Based on first-principles calculations and semiclassical transport theory, we study the nonequilibrium phonon distribution in graphene in the presence of electrical current. In addition to the hot-phonon effect, we observe a cooling of phonon modes at the same time. Interestingly, the presence of electric current along the direction connecting $K$ and $K^{\prime }$ valleys induces an opposite dipolelike temperature distribution in the two valleys and at the $\mathrm{\Gamma }$ point. This leads to a “high-order” valley polarization of phonon distribution between $K$ and $K^{\prime }$. Based on the nonequilibrium phonon distribution, we furthermore study their effect on the lattice parameters and find a negative current-induced expansion coefficient. Similar to graphene's negative thermal expansion, this is rooted in the dominant contribution of out-of-plane acoustic phonons, which do not couple directly to electrons, but gain energy from in-plane phonons through anharmonic scattering.

Dynamics of the imbalance in occupations on even and odd sites of a lattice serves as one of the key characteristics for the identification of the many-body localization transition. In this work, we investigate the long-time behavior of the imbalance in disordered one- and two-dimensional many-body systems in the regime of diffusive or subdiffusive transport. We show that memory effects originating from a coupling between slow and fast modes lead to a power-law decay of the imbalance, with the exponent determined by the diffusive (or subdiffusive) transport law and the spatial dimensionality. Analytical results are supported by numerical simulations performed on a two-dimensional system in the regime of weak localization.

The mobility of carriers, as limited by their scattering with phonons, can now routinely be obtained from first-principles electron-phonon coupling calculations. However, so far, most computations have relied on some form of simplification of the linearized Boltzmann transport equation based on either the self-energy or the momentum relaxation-time or constant relaxation-time approximations. Here, we develop a high-throughput infrastructure and an automatic workflow and we compute 67 phonon-limited mobilities in semiconductors. We compare the results resorting to the approximations with the exact iterative solution. We conclude that the approximate values may deviate significantly from the exact ones and are thus not reliable. Given the minimal computational overhead, our paper encourages reliance on this exact iterative solution.

Helical magnets are extremely promising, as they have fascinating magnetic, electric, and phononic properties. Here, we report a spectrum of simultaneously occurring and highly entangled intriguing phenomena induced by helical spin ordering in a noncentrosymmetric and spin-frustrated system ${\mathrm{Fe}}_3\left(\mathrm{P}{\mathrm{O}}_4\right){\mathrm{O}}_3$. These phenomena include magnetodielectric effect in the form of a frequency-independent pronounced dielectric peak, clear magnetostriction effect manifested as a dramatic downturn in the thermal variation of lattice parameters, and strong spin-phonon coupling (which displays a unique anomalous hardening and softening of various phonon modes) at temperatures as high as $T_N=163\mathrm{K}$. The observed dielectric peak is seemingly associated with a structural distortion via the strong magnetostriction effect.

We report two fundamental inequalities in the Stoner-Wohlfarth model. Specifically, we investigate the theoretical limit for the initial magnetic susceptibility in a system described by the Stoner-Wohlfarth approach. We also find analytical solutions for the magnetization in the low-field regime and obtain the borderline value for the uniaxial anisotropy constant in such an ideal Stoner-Wohlfarth system. We go beyond and introduce a general mean-field theory for interacting Stoner-Wohlfarth-like systems, thus estimating how the initial magnetic susceptibility is affected by the dipolar and exchange interactions inside the system. By means of a simple insight from a fundamental inequality for the magnetic susceptibility of an ideal Stoner-Wohlfarth system, we show its violation is a signature of the existence of exchange interactions between nanoparticles in an interacting Stoner-Wohlfarth-like system.

We performed a first-principles study of the tunneling anisotropic magnetoresistance (TAMR) in Ag(Ir,Pt)/MgO/Fe junctions. An enhanced TAMR of the order of 10% is found in the junctions at the equilibrium state, which shows ideal and skewed fourfold angular dependence for the in-plane and out-of-plane TAMR, respectively. The TAMR effect in the junctions shows a simple barrier thickness dependence with the largest TAMR effect present in the junction with an $\sim 6$-monolayer MgO barrier. The interfacial states due to the spin-orbit coupling effect should be responsible for the complex and enhanced TAMR effect found in these junctions. A little interfacial oxygen vacancy disorder at the Fe/MgO interface can noticeably deteriorate the TAMR effect.

