kc ; The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting Tc in agreement with experiments and predict an anomalous behavior of the specific heat. ; The authors are grateful to Universities and Research of the Basque Government, UPV/EHU (Grant No. IT-366-07) and the Spanish Ministry of Science and Innovation (Grant No. FIS2010-19609-C02-00) for financial support. ; Peer reviewed

5 páginas, 5 figuras.-- PACS number(s): 71.45.Gm. Trabajo presentado a "DIPC10 : Passion for Knowledge" celebrado en Donostia (España) del 27 de septiembre al 1 de octubre de 2010. ; Pressure strongly modifies electronic and optical properties of solids. In this work we report ab initio time-dependent density-functional theory calculations of the dielectric response of the high-pressure metallic phase of aluminum hydride (AlH3) within the random-phase approximation. Besides the conventional free-electronlike plasmon, which is highly damped, low-energy transitions between states near the Fermi level that appear in this metallized phase give rise to a low-energy undamped collective mode. This feature is expected to induce an abrupt edge in the experimentally measured reflectivity just below 1 eV and also affect electronic correlations close to the Fermi energy. Our work shows that AlH3 is basically a hydrogen sublattice weakly perturbed by Al atoms. ; This work was partially supported by the University of the Basque Country UPV/EHU and the Basque Government (Grants No. GIC07IT36607 and No. IT-366-07), as well as the Spanish Ministerio de Educación y Ciencia (Grant No. FIS2009-09631). Computing facilities were provided by the SGI/IZO-SGIker UPV/EHU (supported by the National Program for the Promotion of Human Resources within the National Plan of Scientific Research, Development and Innovation-Fondo Social Europeo, MCyT and the Basque Government) and by the Donostia International Physics Center (DIPC) foundation. ; Peer reviewed

Recent room temperature experiments on TiH2 [Kalita, J. Appl. Phys. 108, 043511 (2010)], an important compound in hydrogen storage research, revealed a cubic (fcc, Fm-3m) to tetragonal (bct, I4/mmm) phase transition at around 0.6 GPa, which was suggested to remain stable up to at least 90 GPa. However, the simulated X-ray diffraction (XRD) pattern of the I4/mmm structure cannot explain all the diffraction peaks observed at 90 GPa. In this article, we apply the recently developed particle swarm optimization algorithm for crystal structure prediction to propose that at 63 GPa TiH2 presents a further phase transition from I4/mmm to P4/nmm, which we have also confirmed is dynamically and enthalpically stable up to 294 GPa. Moreover, the XRD patterns and calculated lattice parameters of the P4/nmm structure are in good agreement with the experimental available data. Above 294 GPa, we predict a monoclinic P21/m structure to be stable. © 2013 American Institute of Physics. ; We acknowledge funding supports from the National Natural Science Foundation of China (under Grant Nos. 91022029 and 11025418), the research fund of Key Laboratory of Surface Physics and Chemistry (No. SPC201103), and the China 973 Program under Grant No. 2011CB808200. A.B. acknowledges the Department of Education, Universities and Research of the Basque Government, UPV/EHU (Grant No. IT-366-07) and the Spanish Ministry of Science and Innovation (Grant No. FIS2010-19609-C02-00). ; Peer Reviewed

Here we present theoretical ab initio calculations based on time-dependent density-functional theory of the macroscopic dielectric function of calcium from 0 to 110 GPa in the fcc, bcc, and sc phases. All the dielectric functions are calculated using very fine reciprocal space meshes since both eigenvalues and matrix elements entering in the density-response functions are interpolated using Wannier functions. Our calculations correctly predict the energies of the low- and high-energy plasmons observed in the fcc phase at room pressure. Moreover, we predict that in the sc phase a very low-energy interband plasmon emerges at around 50 GPa that dramatically modifies the behavior of the reflectivity making it almost vanish at the plasmon energy. As a consequence, calcium becomes an example of how optical properties can become complex under pressure. © 2012 American Physical Society. ; The authors are grateful to the Department of Education, Universities and Research of the Basque Government, UPV/EHU (Grant No. IT-366-07) and the Spanish Ministry of Science and Innovation (Grant No. FIS2010-19609-C02-00) for financial support. ; Peer Reviewed

