Physical review B: condensed matter and materials physics

Vol. 92, p. 224113-

DOI: 10.1103/PhysRevB.92.224113

Date of publication: 2015-12-31

Abstract:

We present a set of three-body interaction models based on the Slater-Kirkwood (SK) potential that are suitable for the study of the energy, structural, and elastic properties of solid He4 at high pressure. Our effective three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced SK three-body potentials reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of ~60 GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts deriving from the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, C44, at pressures 0=P=25 GPa can be quantitatively described by our set of SK three-body potentials. At higher compression, however, the agreement between our C44 calculations and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid He4.]]>

Physical review B: condensed matter and materials physics

Vol. 92, num. 24, p. 1-6

DOI: 10.1103/PhysRevB.92.245305

Date of publication: 2015-12-09

Abstract:

We report the quantum phase diagram of a one-dimensional Coulomb wire obtained using the path-integral Monte Carlo method. The exact knowledge of the nodal points of this system permits us to find the energy in an exact way, solving the sign problem which spoils fermionic calculations in higher dimensions. The results obtained allow for the determination of the stability domain, in terms of density and temperature, of the one-dimensional Wigner crystal. At low temperatures, the quantum wire reaches the quantum-degenerate regime, which is also described by the diffusion Monte Carlo method. Increasing the temperature, the system transforms to a classical Boltzmann gas, which we simulate using classical Monte Carlo. At large enough density, we identify a one-dimensional ideal Fermi gas which remains quantum up to higher temperatures than in two- and three-dimensional electron gases. The obtained phase diagram and the energetic and structural properties of this system are relevant to experiments with electrons in quantum wires and to Coulomb ions in one-dimensional confinement.]]>

Physical review B: condensed matter and materials physics

Vol. 92, num. 9, p. 1-7

DOI: 10.1103/PhysRevB.92.094101

Date of publication: 2015-09-08

Abstract:

Cracks generate the largest strain gradients that any material can withstand. Flexoelectricity (coupling between strain gradient and polarization) must therefore play an important role in fracture physics. Here we use a self-consistent continuum model to evidence two consequences of flexoelectricity in fracture: the resistance to fracture increases as structural size decreases, and it becomes asymmetric with respect to the sign of polarization. The latter phenomenon manifests itself in a range of intermediate sizes where piezo- and flexoelectricity compete. In BaTiO3 at room temperature, this range spans from 0.1 to 50 nm, a typical thickness range for epitaxial ferroelectric thin films.]]>

Physical review B: condensed matter and materials physics

Vol. 92, num. 5, p. 054302-1-054302-6

DOI: 10.1103/PhysRevB.92.054302

Date of publication: 2015-08-06

Abstract:

We report the propagation of high-intensity sound beams in a sonic crystal, under self-collimation or reduced-divergence conditions. The medium is a fluid with elastic quadratic nonlinearity, where the dominating nonlinear effect is harmonic generation. The conditions for the efficient generation of narrow, nondiverging beam of second harmonic are discussed. Numerical simulations are in agreement with the analytical predictions made, based on the linear dispersion characteristics in modulated media and the nonlinear interaction in a quadratic medium under phase matching conditions.]]>

Physical review B: condensed matter and materials physics

Vol. 91, num. 10

DOI: 10.1103/PhysRevB.91.104103

Date of publication: 2015-03-03

Abstract:

Flexoelectricity is a universal property of all dielectrics by which they generate a voltage in response to an inhomogeneous deformation. One of the controversial issues in this field concerns the magnitude of flexoelectric coefficients measured experimentally, which greatly exceed theoretical estimates. Furthermore, there is a broad scatter amongst experimental measurements. The truncated pyramid compression method is one of the common setups to quantify flexoelectricity, the interpretation of which relies on simplified analytical equations to estimate strain gradients. However, the deformation fields in three-dimensional pyramid configurations are highly complex, particularly around its edges. In the present work, using three-dimensional self-consistent simulations of flexoelectricity, we show that the simplified analytical estimations of strain gradients in compressed pyramids significantly overestimate flexoelectric coefficients, thus providing a possible explanation to reconcile different estimates. In fact, the interpretation of pyramid compression experiments is highly nontrivial. We systematically characterize the magnitude of this overestimation, of over one order of magnitude, as a function of the truncated pyramid configuration. These results are important to properly characterize flexoelectricity, and provide design guidelines for effective electromechanical transducers exploiting flexoelectricity.]]>