Two-dimensional magnetic materials hold great promise for applications toward efficient data storage and transfer. It would be a huge advantage if their ground-state properties had strong responses against external stimulations, such as magnetic and electric fields. Here, we report several intriguing discoveries in single-crystal ${\mathrm{Cr}}_2{\mathrm{Te}}_3$. Based on comprehensive specific heat, differential scanning calorimetry, variable-temperature x-ray diffraction, and linear-thermal-expansion measurements, we find that ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ has a low-temperature ferromagnetic (FM) ground-state phase, which changes to an antiferromagnetic (AFM) phase when temperature is increased to $T_{\mathrm{C}}$ = 160 K. This FM-AFM transition is a first-order phase transition, and the transition temperature can be further enhanced to 178 K by a moderate magnetic field. At the same time, the first-order phase transition will transform into a second-order phase transition, indicating strong spin-lattice coupling (SLC). A second-order AFM-paramagnetic phase transition emerges at $T_{\mathrm{N}}=181$ K. This AFM phase is gradually suppressed by the magnetic field and eventually disappears at a critical field of 0.48 T. The SLC results in a notable negative-thermal-expansion coefficient of $-17.2$ ppm/K and a remarkable magnetostriction coefficient of 44.7 ppm/T at 200 K. With the assistance of first-principles density functional theory calculations and Monte Carlo simulations, we conclude that the collinear FM, canted FM, and AFM configurations in ${\mathrm{Cr}}_2{\mathrm{Te}}_3$ depend on the temperature and applied magnetic field. Our work reveals the highly tunable magnetic phases and SLC in ${\mathrm{Cr}}_2{\mathrm{Te}}_3$, which will be helpful for developing the potential functionalities of this material.

Highly sophisticated synthesis methods and experimental techniques allow for precise measurements of magnetic properties of nanoparticles that can be reliably reproduced using theoretical models. Here, we investigate the magnetic properties of ferrite nanoparticles by using theoretical techniques based on Monte Carlo methods. We introduce three stages of sophistication in the macromagnetic model. First, by using tailor-made Hamiltonians we study single nanoparticles. In a second stage, the internal structure of the nanoparticle is taken into consideration by defining an internal (core) and external (shell) region, respectively. In the last stage, an assembly of core-shell nanoparticles is considered. All internal magnetic couplings such as interatomic and intra-atomic exchange interactions or magnetocrystalline anisotropies have been estimated. Moreover, the hysteresis loops of the aforementioned three cases have been calculated and compared with recent experimental measurements. In the case of the assembly of nanoparticles, the hysteresis loops together with the zero-field-cooling and field-cooling curves are shown to be in a very good agreement with the experimental data. The current model provides an important tool to understand the internal structure of the nanoparticles together with the complex internal spin interactions of the core-shell ferrite nanoparticles.

We investigate the dynamic structure factor of the antiferromagnetic Kitaev honeycomb model in a magnetic field by applying perturbative continuous unitary transformations about the high-field limit. One- and two-quasiparticle properties of the dressed elementary spin-flip excitations of the high-field polarized phase are calculated, which account for most of the spectral weight in the dynamic structure factor. We discuss the evolution of spectral features in these quasiparticle sectors in terms of one-quasiparticle dispersions, two-quasiparticle continua, the formation of antibound states, and quasiparticle decay. In particular, a comparably strong spectral feature above the upper edge of the upmost two-quasiparticle continuum represents three antibound states which form due to nearest-neighbor density-density interactions.

Quantum fluctuations enhanced by frustration and subtle interplay between competing degrees of freedom offer an ideal ground to realize novel states with fractional quantum numbers in quantum materials that defy standard theoretical paradigms. Quantum spin liquid (QSL) is a highly entangled state wherein frustration-induced strong quantum fluctuations preclude symmetry-breaking phase transitions down to zero temperature without any order parameter. Experimental realizations of QSL in quantum materials with spin dimensionality greater than one is very rare. Here, we present our thermodynamic, nuclear magnetic resonance, muon spin relaxation, and inelastic neutron scattering studies of a rare-earth hyperkagome compound ${\mathrm{Li}}_3{\mathrm{Yb}}_3{\mathrm{Te}}_2{\mathrm{O}}_{12}$ in which ${\mathrm{Yb}}^{3+}$ ions constitute a three-dimensional spin lattice without any detectable disorder. Our comprehensive experiments evince neither signature of magnetic ordering nor spin freezing down to 38 mK that suggest the realization of dynamic liquid-like ground state in this antiferromagnet. The ground state of this material is interpreted by a low energy $J_{\mathrm{eff}}=1/2$ degrees of freedom with short-range spin correlations. The present results demonstrate a viable basis to explore spin-orbit driven enigmatic correlated quantum states in a class of rare-earth-based three-dimensional frustrated magnets that may open avenues in theoretical and experimental search for spin liquids.