The recent claim of having produced metallic hydrogen in the laboratory relies on measurements of optical spectra. Here, we present first-principles calculations of the reflectivity of hydrogen between 400 and 600 GPa in the I41/amd crystal structure, the one predicted at these pressures, based on both time-dependent density functional and Eliashberg theories, thus, covering the optical properties from the infrared to the ultraviolet regimes. Our results show that atomic hydrogen displays an interband plasmon at around 6 eV that abruptly suppresses the reflectivity, while the large superconducting gap energy yields a sharp decrease of the reflectivity in the infrared region approximately at 120 meV. The experimentally estimated electronic scattering rates in the 0.7–3 eV range are in agreement with our theoretical estimations, which show that the huge electron-phonon interaction of the system dominates the electronic scattering in this energy range. The remarkable features in the optical spectra predicted here encourage extending the optical measurements to the infrared and ultraviolet regions as our results suggest optical measurements can potentially identify high-pressure phases of hydrogen. ; The authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (Grant No. FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (Grant No. IT756- 13). M. B. is also thankful to the Department of Education, Language Policy and Culture of the Basque Government for support (Grant No. PRE-2016-2-0134). ; Peer reviewed

We present first-principles calculations of metallic atomic hydrogen in the 400–600 GPa pressure range in a tetragonal structure with space group I41/amd, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic exchange and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approximation, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We attribute the small impact of anharmonicity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes. ; The authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (FIS2013-48286-C2-2-P), the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13), and French Agence Nationale de la Recherche (Grant No. ANR-13-IS10-0003-01). M.B. is also thankful to the Department of Education, Language Policy and Culture of the Basque Government for a predoctoral fellowship (Grant No. PRE-2014-1-477). Computer facilities were provided by the Donostia Internatinal Physics Center (DIPC). ; Peer reviewed

We present a detailed analysis of the spin-flip excitations induced by a periodic time-dependent electric field in the Rashba prototype Au(111) noble metal surface. Our calculations incorporate the full spinor structure of the spin-split surface states and employ a Wannier-based scheme for the spin-flip matrix elements. We find that the spin-flip excitations associated with the surface states exhibit an strong dependence on the electron momentum magnitude, a feature that is absent in the standard Rashba model. Furthermore, we demonstrate that the maximum of the calculated spin-flip absorption rate is about twice the model prediction. These results show that, although the Rashba model accurately describes the spectrum and spin polarization, it does not fully account for the dynamical properties of the surface states. © 2013 American Physical Society. ; The authors acknowledge financial support from UPV/EHU (Grant No. IT-366-07 and program UFI 11/55), the Spanish Mineco (Grants No. FIS2012-36673-C03-01 and No. FIS2009-12773-C02-01), and the Basque Government (Grant No. IT472-10). ; Peer Reviewed

