Walker-Wang models are fixed-point models of topological order in $3+1$ dimensions constructed from a braided fusion category. For a modular input category $\mathcal{M}$, the model itself is invertible and is believed to be in a trivial topological phase, whereas its standard boundary is supposed to represent a $(2+1$)-dimensional chiral phase. In this work we explicitly show triviality of the model by constructing an invertible domain wall to vacuum as well as a disentangling generalized local unitary circuit in the case where $\mathcal{M}$ is a Drinfeld center. Moreover, we show that if we allow for fermionic (auxiliary) degrees of freedom inside the disentangling domain wall or circuit, the model becomes trivial for a larger class of modular fusion categories, namely, those in the Witt classes generated by the Ising unitary modular tensor category. We also discuss general (noninvertible) boundaries of general Walker-Wang models and describe a simple axiomatization of extended topological quantum field theory in terms of tensors.

We adopt the non-Hermitian Hamiltonian formalism to describe Weyl fermions of types III and IV. The spectrum of Hamiltonian has an unusual type of anisotropy. Namely, the hermiticity of Hamiltonian strongly depends on the direction in momentum space: for some directions, the spectrum is real, in contrast for other directions it becomes complex. It is shown that the type III and IV Weyl points are topologically stable and the Chern number is equal to $\pm 1$ despite to the fact that the Hamiltonian is not Hermitian. Furthermore, we calculate the Landau levels and demonstrate that zero Landau level is real, which means that there is a real spectral flow between electronlike and holelike states. Due to the formal analogy with the index theorem, the presence of such a flow as well indicates a nonzero Chern number. In addition, we illustrate that the non-Hermitian Hamiltonian can be regarded as a one-particle problem in the context of topological band theory.

The transverse and longitudinal magnetoresistance (MR) of two samples of the topological chiral semimetal CoSi with different residual resistivity ratios (RRRs) was studied. It is shown that the Kohler rule works for the transverse MR. The Kohler rule is also fulfilled in the case of longitudinal MR at a low reduced magnetic field. A sharp deviation in the longitudinal MR curve for the sample with low RRR from the Kohler prediction at high fields reveals its tendency to change sign at higher magnetic fields. We probably do not see any specific MR features connected to the topological chiral anomaly in the electron spectra of CoSi. This observation implies the need for certain revisions of the theoretical predictions. Shubnikov–de Haas quantum oscillations were observed and analyzed in both perpendicular and parallel configurations of the current and magnetic field in sample CoSi-1 with $\mathrm{RRR}=9.33$ at low temperatures.

Controlling over the electronic structure of two-dimensional (2D) semiconductors is crucial to unlock their full potential in the applications in future nanodevices. Here, we demonstrate a layer-dependent dielectric modulation of electronic structures in multilayer ${\mathrm{WS}}_2/\mathrm{GaN}$ heterostructures. The results investigated by means of in situ scanning tunneling microscopy/spectroscopy and photoluminescence spectroscopy show that the fluctuations of the dielectric environment induced by local corrugation of the GaN surface dramatically tune the band structure of monolayer ${\mathrm{WS}}_2$ with a large downshift of the valence band maximum by about 1.12 eV. Under the shielding effect of the first-layer ${\mathrm{WS}}_2$, the dielectric environment shows a weak modulation effect on the second-layer, and even has negligible influence on the third- and fourth-layer ${\mathrm{WS}}_2$. The relationship between dielectric environment and interfacial distance is further simulated through first-principles calculations. The electrostatic potential energy, differential charge densities, and charge transfer of the ${\mathrm{WS}}_2/\mathrm{GaN}$ heterostructure under different interfacial distances reveal that the differences of charge redistributions between the ${\mathrm{WS}}_2/\mathrm{GaN}$ interface and ${\mathrm{WS}}_2$ interlayer are responsible for the layer-dependent dielectric modulation. This work offers some references for the design and fabrication of novel 2D optoelectronic devices.

