We study the realizations of topological defects in a one-dimensional quantum Ising model with an open boundary condition at criticality. Applying the construction discussed in M. Hauru, G. Evenbly, W. W. Ho, D. Gaiotto, and G. Vidal, Phys. Rev. B 94, 115125 (2016), we prove that the Ising model on an open chain with multiple topological defects can be transformed to the same model with boundary magnetic fields and noninteracting boundary degrees of freedom. This results in the appearance of a linear combination of Cardy states, which can be interpreted as an edge state of the spin or fermion chain. We show that this edge state with the large boundary entropy can be protected under bulk perturbation, whereas it is fragile to a boundary perturbation. Our formulation suggests the existence of nontrivial edge physics under the existence of topological defects and opens many interesting questions for future analysis related to boundary and bulk physics.

In this work, we study the disorder and phase diagrams of two-dimensional higher-order topological insulators (HOTIs). We concentrate on two cases: (i) bulk-corner correspondence and (ii) edge-corner correspondence. For the bulk-corner correspondence case, we demonstrate the existence of the mobility gaps and clarify the related topological invariant that characterizes the mobility gap. Furthermore, we find that, while the system preserves the bulk-corner correspondence in the presence of disorder, the corner states are protected by the mobility gap instead of the bulk gap. For the edge-corner correspondence case, we show that the bulk mobility gap and edge band gaps of HOTIs are no longer closed simultaneously. Therefore, a rich phase diagram is obtained, including various disorder-induced phase transition processes. Notably, a disorder-induced transition from the nontrivial to trivial phase is realized, distinguishing the HOTIs from the other topological states. Our results deepen the understanding of the topological features and the related phase transitions of disordered HOTIs.

We investigate the dispersion characteristics and the effective properties of acoustic waves propagating in a one-dimensional duct equipped with periodic thermoacoustic coupling elements. Each coupling element consists of a classical thermoacoustic regenerator subject to a static spatial temperature gradient. When acoustic waves pass through the regenerator, thermal-to-acoustic energy conversion takes place and can either amplify or attenuate the wave, depending on the direction of propagation of the wave. The presence of the spatial gradient naturally induces a loss of reciprocity. This paper provides a comprehensive theoretical model as well as an in-depth numerical analysis of the band structure and of the propagation properties of this thermoacoustically coupled, tunable, one-dimensional metamaterial. Among the most significant findings, it is shown that the acoustic metamaterial is capable of supporting nonreciprocal thermoacoustic Bloch waves that are associated with a particular form of unidirectional energy transport. The nonreciprocal nature of the waveguide in the long wavelength limit is well understood by seeing the waveguide as an acoustic Willis material. The homogenized material properties following the Willis approach help both the analysis and the interpretation of the waveguide dynamic behavior in selected frequency ranges. Remarkably, the thermoacoustic coupling also allows achieving a zero refractive index that ultimately leads to phase-invariant propagating sound waves. This zero-index property is shown to have very interesting implications to attain acoustic cloaking.

The properties of ${\mathrm{RbBi}}_2$, a 4.15 K superconductor, were investigated using magnetic field, pressure, and neutron diffraction. Under hydrostatic pressure, an almost 50% reduction of $T_c$ is observed, linked to a low Debye temperature estimated at 165 K. The resistivity and magnetic susceptibility were measured on quenched and slow-cooled polycrystalline samples. The resistivity follows a low temperature power-law dependence in both types of samples, while the diamagnetic susceptibility, $\chi$, is dependent on the sample cooling history. Slow-cooled samples have $\chi =-1$ while quenched samples have $\chi <-1$ due to grain size differences. Evidence of the effects of the cooling rate is also discerned from the local structure, obtained by neutron diffraction and the pair density function analysis. Slow-cooled samples have structurally symmetric Bi hexagons, in contrast to quenched samples in which disorder is manifested in periodic distortions of the Bi hexagonal rings of the kagome sublattice. Disorder may lead to flux pinning that reduces vortex mobility, but $T_c$ remains unaffected by the cooling rate.

