We propose a method to evaluate the diffusion coefficient of vacancy-hydrogen clusters (${\mathrm{VH}}_n)$ in metals. The key is a good separation of time scales between H diffusion and the metal-vacancy exchange. The Ni-H system is investigated in details, using ab initio calculations, but the arguments can be transposed to other systems. It is shown that cluster diffusion can be treated as an uncorrelated random walk and that H is always in equilibrium before the vacancy-metal exchange. Then, the diffusion coefficient is a sum over jump paths of the equilibrium probability of being in a specific ${\mathrm{VH}}_n$ configuration times the corresponding activation terms. The influence of H on the energy barrier is well reproduced by effective pair interactions between the jumping Ni and the H atoms inside the vacancy. This model is motivated by an analysis of the electronic charge redistribution in key saddle configurations. The interaction is repulsive and decreases with distance. The model is used to find easy jump path, reduce the number of saddle searches, and provide an estimate of the error expected from this reduction. The application to the Ni-H system shows that vacancies are drastically slowed down by H. The effects of temperature and bulk H concentration are explored and the origin of the non-Arrhenius behavior is explained. At equilibrium, ${\mathrm{VH}}_n$ clusters always induce a speedup of metal diffusion. The implications concerning H induced damage, in particular in regards to Ni-Cr oxidation, are discussed.

We investigate the validity of the non-Hermitian Hamiltonian approach in describing quantum transport in disordered tight-binding networks connected to external environments, acting as sinks. Usually, non-Hermitian terms are added, on a phenomenological basis, to such networks to summarize the effects of the coupling to the sinks. Here, we consider a paradigmatic model of open quantum network for which we derive a non-Hermitian effective model, discussing its limit of validity by a comparison with the analysis of the full Hermitian model. Specifically, we consider a ring of sites connected to a central one-dimensional lead. The lead acts as a sink that absorbs the excitation initially present in the ring. The coupling strength to the lead controls the opening of the ring system. This model has been widely discussed in literature in the context of light-harvesting systems. We analyze the effectiveness of the non-Hermitian description both in absence and in presence of static disorder on the ring. In both cases, the non-Hermitian model is valid when the energy range determined by the eigenvalues of the ring Hamiltonian is smaller than the energy band in the lead. Under such condition, we show that results about the interplay of opening and disorder, previously obtained within the non-Hermitian Hamiltonian approach, remain valid when the full Hermitian model in presence of disorder is considered. The results of our analysis can be extended to generic networks with sinks, leading to the conclusion that the non-Hermitian approach is valid when the energy dependence of the coupling to the external environments is sufficiently smooth in the energy range spanned by the eigenstates of the network.

Molecular dynamics simulations using a first-principles-derived effective Hamiltonian are conducted on lead zirconium titanate ultrathin films possessing nanoscale ferroelectric domains and experiencing a GHz electric field. Pulses of magnetization are predicted to occur in this system when sudden changes occur in the morphology of these nanodomains. A simple equation relating the magnetization and product between the electric polarization and its time derivative is found to reproduce and explain these magnetization pulses as well as previously observed magnetoelectric effects in moving ferroelectric domain walls and phase boundaries in ferroelectrics and magnetoelectrics.

Thermoelectric energy conversion is a direct but low-efficiency process, which precludes the development of long-awaited wide-scale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling enhancement. The cornerstone of our approach is the observation that heat engines are particularly efficient when their operation involves a phase transition of their working fluid. We derive and compute the thermoelastic coefficients of various systems, including Bose and Fermi gases, and fluctuation Cooper pairs. Combination of these coefficients yields the definition of the thermodynamic figure of merit, the divergence of which at finite temperature indicates that conditions are fulfilled for the best possible use of the thermoelectric working fluid. Here, this situation occurs in the fluctuation regime only, as a consequence of the increased compressibility of the working fluid near its phase transition. Our results and analysis clearly show that efforts in the field of thermoelectricity can now be productively directed towards systems where electronic phase transitions are possible.

