The Lyapunov exponent, serving as an indicator of the localized state, is commonly utilized to identify localization transitions in disordered systems. In non-Hermitian quasicrystals, the non-Hermitian effect induced by nonreciprocal hopping can lead to the manifestation of two distinct Lyapunov exponents on opposite sides of the localization center. Building on this observation, we here introduce a comprehensive approach for examining the localization characteristics and mobility edges of nonreciprocal quasicrystals, referred to as asymmetric transfer matrix analysis. We demonstrate the application of this method to three specific scenarios: the nonreciprocal Aubry-André model, the nonreciprocal off-diagonal Aubry-André model, and the nonreciprocal mosaic quasicrystals. This work may contribute valuable insights to the investigation of non-Hermitian quasicrystal and disordered systems.

We investigate the robustness of the many-body localized (MBL) phase to the quantum-avalanche instability by studying the dynamics of a localized spin chain coupled to a $T=\infty$ thermal bath through its leftmost site. By analyzing local magnetizations we estimate the size of the thermalized sector of the chain and find that it increases logarithmically slowly in time. This logarithmically slow propagation of the thermalization front allows us to lower-bound the slowest thermalization time, and find a broad parameter range where it scales fast enough with the system size that MBL is robust against thermalization induced by avalanches. The further finding that the imbalance—a global quantity measuring localization—thermalizes over a timescale that is exponential both in disorder strength and system size is in agreement with these results.

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using ${\mathrm{TbFeO}}_3$ as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the ${\mathrm{TbFeO}}_3$ single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the “LO-TO rule” due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites.

The perovskite oxide $\mathrm{EuTi}{\mathrm{O}}_3$ (ETO) has attracted increased scientific interest due to its potential multiferroic properties and magnetic activity above and below its structural phase transition at $T_{\mathrm{S}}=282$ K. Various experiments have indirectly evidenced that this transition is neither a cubic tetragonal nor the only one occurring in ETO. Here, we show results demonstrating two further instabilities below $T_{\mathrm{S}}$ based on lattice dynamics and spin-phonon interactions combined with a Landau free energy model with coupled order parameters. The transition temperatures agree with available experimental data where further instabilities have been anticipated.

We propose differential phase contrast (DPC) imaging using energy-filtered electrons to image the magnetic properties of materials at the atomic scale. Compared to DPC measurements with elastic electrons, our simulations predict about two orders of magnitude higher relative magnetic signal intensities and sensitivity to all three vector components of magnetization.

Crystal growth and magnetic and spin-orbit torque (SOT) properties of the systems with polycrystalline ${\mathrm{RuO}}_2$ consisting of ${\mathrm{RuO}}_2/\mathrm{SHM}/\mathrm{Co}/\mathrm{Ir}/\mathrm{MgO}/\mathrm{Ta}$ (SHM = Pt, [Pt/Ir] multilayer) and ${\mathrm{RuO}}_2/\mathrm{NM}/\mathrm{Pt}/\mathrm{Co}/\mathrm{Ir}/\mathrm{MgO}/\mathrm{Ta} (\mathrm{NM}=\mathrm{Ru},\mathrm{Ti})$ and the effects of ${\mathrm{RuO}}_2$ and light or heavy metal (NM and SHM) interfaces on spin current generated from the spin-split band (SSB) are investigated. The Pt layer and [Pt/Ir] multilayer on the polycrystalline ${\mathrm{RuO}}_2$ layer have flat and (111) texture, which induces perpendicular magnetization of Co. Enhancement of dampinglike SOT efficiency $\left({\xi }_{\mathrm{DL}}\right)$ in the thin Pt and [Pt/Ir]-multilayer thickness region and nearly constant magnitude of ${\xi }_{\mathrm{DL}}$ as a function of the thickness of $\mathrm{NM}=\mathrm{Ti}$ are observed. Moreover, field-free switching was observed even in the polycrystalline ${\mathrm{RuO}}_2/\mathrm{Pt}$ heavy metal system in the thin Pt thickness region $(t_{\mathrm{Pt}}\le 2\mathrm{nm})$. The observed field-free switching may be related to the exchange bias from the antiferromagnetic ${\mathrm{RuO}}_2$ layer to the Co layer through a thin Pt layer. The spin-torque ferromagnetic resonance results show that the $x$- and $z$-spin polarizations originating from SSB are canceled out for the polycrystalline ${\mathrm{RuO}}_2$ system. These results indicate that the efficiency of SOT is enhanced by orbital Rashba-Edelstein, spin Hall, and orbital Hall effects. We expect that polycrystalline ${\mathrm{RuO}}_2$-based systems will pave the way to antiferromagnetic spintronics.

