The electron-phonon (e-ph) interaction in ${\mathrm{Ca}}_2\mathrm{N}$ monolayer, the first electrene material with two-dimensional (2D) electron gas floating in free space, is expected to be very weak, which can be used to design weak-scattering transport channels. Therefore, it is highly desirable to quantitatively evaluate the carrier mobility of this electrene material. In this study, the e-ph interaction in ${\mathrm{Ca}}_2\mathrm{N}$ monolayer is investigated by using a precise Wannier interpolation-based first-principles technique. The calculated e-ph coupling matrix elements of ${\mathrm{Ca}}_2\mathrm{N}$ monolayer are indeed small compared with other 2D materials such as graphene, which leads to an intrinsic mobility of $189{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is much higher than those of conventional metals. Other factors affecting mobility are discussed in a comparison with graphene. It is predicted that, based on a momentum mismatch mechanism, mobility of the ${\mathrm{Ca}}_2\mathrm{N}$ monolayer can be increased further to above $3000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ via hole doping. Our results confirm that ${\mathrm{Ca}}_2\mathrm{N}$ electrene is a promising electronic material.

A comprehensive experimental and theoretical study of indirect gap optical absorption in bulk Ge is presented. While this topic has been the subject of intense studies from the early days of semiconductor physics, the resonant aspect of the absorption received very little attention until now. This is a unique property of Ge related to the proximity of the direct and indirect gaps. The absorption coefficient was measured over the entire spectral range between the two gaps for comparison with theory. It is shown that the standard textbook expressions, obtained by assuming intermediate states with constant energy, are in very poor agreement with experiment. A theory first proposed by Hartman [R. L. Hartman, Phys. Rev. 127, 765 (1962)], which takes into account the energy dependence of the intermediate states, provides a much better account of the photon-energy dependence of the absorption, but the prediction of the experimental absorption strength requires the incorporation of excitonic effects. The latter, however, have only been considered by Elliott [R. J. Elliott, Phys. Rev. 108, 1384 (1957)] in the limit of constant intermediate state energy. A generalization to the case of energy-dependent intermediate states, consistent with Hartman's theory, is presented here. The basic qualitative difference with the classical Elliott theory is that the excitonic character of the intermediate states affects the computed absorption, generating an additional resonant enhancement that is confirmed by the experimental data. The generalized theory presented here agrees very well with the experimental absorption using independently determined band structure parameters.

The dynamics of antiferromagnets is a current hot topic in condensed matter physics and spintronics. However, the dynamics of insulating antiferromagnets cannot be excited by an electric current, which is a method usually used to manipulate ferromagnetic metals. Here, we propose to use the voltage-controlled magnetic anisotropy gradient as an excitation source to manipulate insulating antiferromagnetic textures. We analytically and numerically study the dynamics of an antiferromagnetic skyrmion driven by a magnetic anisotropy gradient. Our analytical calculations demonstrate that such a magnetic anisotropy gradient can effectively drive an antiferromagnetic skyrmion towards the area of lower magnetic anisotropy. The micromagnetic simulations are in good agreement with our analytical solution. Furthermore, the magnetic anisotropy gradient induced velocity of an antiferromagnetic skyrmion is compared to that of a ferromagnetic skyrmion. Our results are useful for the understanding of antiferromagnetic skyrmion dynamics and may open an alternative way for the design of antiferromagnetic spintronic devices.

We report measurements of thermal conductivity ($\kappa$) on single crystals of the quasi-one-dimensional antiferromagnetic spin-chain compounds $A{B}_2{\mathrm{O}}_6$ ($A$ = Ni, Co; $B$ = Sb, Ta) in the temperature range $5\le T\le 300$ K. We find that $\kappa$ for all crystallographic directions is substantially suppressed in the Ta compounds in comparison to those of the Sb compounds despite their very similar magnetic and crystallographic structures. The data are well described by resonant scattering of phonons from two-level systems, with energy splittings of $\sim 100–150$ K and scattering strength an order of magnitude greater in the Ta compounds. We distinguish the effects of phonon-spin interactions from those related to changes in the low-energy phonon spectra caused by the different $B$-site ions.