The critical current of a superconductor can depend on the direction of current flow due to magnetochiral anisotropy when both inversion and time-reversal symmetry are broken, an effect known as the superconducting (SC) diode effect. Here, we consider one-dimensional (1D) systems in which superconductivity is induced via the proximity effect. In both topological insulator and Rashba nanowires, the SC diode effect due to a magnetic field applied along the spin-polarization axis and perpendicular to the nanowire provides a measure of inversion symmetry breaking in the presence of a superconductor. Furthermore, a strong dependence of the SC diode effect on an additional component of magnetic field applied parallel to the nanowire as well as on the position of the chemical potential can be used to detect that a device is in the region of parameter space where the phase transition to topological superconductivity is expected to arise.

Recent experiments on superconducting cavities have found that under large rf electromagnetic fields the quality factor can improve with increasing field amplitude, a so-called “anti-$Q$ slope.” Linear theories of dissipation break down under these extreme conditions and are unable to explain this behavior. We numerically solve the Bogoliubov-de Gennes equations at the surface of a superconductor in a parallel AC magnetic field, finding that at large fields there are quasiparticle surface states with energies below the bulk value of the superconducting gap. As the field oscillates, such states emerge and disappear with every cycle. We consider the dissipation resulting from inelastic quasiparticle-phonon scattering into these states and investigate the ability of this mechanism to explain features of the experimental observations, including the field dependence of the quality factor. We find that this mechanism is likely not the dominant source of dissipation and does not produce an anti-$Q$ slope by itself; however, we demonstrate in a modified two-fluid model how these bound states can play a role in producing an anti-$Q$ slope.

We investigate a highly frustrated spin-1 model on the triangular lattice, with nearest- and next-nearest-neighbor antiferromagnetic $S·S$ interactions and nearest-neighbor ${(S·S)}^2$ interactions. Using the density matrix renormalization group technique, we find three magnetically ordered phases, namely ${120}^{\circ }$ spiral order, stripe order, and tetrahedral order, as well as two spin nematic phases: ferroquadrupolar and antiferroquadrupolar. While our data could be consistent with a spin liquid phase between the ${120}^{\circ }$ spiral and antiferroquadrupolar orders, the more likely scenario is a direct continuous transition between these two orders.

We propose a general construction of commuting projector lattice models for 2D and 3D topological phases enriched by U(1) symmetry, with finite-dimensional Hilbert space per site. The construction starts from a commuting projector model of the topological phase and decorates U(1) charges to the state space in a consistent manner. We show that all 2D U(1) symmetry-enriched topological phases, which allow gapped boundaries without breaking the symmetry, can be realized through our construction. We also construct a large class of 3D topological phases with U(1) symmetry fractionalized on particles or loop excitations.

We report a transport, thermodynamic, and spectroscopic study of ${\mathrm{ZrTe}}_5$ with a focus on elucidating the connections between its band structure and unusual thermoelectric properties. Using time and angle resolved photoemission spectroscopy we observe a small electronic band gap and temperature dependent Fermi level which traverses from a single valence to conduction band with lowering temperature, consistent with previous reports. This low temperature Fermi surface closely matches that derived from quantum oscillations, suggesting it is reflective of the bulk electronic structure. The Seebeck and low field Nernst response is characterized by an unusually large and nonmonotonic temperature evolution. We find this can be quantitatively explained using a semiclassical model based on the observed band character and a linear temperature shifting of the Fermi level. Additionally, we observe a large, nonsaturating enhancement of both thermoelectric coefficients in magnetic field. We show this can be captured by the Zeeman energy associated with a large effective $g$ factor of 25.8 consistent with that derived from Lifshitz-Kosevich analysis of the quantum oscillations. Together these observations provide a comprehensive picture of ${\mathrm{ZrTe}}_5$ as a model high mobility small Fermi surface system and potential platform for significant magnetic field driven thermoelectricity.