et al. ; Results of computational investigations of the structural and electronic properties of the ground states of binary compounds LiB x with 0.67 ≤x≤1.00 under pressure are reported. Structure predictions based on evolutionary algorithms and particle swarm optimization reveal that with increasing pressure, stoichiometric 1:1-LiB undergoes a variety of phase transitions, is significantly stabilized with respect to the elements and takes up a diamondoid boron network at high pressures. The Zintl picture is very useful in understanding the evolution of structures with pressure. The experimentally seen finite range of stability for LiB x phases with 0.8≤x≤1.00 is modeled both by boron-deficient variants of the 1:1-LiB structure and lithium-enriched intercalation structures. We find that the finite stability range vanishes at pressures P≥40GPa, where stoichiometric compounds then become more stable. A metal-to-insulator transition for LiB is predicted at P = 70 GPa. © 2012 American Physical Society. ; A. Hermann, N. W. Ashcroft, and R. Hoffmann are grateful for support from EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy (Grant No. DESC0001057 at Cornell), and from the National Science Foundation (Grant No. CHE-0910623 and No.DMR-0907425). Computational resources provided by the Cornell NanoScale Facility (supported by the National Science Foundation through Grant No. ECS-0335765), the XSEDE network (provided by the National Center for Supercomputer Applications through Grant No. TG-DMR060055N), and the KAUST Supercomputing Laboratory (Project ID k128) are gratefully acknowledged. A. Bergara, I. G. Gurtubay, and A. Suarez-Alcubilla are grateful to the Department of Education, Universities and Research of the Basque Government, UPV/EHU (Grant No. IT-366-07) and the Spanish Ministry of Science and Innovation (Grants No. FIS2010-19609-C02-00 and No. FIS2009-09631) for Financial support. ; Peer Reviewed

We present a comprehensive theoretical investigation of the light absorption rate at a Pb/Ge(111)-β√3×√3R30° surface with strong spin-orbit coupling. Our calculations show that electron spin-flip transitions cause as much as 6% of the total light absorption, representing 1 order of magnitude enhancement over Rashba-like systems. Thus, we demonstrate that a substantial part of the light irradiating this nominally nonmagnetic surface is attenuated in spin-flip processes. Remarkably, the spin-flip transition probability is structured in well-defined hot spots within the Brillouin zone, where the electron spin experiences a sudden 90° rotation. This mechanism offers the possibility of an experimental approach to the spin-orbit phenomena by optical means. © 2012 American Physical Society. ; This work was supported by the UPV/EHU (Grant No. IT-366-07 and program UFI 11/55), the Spanish Ministry of Science and Innovation (Grants No. FIS2010-19609-C02-00 and No. FIS2009-12773-C02-01), and the Basque Government (Grant No. IT472-10). ; Peer Reviewed

This article is available under the terms of the Creative Commons Attribution 3.0 License. ; We present an ab initio analysis of a continuous Hamiltonian that maps into the celebrated Haldane model. The tunneling coefficients of the tight-binding model are computed by means of two independent methods—one based on the maximally localized Wannier functions, and the other through analytic expressions in terms of gauge-invariant properties of the spectrum—that provide a remarkable agreement and allow one to accurately reproduce the exact spectrum of the continuous Hamiltonian. By combining these results with the numerical calculation of the Chern number, we are able to draw the phase diagram in terms of the physical parameters of the microscopic model. Remarkably, we find that only a small fraction of the original phase diagram of the Haldane model can be accessed, and that the topological insulator phase is suppressed in the deep tight-binding regime. ; This work has been supported by the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea under Program No. UFI 11/55, the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13), the Ministerio de Economía y Competitividad through Grants No. FIS2013-48286-C2-1-P, No. FIS2013-48286-C2-2-P and No. FIS2012-36673-C03-03, and the Basque Government through Grant No. IT-472-10. J.I.A. would like to acknowledge support from the Helmholtz Gemeinschaft Deutscher-Young Investigators Group Program No. VH-NG-717 (Functional Nanoscale Structure and Probe Simulation Laboratory) and from the Impuls und Vernetzungsfonds der Helmholtz-Gemeinschaft Postdoc Programme. ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; We present two independent approaches for calculating the tight-binding parameters of the Haldane model with ultracold atoms. The tunneling coefficients up to next-nearest neighbors are computed ab initio by using the maximally localized Wannier functions and compared to analytical expressions written in terms of gauge-invariant measurable properties of the spectrum. The two approaches present a remarkable agreement and evidence the breakdown of the Peierls substitution: (i) the phase acquired by the next-nearest tunneling amplitude t1 presents quantitative and qualitative differences with respect to that obtained by the integral of the vector field A, as considered in the Peierls substitution, even in the regime of low amplitudes of A; and (ii) for larger values, also |t1| and the nearest-neighbor tunneling t0 have a marked dependence on A. The origin of this behavior and its implications are discussed. ; This work has been supported by the Universidad del Pais Vasco/Euskal Herriko Unibertsitatea under Programs No. UFI 11/55 and No. UFI IT-366-07, the Ministerio de Economía y Competitividad through Grants No. FIS2010-19609-C02-00 and No. FIS2012-36673-C03-03, and the Basque Government through Grant No. IT-472-10. J.I.A. would like to acknowledge support from the Helmholtz Gemeinschaft Deutscher-Young Investigators Group Program No. VH-NG-717 (Functional Nanoscale Structure and Probe Simulation Laboratory). ; Peer Reviewed

