We use classical molecular dynamics to evaluate the thermal conductivity $\kappa \left(T\right)$ from the heat-flux correlation $⟨\mathbf{j}\left(0\right)\mathbf{j}\left(t\right)⟩$ for a two-dimensional Lennard-Jones triangular lattice. Our work, which follows Ladd, Moran, and Hoover [Phys. Rev. B 34, 5058 (1986)], finds large deviations from the Eucken-Debye result $\kappa \left(T\right)=A/T$ predicted by the phonon-gas model, even though phonon quasiparticles are fairly well defined. The main source of deviations comes from higher order (anharmonic) terms in the heat-flux operator $\mathbf{j}$. By separating different orders of terms $\mathbf{j}={\mathbf{j}}^{\left(2\right)}+{\mathbf{j}}^{\left(3\right)}+\cdots$, we examine various separate contributions to $\kappa \left(T\right)\approx {\kappa }_{22}+{\kappa }_{23}+\cdots$, both from the harmonic and the anharmonic heat fluxes. We find that ${\kappa }_{22}\left(T\right)\approx A/T$ follows quasiparticle theory fairly well but important terms from ${\kappa }_{23}$ and ${\kappa }_{24}$ are independent of $T$ in the classical (high $T$) limit. We use diagrammatic perturbation theory applied to the quantum Kubo formula, to check and explain the $T$ dependence found numerically from anharmonic heat fluxes. We also demonstrate the importance of vertex correction in obtaining the correct quasiparticle coefficient of $1/T$.

We analyze a number of proposed explanations for spectroscopic anomalies observed in atomic hydrogen defects embedded in a solid molecular hydrogen matrix. In particular, we critically evaluate the possibility that these anomalies are related to Bose-Einstein condensation (both global and local). For each proposed mechanism we discuss which aspects of the experiment can be explained and make predictions for future experiments.

We investigate the parametrization and optimization of thermal conductivity in silicon-germanium alloy nanowires by the cluster-expansion technique. ${\text{Si}}_{1-x}{\text{Ge}}_x$ nanowires are of interest for thermoelectric applications and the reduction in lattice thermal conductivity $\left({\kappa }_L\right)$ is desired for enhancing the thermoelectric figure of merit. We seek the minimization of ${\kappa }_L$ with respect to arrangements of Si and Ge atoms in 1.5 nm diameter [111] ${\text{Si}}_{1-x}{\text{Ge}}_x$ nanowires, by obtaining ${\kappa }_L$ from equilibrium classical molecular-dynamics (MD) simulations via the Green-Kubo formalism, and parametrizing the results with a coarse-grained cluster expansion. Using genetic algorithm optimization with the coarse-grained cluster expansion, we are able to predict configurations that significantly decrease ${\kappa }_L$ as verified by subsequent MD simulations. Our results indicate that superlatticelike configurations with planes of Ge show drastically lowered ${\kappa }_L$.

We study a composite quantum quench of the energy gap and the interactions in the interacting ${\varphi }^4$ model using a self-consistent approximation. First we review results for free theories where a quantum quench of the energy gap or mass leads for long times to stationary behavior with thermal characteristics. An exception to this rule is the $2d$ case with zero mass after the quench. In the composite quench, however, we find that the effect of the interactions in our approximation is simply to effectively change the value of the mass. This means on the one hand that the interacting model also exhibits the same stationary behavior and on the other hand that this is now true even for the massless $2d$ case.

