Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: application to platinum and palladium hydrides

Harmonic calculations based on density-functional theory are generally the method of choice for the description of phonon spectra of metals and insulators. The inclusion of anharmonic effects is, however, delicate as it relies on perturbation theory requiring a considerable amount of computer time, fast increasing with the cell size. Furthermore, perturbation theory breaks down when the harmonic solution is dynamically unstable or the anharmonic correction of the phonon energies is larger than the harmonic frequencies themselves.We present a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity at any temperature in the non-perturbative regime. The method is based on the minimization of the free energy with respect to a trial density matrix described by an arbitrary harmonic Hamiltonian. The minimization is performed with respect to all the free parameters in the trial harmonic Hamiltonian, namely, equilibrium positions, phonon frequencies and polarization vectors. The gradient of the free energy is calculated following a stochastic procedure. The method can be used to calculate thermodynamic properties, dynamical properties and anharmonic corrections to the Eliashberg function of the electron-phonon coupling. The scaling with the system size is greatly improved with respect to perturbation theory. The validity of the method is demonstrated in the strongly anharmonic palladium and platinum hydrides. In both cases we predict a strong anharmonic correction to the harmonic phonon spectra, far beyond the perturbative limit. In palladium hydrides we calculate thermodynamic properties beyond the quasiharmonic approximation, while in PtH we demonstrate that the high superconducting critical temperatures at 100 GPa predicted in previous calculations based on the harmonic approximation are strongly suppressed when anharmonic effects are included.Comment: 17 pages, 5 figure

Pairing in 4-component fermion systems: the bulk limit of SU(4)-symmetric Hamiltonians

Fermion systems with more than two components can exhibit pairing condensates of much more complex structure than the well-known single BCS condensate of spin-up and spin-down fermions. In the framework of the exactly solvable SO(8) Richardson-Gaudin model with SU(4)-symmetric Hamiltonians, we show that the BCS approximation remains valid in the thermodynamic limit of large systems for describing the ground state energy and the canonical and quasiparticle excitation gaps. Correlations beyond BCS pairing give rise to a spectrum of collective excitations, but these do not affect the bulk energy and quasiparticle gaps.Comment: 13 pages; 2 figures; 1 tabl

Second order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: theory and stochastic implementation

The self-consistent harmonic approximation is an effective harmonic theory to calculate the free energy of systems with strongly anharmonic atomic vibrations, and its stochastic implementation has proved to be an efficient method to study, from first-principles, the anharmonic properties of solids. The free energy as a function of average atomic positions (centroids) can be used to study quantum or thermal lattice instability. In particular the centroids are order parameters in second-order structural phase transitions such as, e.g., charge-density-waves or ferroelectric instabilities. According to Landau's theory, the knowledge of the second derivative of the free energy (i.e. the curvature) with respect to the centroids in a high-symmetry configuration allows the identification of the phase-transition and of the instability modes. In this work we derive the exact analytic formula for the second derivative of the free energy in the self-consistent harmonic approximation for a generic atomic configuration. The analytic derivative is expressed in terms of the atomic displacements and forces in a form that can be evaluated by a stochastic technique using importance sampling. Our approach is particularly suitable for applications based on first-principles density-functional-theory calculations, where the forces on atoms can be obtained with a negligible computational effort compared to total energy determination. Finally we propose a dynamical extension of the theory to calculate spectral properties of strongly anharmonic phonons, as probed by inelastic scattering processes. We illustrate our method with a numerical application on a toy model that mimics the ferroelectric transition in rock-salt crystals such as SnTe or GeTe

High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: structural and vibrational properties including quantum and anharmonic effects

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation (SSCHA). Within this approach, we determine the transition pressure by calculating the free energy Hessian. We find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Moreover, we observe a prominent isotope effect, as we estimate higher pressure transition for D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. However, for the vibrational spectra at zone center, accessible e.g. by infrared spectroscopy, the broadenings are very small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are well defined

Anharmonic phonon spectra of PbTe and SnTe in the self-consistent harmonic approximation

At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic structure. At low temperature, SnTe undergoes a ferroelectric transition with a critical temperature strongly dependent on the hole concentration, while PbTe is an incipient ferroelectric. By using the stochastic self-consistent harmonic approximation, we investigate the anharmonic phonon spectra and the occurrence of a ferroelectric transition in both systems. We find that vibrational spectra strongly depends on the approximation used for the exchange-correlation kernel in density functional theory. If gradient corrections and the theoretical volume are employed, then the calculation of the free energy Hessian leads to phonon spectra in good agreement with experimental data for both systems. In PbTe, we reproduce the transverse optical mode phonon satellite detected in inelastic neutron scattering and the crossing between the transverse optical and the longitudinal acoustic modes along the $\Gamma$X direction. In the case of SnTe, we describe the occurrence of a ferroelectric transition from the high temperature Fm$\overline{3}$m structure to the low temperature R3m one.Comment: 12 pages, 15 Picture