We discuss and analyze the properties of Casimir forces acting between nonreciprocal objects in thermal equilibrium. By starting from the fluctuation-dissipation theorem and splitting the force into those arising from individual sources, we show that if all temperatures are equal, the resulting force is reciprocal and is derivable as the gradient of a Casimir (free) energy. While the expression for the free energy is identical to the one for reciprocal objects, there are several distinct features: To leading order in reflections, the free energy can be decomposed as the sum of two terms, the first corresponding to two reciprocal objects, and the second corresponding to two antireciprocal objects. The first term is negative and typically yields attraction, while the second can have either sign. For the case of two objects that are each other's mirror images, the second term is positive and yields repulsion. The sum of terms can lead to overall repulsive forces, in agreement with previous observations. Stable configurations, ruled out for reciprocal cases, appear possible for nonreciprocal objects. We show that for three objects, a three-body free energy exists, indicating that previously found persistent heat currents in situations of three objects cannot be used to produce persistent torques.

High-resolution scanning tunneling microscopy and spectroscopy enlighten the formation and composition of self-organized Co-induced structures on Ge(001). Dimer vacancies due to subsurface Co accumulate and lead to the formation of well-ordered embedded structures which consist of two alternating building blocks. A mismatch in the alternation leads to protrusions at both sides of the structures. The deposition of small amounts of Au changes the internal order of the embedded Co structures and results in longer elongated chains. Supported by Monte Carlo simulations, this modification of chain length is attributed to stress relief due to the insertion of Au-induced dimer vacancies in the Ge(001) surface.

Within the framework of the density matrix method, general formulas are obtained that are convenient for describing fast pulsed photoemission that occurs in a time less than or on the order of the times of relaxation processes inside the photocathode. Expressions for the elements of the density matrix are found by solving the kinetic equation that takes into account the alternating electromagnetic field of light pumping and inelastic scattering of electrons. The derived formulas are applied for the numerical-analytical study of a one-dimensional model of wavelike spatiotemporal modulation of a photoelectron pulse of suitable duration during its passage through a double-well quantum-well heterostructure deposited on a volumetric planar photocathode. This modulation is a quantum beat that occurs as a result of excitation and subsequent slow oscillatory decay of the superposition of the doublet of quasistationary states of the heterostructure. It is possible to provide prolongation of generation and even amplification of waves of charge density and current density of photoelectrons when the photocathode is exposed to a periodic sequence of light pulses.

The topological nature of the band structure of a Weyl semimetal leads to a number of unique transport and optical properties. For example, the description of the propagation of an electromagnetic wave in a Weyl semimetal with broken time-reversal and inversion symmetry, for example, requires a modification of the Maxwell equations by the axion field $\theta (\mathbf{r},t)=2\mathbf{b}·\mathbf{r}-2b_{0}t,$ where $2\mathbf{b}$ is the separation in wave vector space between two Weyl nodes of opposite chiralities and $2\hslash b_0$ is their separation in energy. In this paper, we study theoretically how the axion terms $b_0$ and $\mathbf{b}$ modify the frequency behavior of the Kerr rotation and ellipticity angles ${\theta }_K\left(\omega \right)$ and ${\psi }_K\left(\omega \right)$ in a Weyl semimetal. Both the Faraday and Voigt configurations are considered since they provide different information on the electronic transitions and plasmon excitation. We derive the Kerr angles firstly without an external magnetic field where the rotation of the polarization is only due to the axion terms and secondly in a strong magnetic field where these terms compete with the gyration effect of the magnetic field. In this latter case, we concentrate on the ultra-quantum limit where the Fermi level lies in the chiral Landau level and the Kerr and ellipticity angles have more complex frequency and magnetic field behaviors.

We discuss contributions to the thermopower in an electron fluid. A simple argument based on Newton's second law with the pressure gradient as the force suggests that the thermopower is given by a thermodynamic derivative, viz., the entropy per particle, rather than being an independent transport coefficient. The resolution is the existence of an entropic force that results from a coupling between the mass current and the heat current in the fluid. We also discuss and clarify some aspects of a recent paper [Phys. Rev. B 102, 214306 (2020)] that provided a method for exactly solving electronic transport equations in the low-temperature limit.