We revisit the phenomenon of the resonant transmission of fermionic carriers through a quantum device connected to two contacts with different chemical potentials. We show that, besides the traditional Landauer-Büttiker approach in solid-state physics, this phenomenon can also be described by the non-Markovian master equation for the reduced density matrix of the fermions in the quantum device. We identify validity regions for both approaches in the system parameter space and argue that for large relaxation rates the accuracy of the latter approach greatly exceeds the accuracy of the former.

We study the possibility of triply degenerate points (TPs) that can be stabilized in spinless crystalline systems. Based on an exhaustive search over all 230 space groups, we find that the spinless TPs can exist at both high-symmetry points and high-symmetry paths, and they may have either linear or quadratic dispersions. For TPs located at high-symmetry points, they all share a common minimal set of symmetries, which is the point group $T$. The TP protected solely by the $T$ group is chiral and has a Chern number of $\pm 2$. By incorporating additional symmetries, this TP can evolve into chiral pseudospin-1 point, linear TP without chirality, or quadratic contact TP. For accidental TPs residing on a high-symmetry path, they are not chiral but can have either linear or quadratic dispersions in the plane normal to the path. We further construct effective $k·p$ models and minimal lattice models for characterizing these TPs. Distinguished phenomena for the chiral TPs are discussed, including the extensive surface Fermi arcs and the chiral Landau bands.

We developed a theory of electric and thermoelectric conductivity of lightly doped ${\mathrm{SrTiO}}_3$ in the nondegenerate region $k_{B}T\ge E_F$, assuming that the major source of electron scattering is their interaction with soft transverse optical phonons present due to proximity to ferroelectric transition. We have used the kinetic equation approach within relaxation-time parametrization of the collision integral and we have determined energy-dependent transport relaxation time $\tau \left(E\right)$ by the iterative procedure. Using electron effective mass $m$ and electron-transverse phonon coupling constant $\lambda$ as two fitting parameters, we are able to describe quantitatively a large set of the measured temperature dependences of resistivity $R\left(T\right)$ and Seebeck coefficient $\mathcal{S}\left(T\right)$ for a broad range of electron densities studied experimentally in recent paper by Collignon et al. [Phys. Rev. X 10, 031025 (2020)]. In addition, we calculated Nernst ratio $\nu =N/B$ in the linear approximation over weak magnetic field in the same temperature range.

Experimentally, Cr doping in the rutile phase of ${\mathrm{VO}}_2$ is found to stabilize a charge-ordered ferromagnetic insulating state in the doping range of 10% to 20%. In this work, we investigate its origin at 12.5% Cr doping using a combination of ab initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab initio band structure onto a tight binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work, and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.

We describe a many-body theory for interlayer dispersion forces between weakly disordered atomically thin crystals and numerically investigate the role of disorder for different layer-separation distances and for different densities of induced electrons and holes. In contrast to the common wisdom that disorder tends to enhance the importance of Coulomb interactions in Fermi liquids, we find that short-range disorder tends to weaken interlayer dispersion forces. This is in line with previous findings that suggest that transitioning from metallic to insulating propagation weakens interlayer dispersion forces. We demonstrate that disorder alters the scaling laws of dispersion forces and we comment on the role of the maximally crossed vertex-correction diagrams responsible for logarithmic divergences in the resistivity of two-dimensional metals.

