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Strategies for tuning the thermal conductivity of crystals by means of external fields are rare. Here, we predict the existence of large magnetophononic effects in materials that undergo antiferromagnetic (AFM) ↔ ferromagnetic (FM) phase transitions, which allow for the modulation of the lattice heat conductivity, κL, via the application of magnetic fields. Specifically, by using first-principles methods we predict a large and anomalous κL increase of ≈40% for the metamagnetic phase transition occurring in bulk FeRh near room temperature. The disclosed magnetophononic effects are caused by large anharmonic spin-phonon couplings, namely, significant differences in the phase space of allowed phonon-phonon collision processes taking place in the respective AFM and FM phases., We acknowledge financial support by MCIN/AEI/10.13039/501100011033 under Grant No. PID2020-119777GB-I00, the “Ramón y Cajal” fellowship RYC2018-024947-I, and the Severo Ochoa Centres of Excellence Program (CEX2019-000917-S), and by the Generalitat de Catalunya under Grant No. 2017 SGR 1506. Calculations were performed at the Centro de Supercomputación de Galicia (CESGA) within action FI-2021-1-0007 of the Red Española de Supercomputación (RES). We thank Michael Wolloch and Jesús Carrete for useful discussions., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Refrigeration systems based on the compression of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. In the vicinity of a phase transition, caloric materials present large thermal responses to external fields, which makes them promising for developing alternative solid-state cooling devices. Electrocaloric effects are particularly well suited for portable refrigeration applications; however, most electrocaloric materials operate best at nonambient temperatures or require the application of large electric fields. Here, we predict that modest electric fields can yield giant room-temperature electrocaloric effects in multiferroic ${\mathrm{BiCoO}}_{3}$ (BCO) thin films. Depending on the orientation of the applied field, the resulting electrocaloric effect is either direct (heating) or inverse (cooling), which may enable the design of enhanced refrigeration cycles. We show that spin-phonon couplings and phase competition are the underlying causes of the disclosed caloric phenomena. The dual electrocaloric response of BCO thin films can be effectively tuned by means of epitaxial strain, and we anticipate that other control strategies such as chemical substitution are also possible.

The effect of pressure on the structural, vibrational, and electronic properties of Mg-doped Cr bearing spinel $\mathrm{C}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{g}}_{0.5}\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ was studied up to 55 GPa at room-temperature using x-ray diffraction, Raman spectroscopy, electrical transport measurements, and ab initio calculations. We found that the ambient-pressure phase is cubic (spinel-type, $Fd\overline{3}m$) and underwent a pressure-induced structural transition to a tetragonal phase (space group $I\overline{4}m2$) above 28 GPa. The ab initio calculation confirmed this first-order phase transition. The resistivity of the sample decreased at low pressures with the existence of a low-pressure (LP) phase and started to increase with the emergence of a high-pressure (HP) phase. The temperature dependent resistivity experiments at different pressures illustrated the wide band gap semiconducting nature of both the LP and HP phases with different activation energies, suggesting a semiconductor-semiconductor transition at HP. No evidence of chemical decomposition or a semiconductor-metal transition was observed in our studies.

We present a first-principles computational study of solid $^{4}\mathrm{He}$ at $T=0$ K and pressures up to $\ensuremath{\sim}160$ GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schr\"odinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid $^{4}\mathrm{He}$ does not increase appreciably with compression for $P\ensuremath{\ge}85$ GPa. Also, we show that the Lindemann ratio in dense solid $^{4}\mathrm{He}$ amounts to $0.10$ almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed $^{4}\mathrm{He}$. In particular, we disclose huge discrepancies of at least $\ensuremath{\sim}50%$ in the calculated $^{4}\mathrm{He}$ kinetic energies using both the QH DFT and present DFT-DMC methods.

We study the elastic properties of incommensurate solid ${}^{4}$He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (${C}_{11}$, ${C}_{12}$, ${C}_{13}$, ${C}_{33}$, and ${C}_{44}$), longitudinal and transversal speeds of sound, and the $T=0$ Debye temperature of incommensurate and commensurate hcp ${}^{4}$He using the diffusion Monte Carlo method. Our results show that under compression, the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e., $P\ensuremath{\sim}25$ bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (${C}_{12}$ and ${C}_{13}$) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of ${}^{4}$He (${C}_{44}$) undergoes a small reduction.

We study the electrical properties of Pt/LaAlO3/SrTiO3 capacitors under the action of an external bias potential, using first-principles simulations performed at constrained electric displacement field. A complete set of band diagrams, together with the relevant electrical characteristics (capacitance and built-in fields), are determined as a function of LaAlO3 thickness and the applied potential.We find that the internal field in LaAlO3 monotonically decreases with increasing thickness; hence, the occurrence of spontaneous Zener tunneling is ruled out in this system.We discuss the implications of our results in the light of recent experimental observations on biased LaAlO3/SrTiO3 junctions involving metallic top electrodes.