Strain engineering is an effective method to extend the properties of two-dimensional (2D) materials. In this paper, we theoretically studied the electron mobility of 2D hexagonal (H) ${\mathrm{MoS}}_2$ and triangular (T) ${\mathrm{PdSe}}_2$ under different strains. We observe contrary trends of the electron transport with strain. Our study shows it inherently comes from the contrary response of chemical bonding to strain. The covalency of both $\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{T}\text{−}\mathrm{PdS}{\mathrm{e}}_2$ decreases with increasing strain due to the elongated bond length. The ionicity of $\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ is also decreased, resulting in decreased electron-phonon coupling strength. On the contrary, the ionicity of $\mathrm{T}\text{−}\mathrm{PdS}{\mathrm{e}}_2$ is increased, which overwhelms the decrease of the covalency, resulting in enhanced electron-phonon coupling strength. Our work not only uncovers the underlying physics governing the contrary trends of electron mobility with strain in hexagonal and triangular $M{X}_2$, but also offers a paradigm analysis methodology that is applicable to other research areas.

We theoretically investigate the phase transition from a nontrivial topological $p$-wave superconductor to a trivial $s$-wave-like superconducting phase through a gapless phase, driven by different magnetic textures as a one-dimensional spin-chain impurity, e.g., Bloch-type, in-plane, and out-of-plane Néel-type spin chains, etc. In our proposal, the chain of magnetic impurities is placed on a spin-triplet $p$-wave superconductor where we obtain numerically as well as analytically an effective $s$-wave-like pairing due to spin rotation, resulting in gradual destruction of the Majorana zero modes present in the topological superconducting phase. In particular, when the impurity spins are antiferromagnetically aligned, i.e.,  spiral wave vector $G_{\mathrm{s}}=\pi$, the system becomes an effective $s$-wave superconductor without Majorana zero modes in the local density of states. The Shiba bands, on the other hand, formed due to the overlapping of Yu-Shiba-Rusinov states play a crucial role in this topological to trivial superconductor phase transition, confirmed by the sign change in the minigap within the Shiba bands. We also characterize this topological phase transition via gap closing and winding number analysis. Moreover, interference of the Shiba bands exhibiting oscillatory behavior within the superconducting gap $-{\mathrm{\Delta }}_p$ to ${\mathrm{\Delta }}_p$, as a function of $G_s$, also reflects an important evidence for the formation of an effective $s$-wave pairing. Such oscillation is absent in the $p$-wave regime. We also analyze the case of two-dimensional $p$-wave superconductor hosting Majorana edge modes (in absence of any magnetic chain) and show that initially Majorana zero modes (in presence of one-dimensional magnetic chain) can hybridize with such Majorana edge modes. Interestingly, in the topological regime with a fixed $G_s$ value, the Mazorana zero modes survive at the ends of the magnetic chain even when the Majorana edge states disappear at some critical value of the chemical potential and exchange coupling strength.

We introduce a rod-in-a-box model for acoustic resonators. For resonators small compared to the acoustic wavelength, an elastostatic equilibrium approximation yields closed-form expressions for their frequency-dependent effective masses and moments of inertia. The low-frequency bands and gaps are recaptured by an intuitive $6\times 6$ matrix eigenvalue equation, yielding a band structure within $2.3\%$ agreement with the finite-element method. Our model is generalized to complex dumbbell-shaped resonators, revealing a dense collection of flat “slow sound” bands near the resonance band gap.

