Two-dimensional topological insulators and two-dimensional materials with ferroelastic characteristics are intriguing materials and many examples have been reported both experimentally and theoretically. Here, we present the combination of both features—a two-dimensional ferroelastic topological insulator that simultaneously possesses ferroelastic and quantum spin Hall characteristics. Using first-principles calculations, we demonstrate Janus single-layer $M\mathrm{SSe}\left(M=\mathrm{Mo},\mathrm{W}\right)$ stable two-dimensional crystals that show the long-sought ferroelastic topological insulator properties. The material features low switching barriers and strong ferroelastic signals, beneficial for applications in shape memory devices. Moreover, their topological phases harbor sizable nontrivial band gaps, which support the quantum spin Hall effect. The unique coexistence of excellent ferroelastic and quantum spin Hall phases in single-layer $M\mathrm{SSe}$ provides extraordinary platforms for realizing multipurpose and controllable devices.

We have investigated the effect of $B$-site randomness on the antiferroelectric/relaxor nature of the ground state by studying diffuse and inelastic x-ray scattering from ordered and disordered $\mathrm{Pb}\left({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}\right){\mathrm{O}}_3$ (O- and D-PIN) single crystals. The diffuse scattering measurement of O-PIN, which is antiferroelectric at low temperatures, shows that the ferroelectrically interactive local polarization exists in the cubic phase, above the transition temperature $T_{\mathrm{N}}$. Inelastic x-ray scattering analysis of the diffuse scattering shows that the transverse-acoustic (TA) and transverse-optic (TO) modes are dominant at high temperatures ($\sim 800$ K), while the central peak (CP) and TA modes, which are coupled, contribute majorly to diffuse scattering near $T_{{}_{\mathrm{N}}}$ and show critical behavior at temperature close to $T_{{}_{\mathrm{N}}}$. The TO mode shows no anomaly at temperature close to $T_{{}_{\mathrm{N}}}$. Furthermore, the phonon spectra are broad, indicating that a strong damping mechanism exists even in the sample with weak chemical disordering, O-PIN. No clear difference between O- and D-PIN is observed at temperatures above $\sim 500$ K. Here, the major difference between them is the property of the CP mode, which shows no drop and increases rapidly with decreasing temperatures in D-PIN. The CP mode is thought to be directly related to the local polarization and to originate in a combination of Pb flipping and the TO mode. The $B$ site is considered to control the antiferroelectric/ferroelectric instability of lead-based perovskite materials. Finally, the $B$-site randomness is discussed in terms of suppressing the antiferroelectric instability and enhancing the polarization fluctuation.

In this work we use Floquet theory to theoretically study the influence of circularly polarized light on disordered two-dimensional models exhibiting topological transitions. We find circularly polarized light can induce a topological transition in extended Kane-Mele models that include additional hopping terms and on-site disorder. The topological transitions are understood from the Floquet-Bloch band structure of the clean system at high-symmetry points in the first Brillouin zone. The light modifies the equilibrium band structure of the clean system in such a way that the smallest gap in the Brillouin zone can be shifted from the $M$ points to the $K$ $\left(K^{\prime }\right)$ points, the $\mathrm{\Gamma }$ point, or even other lower-symmetry points. The movement of the minimal gap point through the Brillouin zone as a function of laser parameters is explained in the high-frequency regime through the Magnus expansion. In the disordered model, we compute the Bott index to reveal topological phases and transitions. The disorder can induce transitions from topologically nontrivial states to trivial states or vice versa, both examples of Floquet topological Anderson transitions. As a result of the movement of the minimal gap point through the Brillouin zone as a function of laser parameters, the nature of the topological phases and transitions is laser parameter dependent, a contrasting behavior to the Kane-Mele model.

