The effect of confinement on the anharmonic phonon scattering rates and the consequences thereof on the thermal transport properties in ultrathin silicon nanowires with a diameter of 1–4 nm have been characterized using atomistic simulations and the phonon Boltzmann transport equation. The phonon density of states (PDOS) for ultrathin nanowires approaches a constant value in the vicinity of the $\mathrm{\Gamma }$ point and increases with decreasing diameter, which indicates the increasing importance of the low-frequency phonons as heat carriers. The anharmonic phonon scattering becomes dramatically enhanced with decreasing thickness of the nanowires. In the thinnest nanowire, the scattering rates for phonons above 1 THz are one order of magnitude higher than those in the bulk Si. Below 1 THz, the increase in scattering rates is even much more appreciable. Our numerical calculations revealed that the scattering rates for transverse (longitudinal) acoustic modes follow $\sqrt{\omega }$ ($1/\sqrt{\omega }$) dependence at the low-frequency limit, whereas those for the degenerate flexural modes asymptotically approach a constant value. In addition, the group velocities of phonons are reduced compared with bulk Si except for low-frequency phonons ($<1$–2 THz depending on the thickness of the nanowires). The increased scattering rates combined with reduced group velocities lead to a severely reduced thermal conductivity contribution from the high-frequency phonons. Although the thermal conductivity contributed by those phonons with low frequencies is instead increased mainly due to the increased PDOS, the total thermal conductivity is still reduced compared to that of the bulk. This work reveals an unexplored mechanism to understand the measured ultralow thermal conductivity of silicon nanowires.

We present a time-resolved experimental study of the temperature effect on the coherence of traveling polariton condensates. The simultaneous detection of their emission both in real and reciprocal space allows us to fully monitor the condensates’ dynamics. We obtain fringes in reciprocal space as a result of the interference between polariton wave packets (WPs) traveling with the same speed. The periodicity of these fringes is inversely proportional to the spatial distance between the interfering WPs. In a similar fashion, we obtain interference fringes in real space when WPs traveling in opposite directions meet. The visibility of both real- and reciprocal-space interference fringes rapidly decreases with increasing temperature and vanishes. A theoretical description of the phase transition, considering the coexistence of condensed and noncondensed particles, for an out-of-equilibrium condensate such as ours is still missing, yet a comparison with theories developed for atomic condensates allows us to infer a critical temperature for the BEC-like transition when the visibility goes to zero.

We consider the out-of-equilibrium dynamics generated by joining two domains with arbitrary opposite magnetizations. We study the stationary state which emerges by the unitary evolution via the spin-1/2 $XXZ$ Hamiltonian, in the gapless regime, where the system develops a stationary spin current. Using the generalized hydrodynamic approach, we present a simple formula for the space-time profile of the spin current and the magnetization exact in the limit of long times. As a remarkable effect, we show that the stationary state has a strongly discontinuous dependence on the strength of interaction as confirmed by the exact analytic expression of the Drude weight that we compute. These features allow us to give a qualitative estimation for the transient behavior of the current which is in good agreement with numerical simulations. Moreover, we analyze the behavior around the edge of the magnetization profile, and we argue that, unlike the $XX$ free-fermionic point, interactions always prevent the emergence of a Tracy-Widom scaling.

We investigated the specific heat and electrical resistivity of disordered heavy-fermion systems ${\mathrm{Ce}}_2{\mathrm{Co}}_{0.8}{\mathrm{Si}}_{3.2}$ and ${\mathrm{Ce}}_2{\mathrm{Co}}_{0.4}{\mathrm{Rh}}_{0.4}{\mathrm{Si}}_{3.2}$. Results show that pronounced non-Fermi-liquid behavior in these Kondo disordered compounds originates from approaching metal-insulator transition rather than from proximity to magnetic instability. Power-law divergence of the local Kondo temperature distribution, $P\left(T_K\right)$, in the limit of $T_K\to 0$, and clear signature of the quantum interference corrections in the resistivity detected deep below the onset of Kondo coherent state, point to electronic Griffiths phase formation in the studied compounds.

Neutron powder diffraction experiments reveal that $\mathrm{C}{\mathrm{o}}_4\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_9$ forms a noncollinear in-plane magnetic structure with $\mathrm{C}{\mathrm{o}}^{2+}$ moments lying in the $ab$ plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest- and next-nearest-neighbor exchange interactions, in-plane anisotropy, and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of $\mathrm{C}{\mathrm{o}}_4\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_9$ is attributed to the large in-plane anisotropy, while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of $\mathrm{C}{\mathrm{o}}_4\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_9$ in fields can be explained by its special in-plane magnetic structure.

