By proposing a family of two-lengthscale quasicrystalline tilings characterized by even-numbered metallic-mean inflation ratios, we extend the recently introduced notion of aperiodic approximants of triangular and honeycomb lattices to square crystals. The proposed family originates in the eightfold Ammann-Beenker and fourfold Harriss quasicrystals and is based on sets of two square tiles and a parallelogram. We elaborate the higher-dimensional representation of the new tilings and we show that at large inflation ratios they tend to the square lattice just like their triangular and hexagonal counterparts.

Quantum-Hall–superconductor hybrids have been predicted to exhibit various types of topological order, providing possible platforms for intrinsically fault-tolerant quantum computing. In this paper, we investigate disorder effects on the Rashba-coupled quantum-Hall system combined with the type-II superconductor. By diagonalizing the Bogoliubov–de Gennes Hamiltonian projected into a Rashba-coupled Landau level, we demonstrate the emergence of a topological superconducting phase resulting from disorders and proximity-induced pairing. Distinctive gapless modes appear in the real-space entanglement spectrum, which is consistent with topological superconductivity. Historically, the spherical geometry has been commonly used for identifying topologically ordered states, especially quantum-Hall physics, due to its compact and contractible nature. Motivated by this, we develop a formulation to construct this hybrid system on a sphere. Our numerical demonstrations are all performed on the spherical geometry.

Aluminum nitride (AlN) and other wurtzite materials are widely used in piezoelectric microelectromechanical systems and are of great interest for future thin-film ferroelectric devices. Much progress has been made by modeling these materials with quantum mechanical methods such as density functional theory (DFT). However, there are very few existing methods that can model AlN on a larger scale, and none that can model multiple phases and domain walls with the accuracy of DFT. In this work, we present a machine-learned molecular dynamics force field (MLFF) for AlN constructed by fitting an artificial neural network to an underlying DFT dataset. Using our trained MLFF, we can predict the energies, forces, and phonon dispersions of AlN with the accuracy of DFT at dramatically lower computational cost. Accordingly, our MLFF can simulate systems orders of magnitude larger than DFT, enabling the study of emergent and long-range effects, such as the frequency-dependent dielectric function and multiple ferroelectric domains. This method can easily be expanded to other wurtzite nitrides, oxides, and solid solutions.

Light drives offer a potential tool for the dynamical control of magnetic interactions in matter. We theoretically investigate the indirect exchange coupling between two parallel chains of magnetic impurities on the surface of a topological insulator, driven by a time-periodic circularly polarized light field in the high-frequency, off-resonant regime. We derive a closed-form analytic expression for the spin susceptibility of the photon-dressed topological insulator surface states and obtain the irradiation dependence of the Ising, Heisenberg, and Dzyaloshinskii-Moriya exchange couplings between the impurity chains. Our results show a two-pronged modification of these exchange couplings by periodic drives. First, the Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation period of the exchange couplings can be extended by enhancing the driving strength. Second, increasing driving strength enhances the envelope of RKKY oscillations of the Ising type while suppressing those of the Heisenberg type and Dzyaloshinskii-Moriya type. Our work provides useful insights for realizing Floquet engineering of collinear and noncollinear indirect exchange interactions in topological insulating systems.

Utilizing the nonequilibrium Green's function method, we study the local heat current flow of phonons in nanoscale ballistic graphene nanoribbon, where boundary scattering leads to the formation of atomic-scale current vortices. We further map out the atomic temperature distribution in the ribbon with Büttiker's probe approach. From the heat current and temperature distribution, we observe inverted temperature response in the ribbon, where the heat current direction goes from the colder to the hotter region. Moreover, we show that atomic scale defect can generate heat vortex at certain frequency, but it is averaged out when including contributions from all the phonon modes. Meanwhile, our results have recovered residual-resistivity dipole features manifested at the vicinity of local defects. These results extend the study of local heat vortex and negative temperature response in the bulk hydrodynamic regime to the atomic-scale ballistic regime, further confirming boundary scattering is crucial to generate backflow of heat current.

In this paper we study gapped boundary states of ${\mathbb{Z}}_N$ bosonic symmetry-protected topological (SPT) phases in (4+1)d, which are characterized by mixed ${\mathbb{Z}}_N$-gravity response, and the closely related phases protected by $C_N$ rotation symmetry. We show that if $N\notin \{2,4,8,16\}$, any symmetry-preserving boundary theory is necessarily gapless for the root SPT state. We then propose a (3+1)d ${\mathbb{Z}}_2$ gauge theory coupled to fermionic matter as a candidate boundary theory for $N=2,4,8,16$, where the anomalous symmetry is implemented by invertible topological defects obtained from gauging (2+1)d chiral topological superconductors. For the $C_N$ case, we present an explicit construction for the boundary states for $N=2,4,8,16$, and argue that the construction fails for other values of $N$.