We report a theoretical study on the phase transition between superconducting states with and without spontaneous surface current. The phase transition takes place due to the formation of surface Andreev bound states in unconventional superconductors. Based on the quasiclassical theory of superconductivity, we examine the influence of atomic-scale surface roughness on the surface phase transition temperature $T_s$. To describe the surface effect, the boundary condition for the quasiclassical Green's function is parametrized in terms of specularity (the specular reflection probability in the normal state at the Fermi level). This boundary condition allows systematic study of the surface effect ranging from the specular limit to the diffuse limit. We show that diffuse quasiparticle scattering at a rough surface causes substantial reduction of $T_s$ in the $d$-wave pairing state of high-$T_c$ cuprate superconductors. We also consider a $p$-wave pairing state in which Andreev bound states similar to those in the $d$-wave state are generated. In contrast to the $d$-wave case, $T_s$ in the $p$-wave state is insensitive to the specularity. This is because the Andreev bound states in the $p$-wave superconductor are robust against diffuse scattering, as implied from symmetry consideration for odd-frequency Cooper pairs induced at the surface; the $p$-wave state has odd-frequency pairs with $s$-wave symmetry, while the $d$-wave state does not.

We develop a simple and unbiased numerical method to obtain the uniform susceptibility of quantum many-body systems. When a Hamiltonian is spatially deformed by multiplying it with a sine-square function that smoothly decreases from the system center toward the edges, the size-scaling law of the excitation energy is drastically transformed to a rapidly converging one. Then, the local magnetization at the system center becomes nearly size independent; the one obtained for the deformed Hamiltonian of a system length as small as $L\sim 10$ provides the value obtained for the original uniform Hamiltonian of $L\sim 100$. This allows us to evaluate a bulk magnetic susceptibility by using the magnetization at the center by existing numerical solvers without any approximation, parameter tuning, or size-scaling analysis. We demonstrate that the susceptibilities of the spin-1/2 antiferromagnetic Heisenberg chain and square lattice obtained by our scheme at $L\sim 10$ agree within ${10}^{-3}$ with exact analytical and numerical solutions for $L=\infty$ down to temperatures of 0.1 times the coupling constant. We apply this method to the spin-1/2 kagome lattice Heisenberg antiferromagnet which is of prime interest in the search for spin liquids.

We report proximity effects of spin-orbit coupling in ${\mathrm{EuO}}_{1-\mathrm{x}}$ films capped with a Pt overlayer. Transport measurements suggest that current flows along a conducting channel at the interface between the Pt and EuO. The temperature dependence of the resistivity picks up the critical behaviors of EuO, i.e., the metal-to-insulator transition. We also find an unusual enhancement of the magnetic anisotropy in this structure from its bulk value, which results from strong spin-orbit coupling across the $\mathrm{Pt}/{\mathrm{EuO}}_{1-\mathrm{x}}$ interface.

Spin-polarized scanning tunneling microscopy reveals that the spin-spiral period of a two-atomic-layer-thick Fe film on Ir(111) roughly doubles upon covering the system with a monolayer of Pb. The spin spirals have a zigzag shape and are guided by structural dislocation lines. In contrast to the spin spiral in the bare Fe double layer, the magnetic state of the Pb-covered Fe film responds to an external magnetic field: the spin spiral breaks up into segments, reminiscent of the spin-spiral state in the Fe triple layer on Ir(111), where the external magnetic field induces magnetic skyrmions. This demonstrates that the tuning of noncollinear spin textures is not only possible by interface engineering based on transition metals, but also with other metallic adlayers such as Pb with a filled $d$ shell.

Temperature dependent ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy and specific heat measurements for $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ with $x=0$, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (i.e., no static magnetic order) down to 5.5 K was detected for $x=0$ and 0.017 in agreement with the absence of any additional feature below superconducting transition temperature $T_c$ in the specific heat data. The evolution of magnetic hyperfine field with temperature was studied for $x=0.033$ and 0.049. The long-range magnetic order in these two compounds coexists with superconductivity. The magnetic hyperfine field $B_{\text{hf}}$ (ordered magnetic moment) below $T_c$ in $\mathrm{CaK}{\left({\mathrm{Fe}}_{0.967}{\mathrm{Ni}}_{0.033}\right)}_4{\mathrm{As}}_4$ is continuously suppressed with the developing superconducting order parameter. The $B_{\text{hf}}\left(T\right)$ data for $\mathrm{CaK}{\left({\mathrm{Fe}}_{0.967}{\mathrm{Ni}}_{0.033}\right)}_4{\mathrm{As}}_4$ and $\mathrm{CaK}{\left({\mathrm{Fe}}_{0.951}{\mathrm{Ni}}_{0.049}\right)}_4{\mathrm{As}}_4$ can be described reasonably well by Machida's model for coexistence of itinerant spin density wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195 (1981)]. We demonstrate directly that superconductivity suppresses the spin density wave order parameter if the conditions are right, in agreement with the theoretical analysis.