A method to compute anharmonic phonon peaks and anharmonic phonon-dispersion curves in crystals using ab initio calculated Hellmann-Feynman forces created from series of supercells is reported. The supercells are filled with atoms displaced from equilibrium positions in such a way that their configurations correspond to a given temperature. The obtained phonon-dispersion bands are able to represent the positions and shapes of the anharmonic peaks. As illustrations, the anharmonicity in cubic tungsten W, perovskite ${\mathrm{MgSiO}}_3$, and superconductor ${\text{MgB}}_2$ crystals is presented. The method can be applied to search for anharmonic potential-energy landscape of crystals. It includes electron-phonon coupling.

The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A dc bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative dc bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.

The topologically nontrivial magnetic phase with a ${\mathbb{Z}}_2$-vortex (${\mathbb{Z}}_2\mathrm{V}$) lattice is investigated by simulation in the Kitaev-Heisenberg honeycomb model expanded by considering the second- and third-nearest-neighbor Heisenberg interactions ($J_{H2}$ and $J_{H3}$). On the parameter region of the ${\mathbb{Z}}_2\mathrm{V}$ phase, a gradual modulation of vortex density is observed, together with a transition from single-${\mathbb{Z}}_2\mathrm{V}$ to triple-${\mathbb{Z}}_2\mathrm{V}$ state driven by the variation of frustration. Additionally, ${\mathbb{Z}}_2$ vortices are arranged in different manners on the whole honeycomb structure for these two types of ${\mathbb{Z}}_2\mathrm{V}$ states. Moreover, topologically equivalent states are revealed to exist in single-${\mathbb{Z}}_2\mathrm{V}$ dominant and triple-${\mathbb{Z}}_2\mathrm{V}$ dominant styles on different parameter points, which can be controlled to switch between each other without energy consumption. It is worth noting that $J_{H3}$ plays a key role in expanding the ${\mathbb{Z}}_2\mathrm{V}$ phase, and also in stabilizing the single-${\mathbb{Z}}_2\mathrm{V}$ state.

Exact results for time evolution of a many-body quantum system (a transverse field Ising spin chain) under the action of several pulses of an external field are obtained. Unlike the situation with a single pulse, the magnetization depends on time for the sequence of two pulses. For the two-pulse case the magnetization does not depend on the value of the time spacing between pulses, in sharp contrast with the standard spin echo cases. Modulated oscillations of magnetization with time do not decay if the energy of eigenstates of the system before pulses is $k$ independent $(k$ is the quasimomentum), while for $k$-dependent initial eigenstates the modulated oscillations of magnetization decay (without inclusion of the “external” relaxation) to the steady-state value, depending on the parameters of pulses; i.e., the oscillations reveal time irreversibility.

We study magnetization transport in anisotropic spin-$\frac{1}{2}$ chains governed by the integrable XXZ model with and without integrability-breaking perturbations at high temperatures $\left(T\to \infty \right)$ using a hybrid approach that combines exact sum rules with judiciously chosen Ansätze. In the integrable XXZ model we find (i) superdiffusion at the isotropic (Heisenberg) point, with frequency dependent conductivity ${\sigma }^{\prime }(\omega \to 0)\sim {\left|\omega \right|}^{\alpha }$, where $\alpha =-3/7$ in close numerical agreement with recent $t$-DMRG computations; (ii) a continuously drifting exponent from $\alpha =-1^{+}$ in the XY (gapless) limit of the model to $\alpha >0$ within the Ising (gapped) regime; and (iii) a diffusion constant saturating in the XY coupling deep in the Ising limit. We consider two kinds of next-nearest-neighbor integrability breaking perturbations—a simple spin-flip term $\left(J_2\right)$ and a three-spin assisted variant $\left(t_2\right)$, natural in the fermion particle representation of the spin chain. In the first case we discover a remarkable sensitivity of ${\sigma }^{\prime }\left(\omega \right)$ to the sign of $J_2$, with enhanced low frequency spectral weight and a pronounced upward shift in the magnitude of $\alpha$ for $J_2>0$. Perhaps even more surprising, we find subdiffusion $\left(\alpha >0\right)$ over a range of $J_2<0$. By contrast, the effects of the “fermionic” three-spin perturbation are sign symmetric; this perturbation produces a clearly observable hydrodynamic relaxation. At large strength of the integrability breaking term $J_2\to \pm \infty$ the problem is effectively noninteracting (fermions hopping on odd and even sublattices) and we find $\alpha \to -1$ behavior reminiscent of the XY limit of the integrable XXZ chain. Exact diagonalization studies largely corroborate these findings over accessible frequencies.