Physical review B: condensed matter and materials physics

Vol. 91, num. 2, p. 1-9

DOI: 10.1103/PhysRevB.91.024103

Date of publication: 2015-01-07

Abstract:

We present a first-principles computational study of solid He-4 at T = 0 K and pressures up to similar to 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 Schrodinger 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 He-4 does not increase appreciably with compression for P >= 85 GPa. Also, we show that the Lindemann ratio in dense solid He-4 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 He-4. In particular, we disclose huge discrepancies of at least similar to 50% in the calculated He-4 kinetic energies using both the QH DFT and present DFT-DMC methods.]]>

Physical review B: condensed matter and materials physics

Vol. 92, num. 3, p. 035417-1-035417-9

DOI: 10.1103/PhysRevB.92.035417

Date of publication: 2015

Abstract:

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk BaTiO3 ceramics, indicating that there is a “flexoelectric size effect” that mirrors the ferroelectric one]]>

Physical review B: condensed matter and materials physics

Vol. 90, num. 23, p. 1-5

DOI: 10.1103/PhysRevB.90.235439

Date of publication: 2014-12-30

Abstract:

The ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a "Bose gas." a "super-Tonks-Girardeau" regime, and a "quasicrystal." The obtained results are applicable to unpolarized, partially, or fully polarized He-3.

The ground-state properties of one-dimensional He-3 are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well interpolated by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation functions. The density dependence of the Luttinger parameter is explicitly found, and interestingly it shows a nonmonotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a]]>

Physical review B: condensed matter and materials physics

Vol. 90, num. 21, p. 1-5

DOI: 10.1103/PhysRevB.90.214512

Date of publication: 2014-12-11

Abstract:

We study the zero-temperature quantum phase transition between liquid and hcp solid He-4. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.]]>

Physical review B: condensed matter and materials physics

Vol. 89, num. 21, p. 214105-1-214105-8

DOI: 10.1103/PhysRevB.89.214105

Date of publication: 2014-06-06

Abstract:

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of Fe49Rh51. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths (vertical bar Delta S vertical bar/Delta p similar to 12 J kg(-1) K(-1)kbar(-1) and vertical bar Delta S vertical bar/mu(0)Delta H similar to 12 J kg(-1) K-1T-1). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of Fe49Rh51. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths (|¿S|/¿p ~ 12 J kg-1K-1kbar-1 and |¿S|/µ0¿H~ 12 J kg-1K-1T-1). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.]]>

Physical review B: condensed matter and materials physics

Vol. 88, num. 22, p. 224501-1-224501-6

DOI: 10.1103/PhysRevB.88.224501

Date of publication: 2013-12-03

Abstract:

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H2) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2-H2 and Na-H2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H2 ground state is a liquid with a significantly large superfluid fraction of ¿s/¿=0.29(2). The appearance of p-H2 superfluid response is due to the fact that the interactions between Na atoms and H2 molecules are less attractive than between H2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H 2 ) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2 -H 2 and Na-H 2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H 2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H 2 ground state is a liquid with a significantly large superfluid fraction of ρ s /ρ=0.29(2) . The appearance of p-H 2 superfluid response is due to the fact that the interactions between Na atoms and H 2 molecules are less attractive than between H 2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase.]]>

Physical review B: condensed matter and materials physics

Vol. 88, num. 12, p. 1-13

DOI: 10.1103/PhysRevB.88.125416

Date of publication: 2013-09-11

Abstract:

We report the results of a study of 4HeN clusters, for 2=N=40, adsorbed on one and both sides of a graphene sheet. The ground-state properties are determined using variational and diffusion Monte Carlo calculations at zero temperature, and in addition path integral Monte Carlo simulations at finite temperature are performed for some selected cluster sizes. For the interaction of helium atoms with graphene, we compare several models: a smooth He-graphene potential that depends only on the distance to the graphene sheet and potentials constructed as a sum of individual He-C interactions where two possibilities for this He-C interaction are tested. In this way, we assess the effect of corrugation on the binding properties of helium clusters. Furthermore, we study the influence that the graphene-mediated McLachlan dispersion energy has on the He-He interaction. The McLachlan interaction weakens the attraction between helium atoms, which turns out to have a significant effect on the binding energy and the shape of adsorbed 4He clusters. We find that clusters adsorbed on opposite sides of graphene are bound, but according to the He-He pair distribution function across the graphene sheet, pair correlations are very weak. For a large enough number of particles, solidlike v3×v3 structures start to become energetically preferred for the model of anisotropic corrugation without the McLachlan interaction. For the other models, the ground state of the studied clusters is clearly liquidlike. © 2013 American Physical Society.]]>

Physical review B: condensed matter and materials physics

Vol. 87, num. 21, p. 1-5

DOI: 10.1103/PhysRevB.87.214522

Date of publication: 2013-06-28

Abstract:

We study the elastic properties of incommensurate solid 4He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (C11, C12, C13, C33, and C44), longitudinal and transversal speeds of sound, and the T=0 Debye temperature of incommensurate and commensurate hcp 4He 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~25 bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (C12 and C13) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of 4He (C44) undergoes a small reduction.

We study the elastic properties of incommensurate solid 4He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (C11, C12, C13, C33, and C44), longitudinal and transversal speeds of sound, and the T=0 Debye temperature of incommensurate and commensurate hcp 4He 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∼25 bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (C12 and C13) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of 4He (C44) undergoes a small reduction.]]>

Physical review B: condensed matter and materials physics

Vol. 87, num. 16, p. 1-6

DOI: 10.1103/PhysRevB.87.165403

Date of publication: 2013-04-03

Abstract:

We report diffusion Monte Carlo calculations on the phase diagrams of para-H2 and ortho-D2 adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably with available experimental data in both cases.]]>

Physical review B: condensed matter and materials physics

Vol. 88, p. 1-10

DOI: 10.1103/PhysRevB.88.024507

Date of publication: 2013

Abstract:

The ground-state properties of spin-polarized deuterium (D¿) at zero temperature are obtained by means of diffusion Monte Carlo calculations within the fixed-node approximation. Three D¿ species have been investigated (D¿1, D¿2, D¿3), corresponding respectively to one, two, and three equally occupied nuclear-spin states. The influence of the backflow correlations on the ground-state energy of the systems is explored. The equations of state of liquid D¿2 and D¿3 are obtained and compared with the ones obtained in previous approximate predictions. The density and pressure at which D¿1 experiences a gas-liquid transition at T=0 are obtained.]]>

Physical review B: condensed matter and materials physics

Vol. 86, num. 20, p. 1-6

DOI: 10.1103/PhysRevB.86.205313

Date of publication: 2012-11-15

Abstract:

We address stationary patterns in exciton-polariton condensates supported by a narrow external pump beam, and we discover that even in the absence of trapping potentials, such condensates may support stable localized stationary dissipative solutions (quasicompactons), whose field decays faster than exponentially or even vanishes everywhere outside the pump spot. More general conditions lead to dissipative solitons which may display bistability. The bistability in exciton-polariton condensates, which manifests itself in the simultaneous existence of two stable and one unstable localized solitons with different amplitudes, widths, and exciton-photon fractions under the same physical conditions, strongly depends on the width of the pump beam and is found to disappear for sufficiently narrow pump beams.

We address stationary patterns in exciton-polariton condensates supported by a narrow external pump beam, and we discover that even in the absence of trapping potentials, such condensates may support stable localized stationary dissipative solutions (quasicompactons), whose field decays faster than exponentially or even vanishes everywhere outside the pump spot. More general conditions lead to dissipative solitons which may display bistability. The bistability in exciton-polariton condensates, which manifests itself in the simultaneous existence of two stable and one unstable localized solitons with different amplitudes, widths, and exciton-photon fractions under the same physical conditions, strongly depends on the width of the pump beam and is found to disappear for sufficiently narrow pump beams]]>

Physical review B: condensed matter and materials physics

Vol. 86, num. 17, p. 1-5

DOI: 10.1103/PhysRevB.86.174518

Date of publication: 2012-11

Physical review B: condensed matter and materials physics

Vol. 86, num. 16, p. 1-6

DOI: 10.1103/PhysRevB.86.165409

Date of publication: 2012-10-08

Abstract:

The phase diagrams of 4He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42–10.85 Å are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H2 on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved v3×v3 registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.