We study the supercurrent in quasi-one-dimensional Josephson junctions with a weak link involving magnetism, either via magnetic impurities or via ferromagnetism. In the case of weak links longer than the magnetic pair-breaking length, the Josephson effect is dominated by mesoscopic fluctuations. We establish the supercurrent-phase dependence $I\left(\phi \right)$ along with statistics of its sample-dependent properties in junctions with transparent contacts between leads and link. High transparency gives rise to the inverse proximity effect, while the direct proximity effect is suppressed by magnetism in the link. We find that all harmonics are present in $I\left(\phi \right)$. Each harmonic has its own sample-dependent amplitude and phase shift with no correlation between different harmonics. Depending on the type of magnetic weak link, the system can realize a ${\phi }_0$ or $\phi$ junction in the fluctuational regime. Full supercurrent statistics is obtained at arbitrary relation between temperature, superconducting gap, and the Thouless energy of the weak link.

We present results indicating that Chern insulator states can be achieved in the recently synthesized pristine chromium triiodide (${\mathrm{CrI}}_3$) by either electron or hole doping. Our first-principles density-functional-theory calculations confirmed that monolayer ${\mathrm{CrI}}_3$ show nontrivial Chern number $C$ in both the valence and conduction bands. By introducing on-site Coulomb interaction or epitaxial strain, the doped ${\mathrm{CrI}}_3$ exhibit a series of topological quantum phase transitions between multiple Chern insulator phases as well as semimetal-to-insulator transitions. We show that the covalency of Cr $d$-I $p$ bands controlled by the on-site Coulomb interaction $U$ and strain is one of the key ingredients determining the topological phase and semimetal-insulator phase boundary.

Trapping of electrons in localized states strongly affects optoelectronic phenomena in disordered semiconductors. In this paper, it is shown by numerical simulations and by analytical calculations that the release of the trapped electrons into the conduction band can be substantially enhanced by hopping of electrons between the traps. The effect strongly depends on several factors, such as the energy depth of the given trap, the concentration of the assisting traps, and the magnitude of the applied electric field. Recipes are given for theoretical studies of the effect by analytical equations and by kinetic Monte Carlo simulations.

The Casimir effect, a two-body interaction via vacuum fluctuations, is a fundamental property of quantum systems. In solid state physics it emerges as a long-range interaction between two impurity atoms via virtual phonons. In the classical limit for the impurity atoms in $D$ dimensions the interaction is known to follow the universal power law $U\left(r\right)\sim r^{-D}$. However, for finite masses of the impurity atoms on a lattice, it was predicted to be $U\left(r\right)\sim r^{-2D-1}$ at large distances. We examine how one power law can change into another with an increase of the impurity mass and in the presence of an external potential. We provide the exact solution for the system in one dimension. At large distances indeed $U\left(r\right)\sim r^{-3}$ for finite impurity masses, while for the infinite impurity masses or in an external potential it crosses over to $U\left(r\right)\sim r^{-1}$. At short distances the Casimir interaction is not universal and depends on the impurity mass and the external potential.

We investigated the dielectric properties of the charge-ordered phase of $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$ using exact numerical calculations of an extended Hubbard model. The electronic contribution to the electric polarization (electronic polarization) $\overline{\mathit{P}}$ of the charge-ordered ground state is obtained by directly calculating the current when transfer integrals were changed adiabatically from symmetric integrals to integrals for the charge-ordered phase without inversion symmetry. The angle of $\overline{\mathit{P}}$ from the positive $b$ axis is ${36}^{\circ }$, which is consistent with experimental results and previous theoretical results based on density-functional theory. Furthermore, we numerically calculated the dynamics induced by terahertz- (THz-) pulse excitation. Both the THz-pulse-induced variation of the electronic polarization magnitude and that of the charge disproportionation that shows the charge-order amplitude are largest when the electric field of the THz pulse and $\overline{\mathit{P}}$ have almost the same direction. This originates from the charge transfer through bond $b{2}^{\prime }$ being dominant in both the adiabatic flow of the current and the THz-pulse excitation. These results reproduce important features of the experimental results of THz-pulse-induced dynamics.