Much has been learned about the topological transport in real materials. We investigate the interplay between magnetism and topology in the magnetotransport of ${\mathrm{PrRhC}}_2$. The fourfold degeneracy reduces to twofold followed by nondegenerate Weyl nodes when the orientation of the magnetic quantization axis is changed from easy axis to body diagonal through face diagonal. This engenders chirality imbalance between positive and negative chirality Weyl nodes around the Fermi energy. We observe a significant enhancement in the chiral anomaly mediated response such as planar Hall conductivity and longitudinal magnetoconductivity, due to the emergence of chirality imbalance upon orienting the magnetic quantization axis to body diagonal. The angular variations of the above quantities for different magnetic quantization axes clearly refer to the typical signature of planar Hall effect in Weyl semimetals. We further investigate the profiles of anomalous Hall conductivities as a function of Fermi energy to explore the effects of symmetries as well as chirality imbalance on Berry curvature.

We explore the potential of twisted light, a structured beam carrying orbital angular momentum, as a tool to unveil many-body effects in parabolically confined systems. According to the generalized Kohn theorem, the dipole response of such a multiparticle system to a spatially homogeneous probe is indistinguishable from the response of a system of noninteracting particles. Twisted light however can excite internal degrees of freedom, resulting in the appearance of new peaks in the multipole spectrum which are not present when the probe is a plane wave. We also demonstrate the ability of the proposed twisted light probe to capture the transition of interacting fermions into a strongly correlated regime in a one-dimensional harmonic trap. We report that, by suitable choice of the probe's parameters, the transition into a strongly correlated phase manifests itself as an approaching and ultimate superposition of peaks in the second-order quadrupole response. These features are observed in exact calculations for two electrons and well reproduced in adiabatic time-dependent density-functional theory simulations.

Here we show that a coherent random walk in a perfectly periodic chain of bosonic modes with designed loss can exhibit a variety of different anomalous transfer regimes in dependence on the initial state of the chain. In particular, for any given finite initial time-interval there is a set of initial states leading to a hyperballistic transport regime. Also, there are initial states allowing one to achieve a subdiffusive regime or even localization for a given time-interval, or change an asymptotic long-time diffusion rate. We show how these anomalous transport regimes can be practically realized in a laser-written network of single-mode waveguides in balk glass or how a planar system of coupled single-mode waveguides can be realized with an integrated photonic platform.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) promotes homochiral magnetic structures such as Néel-type domain walls (DWs) and skyrmions. iDMI as well as chiral magnetic structures have been demonstrated in metal/garnet heterostructures offering desirable properties for spintronic applications compared to their metallic counterparts. By measuring the motion of DWs as a function of current and in-plane magnetic field, we show that for 6.6 nm thick epitaxial thulium iron garnet films with perpendicular magnetic anisotropy grown on gallium garnet substrates, the iDMI and the DW width depend on the substrate but do not scale linearly with lattice mismatch strain. The largest iDMI was $0.007 \mathrm{mJ} {\mathrm{m}}^{-2}$, obtained for three substrates of different compositions. The total anisotropy, however, does increase linearly with lattice parameter, indicating a dominant magnetoelastic anisotropy contribution. The sensitivity of iDMI and anisotropy to substrate composition provides opportunities for engineering homochiral magnetic structures and their dynamics.

The combined effect of charge transfer, broken symmetries, and spin-orbit coupling (SOC) yields interfacial magnetism and transport in spatially asymmetric quantum oxide heterostructures. The polar nature of such interfaces activates the Rashba effect which provides a platform to modify the electronic band structure, whereas the presence of competing magnetic anisotropies is essential for deriving the topologically nontrivial spin textures. Entanglement of these two effects wherein the Rashba physics can be utilized to manipulate magnetic anisotropy (MA) is contemplated as a pathway to alternative electronic states. Here, using the $\mathrm{CaMn}{\mathrm{O}}_3/\mathrm{CaIr}{\mathrm{O}}_3$ heterostructure as a template, we demonstrate that Rashba SOC modulates the MA and controls the sign and magnitude of anomalous Hall conductivity consequent to the modification in the electronic band structure via the reconstruction of Berry curvature. This approach to tune and control the multipronged magnetotransport phenomena via Rashba SOC (without external bias voltage) is potentially relevant for spin-orbitronics functionalities.