Majorana bound states exist inside a core of the half-quantum vortex in spin-triplet superconductors. Despite intense efforts, they have not been discovered in any spin-triplet superconductor candidate materials. After the success of the topological insulator, another route to achieve Majorana fermions has been suggested: heterostructures of $s$-wave superconductors and the topological insulator or semimetal with strong spin-orbit coupling provide an effective spinless $p+ip$ pairing. This theoretical observation has stimulated experimental searches for Majorana bound states, and further theoretical studies on propagating Majorana fermions beyond bound states. Here we show that high-$T_C$ superconductor-based-heterostructures exhibit both helical Majorana fermions and zero-energy flat edge states. We suggest that an Ir oxide layer of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ in close proximity to high-$T_C$ cuprates would be a good example to explore these boundary states.

We make use of the strong spin-charge coupling in the electron-doped cuprate ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ to probe changes in its spin system via magnetotransport measurements. We present a detailed study of the out-of-plane magnetoresistance in underdoped single crystals of this compound, including the nonsuperconducting, $0.05\le x\le 0.115$, and superconducting, $0.12\le x\le 0.13$, compositions. Special focus is put on the dependence of the magnetoresistance on the field orientation in the plane of the ${\mathrm{CuO}}_2$ layers. In addition to the kink at the field-induced transition between the noncollinear and collinear antiferromagnetic configurations, a sharp irreversible feature is found in the angle-dependent magnetoresistance of all samples in the high-field regime, at field orientations around the Cu-O-Cu direction. The obtained behavior can be explained in terms of field-induced reorientation of ${\mathrm{Cu}}^{2+}$ spins within the collinear antiferromagnetic state. It is therefore considered an unambiguous indication of the long-range magnetic order.

Using a realistic ten-orbital tight-binding model Hamiltonian fitted to the angle-resolved photoemission spectroscopy data on LiFeAs, we analyze the temperature, frequency, and momentum dependencies of quasiparticle interference to identify gap sign changes in a qualitative way, following our original proposal [Phys. Rev. B 92, 184513 (2015)]. We show that all features present for the simple two-band model for the sign-changing $s_{+-}$-wave superconducting gap employed previously are still present in the realistic tight-binding approximation and gap values observed experimentally. We discuss various superconducting gap structures proposed for LiFeAs and identify various features of these superconducting gap functions in the quasiparticle interference patterns. On the other hand, we show that it will be difficult to identify the more complicated possible sign structures of the hole pocket gaps in LiFeAs due to the smallness of the pockets and the near proximity of two of the gap energies.

The unconventional superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ continues to attract considerable interest. Although many measurements can be interpreted on the basis of chiral $p$-wave pairing with intriguing topological character, a number of exceptions hinder an unambiguous verification of such pairing. The pairing mechanism also remains under debate. In this Rapid Communication, with effects of the sizable spin-orbit coupling accounted for, we reexamine the superconducting instabilities in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ through systematic microscopic analysis within the random-phase approximation. Our calculations show that the odd-parity $p$-wave pairing is favored in the regime of extremely weak interactions, but that highly anisotropic even-parity pairings become most leading over a broad range of stronger interactions. These results could shed light on the nature of the enigmatic superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Crystal electric field states in rare earth intermetallics show an intricate entanglement with the many-body physics that occurs in these systems and that is known to lead to a plethora of electronic phases. Here we attempt to trace different contributions to the crystal electric field (CEF) splittings in ${\mathrm{CeIrIn}}_5$, a heavy-fermion compound and member of the $\mathrm{Ce}M{\mathrm{In}}_5$ ($M=$ Co, Rh, Ir) family. To this end, we utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) and present a spectroscopic study of the electronic structure of this unconventional superconductor over a wide temperature range. As a result, we show how ARPES can be used in combination with thermodynamic measurements or neutron scattering to disentangle different contributions to the CEF splitting in rare earth intermetallics. We also find that the hybridization is stronger in ${\mathrm{CeIrIn}}_5$ than ${\mathrm{CeCoIn}}_5$ and the effects of the hybridization on the Fermi volume increase is much smaller than predicted. By providing experimental evidence for $4f_{7/2}^1$ splittings which, in ${\mathrm{CeIrIn}}_5$, split the octet into four doublets, we clearly demonstrate the many-body origin of the so-called $4f_{7/2}^1$ state.