The Ruderman-Kittel-Kasuya-Yosida interaction of magnetic impurities in a superconductor exponentially decreases when the distance $r$ between them is larger than the superconductor's coherence length because this interaction is mediated by quasiparticles, which have a gap in their energy spectra. At the same time, the spin-singlet superconducting condensate was always assumed to stay neutral to magnetic impurities. Due to a spin-orbit coupling (SOC), however, Cooper pairs gain an admixture of spin-triplet correlated states, which provide for a link between impurity spins and an $s$-wave condensate. It is shown that perturbations of its phase mediate the $1/r^2$ interaction of these spins in two-dimensional (2D) systems. This effect is considered within two models: of a clean 2D $s$-wave superconductor with the strong Rashba SOC and of a bilayer system which combines a 2D Rashba coupled electron gas and an $s$-wave superconducting film. The predicted long-range interaction can have a strong effect on spin orders in superconductor-magnetic impurity systems that are expected to host Majorana fermions.

We have fabricated ultrathin $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7–x}$ nanowires with a high critical current density and studied their voltage switching behavior in the 4.2–90 K temperature range. A comparison of our experimental data with theoretical models indicates that, depending on the temperature and nanowire cross section, voltage switching originates from two different mechanisms: hotspot-assisted suppression of the edge barrier by the transport current and the appearance of phase-slip lines in the nanowire. Our observation of hotspot-assisted voltage switching is in good quantitative agreement with predictions based on the Aslamazov-Larkin model for an edge barrier in a wide superconducting bridge.

We study the relaxation of the center of mass or dipole oscillations in the system of interacting fermions confined spatially. With the confinement frequency ${\omega }_{\perp }$ fixed, the particles were considered to freely move along one (quasi-1D) or two (quasi-2D) spatial dimensions. We have focused on the regime of rare collisions, such that the inelastic collision rate $1/{\tau }_{\mathrm{in}}\ll {\omega }_{\perp }$. The dipole oscillation relaxation rate $1/{\tau }_{\perp }$ is obtained at three different levels: by direct perturbation theory, solving the integral Bethe-Salpeter equation, and applying the memory function formalism. As long as anharmonicity is weak, $1/{\tau }_{\perp }\ll 1/{\tau }_{\mathrm{in}}$, the three methods are shown to give identical results. In the quasi-2D case, $1/{\tau }_{\perp }\ne 0$ at zero temperature. In quasi-1D system, $1/{\tau }_{\perp }\propto T^3$ if the Fermi energy $E_F$ lies below the critical value $E_F<3{\omega }_{\perp }/4$. Otherwise, unless the system is close to integrability, the rate $1/{\tau }_{\perp }$ has a temperature dependence similar to that in quasi-2D. In all cases, the relaxation results from the excitation of particle-hole pairs propagating along unconfined directions resulting in the relationship $1/{\tau }_{\perp }\propto 1/{\tau }_{\mathrm{in}}$, with the inelastic rate $1/{\tau }_{\mathrm{in}}\ne 0$ as the phase-space opens up at finite energy of excitation ${\omega }_{\perp }$. While $1/{\tau }_{\perp }\propto {\tau }_{\mathrm{in}}$ in the hydrodynamic regime, ${\omega }_{\perp }\ll 1/{\tau }_{\mathrm{in}}$, in the regime of rare collisions, ${\omega }_{\perp }\gg 1/{\tau }_{\mathrm{in}}$, we obtain the opposite trend $1/{\tau }_{\perp }\propto 1/{\tau }_{\mathrm{in}}$.