In the framework of time-dependent density functional theory (TDDFT), the exact exchange-correlation (xc) kernel $f_{\mathrm{xc}}(n,q,\omega )$ determines the ground-state energy, excited-state energies, lifetimes, and the time-dependent linear density response of any many-electron system. The recently developed MCP07 xc kernel $f_{\mathrm{xc}}(n,q,\omega )$ of Ruzsinszky et al. [Phys. Rev. B 101, 245135 (2020)] yields excellent uniform electron gas (UEG) ground-state energies and plausible plasmon lifetimes. As MCP07 is constructed to describe $f_{\mathrm{xc}}$ of the UEG, it cannot capture optical properties of real materials. To verify this claim, we follow Nazarov et al. [Phys. Rev. Lett. 102, 113001 (2009)] to construct the long-range, dynamic xc kernel, ${lim}_{q\to 0}{f}_{\mathrm{xc}}(n,q,\omega )=-\alpha \left(\omega \right)e^2/q^2$, of a weakly inhomogeneous electron gas, using MCP07 and other common xc kernels. The strong wave-vector and frequency dependence of the “ultranonlocality” coefficient $\alpha \left(\omega \right)$ is demonstrated for a variety of simple metals and semiconductors. We examine how imposing exact constraints on an approximate kernel shapes $\alpha \left(\omega \right)$. Comparisons to kernels derived from correlated-wave-function calculations are drawn.

Microcavity exciton-polaritons, known to exhibit nonequilibrium Bose condensation at high critical temperatures, can also be brought in thermal equilibrium with the surrounding medium and form a quantum degenerate Bose-Einstein distribution. It happens when their thermalization time in the regime of positive detunings—or, alternatively, for high-finesse microcavities—becomes shorter than their lifetime. Here we present the self-consistent finite-temperature Hartree–Fock–Bogoliubov description for such a system of polaritons, universally addressing the excitation spectrum, momentum-dependent interactions, condensate depletion, and the background population of dark excitons that contribute to the system's chemical potential. Employing the derived expressions, we discuss the implications for the Bogoliubov sound velocity, confirmed by existing experiments, and define the critical temperatures of (quasi)condensation and the integral particle lifetime dependencies on the detuning. Large positive detunings are shown to provide conditions for the total lifetime reaching nanosecond timescales. This allows realization of thermodynamically equilibrium polariton systems with Bose-Einstein condensate forming at temperatures as high as tens of Kelvin.

We theoretically study emergent edge- and corner-localized states in monolayer black phosphorene. Using the tight-binding model based on the density functional theory, we find that the multiorbital band structure due to the nonplanar puckered geometry plays an essential role in the formation of the boundary localized modes. In particular, we demonstrate that edge states emerge at a boundary along an arbitrary crystallographic direction, and it can be understood from the fact that the Wannier orbitals associated with $3p_x,3p_y,3p_z$ orbitals occupy all the bond centers of phosphorene. At a corner where two edges intersect, we show that multiple corner-localized states appear due to hybridization of higher-order topological corner state and the edge states nearby. These characteristic properties of the edge and corner states can be intuitively explained by a simple topologically equivalent model where all the bond angles are deformed to ${90}^{\circ }$.

A quantum Hall (QH) interface is different from an ordinary QH edge, as the latter has its location determined by the confining potential, while the former can be unpinned and behave like a free string. In this paper, we demonstrate this difference by studying three different interfaces formed by (i) the Laughlin state and the vacuum, (ii) the Pfaffian state and the vacuum, and (iii) the Pfaffian and the anti-Pfaffian states. We find that stringlike interfaces propagating freely in the QH system lead to very different dynamical properties from edges. This qualitative difference gives rise to fascinating physics and suggests a different direction for future research on QH physics. We also discuss briefly possible analogies between QH interfaces and concepts in string theory.

