arXiv subjects

Felix Yndurain

Publications and source records attributed to Felix Yndurain.

13 recordsLinked to original sources

arXiv2020

Critical analysis of the response function in low dimensional materials

The presence of sharp peaks in the real part of the static dielectric response function are usually accepted as indication of charge or spin instabilities in a material. However, there are misconceptions that Fermi surface (FS) nesting guarantees a peak in the response function like in one-dimensional systems, and, in addition, response function matrix elements between empty and occupied states are usually considered of secondary importance and typically set to unity like in the free electron gas case. In this work, we explicitly show, through model systems and real materials, within the framework of density functional theory, that predictions about the peaks in the response function, using FS nesting and constant matrix elements yields erroneous conclusions. We find that the inclusion of the matrix elements completely alters the structure of the response function. In all the cases studied other than the one-dimensional case we find that the inclusion of matrix elements washes out the structure found with constant matrix elements. Our conclusion is that it is imperative to calculate the full response function, with matrix elements, when making predictions about instabilities in novel materials.

cond-mat.mtrl-sci↗

arXiv2019

Band Unfolding Made Simple

We present a simple view on band unfolding of the energy bands obtained from supercell calculations. It relies on the relationship between the local density of states in reciprocal space (qLDOS) and the fully unfolded band structure. This provides an intuitive and valid approach not only for periodic, but also for systems with no translational symmetry. By refolding into the primitive Brillouin zone of the pristine crystal we recover the conventional unfolded bands. We implement our algorithm in the SIESTA package and apply it to defects on Si and graphene.

cond-mat.mtrl-sci↗

arXiv2018

Pressure induced magnetism in rotated graphene bilayers

Using ab initio methods based on the density functional theory we show that rotated graphene bilayers at angles different from the magic ones can have an electronic spectrum similar to those by applying moderate external pressures. We find that for an angle of $5.08^{\circ}$ and a pressure of 2.19 GPa the spin restricted spectrum displays a flat band at the Fermi level similar to the one found at magic angles. In addition, the spin unrestricted calculations show a correlated ferromagnetic ground state with a total magnetic moment of 3.7 $μ_{B}$ per unit cell being mostly localized in the AA stacking region of the Moiré pattern. The possibility of antiferromagnetic order is considered but not calculated. Doping the system destroys the magnetic moments. The plausibility of BCS superconductivity in the doped system is analyzed.

cond-mat.mes-hall↗

arXiv2017

Selective Hydrogen Adsoprtion in Graphene Rotated Bilayers

The absorption energy of atomic hydrogen at rotated graphene bilayers is studied using ab initio methods based on the density functional theory including van der Waals interactions. We find that, due to the surface corrugation induced by the underneath rotated layer and the perturbation of the electronic density of states near the Fermi energy, the atoms with an almost AA stacking are the preferential ones for hydrogen chemisorption. The adsorption energy difference between different atoms can be as large as 80 meV. In addition, we find that, due to the logarithmic van Hove singularities in the electronic density of states at energies close to the Dirac point, the adsorption energy of either electron or hole doped samples is substantially increased. We also find that the adsorption energy increases with the decrease of the rotated angle between the layers. Finally, the large zero point energy of the C-H bond (${\sim 0.3 eV}$) suggests adsorption and desorption of atomic hydrogen and deuterium should behave differently.

cond-mat.mes-hall↗

arXiv2011

Ab initio energetics and kinetics study of H_2 and CH_4 in the SI Clathrate Hydrate

We present ab initio results at the density functional theory level for the energetics and kinetics of H_2 and CH_4 in the SI clathrate hydrate. Our results complement a recent article by some of the authors [G. Roman-Perez et al., Phys. Rev. Lett. 105, 145901 (2010)] in that we show additional results of the energy landscape of H_2 and CH_4 in the various cages of the host material, as well as further results for energy barriers for all possible diffusion paths of H_2 and CH_4 through the water framework. We also report structural data of the low-pressure phase SI and the higher-pressure phases SII and SH.

cond-mat.mtrl-sci↗

arXiv2010

Electron-phonon interaction in Fe-based superconductors: Coupling of magnetic moments with phonons in LaFeAsO$_{1-x}$F$_{x}$

The coupling of Fe magnetic moments in LaFeAsO$_{1-x}$F$_{x}$ with the As $A_{1g}$ phonon is calculated. We present first principles calculations of the atomic and electronic structure of LaFeAsO as a function of electron doping. We perform calculations using the virtual crystal approximation as well as supercell calculations with F substitutional impurity atoms. The results validate the virtual crystal approximation for the electronic structure near the Fermi level. Its is found that the electronic density of states at the Fermi level is maximum for x=0.125, enhancing the electron-phonon interaction. An additional increase of the electron-phonon parameter $λ$ is obtained if the coupling between the $A_{1g}$ phonon and the Fe magnetic moment is included. It is found that the electron-phonon interaction can be one order of magnitude larger than its value if no spin resolution is included in the calculation. The implications of these results on the superconducting transition are discussed

cond-mat.supr-con↗

arXiv2010

Stability, Adsorption and Diffusion of CH4, CO2 and H2 in Clathrate Hydrates

We present a study of the adsorption and diffusion of CH4, CO2 and H2 molecules in clathrate hydrates using ab initio van der Waals density functional formalism [Dion et al. Phys. Rev. Lett. 92, 246401 (2004)]. We find that the adsorption energy is dominated by van der Waals interactions and that, without them, gas hydrates would not be stable. We calculate the maximum adsorption capacity as well as the maximum hydrocarbon size that can be adsorbed.The relaxation of the host lattice is essential for a good description of the diffusion activation energies, which are estimated to be of the order of 0.2, 0.4, and 1.0 eV for H2, CO2, and CH4, respectively.

