A precise calculation that translates shifts of x-ray $K$ absorption edges to variations of thermodynamic properties allows quantitative characterization of interior thermodynamic properties of warm dense plasmas by x-ray absorption techniques, which provides essential information for inertial confinement fusion and other astrophysical applications. We show that this interpretation can be achieved through an improved first-principles method. Our calculation shows that the shift of $K$ edges exhibits selective sensitivity to thermal parameters and thus would be a suitable temperature index to warm dense plasmas. We also show with a simple model that the shift of $K$ edges can be used to detect inhomogeneity inside warm dense plasmas when combined with other experimental tools.

The valence band structure of $p$-type transparent oxides—crystalline ${\text{Mg}}_x{\text{Cr}}_{2-x}{\text{O}}_3$ and nanocrystalline ${\text{Cu}}_x{\text{CrO}}_y$—is analyzed as a function of incoming photon energy. The valence band of both $p$-type transparent conducting oxides shows striking similarities to measurements on crystalline ${\text{CuCrO}}_2$:Mg with all films showing that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the Cr-${\mathrm{O}}_6$ octahedra. A comparison of the valence band between the best performing $p$-type, crystalline ${\mathrm{CuCrO}}_2$:Mg, with crystalline ${\text{Mg}}_x{\text{Cr}}_{2-x}{\text{O}}_3$ and nanocrystalline ${\text{Cu}}_x{\text{CrO}}_y$ shows that the chromium $3d$ states are fixed irrespective of changes in long-range crystallographic order. This indicates little spatial overlap between adjacent Cr $3d$ states. This further confirms the conduction mechanism via hopping for chromium based $p$-type TCOs as the Cr $3d$ states are localized within the Cr-${\mathrm{O}}_6$ octahedra.

We report on the formation of critical states in disordered graphene, at the origin of variable and unconventional transport properties in the quantum Hall regime, such as a zero-energy Hall conductance plateau in the absence of an energy band gap and Landau-level degeneracy breaking. By using efficient real-space transport methodologies, we compute both the dissipative and Hall conductivities of large-size graphene sheets with random distribution of model single and double vacancies. By analyzing the scaling of transport coefficients with defect density, system size, and magnetic length, we elucidate the origin of anomalous quantum Hall features as magnetic-field-dependent impurity states, which percolate at some critical energies. These findings shed light on unidentified states and quantum-transport anomalies reported experimentally.

We consider the deformation potential mechanism of the electron-phonon coupling in metal films and investigate the intensity of the associated heat transfer between the electron and phonon subsystems. The focus is on the temperature region below dimensional crossover $T<T^{*}$ where the thermally relevant vibrations are described in terms of a quasi-two-dimensional elastic medium, while electron excitations behave as a three-dimensional Fermi gas. We derive an explicit expression for the power $P\left(T\right)$ of the electron-phonon heat transfer which explains the behavior observed in some experiments including the case of metallic film supported by an insulating membrane with different acoustic properties. It is shown that at low temperatures the main contribution is due to the coupling with Lamb's dilatational and flexural acoustic modes.

Knowledge of the electronic band structure of multiferroic oxides, crucial for the understanding and tuning of photoinduced effects, remains very limited even in the model and thoroughly studied ${\mathrm{BiFeO}}_3$. Here, we investigate the electronic band structure of ${\mathrm{BiFeO}}_3$ using Raman scattering with twelve different excitation wavelengths ranging from the blue to the near infrared. We show that resonant Raman signatures can be assigned to direct and indirect electronic transitions, as well as in-gap electronic levels, most likely associated with oxygen vacancies. Their temperature evolution establishes that the remarkable and intriguing variation of the optical band gap can be related to the shrinking of an indirect electronic band gap, while the energies for direct electronic transitions remains nearly temperature independent.

The obtention of spontaneous Raman photons is analyzed in singly charged $p$-doped quantum dots in the absence of an external magnetic field. The use of a far detuned single driving laser allows one to obtain a Raman photon line which exhibits a subnatural linewidth, and whose center can be tuned by changing the detuning and/or the Rabi frequency of the driving field. The Raman photons are produced along the undriven transition and they arise from a weak interaction of the trion states with the nuclear spins. The operating point for the gate voltage of the heterostructure can also be used to modify the linewidth and the peak value of the fluorescent signal.

We have systematically investigated the annealing effect on the structure and composition of the polar surface of $\mathrm{SrTi}{\mathrm{O}}_3$ (111) starting with an ex situ chemical etch. The relative surface concentration between Ti and Sr strongly depends on both the annealing temperature and the oxygen processing. There is a critical annealing temperature at which the maximum concentration ratio of Ti to Sr is achieved while still maintaining a $(1\times 1)$ surface structure. We demonstrate that with proper processing it is possible to avoid surface reconstruction over a broad temperature range. Our results provide an optimal temperature window for epitaxial film growth.