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ interactions $\left(J_2/J_1\approx 0.5\right)$. The double perovskites $\mathrm{S}{\mathrm{r}}_2\mathrm{CuTe}{\mathrm{O}}_6$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{CuW}{\mathrm{O}}_6$ are isostructural spin-1/2 square-lattice antiferromagnets with Néel $(J_1$ dominates) and columnar $(J_2$ dominates) magnetic order, respectively. Here we characterize the full isostructural solid-solution series $\mathrm{S}{\mathrm{r}}_2\mathrm{Cu}\left(\mathrm{T}{\mathrm{e}}_{1\text{−}x}{\mathrm{W}}_x\right){\mathrm{O}}_6\left(0\le x\le 1\right)$ tunable from Néel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the $x=0.5$ phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of $x\approx 0.1–0.6$. This coincides with significant $T$-linear terms in the low-temperature specific heat. However, density-functional theory calculations predict most of the materials are not in the highly frustrated $J_2/J_1\approx 0.5$ region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in $x=0.5$.

We present the novel hydrated layered manganate ${\mathrm{MgMn}}_3{\mathrm{O}}_7·3{\mathrm{H}}_2\mathrm{O}$ as a maple-leaf-lattice (MLL) antiferromagnet candidate. The MLL is obtained by regularly depleting 1/7 of the lattice points from a triangular lattice so that the magnetic connectivity $z=5$ and is thus intermediately frustrated between the triangular $\left(z=6\right)$ and kagome $\left(z=4\right)$ lattices. In ${\mathrm{MgMn}}_3{\mathrm{O}}_7·3{\mathrm{H}}_2\mathrm{O}$, the ${\mathrm{Mn}}^{4+}$ ions carrying Heisenberg spin 3/2 form a regular MLL lattice in the quasi-two-dimensional structure. Magnetization and heat capacity measurements using a hydrothermally prepared powder sample reveal successive antiferromagnetic transitions at 5 and 15 K. A high-field magnetization curve up to 60 T at 1.3 K exhibits a multistep plateaulike anomaly. We discuss the unique frustration of the MLL antiferromagnet in which the chiral degree of freedom may play an important role.

We study fermion-parity-changing quantum phase transitions (QPTs) in platform Josephson junctions. These QPTs, associated with zero-energy bound states, are rather widely observed experimentally. They emerge from numerical calculations frequently without detailed microscopic insight. Importantly, they may incorrectly lend support to claims for the observations of Majorana zero modes. In this paper, we present a fully consistent solution of the Bogoliubov-de Gennes equations for a multicomponent Josephson junction. This provides insights into the origin of the QPTs. It also makes it possible to assess the standard self-energy approximations, which are widely used to understand proximity coupling in topological systems. The junctions we consider are complex and chosen to mirror experiments. Our full proximity calculations associate the mechanism behind the QPT as deriving from a spatially extended, proximity-induced magnetic “defect.” This defect arises because of the insulating region, which effects a local reorganization of the bulk magnetization in the proximitized superconductor. Our results suggest more generally that QPTs in Josephson junctions generally do not require the existence of spin-orbit coupling and should not be confused with, nor are they indicators of, Majorana physics.

We investigate the band structure, nematic state, and superconducting gap structure of two selected FeSe single crystals containing different amounts of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect on the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure is obtained from the three-dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the $k_z$ direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by a small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive nodelike small gap.

Motivated by current research efforts toward exploring the interplay between magnetism and superconductivity in multiband electronic systems, we have investigated the effects of Eu substitution through thermodynamic measurements on the superconducting filled skutterudite alloys ${\mathrm{Pr}}_{1-x}{\mathrm{Eu}}_x{\mathrm{Pt}}_4{\mathrm{Ge}}_{12}$. An increase in Eu concentration leads to a suppression of the superconducting transition temperature consistent with an increase of magnetic entropy due to Eu local moments. While the low-temperature heat capacity anomaly is present over the whole doping range, we find that in alloys with $x\le 0.5$, the Schottky peaks in the heat capacity in the superconducting state appear to be due to Zeeman splitting by an internal magnetic field. Our theoretical modeling suggests that this field is a result of the short-range antiferromagnetic correlations between the europium ions. For the samples with $x>0.5$, the peaks in the heat capacity signal the onset of antiferromagnetic ordering of the Eu moments.