Stimulated by recent experimental synthesis efforts of ${\mathrm{LaB}}_8$, ${\mathrm{WB}}_2$, and $\alpha \text{−}{\mathrm{MoB}}_2$, all characterized by intrinsic superconductivity (SC), we investigated the introduction of cerium (Ce) into various boron (B) concentrations by leveraging first-principles calculations in tandem with crystal-structure prediction techniques. Under ambient and external pressure, Ce atoms transfer electrons to B atoms, enabling the formation of distinct B configurations, namely honeycomb-like B layers in CeB, honeycomb-like B layers in ${\mathrm{CeB}}_2$, ${\mathrm{B}}_{20}$ cages in ${\mathrm{CeB}}_5$, ${\mathrm{B}}_{24}$ cages in ${\mathrm{CeB}}_6$, ${\mathrm{B}}_{23}$ cages in ${\mathrm{CeB}}_7$, ${\mathrm{B}}_{26}$ cages in ${\mathrm{CeB}}_8$, and ${\mathrm{B}}_{24}$ cages in ${\mathrm{CeB}}_{12}$. Except for ${\mathrm{CeB}}_5$ and ${\mathrm{CeB}}_7$, the other Ce-B compounds can be quenched to ambient pressure. Clathrate-like ${\mathrm{CeB}}_{\mathrm{n}} (\mathrm{n}=6, \mathrm{8}, \text{and} 12)$ compounds hold promising prospects as potential incompressible materials, with Vickers hardness $\left(H_v\right)$ of $20\sim 39$ GPa. Based on the Migdal-Eliashberg (ME) theory, we discovered that both ${\mathrm{CeB}}_2$ and ${\mathrm{CeB}}_8$ are typical anisotropic single-gap superconductors with superconducting critical temperature $\left(T_c\right)$ values of 29 and 27 K, respectively. Because of the 29-K SC originating from the coupling of both $\mathrm{B}\text{−}2p$ and $\mathrm{Ce}\text{−}5d$ orbital states with several soft phonon modes induced by Fermi surface nesting and Kohn anomalies—fundamentally different from that in isostructural ${\mathrm{MgB}}_2$, ${\mathrm{WB}}_2$, and $\alpha \text{−}{\mathrm{MoB}}_2$—${\mathrm{CeB}}_2$ thus can be classified as another high-$T_c$ superconductor.

Ice systems are prototypes of locally constrained dynamics. This is exemplified in Coulomb-liquid phases where a large space of configurations is sampled, each satisfying local ice rules. Dynamics proceeds through “flipping” rings, i.e., through reversing arrows running along the edges of a polygon. We examine the role of defect rings in such phases, with square ice as a testing ground. When placed on a curved surface, the underlying square lattice will form defects such as triangles or pentagons. We show that triangular defects are statistically more “flippable” than the background. In contrast, pentagons and larger polygons are less flippable. In fact, flippability decreases monotonically with ring size, as seen from a Pauling-like argument. As an explicit demonstration, we wrap the square-ice model on a sphere. We start from an octahedron and perform repeated rectifications, producing a series of clusters with sphere-like geometry. They contain a fixed number of defect triangles in an otherwise square lattice. We numerically enumerate all ice-rule-satisfying configurations. Indeed, triangles are flippable in a larger fraction of configurations than quadrilaterals. The obtained flippabilities are in broad agreement with our Pauling-like estimates. We construct two minimal models for dynamics: (i) a classical model with stochastic dynamics and (ii) a quantum model with tunneling terms that flip rings. The steady state in the former (and the ground state in the latter) has contributions from all ice configurations. The dominant contribution to dynamics (or kinetic energy) comes from localized resonance within triangles. Our results suggest local dynamics as a promising observable for experiments in spin-ice and artificial-ice systems. They also point to hierarchical dynamics in materials such as ice V that contain rings of multiple sizes.