Non-Hermitian systems have garnered significant attention due to the emergence of novel topology of complex spectra and skin modes. However, investigating transport phenomena in such systems faces obstacles stemming from the nonunitary nature of time evolution. Here, we establish the continuity equation for a general non-Hermitian Hamiltonian in the Schrödinger picture. It attributes the universal nonconservativity to the anticommutation relationship between particle number and non-Hermitian terms. Our paper derives a comprehensive current formula for non-Hermitian systems using Green's function, applicable to both time-dependent and steady-state responses. To demonstrate the validity of our approach, we calculate the local current of models with one-dimensional and two-dimensional settings, incorporating scattering potentials. The spatial distribution of local current highlights the widespread non-Hermitian phenomena, including skin modes, nonreciprocal quantum dots, and corner states. Additionally, we revisit a recent experiment within the quantum Hall regime and propose a set-up for experimentally detecting non-Hermitian current. Our findings offer valuable insights for advancing theoretical and experimental research in the transport of non-Hermitian systems.

A prototypical charge-transfer type Mott insulator ${\mathrm{NiS}}_2$ pyrite exhibits a metal-insulator transition with bandwidth control. Recent discoveries on surface-specific electronic states on other $3d$ transition-metal disulfide pyrites motivate us to further investigate the surface of ${\mathrm{NiS}}_2$, where metallic surface conduction is discussed. Here, the spectroscopic-imaging scanning-tunneling-microscopy observations revealed that the surface is not metallic, contrary to the expectation. Instead, the Mott gap is closed near step edges, suggesting possible electrical conduction from one-dimensional channels. The edge anomaly was observed irrespective of its magnetic order and is limited to the insulator phases.

We predict surface induced odd-frequency (odd-$\nu$) spin-triplet superconducting pairing can be a veritable signature of Majorana bound states (MBS) in a Josephson nodal $p$-wave superconductor ($p_x$)-spin flipper (SF)-nodal $p$-wave superconductor ($p_x$) junction. Remarkably, in a $p_x$-SF-$p_x$ Josephson junction three distinct phases emerge: the topological phase featuring MBS, the topological phase without MBS, and the trivial phase devoid of MBS. Surface odd-$\nu$ spin-triplet pairing is induced only in the topological regime when MBS appears. In contrast, surface induced even-frequency (even-$\nu$) spin-triplet pairing is finite regardless of the existence of MBS. Importantly, we find the surface induced odd-$\nu$ spin-triplet pairing is immune to disorder in the topological phase featuring MBS, while in the trivial phase the surface induced even-$\nu$ spin-triplet pairing is affected by disorder. Our study offers a potential means for distinguishing the topological phase featuring MBS from both the trivial phase as well as the topological phase devoid of MBS, primarily through the observation of induced surface odd-$\nu$ spin-triplet superconductivity.

Plastic deformation in metallic glasses at room temperature leads to the development of shear bands due to shear localization. In many experiments, shear bands have shown local density variations along their path, with a distinct imbalance in magnitude between local densification and dilation. However, a comprehensive mechanistic understanding or theory to explain this asymmetry has been lacking until now. Here, we introduce a model that consists of a sequential arrangement of alternating topological “charges,” generating a dipolar field. The resulting microscopic displacement field, when integrated into the deformation gradient tensor, provides an accurate analytical solution for the observed imbalances in the density variations. The implications of this method are discussed, highlighting the potential to elucidate a broader range of observations in shear bands.

The elastic constants of an epitaxial film of FeRh have been determined experimentally in both ferromagnetic (FM) and antiferromagnetic (AF) phases, using a combination of Brillouin light scattering and picosecond acoustics experiments. The $C_{11}$ constant is noticeably larger in the FM phase than in the AF phase, while $C_{12}$ and $C_{44}$ are both lower, leading to larger Rayleigh wave velocities in the FM phase than in the AF phase. The elastic constants were calculated numerically using first-principles anharmonic modeling and machine-learned interatomic potentials. We find that using a temperature-dependent effective potential is indispensable to correctly reproduce the experimental values to within 80–100%. The accurate knowledge of the temperature- and phase-dependences of the elastic constants of crystalline FeRh are valuable ingredients for the predictive modeling of the acoustic and magnetoacoustic properties of this magnetostrictive material.