We use first-principles techniques to re-examine the suggestion that transitions seen in high-P experiments on Mo are solid-solid transitions from the bcc structure to either the fcc or hcp structures. We confirm that in the harmonic approximation the free energies of fcc and hcp structures become lower than that of bcc at P > 325 GPa and T below the melting curve, as reported recently. However, we show that if anharmonic effects are fully included this is no longer true. We calculate fully anharmonic free energies of high-T crystal phases by integration of the thermal average stress with respect to strain as structures are deformed into each other, and also by thermodynamic integration from harmonic reference systems to the fully anharmonic system. Our finding that fcc is thermodynamically less stable than bcc in the relevant high-P/high-T region is supported by comparing the melting curves of the two structures calculated using the first-principles reference-coexistence technique. We present first-principles simulations based on the recently proposed Z method which also support the stability of bcc over fcc., 33 pages, 10 figures

Equation of state of $^{4}\text{H}\text{e}$ hcp crystals with vacancies is determined at zero temperature using the diffusion Monte Carlo technique, an exact ground-state zero-temperature method. This allows us to extract the formation enthalpy and isobaric formation energy of a single vacancy in otherwise perfect helium solid. Results are obtained for pressures up to 160 bar. The isobaric formation energy is found to reach a minimum near 57 bar where it is equal to $10.5\ifmmode\pm\else\textpm\fi{}1.2\text{ }\text{K}$. At the same pressure, the vacancy formation volume exhibits a maximum and reaches the volume of the unit cell. This pressure coincides with the pressure interval over which a peak in the supersolid fraction of $^{4}\text{H}\text{e}$ was observed in a recent experiment.

In view of the interest in calcium-decorated carbon nanostructures motivated by potential biotechnological and nanotechnological applications, we have carried out a systematic and thorough first-principles computational study of the energetic and structural properties of these systems. We use density-functional theory (DFT) and ab initio molecular dynamic simulations to determine minimum energy configurations, binding energy profiles and the thermodynamic stability of Ca-decorated graphene and carbon nanotubes (CNT) as function of doping concentration. In graphene, we predict the existence of an equilibrium $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\text{ }\text{R}30\ifmmode^\circ\else\textdegree\fi{}$ commensurate ${\text{CaC}}_{6}$ monolayer that remains stable without clustering at low and room temperatures. For carbon nanotubes, we demonstrate that uniformly Ca-decorated zigzag $(n\ensuremath{\le}10,0)$ CNT become stable against clustering at moderately large doping concentrations while Ca-coated armchair $(n,n)$ CNT exhibit a clear thermodynamic tendency for Ca aggregation. In both Ca-doped graphene and CNT systems, we estimate large energy barriers $(\ensuremath{\sim}1\text{ }\text{eV})$ for atomic aggregation processes, which indicates that Ca clustering in carbon nanosurfaces may be kinematically hindered. Finally, we demonstrate via comparison of DFT and M\o{}ller-Plesset second-order perturbation calculations that DFT underestimates significantly the weak interaction between a Ca dopant and a coronene molecule, and also that the Ca-coronene system is not physically comparable to Ca-doped graphene due to lack of electronic $\ensuremath{\pi}\text{\ensuremath{-}}d$ orbitals hybridization near the Fermi energy level.

We study the structural stability and dynamical properties of $\text{Ar}{({\text{H}}_{2})}_{2}$ under pressure using first-principles and ab initio molecular-dynamics techniques. At low temperatures, $\text{Ar}{({\text{H}}_{2})}_{2}$ is found to stabilize in the cubic C15 Laves structure $({\text{MgCu}}_{2})$ and not in the hexagonal C14 Laves structure $({\text{MgZn}}_{2})$ as it has been assumed previously. Based on enthalpy energy and phonon calculations, we propose a temperature-induced ${\text{MgCu}}_{2}\ensuremath{\rightarrow}{\text{MgZn}}_{2}$ phase transition that may rationalize the existing discrepancies between the sets of Raman and infrared vibron measurements. Our AIMD simulations suggest that the melting line of $\text{Ar}{({\text{H}}_{2})}_{2}$ presents negative slope in the interval $60\ensuremath{\le}P\ensuremath{\le}110\text{ }\text{GPa}$. We explain the origin of this intriguing physical phenomenon in terms of decoupling of the Ar and ${\text{H}}_{2}$ degrees of freedom and effective thermal-like excitations arising from coexisting liquid ${\text{H}}_{2}$ and solid Ar phases.