The joint effect of exchange and Rashba spin-orbit interactions is examined on the surface and quantum well states of ${\mathrm{Ag}}_2\mathrm{Bi}$-terminated Ag films grown on ferromagnetic Fe(110). The system displays a particular combination of time-reversal and translational symmetry breaking that strongly influences its electronic structure. Angle-resolved photoemission reveals asymmetric band-gap openings, due to spin-selective hybridization between Rashba-split surface states and exchange-split quantum well states. This results in an unequal number of states along positive and negative reciprocal space directions. We suggest that the peculiar asymmetry of the discovered electronic structure can have significant influence on spin-polarized transport properties.

Hydrodynamic flow occurs in an electron liquid when the mean free path for electron-electron collisions is the shortest length scale in the problem. In this regime, transport is described by the Navier-Stokes equation, which contains two fundamental parameters, the bulk and shear viscosities. In this paper, we present extensive results for these transport coefficients in the case of the two-dimensional massless Dirac fermion liquid in a doped graphene sheet. Our approach relies on microscopic calculations of the viscosities up to second order in the strength of electron-electron interactions and in the high-frequency limit, where perturbation theory is applicable. We then use simple interpolation formulas that allow to reach the low-frequency hydrodynamic regime where perturbation theory is no longer directly applicable. The key ingredient for the interpolation formulas is the “viscosity transport time” ${\tau }_{\mathrm{v}}$, which we calculate in this paper. The transverse nature of the excitations contributing to ${\tau }_{\mathrm{v}}$ leads to the suppression of scattering events with small momentum transfer, which are inherently longitudinal. Therefore, contrary to the quasiparticle lifetime, which goes as $-1/[T^{2}ln(T/T_{\mathrm{F}})]$, in the low-temperature limit we find ${\tau }_{\mathrm{v}}\sim 1/T^2$.

The phonovoltaic (pV) cell is similar to the photovoltaic. It harvests nonequilibrium (hot) optical phonons $\left(E_{p,\mathrm{O}}\right)$ more energetic than the band gap $\left(\mathrm{\Delta }{E}_{e,g}\right)$ to generate power in a p-n junction. We examine the theoretical electron-phonon and phonon-phonon scattering rates, the Boltzmann transport of electrons, and the diode equation and hydrodynamic simulations to describe the operation of a pV cell and develop an analytic model predicting its efficiency. Our findings indicate that a pV material with $E_{p,\mathrm{O}}\simeq \mathrm{\Delta }{E}_{e,g}\gg k_{\mathrm{B}}T$, where $k_{\mathrm{B}}T$ is the thermal energy, and a strong interband electron-phonon coupling surpasses the thermoelectric limit, provided the optical phonon population is excited in a nanoscale cell, enabling the ensuing local nonequilibrium. Finding and tuning a material with these properties is challenging. In Paper II [C. Melnick and M. Kaviany, Phys. Rev. B 93, 125203 (2016)], we tune the band gap of graphite within density functional theory through hydrogenation and the application of isotropic strains. The band gap is tuned to resonate with its energetic optical phonon modes and calculate the ab initio electron-phonon and phonon-phonon scattering rates. While hydrogenation degrades the strong electron-phonon coupling in graphene such that the figure of merit vanishes, we outline the methodology for a continued material search.

We investigate the classical Rényi entropy $S_n$ and the associated mutual information ${\mathcal{I}}_n$ in the Sherrington-Kirkpatrick (S-K) model, which is the paradigm model of mean-field spin glasses. Using classical Monte Carlo simulations and analytical tools we investigate the S-K model in the $n$-sheet booklet. This is achieved by gluing together $n$ independent copies of the model, and it is the main ingredient for constructing the Rényi entanglement-related quantities. We find a glassy phase at low temperatures, whereas at high temperatures the model exhibits paramagnetic behavior, consistent with the regular S-K model. The temperature of the paramagnetic-glassy transition depends nontrivially on the geometry of the booklet. At high temperatures we provide the exact solution of the model by exploiting the replica symmetry. This is the permutation symmetry among the fictitious replicas that are used to perform disorder averages (via the replica trick). In the glassy phase the replica symmetry has to be broken. Using a generalization of the Parisi solution, we provide analytical results for $S_n$ and ${\mathcal{I}}_n$ and for standard thermodynamic quantities. Both $S_n$ and ${\mathcal{I}}_n$ exhibit a volume law in the whole phase diagram. We characterize the behavior of the corresponding densities, $S_n/N$ and ${\mathcal{I}}_n/N$, in the thermodynamic limit. Interestingly, at the critical point the mutual information does not exhibit any crossing for different system sizes, in contrast with local spin models.

We propose inducing Bose-Einstein condensation of magnons in a magnetic insulator by a heat flow oriented toward its boundary. At a critical heat flux, the oversaturated thermal gas of magnons accumulated at the boundary precipitates the condensate, which then grows gradually as the thermal bias is dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin) Seebeck effect must generally overcome both the local Gilbert damping associated with the coherent magnetic dynamics as well as the radiative spin-wave losses toward the magnetic bulk, in order to achieve the threshold of condensation. We quantitatively estimate the requisite bias in the case of the ferrimagnetic yttrium iron garnet, discuss different physical regimes of condensation, and contrast it with the competing (so-called Doppler-shift) bulk instability.