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $\kappa$-type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge-carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even on the very edge of its existence, the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.

Excitons and free charge carriers with large wave vectors form a nonradiative reservoir, which can strongly affect properties of bright excitons due to the exciton-exciton and exciton-carrier interactions. In the present work, the dynamics of quasiparticles in the reservoir at different areal densities is experimentally studied in a GaAs/AlGaAs quantum well using a time-resolved reflectance spectroscopy of nonradiative broadening of exciton resonances. The population of the reservoir is controlled either by the excitation power or by the temperature of the structure under study. The dynamics is quantitatively analyzed in the framework of the model developed earlier [A. S. Kurdyubov et al., Phys. Rev. B 104, 035414 (2021)]. The model considers several dynamic processes, such as scattering of photoexcited excitons into the reservoir, dissociation of excitons into free charge carriers and the reverse process, carrier-induced exciton scattering into the light cone depleting the reservoir, thermally activated nonradiative losses of charge carriers. We have found that competition of these processes leads to highly nonlinear dynamics of reservoir excitons, although their density is far below the exciton Mott transition densities. Characteristic rates of the processes and their dependences on the excitation power and the temperature are determined.

We construct microscopical models of one-dimensional noninteracting topological insulators in all of the chiral universality classes. Specifically, we start with a deformation of the Su-Schrieffer-Heeger (SSH) model that breaks time-reversal symmetry, which is in the AIII class. We then couple this model to its time-reversal counterpart in order to build models in the classes BDI, CII, DIII, and CI. We find that the $\mathbb{Z}$ topological index (the winding number) in individual chains is defined only up to a sign. This comes from noticing that changing the sign of the chiral symmetry operator changes the sign of the winding number. This freedom to choose the sign of the chiral symmetry operator on each chain independently allows us to construct two distinct possible chiral symmetry operators when the chains are weakly coupled—in one case, the total winding number is given by the sum of the winding number of individual chains, while in the second case, the difference is taken. We also find that the chiral models that belong to $\mathbb{Z}$ classes, AIII, BDI, and CII are topologically equivalent, so they can be adiabatically deformed into one another without the change of topological invariant, so long as the chiral symmetry is preserved. We study the properties of the edge states in the constructed models and prove that topologically protected edge states must all be localized on the same sublattice (on any given edge). We also discuss the role of particle-hole symmetry on the protection of edge states and explain how it manages to protect edge states in ${\mathbb{Z}}_2$ classes, where the integer invariant vanishes and chiral symmetry alone does not protect the edge states anymore. We generalize our results to the case of an arbitrary number of coupled chains, by constructing possible chiral symmetry operators for the multiple chain case, and briefly discuss the applications to any odd number of dimensions.

Manipulation of spin orientation in magnetic material has been the focus of increasing research interest, as it brings fascinating perspectives with regard to the exploration of underlying magnetic interactions and the design of electronic devices. Here we report on the field-induced spin reorientation below Néel temperature in antiferromagnet ${\mathrm{MnPS}}_3$. The ferromagnetic ordering emerges in the antiferromagnetic phase in ${\mathrm{MnPS}}_3$ at temperatures below $\sim 34$ K with external magnetic fields up to $\sim 2$ kOe. Such a coexistence of the ferromagnetic and antiferromagnetic phases is obtained in both perpendicular $\left(H\perp ab\right)$ and parallel $\left(H//ab\right)$ configurations, even with strong anisotropy. The $H\text{−}T$ phase diagram for ${\mathrm{MnPS}}_3$ single crystal is further established with multiple magnetic phases being demonstrated. A phenomenological picture of the evolution of spin reorientation under external magnetic fields is proposed as physical insight into the rich magnetic properties of ${\mathrm{MnPS}}_3$. Our findings examine an exciting frontier in fundamental investigations of low-dimensional antiferromagnets and open a path towards the realization of conceptual electronic devices.