cond-mat.mtrl-sci↗

arXiv2008

Anomalous electron-phonon interaction in doped LaOFeAs: a First Principles calculation

We present first principles calculations of the atomic and electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the $A_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular for Fe-Fe direct interactions, thus avoiding frustration. This coupling of magnetism with vibrations induces anharmonicities and an electron-phonon interaction much larger than in the pure paramagnetic case. We propose that such enhanced interactions play an essential role in superconducting compounds close to an atiferromagnetic phase transition.

cond-mat.supr-con↗

arXiv2007

Superconductivity and magnetic order in CeRhIn$_{5}$; spectra of coexistence

We discuss the fixed-point Hamiltonian and the spectrum of excitations of a quasi-bidimensional electronic system supporting simultaneously antiferromamagnetic ordering and superconductivity. The coexistence of these two order parameters in a single phase is possible because the magnetic order is linked to the formation of a spin density wave, and its order parameter is not associated to a spectral gap but to an energy shift of the paramagnetic bands. This peculiarity entails several distinct features in the phase diagram and the spectral properties of the model, which may have been observed in CeRhIn$_5$. Apart from the coexistence, we find an abrupt suppression of the spin density wave when the superconducting and magnetic ordering temperatures are equal. The divergence of the cyclotron mass extracted from de Haas-van Alphen experiments is also analyzed in the same framework.

cond-mat.str-el↗

arXiv2006

Magnetism of two-dimensional defects in Pd: stacking faults, twin boundaries and surfaces

Careful first-principles density functional calculations reveal the importance of hexagonal versus cubic stacking of closed packed planes of Pd as far as local magnetic properties are concerned. We find that, contrary to the stable face centered cubic phase, which is paramagnetic, the hexagonal close-packed phase of Pd is ferromagnetic with a magnetic moment of 0.35 $μ_{B}$/atom. Our results show that two-dimensional defects with local hcp stacking, like twin boundaries and stacking faults, in the otherwise fcc Pd structure, increase the magnetic susceptibility. The (111) surface also increases the magnetic susceptibility and it becomes ferromagnetic in combination with an individual stacking fault or twin boundary close to it. On the contrary, we find that the (100) surface decreases the tendency to ferromagnetism. The results are consistent with the magnetic moment recently observed in small Pd nanoparticles, with a large surface area and a high concentration of two-dimensional stacking defects.

cond-mat.mtrl-sci↗

arXiv1996

Low-energy quantum dynamics of atoms at defects. Interstitial oxygen in silicon

The problem of the low-energy highly-anharmonic quantum dynamics of isolated impurities in solids is addressed by using path-integral Monte Carlo simulations. Interstitial oxygen in silicon is studied as a prototypical example showing such a behavior. The assignment of a "geometry" to the defect is discussed. Depending on the potential (or on the impurity mass), there is a "classical" regime, where the maximum probability-density for the oxygen nucleus is at the potential minimum. There is another regime, associated to highly anharmonic potentials, where this is not the case. Both regimes are separated by a sharp transition. Also, the decoupling of the many-nuclei problem into a one-body Hamiltonian to describe the low-energy dynamics is studied. The adiabatic potential obtained from the relaxation of all the other degrees of freedom at each value of the coordinate associated to the low-energy motion, gives the best approximation to the full many-nuclei problem.

cond-mat↗

arXiv1995

Theory of interstitial oxygen in silicon and germanium

The interstitial oxygen centers in silicon and germanium are reconsidered and compared in an analysis based on the first-principles total-energy determination of the potential-energy surface of the centers, and a calculation of their respective low energy excitations and infrared absorption spectra. The total-energy calculations reveal unambiguously that interstitial oxygen is quantum delocalized, the delocalization being essentially different in silicon and in germanium. Oxygen in silicon lies at the bond center site in a highly anharmonic potential well, whereas in germanium it is found to rotate almost freely around the original Ge-Ge bond it breaks. This different delocalization is the origin of the important differences in the low energy excitation spectra: there is a clear decoupling in rotation and vibration excitations in germanium, giving different energy scales (1 cm$^{-1}$ for the rotation, 200 cm$^{-1}$ for the $ν_2$ mode), whereas both motions are non-trivially mixed in silicon, in a common energy scale of around 30 cm$^{-1}$. The calculation of the vibrational spectra of the defect reveals the existence of vibrational modes (related to the $ν_1$ mode) never been experimentally observed due to their weak infrared activity. It is found that the combination of these modes with the well established $ν_3$ asymmetric stretching ones is the origin of the experimentally well characterized modes at frequencies above the $ν_3$ mode frequency.

cond-mat↗

arXiv1994

Geometry and quantum delocalization of interstitial oxygen in silicon

The problem of the geometry of interstitial oxygen in silicon is settled by proper consideration of the quantum delocalization of the oxygen atom around the bond-center position. The calculated infrared absorption spectrum accounts for the 517 and 1136 cm$^{-1}$ bands in their position, character, and isotope shifts. The asymmetric lineshape of the 517 cm$^{-1}$ peak is also well reproduced. A new, non-infrared-active, symmetric-stretching mode is found at 596 cm$^{-1}$. First-principles calculations are presented supporting the nontrivial quantum delocalization of the oxygen atom.

cond-mat↗