We use inelastic neutron scattering to study the fate of the two spin resonance modes in underdoped superconducting ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}\left(x=0.0175\right)$ under applied magnetic fields. While an in-plane magnetic field of $B=12$ T only modestly suppresses superconductivity and enhances static antiferromagnetic order, the two spin resonance modes display disparate responses. The spin resonance mode at higher energy is mildly suppressed, consistent with the field effect in other unconventional superconductors. The spin resonance mode at lower energy, on the other hand, is almost completely suppressed. Such dramatically different responses to applied magnetic field indicate distinct origins of the two spin resonance modes, resulting from the strongly orbital-selective nature of spin excitations and Cooper pairing in iron-based superconductors.

We review the use of continuum quantum Monte Carlo (QMC) methods for the calculation of energy gaps from first principles, and present a broad set of excited-state calculations carried out with the variational and fixed-node diffusion QMC methods on atoms, molecules, and solids. We propose a finite-size-error correction scheme for bulk energy gaps calculated in finite cells subject to periodic boundary conditions. We show that finite-size effects are qualitatively different in two-dimensional materials, demonstrating the effect in a QMC calculation of the band gap and exciton binding energy of monolayer phosphorene. We investigate the fixed-node errors in diffusion Monte Carlo gaps evaluated with Slater-Jastrow trial wave functions by examining the effects of backflow transformations, and also by considering the formation of restricted multideterminant expansions for excited-state wave functions. For several molecules, we examine the importance of structural relaxation in the excited state in determining excited-state energies. We study the feasibility of using variational Monte Carlo with backflow correlations to obtain accurate excited-state energies at reduced computational cost, finding that this approach can be valid. We find that diffusion Monte Carlo gap calculations can be performed with much larger time steps than are typically required to converge the total energy, at significantly diminished computational expense, but that in order to alleviate fixed-node errors in calculations on solids the inclusion of backflow correlations is sometimes necessary.

Weyl semimetals (WSMs) have Weyl nodes where conduction and valence bands meet in the absence of inversion or time-reversal symmetry (TRS) or both. The TRS-broken WSM phase can be driven from a topological Dirac semimetal by magnetic field $\mathbf{B}$ or magnetic dopants, considering that Dirac semimetals have degenerate Weyl nodes stabilized by rotational symmetry, i.e., Dirac nodes. Here we develop a Wannier-function-based tight-binding (WF-TB) model to investigate the formation of Weyl nodes and nodal rings induced by $\mathbf{B}$ field in the topological Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$. The field is applied along the rotational axis. So far, studies of $\mathbf{B}$ field induced WSMs have been limited to cases with effective models. Remarkably, our study based on the WF-TB model shows that upon $\mathbf{B}$ field each Dirac node is split into four separate Weyl nodes along the rotational axis near the Fermi level; two nodes with Chern number $\pm 1$ (single Weyl nodes) and two with Chern number $\pm 2$ (double Weyl nodes). This result is in contrast to the common belief that each Dirac node consists of only two single Weyl nodes with opposite chirality. In the context of the $4\times 4$ effective models, the existence of double Weyl nodes ensures nonzero cubic terms in momentum. We examine the evolution of Fermi arcs at a side surface as a function of chemical potential. The number of Fermi arcs at a given chemical potential is consistent with the corresponding Fermi surface Chern numbers. Our study also reveals the existence of nodal rings in the mirror plane near the Fermi level upon $\mathbf{B}$ field. These nodal rings persist with spin-orbit coupling. Our WF-TB model can be used to compute interesting features such as anomalous Hall and thermal conductivities, and our findings can be applied to other topological Dirac semimetals like ${\mathrm{Cd}}_3{\mathrm{As}}_2$.