We discuss how to construct tight-binding models for ultracold atoms in honeycomb potentials, by means of the maximally localized Wannier functions (MLWFs) for composite bands introduced by Marzari and Vanderbilt. In particular, we work out the model with up to third-nearest neighbors, and provide explicit calculations of the MLWFs and of the tunneling coefficients for the graphenelike potential with two degenerate minima per unit cell. Finally, we discuss the degree of accuracy in reproducing the exact Bloch spectrum of different tight-binding approximations, in a range of typical experimental parameters. © 2013 American Physical Society. ; This work has been supported by the UPV/EHU under programs UFI 11/55 and IT-366-07, the Spanish Ministry of Science and Innovation through Grant No. FIS2010-19609-C02-00, and the Basque Government through Grant No. IT-472-10. ; Peer Reviewed

We present an accurate ab initio tight-binding model, capable of describing the dynamics of Dirac points in tunable honeycomb optical lattices following a recent experimental realization. Our scheme is based on first-principle maximally localized Wannier functions for composite bands. The tunneling coefficients are calculated for different lattice configurations, and the spectrum properties are well reproduced with high accuracy. In particular, we show which tight-binding description is needed in order to accurately reproduce the position of Dirac points and the dispersion law close to their merging, for different laser intensities. © 2013 American Physical Society. ; This work has been supported by the UPV/EHU under programs UFI 11/55 and IT-366-07, the Spanish Ministry of Science and Innovation through Grants No. FIS2010-19609-C02-00 and No. FIS2012-36673-C03-03, and the Basque Government through Grant No. IT-472-10. ; Peer Reviewed

Pressure has become an effective way to obtain new materials with desirable properties. Considering the various stoichiometries, wide applications, and unique structures of binary H-S, H-B, and B-S compounds, especially at high pressures, it is worth expecting that they might form ternary compounds with interesting properties as well. Here, phase stabilities of H-B-S ternary compounds, in the pressure range from 0 to 200 GPa, are reliably determined through first-principles unbiased structural search calculations. A hitherto unknown orthorhombic HBS compound with Ama2 symmetry is identified to be stable above 25 GPa, in which B and S atoms alternate to form distorted chains and H atoms are covalently bonded with Bs. With pressure, Ama2 HBS undergoes a semiconductor-to-metal electronic transition and even becomes superconducting. The exploration of the binary B-S phase diagram indicates that B-S compounds tend to decompose into elemental solids above 208 GPa. However, two S-rich phases, B2S3 and BS2, are predicted at low pressures, exhibiting semiconducting and metallic properties, respectively. BS2 shows a calculated Tc value of 21.9 K at 200 GPa, becoming a superconductor among bulk binary B-S compounds. ; The authors acknowledge funding support from the Natural Science Foundation of China under Grants No. 21573037, No. 21873017, No. 11704062, and No. 51732003, the Postdoctoral Science Foundation of China under Grant No. 2013M541283, the Natural Science Foundation of Jilin Province (Grant No. 20190201231JC), the "111" Project (No. B13013), and the Fundamental Research Funds for the Central Universities (2412017QD006). The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).