The low-temperature magnetic properties of a quantum spin liquid candidate $\kappa \text{−}(\mathrm{BEDT}\text{−}{\mathrm{TTF})}_2{\mathrm{Cu}[\mathrm{Au}\left(\mathrm{CN}\right)}_2]\text{Cl}$ with a nearly regular triangular spin lattice $(t^{\prime }/t\approx 1.19)$ and ordered anions were investigated by SQUID, ${}^1\mathrm{H}$ NMR, and ESR measurements. The magnetic susceptibility is described by a triangular lattice $S=1/2$ antiferromagnetic Heisenberg model with an exchange interaction of $J=210$ K. The ${}^1\mathrm{H}$ NMR indicated no magnetic order down to 0.45 K, which was about three orders of magnitude smaller than $J$. The ESR study clarified the absence of a spin gap at least down to 4 K, indicating a gapless spin liquid state. We also identified the ${\mathrm{Cu}}^{2+}$ magnetic impurity and its effect on the gapless spin liquid.

We propose a scheme for achieving long-distance entanglement between an optomagnonics system, consisting of a ferromagnetic yttrium iron garnet sphere, and a spatially separated Fabry-Pérot cavity, based on magnon-induced Brillouin light scattering. In this scheme, the establishment of nonlocal entanglement relies on two conditions: the triple-resonance condition determined by Brillouin light scattering and the parametric-type interaction between the magnon and photon modes. Consequently, precise control over the activation and deactivation of quantum entanglement can be achieved through manipulation of polarization and frequency of the pump field, effectively functioning as a unilateral ‘switch’ for quantum channels. Specifically, when exciting the ${\sigma }^{+}$-polarized transverse magnetic mode, even under a weak bias magnetic field that induces a low-frequency magnon, long-distance entanglement reaches its peak as long as the triple-resonance condition is matched. This feature is significant for mitigating strong magnetic interference among integrated elements. Conversely, pumping to $\pi$-polarized transverse electric mode leads to disruption of entanglement. Furthermore, one-way quantum steering enables the party holding magnons to unilaterally control the spatially separated party of Fabry-Pérot cavity. This pump modulation scheme offers an effective approach for establishing quantum correlations between materials across frequency domains and spatial distances.

Based on the notion of emergent elasticity, we theoretically demonstrate that wavelike to particle-like crossover occurs in the chiral soliton lattice (CSL) formed in a monoaxial chiral helimagnet. We use the principle of virtual work and analyze the reaction of the CSL to the external disturbances, which is described in terms of the elastic continuum with the Voigt constant $c_{11}$. We also analyze dependence of “spring constant” of the CSL on the magnetic field strengths, when the magnetic field is applied perpendicular to the chiral axis. As a result, we found that the CSL exhibits crossover phenomena from a ductile-material-like behavior at small and intermediate magnetic field strengths to an elastic-medium-like behavior at higher magnetic fields for $0.65H_c\lesssim H<H_c$, with $H_c$ being the critical field above which the forced ferromagnetic state appears. This ductile-to-elastic crossover is characterized by the emergence of the restoring elastic force and corresponds to the crossover from the wavelike collective soliton lattice dynamics to the particle-like dynamics of the individual kinks. This result may explain the crossover from “weakly nonlinear” to “highly nonlinear regimes” in the CSL formation, found in ${\mathrm{CrNb}}_3{\mathrm{S}}_6$, and the nature of which has remained unclear as yet.

We study two-dimensional $s$-wave topological superconductivity with Rashba spin-orbit coupling and Zeeman field in Penrose and Ammann-Beenker quasicrystals. By solving the Bogoliubov–de Gennes equations self-consistently for not only the superconducting order parameter, but also the spin-dependent Hartree potential, we show the stable occurrence of topological superconductivity with broken time-reversal symmetry in both Penrose and Ammann-Beenker quasicrystals. The topological nature of the quasicrystalline system is signified by the Bott index $B$. Topological phase transitions are found to occur, where $B$ changes between 0 and $\pm 1$, as the chemical potential or Zeeman field is varied. In terms of self-consistent solutions, we demonstrate the existence of a Majorana zero mode per edge or vortex when $B=\pm 1$, consistently with the bulk-edge/defect correspondence for periodic systems.