We report magnetodielectric coupling in rhombohedral ilmenites ${\mathrm{CoTiO}}_3$ and ${\mathrm{NiTiO}}_3$ along with heat capacity and magnetic measurements. ${\mathrm{CoTiO}}_3$ and ${\mathrm{NiTiO}}_3$ undergo antiferromagnetic ordering at $\approx$ 38 and $\approx$ 22 K, respectively. The dielectric permittivity of ${\mathrm{CoTiO}}_3$ decreases below its magnetic ordering temperature while the dielectric permittivity of ${\mathrm{NiTiO}}_3$ increases below the Néel temperature. The dielectric permittivity of ${\mathrm{CoTiO}}_3$ and ${\mathrm{NiTiO}}_3$ are dependent on the applied magnetic field and temperature with the strongest dependence observed close to the magnetic-ordering temperature. We analyze magnetodielectric coupling in ${\mathrm{CoTiO}}_3$ and ${\mathrm{NiTiO}}_3$ on the basis of spin-phonon coupling and suggest other possible mechanisms such as exchange coupling and magnetostrictive effects.

We study theoretically a one-dimensional quasiperiodic Fermi system with topological $p$-wave superfluidity, which can be deduced from a topologically nontrivial tight-binding model on the square lattice in a uniform magnetic field and subject to a non-Abelian gauge field. The system may be regarded as a non-Abelian generalization of the well-known Aubry-André-Harper model. We investigate its phase diagram as a function of the strength of the quasidisorder and the amplitude of the $p$-wave order parameter through a number of numerical investigations, including a multifractal analysis. There are four distinct phases separated by three critical lines, i.e., two phases with all extended wave functions [(I) and (IV)], a topologically trivial phase (II) with all localized wave functions, and a critical phase (III) with all multifractal wave functions. Phase (I) is related to phase (IV) by duality. It also seems to be related to phase (II) by duality. Our proposed phase diagram may be observable in current cold-atom experiments, in view of simulating non-Abelian gauge fields and topological insulators/superfluids with ultracold atoms.

The pulsed flow of solid ${}^4\mathrm{He}$ through a narrow capillary in a flow system which issues into vacuum is investigated at temperatures between 1.64 and 2.66 K and pressures between 54 and 104 bars. After each pulse three different capillary flow regimes are observed as the upstream pressure decreases: an oscillatory [mini-geyser (MG)] regime, a constant flow (CF) regime with a linearly decreasing pressure difference, and a nonresistant (NR) regime. A quantitative analysis of the three regimes suggests that the flow of solid ${}^4\mathrm{He}$ is driven by a counterflow of excess vacancies, which are injected downstream of the capillary at the solid/liquid interface near the micrometric orifice exposed to vacuum. The CF regime, where the flow velocity is found to be independent of the pressure difference, and the NR regime, where the solid flows as a Bernoulli fluid, suggest a new dynamic phase of solid helium induced by a steady influx of vacancies.

Motivated by exploring the effect of interactions on Chern insulators, and by recent experiments realizing topological bands for ultracold atoms in synthetic gauge fields, we study the honeycomb lattice Haldane-Hubbard model of spin-$\frac{1}{2}$ fermions. Using an unrestricted mean field approach, we map out the instability of the topological band insulator towards magnetically ordered insulators which emerge with increasing Hubbard repulsion. In addition to the topological Néel phase, we recover various chiral noncoplanar magnetic orders previously identified within a strong-coupling approach. We compute the band structure of these ordered phases, showing that the triple-$Q$ tetrahedral phase harbors topological Chern bands with large Chern numbers.

The functional renormalization group (RG) in combination with Fermi surface patching is a well-established method for studying Fermi liquid instabilities of correlated electron systems. In this paper, we further develop this method and combine it with mean-field theory to approach multiband systems with spin-orbit coupling, and we apply this to a tight-binding Rashba model with an attractive, local interaction. The spin dependence of the interaction vertex is fully implemented in a RG flow without SU(2) symmetry, and its momentum dependence is approximated in a refined projection scheme. In particular, we discuss the necessity of including in the RG flow contributions from both bands of the model, even if they are not intersected by the Fermi level. As the leading instability of the Rashba model, we find a superconducting phase with a singlet-type interaction between electrons with opposite momenta. While the gap function has a singlet spin structure, the order parameter indicates an unconventional superconducting phase, with the ratio between singlet and triplet amplitudes being plus or minus one on the Fermi lines of the upper or lower band, respectively. We expect our combined functional RG and mean-field approach to be useful for an unbiased theoretical description of the low-temperature properties of spin-based materials.

Resonant graphene dopants, such as hydrogen adatoms, experience long-range effective interaction mediated by conduction electrons. As a result of this interaction, when several adatoms are present in the sample, hopping of adatoms between sites belonging to different sublattices involves significant energy changes. Different inelastic mechanisms facilitating such hopping, coupling to phonons and conduction electrons, are considered. It is estimated that the diffusion of hydrogen adatoms is rather slow, amounting to roughly one hop to a nearest neighbor per millisecond.