Recent work by Wu et al. [arXiv:1910.11011] proposed a numerical method, so-called matrix product operator–matrix product state (MPO-MPS) method, by which several types of quantum many-body wave functions, in particular, the projected Fermi sea state, can be efficiently represented as a tensor network. In this paper, we generalize the MPO-MPS method to study Gutzwiller projected paired states of fermions, where the maximally localized Wannier orbitals for Bogoliubov quasiparticles/quasiholes have been adapted to improve the computational performance. The study of $SO\left(3\right)$-symmetric spin-1 chains reveals that this new method has better performance than variational Monte Carlo for gapped states and similar performance for gapless states. Moreover, we demonstrate that dynamic correlation functions can be easily evaluated by this method cooperating with other MPS-based accurate approaches, such as the Chebyshev MPS method.

The equality between the spectral directional emittance and absorptance of an object under local thermodynamic equilibrium is known as Kirchhoff's law of radiation. The breakdown of Kirchhoff's law of radiation is physically allowed by breaking time reversal symmetry and can open opportunities for nonreciprocal light emitters and absorbers. Large anomalous Hall conductivity and angle recently observed in topological Weyl semimetals, particularly type-I magnetic Weyl semimetals and type-II Weyl semimetals, are expected to create large nonreciprocal electromagnetic wave propagation. In this work, we focus on type-I magnetic Weyl semimetals and show via modeling and simulation that nonreciprocal surface plasmon polaritons can result in pronounced nonreciprocity without an external magnetic field. The modeling in this work begins with a single pair of Weyl nodes, followed by a more realistic model with multiple paired Weyl nodes. Fermi-arc surface states are also taken into account through the surface conductivity. This work points to the promising applicability of topological Weyl semimetals for magneto-optical and energy applications.

Fluorine NMR is performed on two ${\mathrm{ReF}}_6$-based single-molecule magnets, a conductor, ${\left(\mathrm{BEDO}\right)}_4\left[{\mathrm{ReF}}_6\right]·6{\mathrm{H}}_2\mathrm{O}$, and its parent substance, an insulator, ${\left({\mathrm{PPh}}_4\right)}_2\left[{\mathrm{ReF}}_6\right]·2{\mathrm{H}}_2\mathrm{O}$. The correlation frequency of electronic spin fluctuations, ${\omega }_c$, is derived from the spin-lattice relaxation rate. These data, combined with the inverse decay time of the magnetization, $1/{\tau }_c$, determined from the low-frequency ac susceptibility measurements, allows tracking of the spin dynamics span over seven orders of magnitude. We demonstrate that the two data sets, $1/{\tau }_c$ and the low-temperature region of ${\omega }_c$, can be described by the same model, which includes Orbach, Raman, and “quantum tunneling of magnetization” contributions. This indicates that the two techniques characterize the same process. The influence of the conduction electrons on spin-lattice relaxation in ${\left(\mathrm{BEDO}\right)}_4\left[{\mathrm{ReF}}_6\right]·6{\mathrm{H}}_2\mathrm{O}$ is revealed below 25 K as a trend of the relaxation to follow a linear temperature dependence.

This comment concerns the contribution of configurational mixing entropy to the change in the total Gibbs free energy in the process of vacancy formation and the consequent effect on the thermal equilibrium vacancy concentration in multicomponent alloys. A different derivation is shown than that in [Phys. Rev. B 93, 134115 (2016)], correcting an error that may come from using Gibbs free-energy per site. The derivation is further generalized to systems beyond binary alloys.

We study the stability of the Wilson-Fisher fixed point of the quantum $\text{O}\left(2N\right)$ vector model to quenched disorder in the large-$N$ limit. While a random mass is strongly relevant at the Gaussian fixed point, its effect is screened by the strong interactions of the Wilson-Fisher fixed point. This enables a perturbative renormalization group study of the interplay of disorder and interactions about this fixed point. We show that, in contrast to the spiralling flows obtained in earlier double-$\epsilon$ expansions, the theory flows directly to a quantum critical point characterized by finite disorder and interactions. The critical exponents we obtain for this transition are in remarkable agreement with numerical studies of the superfluid-Mott glass transition. We additionally discuss the stability of this fixed point to scalar and vector potential disorder and use proposed boson-fermion dualities to make conjectures regarding the effects of weak disorder on dual Abelian Higgs and Chern-Simons-Dirac fermion theories when $N=1$.