While the thermal conductivity of the filled skutterudites has been of great interest it had not been calculated within a microscopic theory. Here a central force, Guggenheim-McGlashen, model with parameters largely extracted from first-principles calculations and from spectroscopic data, specific to ${\text{LaFe}}_4{\text{Sb}}_{12}$ or ${\text{CoSb}}_3$, is employed in a Green-Kubo/molecular dynamics calculation of thermal conductivity as a function of temperature. We find that the thermal conductivity of a filled solid is more than a factor of two lower than that of an unfilled solid, assuming the “framework” interatomic force parameters are the same between filled and unfilled solids, and that this decrease is almost entirely due to the cubic anharmonic interaction between filling and framework atoms. In addition, partially as a test of our models, we calculate thermal expansivity and isotropic atomic mean-square displacements using both molecular dynamics and lattice dynamics methods. These quantities are in reasonable agreement with experiment, increasing our confidence in the anharmonic parameters of our models. We also find an anomalously large filling-atom mode Gruneisen parameter that is apparently observed for a filled skutterudite and is observed in a clathrate.

The effective vibrational density of states (VDOS) has been derived from inelastic neutron-scattering data, for isotopically substituted $\text{Si}{{}^{18}\text{O}}_2$ and $\text{Si}{{}^{16}\text{O}}_2$ glasses, to gain information about the relative contribution to the Si and O partial VDOS. This is a necessary point of comparison for vibrational mode analyses of molecular-dynamics models. The mass contrast has led to a measurable shift between vibrational mode frequencies in the effective VDOS of $\text{Si}{{}^{18}\text{O}}_2$ and $\text{Si}{{}^{16}\text{O}}_2$, which is well reproduced in an ab initio simulation. The vibrational band centered at 100.2 meV is confirmed to have significantly lower contribution from the oxygen partial VDOS, than the higher (150.3 and 135.8 meV) and lower energy bands (53.3 meV).

The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant $C_{33}$ and thermal conductivity $\Lambda$ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precision measurements of $C_{33}$ and $\Lambda$, respectively, of micron-sized samples within a diamond-anvil cell; $\Lambda$ changes from the anomalously low value of $0.46\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$ at ambient pressure to a value more typical of oxides crystals with large unit cells, $6.6\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$, at $P=24\text{GPa}$. Most of the pressure dependence of $\Lambda$ can be accounted for by the pressure dependence of the cross-plane sound velocities.

By use of an experimental design that provides for superfluid helium in contact with bulk hcp ${}^4\text{H}\text{e}$ off the melting curve, we have observed the dc transport of mass through a cell filled with solid ${}^4\text{H}\text{e}$ in the hcp region of the phase diagram. Flow, which shows characteristics of a superflow, is seen to be independent of the method used to grow the solid but depends on pressure and temperature. The temperature dependence suggests the possibility of hysteresis.

Inelastic neutron measurements of the high-temperature lattice excitations in NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in three dimensions, occurs at a single frequency near the center of the spectral phonon gap, polarized along [111]. At higher temperatures the intrinsic localized mode gains intensity. Higher energy inelastic neutron and x-ray scattering measurements on a room-temperature NaI crystal indicate that the creation energy of the ground state of the intrinsic localized mode is 299 meV.

Activation energies for vacancy-mediated impurity diffusion in face-centered-cubic aluminum have been computed ab initio for all technologically important alloying elements, as well as for most of the lanthanides. The so-called five-frequency rate model is used to establish the limiting vacancy interchange process. Many elements were shown to be limited by Al-vacancy interchanges. For these elements we showed that the diffusion activation energy is rather close to that for Al self-diffusion, and additionally the diffusion pre-exponential factor is of the same order as that for Al self-diffusion. The diffusion activation energy is shown to exhibit a linear relation with the solute partial molar volume in Al. In contrast, transition metals are shown to deviate strongly from these generalities. Diffusion of transition-metal atoms is limited by solute-vacancy interchanges that require remarkably high activation energies. Transition-metal diffusivities in Al show strong trends with the number of $d$-valence electrons but not with partial molar volume.

We consider a nanomechanical resonator coupled to a double quantum dot. We demonstrate how the finite-frequency current-noise spectrum through the double quantum dot can be used to distinguish classical and quantum behavior in the nearby nanoelectromechanical resonator. We also show how the full-frequency current-noise spectrum gives important information on the combined double quantum dot-resonator energy spectrum. Finally, we point out regimes where the quantum state of the resonator becomes squeezed and also examine the cross-correlated electron-phonon current noise.