The phase diagrams of 4He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42–10.85 Å are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H2 on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved √3×√3 registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.]]>

Physical review B: condensed matter and materials physics

Vol. 86, p. 035316-1-035316-6

DOI: 10.1103/PhysRevB.86.035316

Date of publication: 2012-07-17

Abstract:

The thermal emission of cross-slit silicon carbide grating is studied in the Restrahlen region over all emission angles. We show experimentally that the thermal excitation of surface-phonon polaritons on the surface of 2D grating allows us to get a high emissivity in both polarizations, which is collimated in p polarization for a specific wavelength determined by the periodicity of the grating. We also show numerically that 2D gratings optimized to efficiently out-couple thermally excited surface-phonon polaritons of the flat part of the dispersion relation can have a high efficiency for all emission directions for both polarizations.]]>

Physical review B: condensed matter and materials physics

Vol. 85, num. 22, p. 1-10

DOI: 10.1103/PhysRevB.85.224513

Date of publication: 2012-06-13

Abstract:

Molecular para-hydrogen has been proposed theoretically as a possible candidate for superfluidity, but the eventual superfluid transition is hindered by its crystallization. In this work, we study a metastable non crystalline phase of bulk p-H2 by means of the Path Integral Monte Carlo method in order to investigate at which temperature this system can support superfluidity. By choosing accurately the initial configuration and using a non commensurate simulation box, we have been able to frustrate the formation of the crystal in the simulated system and to calculate the temperature dependence of the one-body density matrix and of the superfluid fraction. We observe a transition to a superfluid phase at temperatures around 1 K. The limit of zero temperature is also studied using the diffusion Monte Carlo method. Results for the energy, condensate fraction, and structure of the metastable liquid phase at T=0 are reported and compared with the ones obtained for the stable solid phase.]]>

Physical review B: condensed matter and materials physics

Vol. 85, num. 19, p. 1-6

DOI: 10.1103/PhysRevB.85.195457

Date of publication: 2012-05-25

Abstract:

The phase diagram at zero temperature of 4He adsorbed on a helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of 4He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163±0.005 Å−2 and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186±0.003 Å−2. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density.]]>

Physical review B: condensed matter and materials physics

Vol. 85, num. 13, p. 134201-1-134201-7

DOI: 10.1103/PhysRevB.85.134201

Date of publication: 2012-04-04

Abstract:

The dynamics of a simple rigid pseudoglobular molecule (2-adamantanone) has been studied by means of dielectric spectroscopy and examined under the constraints imposed by the space group of the crystal structure determined by x-ray powder diffraction. The low-temperature monoclinic structure of 2-adamantanone, with one molecule per asymmetric unit (Z'= 1), displays a statistical intrinsic disorder, concerning the site occupancy of the oxygen atom along three different sites. Such a physically identifiable disorder gives rise to large-angle molecular rotations which inherently lead to time-average fluctuations of the molecular dipole, thus contributing to the dielectric susceptibility. The dielectric spectra for the low-temperature “ordered” phase displays a universal feature of glassy-like materials, i.e., coexistence of α- and β-relaxation processes. The former is clearly identified with the strongly restricted reorientational motions within the long-range “ordered” crystalline lattice. The latter, never observed before in fully translationally and highly orientationally ordered phases, displays all the properties of an original Johari-Goldstein β-relaxation, in spite of the strong character of this glass-like phase. These findings can be explained according to the coupling model, applied to such “ordered” phases.]]>

Physical review B: condensed matter and materials physics

Vol. 85, num. 1, p. 014206-1-014206-10

DOI: 10.1103/PhysRevB.85.014206

Date of publication: 2012-01-30

Abstract:

The thermal conductivity κ(T ) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C6H11OH) has been measured under equilibrium vapor pressure within the 2–200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C6H11CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar.]]>