A Josephson junction can be subjected to a local, strongly inhomogeneous magnetic field in various experimental situations. Here this problem is analyzed analytically and numerically. A modified sine-Gordon type equation in the presence of time-dependent local field is derived and solved numerically in static and dynamic cases. Two specific examples of local fields are considered: induced either by an Abrikosov vortex, or by a tip of a magnetic force microscope (MFM). It is demonstrated that a time-dependent local field can induce a dynamic flux-flow state in the junction with shuttling or unidirectional ratchetlike Josephson vortex motion. This provides a mechanism for detection and manipulation of Josephson vortices by an oscillating MFM tip. In a static case, the local field leads to a distortion of the critical current versus magnetic field, $I_c\left(H\right)$, pattern. The distortion is sensitive to both the shape and the amplitude of the local field. Therefore, the $I_c\left(H\right)$ pattern carries information about the local field distribution within the junction. This opens the possibility for employing a single Josephson junction as a scanning probe sensor with spatial resolution not limited by its geometrical size, thus obviating a known problem of a trade-off between the field sensitivity and the spatial resolution of a sensor.

Rutile ($R$) phase ${\mathrm{VO}}_2$ is a quintessential example of a strongly correlated bad metal, which undergoes a metal-insulator transition (MIT) concomitant with a structural transition to a V-V dimerized monoclinic ($M_1$) phase below $T_{\text{MIT}}\sim 340\text{K}$. It has been experimentally shown that one can control this transition by doping ${\mathrm{VO}}_2$. In particular, doping with oxygen vacancies ($V_{\text{O}}$) has been shown to completely suppress this MIT without any structural transition. We explain this suppression by elucidating the influence of oxygen vacancies on the electronic structure of the metallic $R$ phase ${\mathrm{VO}}_2$, explicitly treating strong electron-electron correlations using dynamical mean-field theory (DMFT) as well as diffusion Monte Carlo (DMC) flavor of quantum Monte Carlo (QMC) techniques. DMC calculations show a gap closure in the $M_1$ phase when vacancies are present, suggesting that when vacancies are introduced in the high-temperature rutile phase, the dimerized insulating phase cannot be reached when temperature is lowered. Both DMFT and DMC calculations of nonstoichiometric metallic rutile phase shows that this tendency not to dimerize in the presence of vacancies is because $V_{\text{O}}$'s tend to change the $\mathrm{V}\text{−}3d$ filling away from its nominal half-filled value, with the $e_g^{\pi }$ orbitals competing with the otherwise dominant $a_{1g}$ orbital. Loss of this near orbital polarization of the $a_{1g}$ orbital is associated with a weakening of electron correlations, especially along the V-V dimerization direction. This removes a charge-density wave (CDW) instability along this direction above a critical doping concentration, which further suppresses the metal-insulator transition. Our study also suggests that the MIT is predominantly driven by a correlation-induced CDW instability along the V-V dimerization direction.

Nano-size confinement induces many intriguing non-Fourier heat conduction phenomena, such as nonlinear temperature gradients, temperature jumps near the contacts, and size-dependent thermal conductivity. Over the past decades, these effects have been studied and interpreted by nonequilibrium molecular dynamics (NEMD) and phonon Boltzmann transport equation (BTE) simulations separately, but no theory that unifies these two methods has ever been established. In this work, we unify these methods using a quantitative mode-level comparison and demonstrate that they are equivalent for various thermostats. We show that different thermostats result in different non-Fourier thermal transport characteristics due to the different mode-level phonon excitations inside the thermostats, which explains the different size-dependent thermal conductivities calculated using different reservoirs, even though they give the same bulk thermal conductivity. Specifically, the Langevin thermostat behaves like a thermalizing boundary in phonon BTE and provides mode-level thermal-equilibrium phonon outlets, while the Nose-Hoover chain thermostat and velocity rescaling method behave like biased reservoirs, which provide a spatially uniform heat generation and mode-level nonequilibrium phonon outlets. These findings explain why different experimental measurement methods can yield different size-dependent thermal conductivity. They also indicate that the thermal conductivity of materials can be tuned for various applications by specifically designing thermostats. The unification of NEMD and phonon BTE will largely facilitate the study of thermal transport in complex systems in the future by, e.g., replacing computationally unaffordable first-principles NEMD simulations with computationally less expensive spectral BTE simulations.