We investigate the magnetization dynamics of ferromagnetic nanoparticles in the atomistic approach taking account of surface anisotropy and the spin misalignment it causes. We demonstrate that such inhomogeneous spin configurations induce nutation in the dynamics of the particle's magnetization. More precisely, in addition to the ordinary precessional motion with frequency $f_p\sim 10\mathrm{GHz}$, we find that the dynamics of the net magnetic moment exhibits two more resonance peaks with frequencies $f_c$ and $f_n$ which are higher than the frequency $f_p$: $f_c=4\times f_p\sim 40\mathrm{GHz}$ is related with the oscillations of the particle's magnetic moment between the minima of the effective potential induced by weak surface anisotropy. On the other hand, the much higher frequency $f_n\sim 1\mathrm{THz}$ is attributed to the magnetization fluctuations at the atomic level driven by exchange interaction. We have compared our results on nutation induced by surface effects with those rendered by the macroscopic approach based on the Landau-Lifshitz-Gilbert equation augmented by an inertial term (proportional to the second-order time derivative of the macroscopic moment) with a phenomenological coefficient. The good agreement between the two models has allowed us to estimate the latter coefficient in terms of the atomistic parameters such as the surface anisotropy constant. We have thus proposed a different origin for the magnetization nutations as being induced by surface effects and have interpreted the corresponding resonance peaks and their frequencies.

We investigate the potential of a quantum Boltzmann equation without momentum conservation for description of strongly correlated electron systems out of equilibrium. In a spirit similar to dynamical mean field theory (DMFT), the momentum conservation of the electron-electron scattering is neglected, which yields a time-dependent occupation function for the equilibrium spectral function, even in cases where well-defined quasiparticles do not exist. The main assumption of this method is that the spectral function remains sufficiently rigid under the nonequilibrium evolution. We compare the result of the quantum Boltzmann equation to nonequilibrium DMFT simulations for the case of photocarrier relaxation in Mott insulators, where processes on very different timescales emerge, i.e., impact ionization, intra-Hubbard-band thermalization, and full thermalization. Since quantum Boltzmann simulations without momentum conservation are computationally cheaper than nonequilibrium DMFT, this method allows the simulation of more complicated systems or devices, and to access much longer times.

Extensive thermal conductivity measurements have been conducted on two single crystals of ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ from 60 mK to 60 K. The samples are differentiated by their levels of disorder, which are determined through a combination of x-ray diffraction measurements and thermal conductivity data. We report clear evidence of a magnetic contribution to the thermal conductivity, which we attribute to magnetic monopoles. Furthermore, the magnetic conductivity increases with increased disorder due to the impact of disorder on the monopole density.

A series of $\mathrm{Pr}{\left(\mathrm{TM}\right)}_2{\mathrm{X}}_{20}$ (with TM = Ti, V, Rh, Ir and X = Al, Zn) Kondo materials, containing non-Kramers ${\mathrm{Pr}}^{3+}4f^2$ moments on a diamond lattice, have been shown to exhibit intertwined orders such as quadrupolar order and superconductivity. Motivated by these experiments, we propose and study a Landau theory of multipolar order to capture the phase diagram and its field dependence. In zero magnetic field, we show that different quadrupolar states, or the coexistence of quadrupolar and octupolar orderings, may lead to ground states with multiple broken symmetries. Upon heating, such states may undergo two-step thermal transitions into the symmetric paramagnetic phase, with partial restoration of broken symmetries in the intervening phase. For nonzero magnetic field, we show the evolution of these thermal phase transitions strongly depends on the field direction, due to clock anisotropy terms in the free energy. Our findings shed substantial light on experimental results in the $\mathrm{Pr}{\left(\mathrm{TM}\right)}_2{\mathrm{Al}}_{20}$ materials. We propose further experimental tests to distinguish purely quadrupolar orders from coexisting quadrupolar-octupolar orders.

We reveal that topological Josephson junctions provide a natural platform for the interplay between the Josephson effect and the Landau-Zener effect through a two-level system formed by coupled Majorana modes. We build a quantum resistively shunted junction (RSJ) model by modifying the standard textbook RSJ model to take account of the two-level system from the Majorana modes at the junction. We show that the dynamics of the two-level system is governed by a nonlinear Schrödinger equation and solve the equations analytically via a mapping to a classical dynamical problem. This nonlinear dynamics leads to hysteresis in the I-V characteristics, which can give a quantitative explanation to recent experiments. We also predict the coexistence of two interference patterns with periods $h/e$ and $h/2e$ in topological superconducting quantum interference devices.