Physical review B: condensed matter and materials physics

Vol. 85, num. 1, p. 014202-1-014202-9

DOI: 10.1103/PhysRevB.85.014202

Date of publication: 2012-01-01

Abstract:

The study of how both steric and electrostatic interactions affect the structure of liquids formed by quasitetrahedral molecules has been undertaken in this work. We have studied trichlorobromomethane (CBrCl3) and dibromodichloromethane (CBr2Cl2), both displaying a dipole along their C3v and C2v molecular symmetry axes, respectively. The short-range order of the liquid state has been determined using neutron diffraction experiments that were modeled through the reverse Monte Carlo (RMC) technique. To study changes in steric effects due to the distortion of the tetrahedral symmetry, we have compared our results with a previous RMC modeling of carbon tetrachloride (CCl4). The subtle effects of the dipole in the structure of the liquid have been determined using a set of molecular dynamics simulations with and without atomic partial charges, being the force field validated via comparison with the diffraction data. In a first approximation, neither steric nor electrostatic interactions are able to modify the molecular ordering of a fully tetrahedral liquid such as CCl4. A more detailed analysis indicates that, although the interaction between dipoles does not have appreciable effects when aligned along the C3v molecular axes, as for the CBrCl3, it enhances the antiparallel orientation of dipoles when it is oriented along the C2v axes, as in the case of CBr2Cl2.]]>

Physical review B: condensed matter and materials physics

Vol. 84, num. 22, p. 224116

DOI: 10.1103/PhysRevB.84.224116

Date of publication: 2011-12-01

Physical review B: condensed matter and materials physics

Vol. 6, num. 84, p. 064202-1-064202-12

DOI: 10.1103/PhysRevB84.064202

Date of publication: 2011-08-18

Physical review B: condensed matter and materials physics

Vol. 83, num. 15, p. 1-4

DOI: 10.1103/PhysRevB.83.153303

Date of publication: 2011-04-15

Abstract:

The ground-state properties of a single-component one-dimensional Coulomb gas are investigated. We use Bose-Fermi mapping for the ground-state wave function which permits solution of the Fermi sign problem in the following respects: (i) the nodal surface is known, permitting exact calculations; and (ii) evaluation of determinants is avoided, reducing the numerical complexity to that of a bosonic system and, thus, allowing simulation of a large number of fermions. Due to the mapping, the energy and local properties in one-dimensional Coulomb systems are exactly the same for Bose-Einstein and Fermi-Dirac statistics. The exact ground-state energy is calculated in homogeneous and trapped geometries using the diffusion Monte Carlo method. We show that in the low-density Wigner crystal limit an elementary low-lying excitation is a plasmon, which is to be contrasted with the high-density ideal Fermi gas/Tonks-Girardeau limit, where low-lying excitations are phonons. Exact density profiles are compared to the ones calculated within the local density approximation, which predicts a change from a semicircular to an inverted parabolic shape of the density profile as the value of the charge is increased.]]>

Physical review B: condensed matter and materials physics

Vol. 83, num. 12, p. 1-4

DOI: 10.1103/PhysRevB.83.121406

Date of publication: 2011-03-17

Abstract:

Using quantum Monte Carlo we have studied the superfluid density of the first layer of 4He and H2 adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered √3 × √3 phase. The perfect solid phase of H2 shows no superfluid signal, whereas 4He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in 4He without destroying the spatial solid order.]]>

Physical review B: condensed matter and materials physics

Vol. 83, num. 8, p. 1-4

DOI: 10.1103/PhysRevB.83.081413

Date of publication: 2011-02-23

Physical review B: condensed matter and materials physics

Vol. 85, num. 2, p. 1-8

DOI: 10.1103/PhysRevB.85.024101

Date of publication: 2011-01-04

Physical review B: condensed matter and materials physics

Vol. 84, num. 3, p. 1-4

DOI: 10.1103/PhysRevB.84.033406

Date of publication: 2011

Physical review B: condensed matter and materials physics

Vol. 82, num. 18, p. 1-4

DOI: 10.1103/PhysRevB.82.180506

Date of publication: 2010-08-09

Physical review B: condensed matter and materials physics

Vol. 82, num. 1

DOI: 10.1103/PhysRevB.82.014508

Date of publication: 2010-07-09

Physical review B: condensed matter and materials physics

Vol. 81, num. 21, p. 214105-1-214105-8

DOI: 10.1103/PhysRevB.81.214105

Date of publication: 2010-06-07

Abstract:

Stress-induced stress-strain constitutive behavior has been studied in detail in ferroelastic martensites. It has been found that the weights of the long-range anisotropic interactions and of the disorder are important to determine the fine structure of the stress-strain curves. As experiments show, a wide variety of behaviors has been observed. A decrease of anisotropy and/or increase in the disorder results in changes in the temperature range where pseudoplastic and superelastic regimes are observed. Also, a smoothing of the stress-induced ferroelastic transition, accompanied with a decrease in the transition stress and the hysteresis area, is found. However, Clausius-Clapeyron slope has been observed not to depend on the specific values of anisotropy and disorder. This is in general agreement with experimental results in different alloy families, although some particular features differ from those in experiments. Elastocaloric effect has been studied as well. Varying the anisotropy slightly modifies the shape of the entropy change-temperature curve but the magnitude of the entropy change remains essentially constant.]]>

Physical review B: condensed matter and materials physics

Vol. 81, num. 17

DOI: 10.1103/PhysRevB.81.174304

Date of publication: 2010-05-01

Physical review B: condensed matter and materials physics

Vol. 81, num. 15, p. 1-5

DOI: 10.1103/PhysRevB.81.155435

Date of publication: 2010-04-15

Abstract:

The phase diagram of the first layer of H2 adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo simulations. We have found that, as in the case of 4He, the ground state of molecular hydrogen is a √3×√3 commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.]]>

Physical review B: condensed matter and materials physics

Vol. 81, num. 9, p. 094205-1-094205-6

DOI: 10.1103/PhysRevB.81.094205

Date of publication: 2010-03-30

Physical review B: condensed matter and materials physics

Vol. 81, num. 092202, p. 092202-1-092202-4

DOI: 10.1103/PhysRevB.81.092202

Date of publication: 2010-03-08

Physical review B: condensed matter and materials physics

Vol. 81, num. 8, p. 5104

DOI: 10.1103/PhysRevB.81.085104

Date of publication: 2010-02

Physical review B: condensed matter and materials physics

Vol. 82, num. 8, p. 085401-

DOI: 10.1103/PhysRevB.82.085401

Date of publication: 2010

Abstract:

Stable cross sections of multiwalled carbon nanotubes subjected to electron-beam irradiation are investigated in the realm of the continuum mechanics approximation. The self-healing nature of sp2 graphitic sheets implies that selective irradiation of the outermost walls causes their radial shrinkage with the remaining inner walls undamaged. The shrinking walls exert high pressure on the interior part of nanotubes, yielding a wide variety of radial-corrugation patterns (i.e. circumferentially wrinkling structures) in the cross section. All corrugation patterns can be classified into two deformation phases for which the corrugation amplitudes of the innermost wall differ significantly.]]>

Physical review B: condensed matter and materials physics

Vol. 80, num. 9, p. 094412-094417

DOI: 10.1103/PhysRevB.80.094412

Date of publication: 2009-09-21

Physical review B: condensed matter and materials physics

Vol. 80, num. 5, p. 054107-1-054107-7

DOI: 10.1103/PhysRevB.80.054107

Date of publication: 2009-08-17

Abstract:

We study the combined effect of elastic anisotropy and disorder on the microstructure and thermodynamic behavior in alloys undergoing a martensitic transformation. Within a Ginzburg-Landau free-energy framework we find the region in the parameter space where a ferroelastic glassy state exists without twinning. We find that such a glassy state is of kinetic origin rather than due to geometrical frustration. The glassy behavior is characterized by simulating zero-field-cooling/field-cooling curves for different values of anisotropy and disorder. Finally, we discuss experimental implications for Fe-Pd and Ni-Ti alloy]]>