We study the stability, magnetic order, charge segregation, and electronic properties of a novel three-layered ${\mathrm{Fe}}_3{\mathrm{O}}_4$ film by means of Hubbard-corrected density functional theory calculations. The stable film is predicted to consist of close-packed iron and oxygen layers, comprising a center layer with octahedrally coordinated Fe sandwiched between two layers with tetrahedrally coordinated Fe. The film exhibits an antiferromagnetic type I spin order. A charge analysis confirms that the stable structure has distinct charge segregation, with ${\mathrm{Fe}}^{2+}$ ions in the center layer and ${\mathrm{Fe}}^{3+}$ in the tetrahedral surface layers. Examination of the electronic band structures and densities of states shows that the bandgap is substantially reduced, from 2.4 eV for the bulk rocksalt to 0.3 eV for the film. The reduction in the bandgap is a consequence of the $2+$ to $3+$ change in oxidation state of Fe in the surface layers.

Based on first principles, we predict a new low-energy Stone-Wales graphene SW40, which has an orthorhombic lattice with $Pbam$ symmetry and 40 carbon atoms in its crystalline cell forming well-arranged Stone-Wales patterns. The calculated total energy of SW40 is just about 133 meV higher than that of graphene, indicating that its excellent stability exceeds all the previously proposed graphene allotropes. We find that SW40 exhibits an intrinsic type-III Dirac cone [Phys. Rev. Lett. 120, 237403 (2018)] formed by band crossing of local linear and flat bands, which can result in highly anisotropic fermions in the system. Interestingly, such an intrinsic type-III Dirac cone can be effectively tuned by inner-layer strain and it will be transferred into type-I and type-II Dirac cones under tensile and compressed strain, respectively. Finally, a general tight-binding model was constructed to understand the electronic properties near the Fermi level in SW40. The results show that type-III Dirac cone features can be well understood by the $\pi$-electron interactions between adjacent Stone-Wales defects.

The newly discovered magnetic topological insulators ${\left(\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4\right)}_m{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ are predicted to be a versatile platform for exploring novel topological states. Here, we report angle-resolved photoemission spectroscopy studies on a series of ${\left(\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4\right)}_m{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ heterostructures. An unexpected but universal gapless Dirac cone is observed on the ($\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$) terminated (0001) surfaces in all systems, indicating an altered magnetic structure near the surface. The specific band dispersion of the surface states, presumably dominated by the top surface, is found to be sensitive to different stackings of the underlying $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ and $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ layers. Our results suggest the high tunability of both magnetic and electronic structures of the topological surface states in ${\left(\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4\right)}_m{\left(\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3\right)}_n$ heterostructures, which is essential in realizing and manipulating various topological states.

We study equilibrium and nonequilibrium properties of the single-impurity Anderson model with a power-law pseudogap in the density of states. In equilibrium, the model is known to display a quantum phase transition from a generalized Kondo to a local moment phase. In the present work, we focus on the extension of these phases beyond equilibrium, i.e., under the influence of a bias voltage. Within the auxiliary master equation approach combined with a scheme based on matrix product states (MPS) we are able to directly address the current-carrying steady state. Starting with the equilibrium situation, we first corroborate our results by comparing to a direct numerical evaluation of ground-state spectral properties of the system by MPS. Here, a scheme to locate the phase boundary by extrapolating the power-law exponent of the self energy produces a very good agreement with previous results obtained by the numerical renormalization group. Our nonequilibrium study as a function of the applied bias voltage is then carried out for two points on either side of the phase boundary. In the Kondo regime the resonance in the spectral function is split as a function of the increasing bias voltage. The local moment regime, instead, displays a dip in the spectrum near the position of the chemical potentials. Similar features are observed in the corresponding self energies. The Kondo split peaks approximately obey a power-law behavior as a function of frequency whose exponents depend only slightly on voltage. Finally, the differential conductance in the Kondo regime shows a peculiar maximum at finite voltages, whose height, however, is below the accuracy level.

The Kondo-Heinsberg chain is an interesting model of a strongly correlated system which has a broad superconducting state with pair-density wave (PDW) order. Some of us have recently proposed that this PDW state is a symmetry-protected topological (SPT) state, and the gapped spin sector of the model supports Majorana zero modes. In this paper, we reexamine this problem using a combination of numeric and analytic methods. In extensive density-matrix renormalization group calculations, we find no evidence of a topological ground state degeneracy or the previously proposed Majorana zero modes in the PDW phase of this model. This result motivated us to reexamine the original arguments for the existence of the Majorana zero modes. A careful analysis of the effective continuum field theory of the model shows that the Hilbert space of the spin sector of the theory does not contain any single Majorana fermion excitations. This analysis shows that the PDW state of the doped 1D Kondo-Heisenberg model is not an SPT with Majorana zero modes.