We have carried out a comprehensive first-principles study of the energetic, structural, and electronic properties of solid rare-gas RG-helium binary compounds, in particular, NeHe2 and ArHe2, under pressure and at temperatures within the range of 0T2000 K. Our approach is based on density-functional theory and the generalized gradient approximation for the exchange-correlation energy; we rely on total Helmholtz freeenergy calculations performed within the quasiharmonic approximation for most of our analysis. In NeHe2, we find that at pressures of around 20 GPa the system stabilizes in the MgZn2 Laves structure, in accordance to what was suggested in previous experimental investigations. In the same compound, we predict a solid-solid phase transition among structures of the Laves family of the type MgZn2 →MgCu2, at a pressure of Pt =1201 GPa. In ArHe2, we find that the system stabilizes in the MgCu2 Laves phase at low pressures but it transitates toward the AlB2-type structure by effect of compression at Pt=13.84 GPa. The phonon spectra of the NeHe2 crystal in the MgZn2 and MgCu2 Laves structures, and that of ArHe2 in the AlB2-type phase, are reported. We observe that the compressibility of RG-RG and He-He bond distances in RGHe2 crystals is practically identical to that found in respective RG and He pure solids. This behavior emulates that of a system of noninteracting hard spheres in closed-packed configuration and comes to show the relevance of short-range interactions on this type of mixtures. Based on size-ratio arguments and empirical observations, we construct a generalized phase diagram for all RGHe2 crystals up to a pressure of 200 GPa where we map out systematic structural trends. Excellent qualitative agreement between such generalized phase diagram and accurate ab initio calculations is proved. A similar construction is done for RGH22 crystals; we find that the MgCu2 Laves structure, which has been ignored in all RG-H2 works so far, might turn out to be competitive with respect to the MgZn2 and AlB2-type structures. Furthermore, we explore the pressure evolution of the energy-band gap in RGHe2 solids and elaborate an argument based on electronic-band theory which explains the observed trends.

We study molecular para-hydrogen (p-${\rm H_{2}}$) and ortho-deuterium (o-${\rm D_{2}}$) in two dimensions and in the limit of zero temperature by means of the diffusion Monte Carlo method. We report energetic and structural properties of both systems like the total and kinetic energy per particle, radial pair distribution function, and Lindemann's ratio in the low pressure regime. By comparing the total energy per particle as a function of the density in liquid and solid p-${\rm H_{2}}$, we show that molecular para-hydrogen, and also ortho-deuterium, remain solid at zero temperature. Interestingly, we assess the quality of three different symmetrized trial wave functions, based on the Nosanow-Jastrow model, in the p-${\rm H_{2}}$ solid film at the variational level. In particular, we analyze a new type of symmetrized trial wave function which has been used very recently to describe solid $^{4}$He and found that also characterizes hydrogen satisfactorily. With this wave function, we show that the one-body density matrix $\varrho_{1} (r)$ of solid p-${\rm H_{2}}$ possesses off-diagonal long range order, with a condensate fraction that increases sizably in the negative pressure regime., 11 pages, 9 figures

There has been a major controversy over the past seven years about the high-pressure melting curves of transition metals. Static compression (diamond-anvil cell: DAC) experiments up to the Mbar region give very low melting slopes dT_m/dP, but shock-wave (SW) data reveal transitions indicating much larger dT_m/dP values. Ab initio calculations support the correctness of the shock data. In a very recent letter, Belonoshko et al. propose a simple and elegant resolution of this conflict for molybdenum. Using ab initio calculations based on density functional theory (DFT), they show that the high-P/high-T phase diagram of Mo must be more complex than was hitherto thought. Their calculations give convincing evidence that there is a transition boundary between the normal bcc structure of Mo and a high-T phase, which they suggest could be fcc. They propose that this transition was misinterpreted as melting in DAC experiments. In confirmation, they note that their boundary also explains a transition seen in the SW data. We regard Belonoshko et al.'s Letter as extremely important, but we note that it raises some puzzling questions, and we believe that their proposed phase diagram cannot be completely correct. We have calculated the Helmholtz and Gibbs free energies of the bcc, fcc and hcp phases of Mo, using essentially the same quasiharmonic methods as used by Belonoshko et al.; we find that at high-P and T Mo in the hcp structure is more stable than in bcc or fcc., Comment: 1 page, 1 figure. submitted to Phys. Rev. Lett

We present a zero-temperature quantum Monte Carlo calculation of liquid $^{4}\mathrm{He}$ immersed in an array of confining potentials. These external potentials are centered in the lattice sites of a fcc solid geometry and, by modifying their well depth and range, the system evolves from a liquid phase towards a progressively localized system which mimics a solid phase. The superfluid density decreases with increasing order, reaching a value of ${\ensuremath{\rho}}_{s}∕\ensuremath{\rho}=0.079(16)$ when the Lindemann's ratio of the model equals the experimental value for solid $^{4}\mathrm{He}$.