The skyrmion crystal (SkX) has attracted much attention in condensed matter physics since topologically nontrivial structures induce fascinating physical phenomena. The SkXs have been experimentally observed in a variety of materials, where the Zeeman coupling to the static magnetic field plays an important role in the formation of the SkXs. In this study, we theoretically propose another route to generate the SkXs by using a circularly polarized electric field. We investigate a nonequilibrium steady state in a classical frustrated Heisenberg magnet under the circularly polarized electric field, where the electric field is coupled to the electric polarization via the spin-current mechanism. By numerically solving the Landau-Lifshitz-Gilbert equation at zero temperature, we show that the electric field radiation generates a SkX with a high topological number in the high-frequency regime, where the sign of the skyrmion number is fixed to be negative (positive) under the left (right) circularly polarized field. The intense electric field melts these SkXs and generates isolated skyrmions. We clarify that the microscopic origin is effective electric-field-induced three-spin interactions by adopting the high-frequency expansion in the Floquet formalism. Furthermore, we find that the electric field radiation generates another type of SkXs, a bimeron crystal, in the low-frequency regime. Our results provide a way to generate the SkXs and control the topology by the circularly polarized electric field.

Disordered hyperuniform materials are very promising for applications but the successful route for synthesizing them requires understanding the interactions induced by the host media that can switch off this hidden order. With this aim we study the model system of vortices in the $\beta \text{−}{\mathrm{Bi}}_2\mathrm{Pd}$ superconductor where correlated defects seem to play a determinant role for the nucleation of a gel vortex phase at low densities. We directly image vortices in extended fields of view and show that the disordered vortex structure in this material is anti-hyperuniform, contrasting with the case of vortex structures nucleated in samples with pointlike disorder. Based on numerical simulations, we show that this anti-hyperuniform structure arises both from the interaction of a diluted vortex structure with a fourfold-symmetric correlated disorder quite likely generated when cleaving the samples and from the out-of-equilibrium nature of the quenched configuration.

We explore quantum criticality and Kibble-Zurek scaling (KZS) in the Aubry-André-Stark (AAS) model, where the Stark field of strength $\varepsilon$ is added onto the one-dimensional quasiperiodic lattice. We perform scaling analysis and numerical calculations of the localization length, the inverse participation ratio (IPR), and the energy gap between the ground and first excited states to characterize critical properties of the delocalization-localization transition. Remarkably, our scaling analysis shows that, near the critical point, the localization length $\xi$ scales with $\varepsilon$ as $\xi \propto {\varepsilon }^{-\nu }$ with $\nu \approx 0.3$, a new critical exponent for the AAS model, which is different from the counterparts for both the pure Aubry-André (AA) model and the pure Stark model. The IPR $\mathcal{I}$ scales as $\mathcal{I}\propto {\varepsilon }^s$ with the critical exponent $s\approx 0.098$, which is also different from both of the pure models. The energy gap $\mathrm{\Delta }E$ scales as $\mathrm{\Delta }E\propto {\varepsilon }^{\nu z}$ with the same critical exponent $z\approx 2.374$ as that for the pure AA model. We further reveal hybrid scaling functions in the overlap between the critical regions of the Anderson and Stark localizations. Moreover, we investigate the driven dynamics of the localization transitions in the AAS model. By linearly changing the Stark (quasiperiodic) potential, we calculate the evolution of the localization length and the IPR, and we study their dependence on the driving rate. We find that the driven dynamics from the ground state is well described by the KZS with the critical exponents obtained from the static scaling analysis. When both the Stark and the quasiperiodic potentials are relevant, the KZS form includes the two scaling variables. This work extends our understanding of critical phenomena on localization transitions and generalizes the application of the KZS to hybrid models.

The dynamics of magnetization and current-voltage characteristics of the superconductor-ferromagnet-superconductor ${\phi }_0$ Josephson junction in the presence of external electromagnetic radiation have been studied. Effects of the electric and magnetic components of radiation on the magnetic precession in the ferromagnetic layer are investigated. The implementation of two types of dynamical states of magnetization is demonstrated. These states have a phase shift of $\pi$ in the synchronization region of magnetic precession and Josephson oscillations and differ in the nature of their time dependence. Transitions between these states with increasing and decreasing bias current show hysteresis, which is reflected in the bifurcation diagram and as spikes in the current-voltage characteristics. We also provide an experimental way to test the obtained results by measuring the phase shift in voltage temporal dependence at fixed current value for both sweeping directions. The results obtained can find application in various fields of superconducting spintronics and quantum computing.