The dielectric and piezoelectric properties of an epitaxial $\mathrm{PbZ}{\mathrm{r}}_{0.8}\mathrm{T}{\mathrm{i}}_{0.2}{\mathrm{O}}_3/\mathrm{PbZ}{\mathrm{r}}_{0.6}\mathrm{T}{\mathrm{i}}_{0.4}{\mathrm{O}}_3$ ferroelectric bilayer film were studied. Time-resolved synchrotron x-ray microdiffraction provided access to layer-specific structural information during electric-field-induced changes. The observed dielectric and electromechanical responses are consistent with a weak electrostatic polarization coupling and can be described using a thermodynamic model of epitaxial ferroelectric bilayers. The weak electrostatic coupling between ferroelectric layers can enable unusual tail-to-tail and head-to-head polarization configurations. X-ray measurements of the piezoelectric response of the ferroelectric bilayer at a microsecond time scale confirmed a possible tail-to-tail polarization domain configuration.

We use the concept of typicality to study the real-time dynamics of spin and energy currents in spin-$1/2$ models in one dimension and at nonzero temperatures. These chains are the integrable XXZ chain and a nonintegrable modification due to the presence of a staggered magnetic field oriented in the $z$ direction. In the framework of linear response theory, we numerically calculate autocorrelation functions by propagating a single pure state, drawn at random as a typical representative of the full statistical ensemble. By comparing to small-system data from exact diagonalization (ED) and existing short-time data from time-dependent density matrix renormalization group, we show that typicality is satisfied in finite systems over a wide range of temperature and is fulfilled in both integrable and nonintegrable systems. For the integrable case, we calculate the long-time dynamics of the spin current and extract the spin Drude weight for large systems outside the range of ED. We particularly provide strong evidence that the high-temperature Drude weight vanishes at the isotropic point. For the nonintegrable case, we obtain the full relaxation curve of the energy current and determine the heat conductivity as a function of magnetic field, exchange anisotropy, and temperature.

The electron pairing in the topological superconductor has been predicted to be spin-triplet pairing with odd parity. If the superconducting carries have a magnetic moment, the magnetization arising from it reduces the repulsive interaction between the vortices, thus the vortex density drastically increases in the region above the lower critical magnetic field $H_{c1}$. The comparison of the theoretical calculation with the experimental result by Das et al. [Phys. Rev. B 83, 220513(R) (2011)] indicates that the magnetization is small, suggesting that it is not a spin-triplet superconductor. The theory suggests that if the nonzero magnetization arises from the supercurrent carriers a hysteresis in the magnetization curve around $H_{c1}$ may be observed. The observation (or nonobservation) of it will narrow down the mechanism of the superconductivity in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

In this Comment we would like to emphasize that in Phys. Rev. B 90, 104506 (2014) the calculated energy spectrum takes into account only the small interaction (cross) term and, additionally, this term is only calculated at the instant when the two vortices reconnect. The majority of the kinetic energy is contained in the self-energy term which has a characteristic spectrum of $1/k$. If this, and the additional average over time, is taken into account the suggested Kolmogorov-type $k^{-5/3}$ spectrum is likely not visible in the kinetic energy spectrum which contains both terms. Therefore, we find the suggestion misleading that the Kolmogorov spectrum in superfluids arises from the reconnection of vortices.

We analyze the strongly correlated regime of a two-component trapped ultracold fermionic gas in a synthetic non-Abelian $U\left(2\right)$ gauge potential, that consists of both a magnetic field and a homogeneous spin-orbit coupling. This gauge potential deforms the Landau levels (LLs) with respect to the Abelian case and exchanges their ordering as a function of the spin-orbit coupling. In view of experimental realizations, we show that a harmonic potential combined with a Zeeman term gives rise to an angular momentum term, which can be used to test the stability of the correlated states obtained through interactions. We derive the Haldane pseudopotentials (HPs) describing the interspecies contact interaction within a lowest LL approximation. Unlike ordinary fractional quantum Hall systems and ultracold bosons with short-range interactions in the same gauge potential, the HPs for sufficiently strong non-Abelian fields show an unconventional nonmonotonic behavior in the relative angular momentum. Exploiting this property, we study the occurrence of new incompressible ground states as a function of the total angular momentum. In the first deformed Landau level (DLL) we obtain Laughlin and Jain states. Instead, in the second DLL three classes of stabilized states appear: Laughlin states, a series of intermediate strongly correlated states, and finally vortices of the integer quantum Hall state. Remarkably, in the intermediate regime, the nonmonotonic HPs of the second DLL induce two-particle correlations which are reminiscent of paired states such as the Haffnian state. Via exact diagonalization in the disk geometry, we compute experimentally relevant observables such as density profiles and correlations, and we study the entanglement spectra as a further tool to characterize the obtained strongly correlated states.