Physical review B: condensed matter and materials physics

Vol. 80, num. 8, p. 5423-

DOI: 10.1103/PhysRevB.80.085423

Date of publication: 2009-08

Physical review B: condensed matter and materials physics

Vol. 79, num. 21, p. 214117-1-214117-22

DOI: 10.1103/PhysRevB.79.214117

Date of publication: 2009-06-29

Abstract:

We develop a strain-based approach to study the transformation of a finite martensite domain within an austenite host matrix. Analytical and numerical solutions are obtained for the fringing fields in the austenite and in the martensite and we test how well the stress and strain matching conditions are obeyed at the habit planes. We investigate the scaling of the energy of the fringing fields and show how simulations on relaxed microstructures corroborate the 1/||ky|| behavior for the energy in Fourier space. Our results show that the functional form F=F0+aL1¿+bLL1/¿ for the total elastic energy provides an excellent fit to the numerical simulations, thus demonstrating that ¿~L--v, where ¿ is the twin width for a martensite region L×L1 with length of the habit plane L1 and where aL1¿, bLL1/¿, and F0 are the energies of the decaying strain field at the habit plane, twin-boundary energy, and energy of a single martensite variant, respectively. However, the result is only true for sufficiently large L and we provide insight into the breakdown of the ¿~L--v scaling at the nanoscale. Our approach allows us to investigate the effect of varying the finite distance between habit planes, L, and our key finding is that there is a minimum length, Lmin, for the nucleation of the twinned martensite structure which depends on temperature. As the temperature is lowered, Lmin decreases, and at temperatures close to the stability limit of the austenite phase a lattice martensite structure in which the parent and product phases spatially alternate in a checker-board pattern is stable in a narrow region of the temperature versus L phase diagram. Such patterns have been seen at the nanoscale in lithium-based perovskites and inorganic spinels, as well as in coherent decomposition of precipitates in Co-Pt alloys. Finally, we show how the nature of the fringing fields due to an inclusion within an austenite matrix sensitively depends on its shape, size, and orientation and determines whether twinning or lattice martensite are the stable structures.]]>

Physical review B: condensed matter and materials physics

Vol. 80, num. 134303, p. 1-7

DOI: 10.1103/PhysRevB.80.134303

Date of publication: 2009

Physical review B: condensed matter and materials physics

Vol. 80, num. 13, p. 1-8

DOI: 10.1103/PhysRevB.80.134506

Date of publication: 2009

Abstract:

The ground-state properties of spin-polarized tritium T↓ at zero temperature are obtained by means of diffusion Monte Carlo calculations. Using an accurate ab initio T↓-T↓ interatomic potential we have studied its liquid phase, from the spinodal point until densities above its freezing point. The equilibrium density of the liquid is significantly higher and the equilibrium energy of −3.664(6) K significantly lower than in previous approximate descriptions. The solid phase has also been studied for three lattices up to high pressures and we find that hcp lattice is slightly preferred. The liquid-solid phase transition has been determined using the double-tangent Maxwell construction; at zero temperature, bulk tritium freezes at a pressure of P=9(1) bar.]]>

Physical review B: condensed matter and materials physics

Vol. 78, num. 224302, p. 1-9

Date of publication: 2008-12

Physical review B: condensed matter and materials physics

Vol. 78, num. 155433, p. 1-9

DOI: 10.1103/PhysRevB.78.155433

Date of publication: 2008-10

Abstract:

Magnetotransport experiments on antidot lattices show a rich variety of physical phenomena. Depending on the value of the particle mean-free path l in relation to the period lambda of regular scatterers, two very different regimes can be distinguished: the strongly diffusive (l

Physical review B: condensed matter and materials physics

Vol. 78, p. 075432-1-075432-5

DOI: 10.1103/PhysRevB.78.075432

Date of publication: 2008-08

Abstract:

We present the result of molecular-dynamics simulations of water adsorbed on top of a single graphene layer at temperatures between 25 and 50°C. The analysis of the energy per particle and the density profiles indicate that the behavior of the adsorbed liquid is similar to the case of multiple graphene layers (graphite) with the only difference being the values of configurational energy. Other structural properties, such as stability ranges, hydrogen bond distributions, and molecular orientations are also presented.]]>

Physical review B: condensed matter and materials physics

Vol. 77, num. 20, p. 1

Date of publication: 2008-05