Anharmonic stabilization of the high-pressure simple cubic phase of calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approx- imation, shows imaginary branches that make it mechanically unstable. In this letter, the phonon spectrum is recalculated using density-functional theory (DFT) ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that 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.Comment: 5 pages, 3 figure

Preferencias por lugar de compra de carne vacuna y atributos de inocuidad

Las expectativas de los consumidores por atributos de calidad en los alimentos se han incrementado en las últimas décadas, en concordancia con el incremento en el nivel de ingreso de los países desarrollados y en vías de desarrollo. La provisión de información permite a las empresas señalizar atributos de calidad en los alimentos que comercializan y obtener un precio diferencial. El acceso a esta información permite a los consumidores seleccionar los alimentos con las características de calidad que prefieren. Para investigar la forma en que los consumidores domésticos infieren sobre la calidad, e inocuidad en particular, de los alimentos que se comercializan sin marca, se considera el caso de la carne vacuna. En nuestro país, los consumidores manifiestan preferencia por adquirirla en carnicerías, y en esos puntos de venta, el producto no posee marcas o etiquetas que exhiban información sobre su origen, su composición química. Este trabajo, de carácter exploratorio, tiene por objetivo indagar sobre los atributos del lugar de compra (carnicería), vinculados a la inocuidad, que los consumidores argentinos de carne valoran al momento de decidir la compra. Pese a que la carne se encuentra entre los alimentos que más preocupan a los individuos en relación con exposición a riesgos para la salud, al igual que la etapa de venta al por menor, los atributos que más evalúan y se recuerdan, son atributos de búsqueda con bajo costo de verificación: la percepción general de limpieza y la presencia de una persona que cobre con independencia de quien/ quienes manipulan la mercadería. Otras características que implican evaluar con mayor atención, como el uso de herramientas apropiadas y guantes, coexisten consumidores con especial preocupación al respecto y otros, incapaces de recordar esos "detalles". Este comportamiento permite suponer una alta sensibilidad al nivel de información que cada individuo posee.Consumer preference for food quality attributes have increased in the last decades, associated with the increase in the income level of developed and developing countries. The provision of information allows companies to signalize food quality attributes that sell and get a price differential. Access to this information allows consumers to select foods with the quality characteristics they prefer. To investigate the way that domestic consumers infer about the quality and safety, in particular, foods that are sold without brand, it considers the case of beef. In our country, consumers reveal a preference for acquiring beef in butcher's, and in these points of sale, the product has no signals or labels that display information about its origin or its chemical composition. This exploratory work aims to investigate how the argentine beef consumers value the attributes of the place of purchase (butcher's), linked to safety, when deciding the purchase. In spite of being the beef one of the foods that most concern to consumers in relation to exposure to health risks, as well as the stage of retail, the evaluating attributes and those attributes most remembered are those related to search inexpensive verification: the general perception of cleanliness and the presence of a person who independently collects the money and other person who handles merchandise. Other features that involve evaluating more closely, for example, the use of appropriate tools and gloves, consumers coexist with particular concern and others are unable to remember those "details". This behavior suggests a high sensitivity to the level of information that each individual possess.Fil: Berges, Miriam. Universidad Nacional de Mar del Plata. Facultad de Ciencias Económicas y Sociales; Argentina.Fil: Errea, Damián. Universidad Nacional de Mar del Plata. Facultad de Ciencias Económicas y Sociales; Argentina.Fil: Casellas, Karina. Instituto Nacional de Tecnología Agropecuaria (INTA). Instituto de Economía y Sociología; Argentina

Caracterización, adaptación y mejora de material vegetal para una fruticultura sostenible

Presentación del Grupo Consolidado A-12 del Instituto Universitario de Investigación Mixto Agroalimentario de Aragón - IA

Breached pairing in trapped three-color atomic Fermi gases

We introduce an exactly solvable model for trapped three-color atom gases. Applications to a cigar-shaped trapped cold fermions reveal a complex structure of breached pairing phases. We find two competing superfluid phases at weak and intermediate couplings, each one with two color pair condensates, that can be distinguished from density profile measurements.Comment: 4 pages, 5 figure