We present a generalized formalism to describe the optical energy flow and spatially resolved absorption in arbitrarily anisotropic layered structures. The algorithm is capable of treating any number of layers of arbitrarily anisotropic, birefringent, and absorbing media and is implemented in an open-access computer program. We derive explicit expressions for the transmitted and absorbed power at any point in the multilayer structure, using the electric field distribution from a $4\times 4$ transfer matrix formalism. As a test ground, we study three nanophotonic device structures featuring unique layer-resolved absorption characteristics, making use of (i) in-plane hyperbolic phonon polaritons, (ii) layer-selective, cavity-enhanced exciton absorption in transition-metal dichalcogenide monolayers, and (iii) intersubband-cavity polaritons in quantum wells. Covering such a broad spectral range from the far infrared to the visible, the case studies demonstrate the generality and wide applicability of our approach.

Field-induced excitation gaps in quantum spin chains are an interesting phenomenon related to confinements of topological excitations. In this paper I present a type of this phenomenon. I show that an effective magnetic field with a fourfold screw symmetry induces the excitation gap accompanied by dimer orders. The dimer order parameter and the excitation gap exhibit characteristic power-law dependence on the fourfold screw-symmetric field. Moreover, the field-induced dimer order and the field-induced Néel order coexist when the external uniform magnetic field, the fourfold screw-symmetric field, and the twofold staggered field are applied. This situation is in close connection with a compound $\left[\mathrm{Cu}\left(\mathrm{pym}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]{\mathrm{SiF}}_6·{\mathrm{H}}_2\mathrm{O}$ [J. Liu et al., Phys. Rev. Lett. 122, 057207 (2019)]. In this paper I discuss a mechanism of field-induced dimer orders by using a density-matrix renormalization group method, a perturbation theory, and quantum field theories.

A generalized rigorous Floquet scattering-matrix method for stratified anisotropic optical media, subject to a periodic spatiotemporal modulation, is formulated and implemented. The method is applied for studying an optomagnonic cavity formed by an in-plane magnetized ferrite film, in which a magnetostatic surface spin wave propagates, sandwiched between two nonmagnetic dielectric Bragg mirrors. Our results provide unambiguous evidence that externally incident light, when trapped in a cavity mode, experiences a strongly enhanced interaction with the spin wave due to the increased coupling time, which can give rise to pronounced effects if the appropriate selection rules are fulfilled. By means of systematic calculations we reveal and explain some remarkable features of this interaction, such as formation of spectral gaps, controllable transmission, and the emergence of inelastic diffracted beams, and show that efficient conversion of the optical wave can be achieved by triply resonant inelastic scattering through (multi)magnon absorption and emission processes.

A nonuniform in-plane Zeeman field can induce spontaneous supercurrents of spin-orbit-coupled electrons in superconducting two-dimensional systems and thin films. In this work it is shown that current vortices can be created at the ends of a long homogeneously magnetized strip of a ferromagnetic insulator, which is deposited on the surface of a three-dimensional topological insulator. The $s$-wave superconductivity on its surface is assumed to have an intrinsic origin, or to be induced by the proximity effect. It is shown that vortices with the odd vorticity can localize Majorana zero modes. The latter may also be induced by sufficiently narrow domain walls inside the strip, which opens a way for manipulating these modes by moving the walls. It is shown that the vorticity can be tuned by varying the magnetization and width of the strip. A stability of the strip magnetization with respect to the Berezinsky-Kosterlitz-Thouless transition has been analyzed.

We present large-scale atomic simulations of shock-induced phase transition in Zr assisted by the machine learning method. The results indicate that there exists a critical piston velocity of $U_{\mathrm{p}}\sim 0.85\mathrm{km}/\mathrm{s}$, above which the product phase has changed from ω to bcc. Unlike the case in Fe, the shock-induced $\mathrm{hcp}\to \mathrm{bcc}$ nucleation mechanism in hcp-Zr single-crystal shows significant dependence on crystal orientation. For shock along the $\left[10\overline{1}0\right]$ direction, the hcp phase directly transforms into bcc as expected. However, for shock compression along [0001] and $\left[1\overline{2}10\right]$ directions, the $\mathrm{hcp}\to \mathrm{bcc}$ transformation occurs in quite a different manner, i.e., the Zr single crystal transforms into a disordered intermediate that subsequently exhibits ultrafast crystallization of the bcc phase within the timescales of subnanoseconds. We associate such presence of disordered intermediate structure with the sluggishness of shear stress relaxation, which leads to an elastic unstable condition of the crystal during the first few picoseconds of uniaxial compression, and suggests that the fewer possible shear planes (related to Burgers mechanism) for [0001] and $\left[1\overline{2}10\right]$ shock loading is an underlying factor for the orientation dependence.