Inspired by the topological insulator circuit experimentally proposed by Jia Ningyuan et al. [Phys. Rev. X 5, 021031 (2015)], we theoretically realize the topological Lieb lattice, a line-centered square lattice with rich topological properties, in a radio-frequency circuit. We design a specific capacitor-inductor connection to resemble the intrinsic spin-orbit coupling and construct the analog spin by mixing degrees of freedom of voltages. As such, we are able to simulate the quantum spin Hall effect in the topological Lieb lattice of linear circuits. We then investigate the spin-resolved topological edge mode and the topological phase transition of the band structure varied with capacitances. Finally, we discuss the extension of the $\pi /2$ phase change of hopping between sites to arbitrary phase values. Our results may find implications in engineering microwave topological metamaterials for signal transmission and energy harvesting.

We use nonequilibrium dynamical mean-field theory with iterative perturbation theory as an impurity solver to study the recovery of SU(2) symmetry in real time following a hopping integral parameter quench from a mass-imbalanced to a mass-balanced single-band Hubbard model at half filling. A dynamical order parameter $\gamma \left(t\right)$ is defined to characterize the evolution of the system towards SU(2) symmetry. By comparing the momentum-dependent occupation from an equilibrium calculation [with the SU(2) symmetric Hamiltonian after the quench at an effective temperature] with the data from our nonequilibrium calculation, we conclude that the SU(2) symmetry recovered state is a thermalized state. Further evidence from the evolution of the density of states supports this conclusion. We find the order parameter in the weak Coulomb interaction regime undergoes an approximate exponential decay. We numerically investigate the interplay of the relevant parameters (initial temperature, Coulomb interaction strength, initial mass-imbalance ratio) and their combined effect on the thermalization behavior. Finally, we study evolution of the order parameter as the hopping parameter is changed with either a linear ramp or a pulse. Our results can be useful in strategies to engineer the relaxation behavior of interacting quantum many-particle systems.

We present here the minimal tight-binding model for a single layer of transition metal dichalcogenides (TMDCs) ${MX}_2(M$, metal; $X$, chalcogen) which illuminates the physics and captures band nesting, massive Dirac fermions, and valley Landé and Zeeman magnetic field effects. TMDCs share the hexagonal lattice with graphene but their electronic bands require much more complex atomic orbitals. Using symmetry arguments, a minimal basis consisting of three metal $d$ orbitals and three chalcogen dimer $p$ orbitals is constructed. The tunneling matrix elements between nearest-neighbor metal and chalcogen orbitals are explicitly derived at $K,-K$, and $\mathrm{\Gamma }$ points of the Brillouin zone. The nearest-neighbor tunneling matrix elements connect specific metal and sulfur orbitals yielding an effective $6\times 6$ Hamiltonian giving correct composition of metal and chalcogen orbitals but not the direct gap at $K$ points. The direct gap at $K$, correct masses, and conduction band minima at $Q$ points responsible for band nesting are obtained by inclusion of next-neighbor Mo-Mo tunneling. The parameters of the next-nearest-neighbor model are successfully fitted to ${MX}_2(M=\text{Mo}$; $X=\text{S})$ density functional ab initio calculations of the highest valence and lowest conduction band dispersion along $K-\mathrm{\Gamma }$ line in the Brillouin zone. The effective two-band massive Dirac Hamiltonian for ${\mathrm{MoS}}_2$, Landé $g$ factors, and valley Zeeman splitting are obtained.

We apply the method of irreversible thermodynamics to obtain, for a semiconductor with electron and hole carriers adjacent to a single-carrier material, the recombination velocity and the rate of surface heating. The lack of equilibrium across the interface drives both a charged number current flux (the difference between electron and hole fluxes) and an uncharged number current flux (the sum of electron and hole fluxes). The latter provides a generalization of Shockley's recombination velocity. There are two contributions to the surface heating rate, one proportional to the charged number current flux times a difference in electrochemical potentials across the interface, and the other proportional to the uncharged number current flux times its associated difference in electrochemical potential. We apply our results to photoabsorption at a uniform volume rate $G$ within a distance $d$ of a vacuum semiconductor surface, finding the distributions of number current density, charge, and voltage as a function of position.

A phase-field model of a crystalline material is introduced to develop the necessary theoretical framework to study plastic flow due to dislocation motion. We first obtain the elastic stress from the phase-field crystal free energy under weak distortion and show that it obeys the stress-strain relation of linear elasticity. We focus next on dislocations in a two-dimensional hexagonal lattice. They are composite topological defects in the weakly nonlinear amplitude equation expansion of the phase field, with topological charges given by the standard Burgers vector. This allows us to introduce a formal relation between the dislocation velocity and the evolution of the slowly varying amplitudes of the phase field. Standard dissipative dynamics of the phase-field crystal model is shown to determine the velocity of the dislocations. When the amplitude expansion is valid and under additional simplifications, we find that the dislocation velocity is determined by the Peach-Koehler force. As an application, we compute the defect velocity for a dislocation dipole in two setups, pure glide and pure climb, and compare it with the analytical predictions.