We describe quantum entanglement inherent to the polaron ground states of coupled electron-phonon (or, more generally, particle-phonon) systems based on a model comprising both local (Holstein-type) and nonlocal (Peierls-type) couplings. We study this model using a variational method supplemented by the exact numerical diagonalization on a system of finite size. By way of subsequent numerical diagonalization of the reduced density matrix, we determine the particle-phonon entanglement as given by the von Neumann and linear entropies. Our results are strongly indicative of the intimate relationship between the particle localization/delocalization and the particle-phonon entanglement. In particular, we find a compelling evidence for the existence of a nonanalyticity in the entanglement entropies with respect to the Peierls-coupling strength. The occurrence of such nonanalyticity—not accompanied by an actual quantum phase transition—reinforces analogous conclusion drawn in several recent studies of entanglement in the realm of quantum-dissipative systems. In addition, we demonstrate that the entanglement entropies saturate inside the self-trapped region where the small-polaron states are nearly maximally mixed.

A superglass is a phase of matter which is characterized at the same time by superfluidity and a frozen amorphous structure. We introduce a model of interacting bosons in three dimensions that displays this phase unambiguously and that can be analyzed exactly or using controlled approximations. Employing a mapping between quantum Hamiltonians and classical Fokker-Planck operators, we show that the ground-state wave function of the quantum model is proportional to the Boltzmann measure of classical hard spheres. This connection allows us to obtain quantitative results on static and dynamic quantum correlation functions. In particular, by translating known results on the glassy dynamics of Brownian hard spheres we work out the properties of the superglass phase and of the quantum phase transition between the superfluid and the superglass phase.

Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet ${\text{FeBO}}_3$. Impulsive stimulated Raman scattering (ISRS) is proven to be the microscopic mechanism of the excitation. It is shown that coherent magnons can be excited by both linearly and circularly polarized laser pulses where the efficiency of the process depends on the mutual orientation of the magnetic and crystallographic axes and the light propagation direction. The strong ellipticity of the ferromagnetic magnon mode is demonstrated, both experimentally and theoretically, to be essential for the excitation and observation of such coherent magnons. Because of this ellipticity, the amplitude of the coherent magnons excited by linearly polarized light may exceed by 2 orders of magnitude the amplitude of those excited by circularly polarized light. The primary difference between the excitation of coherent magnons by linearly polarized pulses via ISRS and via the earlier reported process of photoinduced magnetic anisotropy is discussed. Furthermore, the ISRS process is found to be responsible for the excitation of two optical phonon branches (8.4 and 12.1 THz) observed in our experiments. A coherent excitation, with a temperature-independent frequency of 0.7 THz, has also been observed in the magnetically ordered phase but could not be assigned to any optical phonon modes known in ${\text{FeBO}}_3$. The well-pronounced dependence of the amplitude of this mode on temperature suggests that this mode of nonmagnetic origin becomes Raman active only in the magnetically ordered phase and, therefore, can be excited and observed only below the Néel temperature.

We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent and thermal quantities in quantum systems. For time-dependent systems, we modify a previous mapping to quantum circuits to significantly reduce the computer resources required. This modification is based on a principle of “observing” the system outside the light cone. We apply this method to study spin relaxation in systems started out of equilibrium with initial conditions that give rise to a very rapid entanglement growth. We also show that it is possible to approximate time evolution under a local Hamiltonian by a quantum circuit whose light cone naturally matches the Lieb-Robinson velocity. Asymptotically, these modified methods allow a doubling of the system size that one can obtain as compared to a direct simulation. We then consider a different problem of thermal properties of disordered spin chains and use quantum belief propagation to average over different configurations. We test this algorithm on one-dimensional systems with mixed ferromagnetic and antiferromagnetic bonds, where we can compare to quantum Monte Carlo, and then we apply it to the study of disordered, frustrated spin systems.