In this paper, we compute the electronic structure of acceptor clusters in silicon by using three different methods to take into account electron correlations: the full configuration interaction (full CI calculation), the Heitler-London approximation (HL approximation), and the unrestricted Hartree-Fock method (UHF method). We show that both the HL approach and the UHF method are good approximations to the ground state of the full CI calculation for a pair of acceptors and for finite linear chains along [001], [110], and [111]. The total energies for finite linear chains show the formation of a fourfold-degenerate ground state (lying highest in energy), below which there are characteristic low-lying eightfold and fourfold degeneracies, when there is a long (weak) bond at the end of the chain. We present evidence that this is a manifold of topological edge states. We identify a change in the angular momentum composition of the ground state at a critical pattern of bond lengths, and show that it is related to a crossing in the Fock matrix eigenvalues. We also test the symmetry of the self-consistent mean-field UHF solution and compare it to the full CI; the symmetry is broken under almost all the arrangements by the formation of a magnetic state in UHF, and we find further broken symmetries for some particular arrangements related to crossings (or potential crossings) between the Fock-matrix eigenvalues in the [001] direction. We also compute the charge distributions across the acceptors obtained from the eigenvectors of the Fock matrix; we find that, with weak bonds at the chain ends, two holes are localized at either end of the chain while the others have a nearly uniform distribution over the middle; this also implies the existence of the nontrivial edge states. We also apply the UHF method to treat an infinite linear chain with periodic boundary conditions, where the full CI calculation and the HL approximation cannot easily be used. We find the band structures in the UHF approximation, and compute the Zak phases for the occupied Fock-matrix eigenvalues; however, we find they do not correctly predict the topological edge states formed in this interacting system. On the other hand, we find that direct study of the quantum numbers characterizing the edge states, introduced by Turner et al. [Phys. Rev. B 83, 075102 (2011)], provides a better insight into their topological nature.

An interacting electron liquid in two (2D) and three (3D) dimensions may undergo a paramagnetic-to-ferromagnetic quantum spin polarization transition at zero applied magnetic field, driven entirely by exchange interactions, as the system density ($n$) is decreased. This is known as Bloch ferromagnetism. We show theoretically that the application of an external magnetic field ($B$), which directly spin polarizes the system through Zeeman spin splitting, has an interesting effect on Bloch ferromagnetism if the applied field and carrier density are both decreased (from some initial applied high magnetic field at a high carrier density) in a power-law manner, $B\sim n^p$. For $p<p_c$, with $p_c=1(2/3)$ in $2\left(3\right)\mathrm{D}$, the system remains either fully spin polarized or undergoes a single transition from a partially spin-polarized (with two Fermi surfaces corresponding to spin up and down electrons) to a fully spin-polarized state (with a single Fermi surface of one spin) as the density and field decrease, depending on whether the starting point is partially spin polarized or fully spin polarized. However, for $p>p_c$, the system may undergo two transitions if starting from the fully spin-polarized state: first, a weak second-order transition at high density and field from the field-induced fully polarized phase to the partially polarized phase; and then, at a lower field and density, a reentrant first-order transition back to the fully spin-polarized phase again with a single Fermi surface.

Pressure-induced polymerization of carbon monoxide (CO) molecules may lead to next-generation high energy density materials. By combining structural search method, first-principles calculations and ab initio molecular dynamics (AIMD) simulations, we predict several polymeric carbon oxide $C_x{O}_y$, i.e., ${\mathrm{C}}_5{\mathrm{O}}_2$ $\left(P\text{−}4m2\right)$, ${\mathrm{C}}_6{\mathrm{O}}_4$ ($P\text{−}4m2$ and $I\text{−}4m2$) and ${\mathrm{C}}_8{\mathrm{O}}_8$ $\left(I\text{−}4m2\right)$, which are all layered semiconductors with high energy density, large bulk modulus, and high hardness. For the $\mathrm{C}:\mathrm{O}\mathrm{ratio}=1:1$, the ${\mathrm{C}}_8{\mathrm{O}}_8\text{−}I\text{−}4m2$ phase is energetically more stable than the reported $Cmca$ and $Cmcm$ phases above 90 GPa, and its energy density is 4.51 kJ/g, which is higher than trinitrotoluene (TNT). Considering the mechanism of ${\mathrm{CO}}_2$ release, $C_x{O}_y(x>y)$ crystals, namely ${\mathrm{C}}_5{\mathrm{O}}_2$ $\left(P\text{−}4m2\right)$ and ${\mathrm{C}}_6{\mathrm{O}}_4$ ($P\text{−}4m2$ and I-4m2) are predicted above $40\sim 50\mathrm{GPa}$. At 100 GPa, phonon spectrum calculations and AIMD simulations indicate that they have good mechanical and dynamic stability. At 0 GPa, AIMD simulations also show the possible phase transition path of the four $C_x{O}_y$ structures by releasing ${\mathrm{CO}}_2$: ${\mathrm{C}}_8{\mathrm{O}}_8\text{−}I\text{−}4m2\to {\mathrm{C}}_6{\mathrm{O}}_4\text{−}I\text{−}4m2$ and ${\mathrm{C}}_6{\mathrm{O}}_4\text{−}P\text{−}4m2\to {\mathrm{C}}_5{\mathrm{O}}_2\text{−}P\text{−}4m2$, and ${\mathrm{C}}_6{\mathrm{O}}_4\text{−}I\text{−}4m2$ and ${\mathrm{C}}_5{\mathrm{O}}_2\text{−}P\text{−}4m2$ can remain stable at ambient conditions. Those structures may enrich the phase diagram of high-pressure $C_x{O}_y$, and provide clues for synthesis and exploration of new stable energetic materials besides polymeric nitrogen.