We study anomalous Hall conductivity (${\sigma }_{\mathrm{AHC}}$) and electronic band structures of Si-substituted ${\mathrm{Mn}}_2\mathrm{CoAl}$ (${\mathrm{Mn}}_2{\mathrm{CoAl}}_{1-x}{\mathrm{Si}}_x$). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to $x=0.2$ in ${\mathrm{Mn}}_2{\mathrm{CoAl}}_{1-x}{\mathrm{Si}}_x$. This means that the Si substitution enables the Fermi-level shift without largely changing the electronic structures in ${\mathrm{Mn}}_2\mathrm{CoAl}$. By using molecular beam epitaxy techniques, ${\mathrm{Mn}}_2{\mathrm{CoAl}}_{1-x}{\mathrm{Si}}_x$ epitaxial films can be grown, leading to the systematic control of $x$ ($0⩽x⩽0.3$). In addition to the electrical conductivity, the values of ${\sigma }_{\mathrm{AHC}}$ for the ${\mathrm{Mn}}_2{\mathrm{CoAl}}_{1-x}{\mathrm{Si}}_x$ films are similar to those in ${\mathrm{Mn}}_2\mathrm{CoAl}$ films shown in previous reports. We note that a very small ${\sigma }_{\mathrm{AHC}}$ of $\sim 1.1$ S/cm is obtained for $x=$ 0.225, and the sign of ${\sigma }_{\mathrm{AHC}}$ is changed from positive to negative at around $x=$ 0.25. We discuss the origin of the sign reversal of ${\sigma }_{\mathrm{AHC}}$ as a consequence of the Fermi-level shift in ${\mathrm{Mn}}_2\mathrm{CoAl}$. Considering the presence of the structural disorder in the ${\mathrm{Mn}}_2{\mathrm{CoAl}}_{1-x}{\mathrm{Si}}_x$ films, we can conclude that the small value and sign reversal of ${\sigma }_{\mathrm{AHC}}$ are not related to the characteristics of spin-gapless semiconductors.

 ${}^7\mathrm{Li}$ NMR spectra were measured in a magnetic field up to 17 T at temperatures 5–30 K on single crystalline ${\mathrm{LiCu}}_2{\mathrm{O}}_2$. Earlier reported anomalies on magnetization curves correspond to magnetic field values where we observe changes of the NMR spectral shape. For the interpretation of the field and temperature evolutions of our NMR spectra, the magnetic structures were analyzed in the frame of the phenomenological theoretical approach of the Dzyaloshinskii-Landau theory. A set of possible planar and collinear structures was obtained. Most of these structures have an unusual configuration; they are characterized by a two-component order parameter and their magnetic moments vary harmonically not only in direction, but also in size. From the modeling of the observed spectra, a possible scenario of magnetic structure transformations is obtained.

The nuclear and magnetic structure and full magnon dispersions of yttrium iron garnet ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ have been studied using neutron scattering. The refined nuclear structure is distorted to a trigonal space group of $R\overline{3}$. The highest-energy dispersion extends up to 86 meV. The observed dispersions are reproduced by a simple model with three nearest-neighbor-exchange integrals between $16a$ (octahedral) and $24d$ (tetrahedral) sites, $J_{aa}$, $J_{ad}$, and $J_{dd}$, which are estimated to be $0.00\pm 0.05$, $-2.90\pm 0.07$, and $-0.35\pm 0.08$ meV, respectively. The lowest-energy dispersion below 14 meV exhibits a quadratic dispersion as expected from ferromagnetic magnons. The imaginary part of $q$-integrated dynamical spin susceptibility ${\chi }^{″}(E$) exhibits a square-root energy dependence at low energies. The magnon density of state is estimated from ${\chi }^{″}(E$) obtained on an absolute scale. The value is consistent with the single chirality mode for the magnon branch expected theoretically.

The possible superfluid transition of supercooled liquid parahydrogen is investigated by quantum Monte Carlo simulations. The cooling protocol adopted here allows for the study of a fluid phase down to a temperature $T=0.25$ K. No evidence of superfluidity is found, as exchanges of identical particles are strongly suppressed even at the lowest temperature. It is shown that, contrary to a commonly held belief, it is not the well depth of the pairwise interaction but rather its relatively large, hard-core diameter that physically hinders superfluidity in parahydrogen.