Thermoelectric effects in graphene are considered theoretically with particular attention paid to the role of resonant scattering on impurities. Using the $T$-matrix method we calculate the impurity resonant states and the momentum relaxation time due to scattering on impurities. The Boltzmann kinetic equation is used to determine the thermoelectric coefficients. It is shown that the resonant impurity states near the Fermi level give rise to a resonant enhancement of the Seebeck coefficient and figure of merit $ZT$. The Wiedemann-Franz ratio deviates from that known for ordinary metals, where this ratio is constant and equal to the Lorentz number. This deviation appears for small chemical potentials and in the vicinity of the resonant states. In the limit of a constant relaxation time, this ratio has been calculated analytically for $\mu =0$.

Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semianalytical solutions of nonlocal Maxwell's equations for graphene nanoribbon arrays with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as an additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measure of nonlocal effects, and we quantify their dependence on the ribbon width, doping, and wavelength.

We have studied the low-lying electronic structure of a new ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type superconductor ${\mathrm{KNi}}_2{\mathrm{Se}}_2$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor ${\mathrm{K}}_x\mathrm{Fe}{}_{2-y}{\mathrm{Se}}_2$. The Fermi surface shows weak variation along the $k_z$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.

We consider rather general spin-$1/2$ lattices with large coordination numbers $Z$. Based on the monogamy of entanglement and other properties of the concurrence $C$, we derive rigorous bounds for the entanglement between neighboring spins, such as $C\le 1/\sqrt{Z}$, which show that $C$ decreases for large $Z$. In addition, we demonstrate that the concurrence $C$ measures the deviation from mean-field behavior and can only vanish if the mean-field ansatz yields an exact ground state of the Hamiltonian. Motivated by these findings, we propose an improved mean-field ansatz by adding entanglement.

Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around $P_{\mathrm{c}}\sim$ 5.2 GPa. Near $P_{\mathrm{c}}$, the $A$ coefficient of the $A{T}^2$ Fermi-liquid resistivity term below $T^{*}$ is largely enhanced with a maximum around 5.2–5.5 GPa. Above $P_{\mathrm{c}}$, the exponent of the resistivity $\rho \left(T\right)$ deviates from 2. At $P_{\mathrm{c}}$, it is close to $n=5/3$, which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a second-order quantum-critical point (QCP). However, $T_{\mathrm{C}}\left(P\right)$ disappears as a first-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a second-order QCP. The first-order nature of the phase transition is weak, and the electron system in URhAl is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tricritical point (TCP) at $\sim 4.8–4.9$ GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through $P_{\mathrm{c}}$, the nature of the interaction itself may change and lead to the observed unconventional behavior.

To define the conductance of single-molecule junctions with a redox functionality in an electrochemical cell, two conceptually different electron transport mechanisms, namely, coherent tunneling and vibrationally induced hopping, compete with each other, where implicit parameters of the setup such as the length of the molecule and the applied gate voltage decide which mechanism is the dominant one. Although coherent tunneling is most efficiently described within Landauer theory and the common theoretical treatment of electron hopping is based on Marcus theory, both theories are adequate for the processes they describe without introducing accuracy-limiting approximations. For a direct comparison, however, it has to be ensured that the crucial quantities obtained from electronic structure calculations, i.e., the transmission function $T\left(E\right)$ in Landauer theory and the transfer integral $V$, the reorganization energy $\lambda$, and the driving force $\Delta G^0$ in Marcus theory, are derived from similar grounds, as pointed out by Nitzan and coworkers in a series of publications. In this paper our framework is a single-particle picture, for which we perform density functional theory calculations for the conductance corresponding to both transport mechanisms for junctions with the central molecule containing one, two, or three Ruthenium centers, from which we extrapolate our results in order to define the critical length of the transition point of the two regimes which we identify at 5.76nm for this type of molecular wire. We also discuss trends in the dependence on an electrochemically induced gate potential.