We study a magnon-photon coupling system consisting of a layered antiferromagnetic insulator ${\mathrm{CrCl}}_3$ and a planar superconducting cavity. By changing the position of the ${\mathrm{CrCl}}_3$ crystal on the cavity, we separately demonstrate the cavity mode coupled with an acoustic mode and an optical mode when the magnetic moments are in a spin-flop transition. Based on the coupled Landau-Lifshitz-Gilbert equations and Kirchhoff's equation, we predict that both the net magnetization and the Néel vector contribute to the coupling between the optical mode and cavity mode, and only the net magnetization participates in the coupling of the acoustic mode and cavity mode. This paper highlights the vital role played by the Néel vector as the key parameter of an antiferromagnet in magnon-photon coupling, which offers insight into designing antiferromagnet spintronic devices.

Competition between unitary dynamics that scrambles quantum information nonlocally and local measurements that probe and collapse the quantum state can result in a measurement-induced entanglement phase transition. Here we study this phenomenon in an analytically tractable all-to-all Brownian hybrid circuit model composed of qubits. The system is initially entangled with an equal sized reference, and the subsequent hybrid system dynamics either partially preserves or totally destroys this entanglement depending on the measurement rate. Our approach can access a variety of entropic observables which are distinguished by the averaging procedure, and for concreteness we focus on a particular purity quantity for which the averaging is particularly simple. We represent the purity as a path integral coupling four replicas with twisted boundary conditions. Saddle-point analysis reveals a second-order phase transition corresponding to replica permutation symmetry breaking below a critical measurement rate. The transition is mean-field-like and we characterize the critical properties near the transition in terms of a simple Ising field theory in $0+1$ dimensions. In addition to studying the purity of the entire system, we study subsystem purities and relate these results to manifestations of quantum error correction in the model. We also comment on the experimental feasibility for simulating this averaged purity, and corroborate our results with exact diagonalization for modest system sizes.

A general, symmetry-allowed model of nearest-neighbor interactions for MnSi-like magnets is presented. A left-handed helical magnet phase occurs within a large parameter space of the model, which is explored via numerical simulation. The relations between microscopic features of the spiral structure and various model parameters, including an external magnetic field, are determined and show good agreement with predictions from free energy considerations. A skyrmion structure is stabilized near the boundary.

Although the magnetic relaxation mechanism in ferromagnetic/nonmagnetic (FM/NM) bilayers has been commendably established, the nonlocal Gilbert damping has been much less addressed in spin valve configurations of FM/NM/FM associated with both the static interlayer exchange coupling and the dynamic coupling. Here, we report the dimensional crossover role of the Pd layer on magnetic relaxation in Fe/Pd/Fe trilayer structures. We identify a pronounced jump of Gilbert damping across the characteristic static interlayer exchange coupling length of Fe/Pd/Fe. The significant enhancement and suppression of Gilbert damping values are ascribed to the entanglement of the dynamics of the two Fe layers mediated by the static exchange coupling and the dynamic exchange coupling. Our work deepens the understanding to manipulate the spin transport and magnetic relaxation via the coherence of spin current.