This study explores transitions between states with different winding number in two-band superconducting rings. From the time-dependent Ginzburg-Landau equations for two-component superconductors, we apply linear instability theory and develop a semianalytical method that provides the critical flux for phase-slip occurrence. The developed method was applied to investigate how the critical flux depends on physical properties, such as band parameters and temperature. Finally, we show the possible existence of a soliton state, in which the phase winding number is different in each condensate.

We describe a hybrid quantum-classical approach to treat quantum many-body systems in the thermodynamic limit. This is done by combining numerical linked-cluster expansions (NLCE) with the variational quantum eigensolver (VQE). Here, the VQE algorithm is used as a cluster solver within the NLCE. We test our hybrid quantum-classical algorithm ($\mathrm{NLCE}+\mathrm{VQE}$) for the ferromagnetic transverse-field Ising model on the one-dimensional chain and the two-dimensional square lattice. The calculation of ground-state energies on each open cluster demands a modified Hamiltonian variational ansatz for the VQE. One major finding is convergence of $\mathrm{NLCE}+\mathrm{VQE}$ to the conventional NLCE result in the thermodynamic limit when at least $N/2$ circuit repetitions (known as layers) are used in the VQE ansatz for each cluster with $N$ sites. Our approach demonstrates the fruitful connection of techniques known from correlated quantum many-body systems with hybrid algorithms explored on existing quantum-computing devices.

Exact solutions of model problems are elusive but potent tools for understanding many-body interacting systems. We study a system of dissipative qubits with the Heisenberg interaction and obtain, for qubits under a certain condition, an exact steady-state solution to the Lindblad master equation describing its dynamics. The physical content of such a solution is a remarkable no-go theorem, which states that for qubits possessing identical ratios of the damping and gain rates, no correlations can emerge between them in the steady state, provided their interaction is in the Heisenberg XXZ form. Notably, this even holds for the more general Heisenberg XYZ interaction if the damping rate equals the gain rate. Two consequences of this theorem are discussed in the context of quantum synchronization of qubits. The first is a complete blockade of quantum synchronization of qubits under the aforementioned condition, an effect reminiscent of, but having a much broader scope than, that found in dissipated Kerr-anharmonic oscillators. The second, and a more important consequence is the possibility of reducing a complex all-to-all qubit network to a much simpler one-to-all network by engineering the dissipation. Such a reduction is desired because it provides an effective tool to optimize the quantum synchronization of a complex qubit network. Finally, we propose two concrete experimental schemes to implement our model and to test our predictions. The no-go theorem opens a new avenue for exactly toggling the correlations of quantum many-body systems on and off without disrupting their interaction.

We investigate the low-energy effective theory of a Fermi surface (FS) coupled to an Ising-nematic quantum critical point in (2 + 1) spacetime dimensions with translation symmetry. We formulate the system using the large $N$ Yukawa-Sachdev-Ye-Kitaev model, whose saddle point is described by the Migdal-Eliashberg equations. The low-energy physics can be revealed by studying the Gaussian fluctuation spectrum around the saddle point, which is generated by the Bethe-Salpeter (BS) kernel $K_{\mathrm{BS}}$. Based on the Ward identities, we propose an inner product on the space of two point functions, which reveals a large number of soft modes of $K_{\mathrm{BS}}$. These soft modes parametrize deformation of the Fermi surface, and their fluctuation eigenvalues describe their decay rates. We analytically compute these eigenvalues for a circular Fermi surface, and we discover the odd-parity modes to be parametrically longer-lived than the even-parity modes, due to the kinematic constraint of fermions scattering on a convex FS. The sign of the eigenvalues signals an instability of the Ising-nematic quantum critical point at zero temperature for a convex Fermi surface. At finite temperature, the system can be stabilized by thermal fluctuations of the critical boson. We derive an effective action that describes the soft-mode dynamics, and it leads to a linearized Boltzmann equation, where the real part of the soft-mode eigenvalues can be interpreted as the collision rates. The structure of the effective action is similar to the theory of linear bosonization of a Fermi surface. As an application of the formalism, we investigate the hydrodynamic transport of non-Fermi liquid. Analyzing the Boltzmann equation, we obtain a conventional hydrodynamic transport regime and a tomographic transport regime. In both regimes, the conductance of the system in finite geometry can be a sharp indicator for the soft-mode dynamics and non-Fermi liquid physics.