Topological spin liquids in two spatial dimensions are stable phases in the presence of a small magnetic field, but may give way to field-induced phenomena at intermediate field strengths. Sandwiched between the low-field spin liquid physics and the high-field spin-polarized phase, the exploration of magnetic phenomena in this intermediate regime, however, often remains elusive to controlled analytical approaches. Here we numerically study such intermediate-field magnetic phenomena for two representative Kitaev models (on the square-octagon and decorated honeycomb lattice) that exhibit either Abelian or non-Abelian topological order in the low-field limit. Using a combination of exact diagonalization and density matrix renormalization group techniques, as well as linear spin-wave theory, we establish the generic features of Kitaev spin liquids in an external magnetic field. While ferromagnetic models typically exhibit a direct transition to the polarized state at a relatively low field strength, antiferromagnetic couplings not only substantially stabilizes the topological spin liquid phase, but generically lead to the emergence of a distinct field-induced intermediate regime, separated by a crossover from the high-field polarized regime. Our results suggest that, for most lattice geometries, this regime generically exhibits significant spin canting, antiferromagnetic spin-spin correlations, and an extended proximate spin liquid regime at finite temperatures. Notably, we identify a symmetry obstruction in the original honeycomb Kitaev model that prevents, at least for certain field directions, the formation of such canted magnetism without breaking symmetries—consistent with the recent numerical observation of an extended gapless spin liquid in this case.

We observe a large negative magnetoresistance and a decrease of resistivity with increasing temperature, known as the Gurzhi effect, in a bilayer (BL) electron system formed by a wide GaAs quantum well. A hydrodynamic model for the single fluid transport parameters in narrow channels is employed and successfully describes our experimental findings. We find that the electron-electron scattering in the bilayer is more intensive in comparison with a single-band well (SW). The hydrodynamic assumption implies a strong dependence on boundary conditions, which can be characterized by slip length, describing the behavior of a liquid near the edge. Our results reveal that slip length in a BL is shorter than in a SW, and that the BL system goes deeper into the hydrodynamic regime. This is in agreement with the model proposed where the slip length is of the order of the electron-electron mean free path.

Recently, it has been demonstrated that the absorption of moderately doped two-dimensional semiconductors can be described in terms of exciton polarons. In this scenario, attractive and repulsive polaron branches are formed due to interactions between a photoexcited exciton and a Fermi sea of excess charge carriers. These interactions have previously been treated in a phenomenological manner. Here, we present a microscopic derivation of the electron-exciton interactions which utilizes a mixture of variational and perturbative approaches. We find that the interactions feature classical charge-dipole behavior in the long-range limit, and that they are only weakly modified for moderate doping. We apply our theory to the absorption properties and show that the dependence on doping is well captured by a model with a phenomenological contact potential.

The orbital Hall effect (OHE) is the transverse flow of orbital moment in a solid in response to an applied electric field, analogous to the flow of spin moment in the spin Hall effect (SHE). Although the effect has not been directly observed, there is ample indirect evidence for its existence in a number of experiments. Here, we show that the OHE is enhanced in solids with broken inversion symmetry, which may be more suitable to observe the effect. The mechanism of the OHE is fundamentally different in solids with inversion symmetry, where the orbital moment is quenched in the Brillouin zone (BZ), from that in a solid with broken inversion symmetry, where an intrinsic orbital moment is already present, the motion of which under the applied electric field could lead to a robust OHE. Using a tight-binding model Hamiltonian of a simple cubic lattice with two atoms in the unit cell, we study the effect of the inversion symmetry breaking on the OHE. We show that with the increase in the strength of the broken symmetry, the magnitude of the intrinsic orbital moment in the Brillouin zone increases. This, in turn, enhances the orbital Hall conductivity, in particular, the part that is directly proportional to the orbital moment in the BZ, which we call the “noncentrosymmetric contribution.” If the spin-orbit coupling is present, which couples the orbital and spin moments, the OHE leads to the SHE, which also becomes enhanced by the broken inversion symmetry. Our work has important implications for experimenters, suggesting that noncentrosymmetric solids may be more suitable for direct observation of the OHE.

The microscopic origin of a magnetic phase diagram under pressure in the heavy-fermion antiferromagnet ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ was investigated using the ${}^{29}\mathrm{Si}$ nuclear magnetic resonance (NMR) technique. Owing to the characteristics of the hyperfine form factor at the Si site, which filters specific $\mathit{q}$-dependent fluctuations around antiferromagnetic $\mathit{q}$ vectors, we investigated the temperature and pressure dependences of $\mathit{q}$-independent local fluctuations caused by the single-site Kondo effect. A universal scaling behavior observed on the energy scale of the local fluctuations in the entire pressure region demonstrates that the characteristic energy scale $T_0$ of Kondo interactions monotonically increases by applying pressure without a critical anomaly around the quantum critical pressure $P_{\mathrm{c}}$ of antiferromagnetism. Our NMR result agrees with the Doniach picture of the heavy-fermion phase diagram, where the progressive delocalization of the $f$ electrons occurs across $P_{\mathrm{c}}$, accompanied by the development of antiferromagnetic correlations among the $f$ electrons.

Simulating reconstructive phase transition requires an accurate description of potential energy surface (PES). Density-functional-theory (DFT) based molecular dynamics can achieve the desired accuracy but it is computationally unfeasible for large systems and/or long simulation times. Here we introduce an approach that combines the metadynamics simulation and machine learning representation of PES at the accuracy close to the DFT calculations, but with the computational cost several orders of magnitude less, and scaling with system size approximately linear. The high accuracy of the method is demonstrated in the simulation of pressure-induced $B4\text{−}B1$ phase transition in gallium nitride (GaN). The large-scale simulation using a 4096-atom simulation box reveals the phase transition with excellent detail, revealing different simulated transition paths under particular stress conditions. With well-trained machine learning potentials, this method can be easily applied to all types of systems for accurate scalable simulations of solid-solid reconstructive phase transition.

Many transition-metal dichalcogenides show competing metallic, superconducting, charge density waves (CDWs), or Mott-insulating phases. Understanding the interplay between these phases is of fundamental interest. One approach to understand this relationship is to suppress one or more of the competing phases through systematic introduction of disorder. Intercalation is one of several approaches used to introduce this disorder. However, the intercalation process itself and the resulting changes in the atomic and electronic structure of the host-intercalant systems are not completely understood. Here, we characterize the structural and spectral signatures accompanying the intercalation of triethylenediamine [TED, (${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_2$)] into the CDW material $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$. Electron diffraction and electron microscopy imaging reveal that the intercalation of TED into $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ leads to a change of the layer stacking in the intercalated $\mathrm{Ta}{\mathrm{S}}_2\text{−}\mathrm{TED}$ structure. Increased peak intensity is also observed between 3–5 eV in the valence electron-energy-loss spectra of the intercalated sample. Using electronic structure and theoretical spectra calculations we argue that these spectral features are a result of the band-structure changes due to structural phase transformation accompanying intercalation.

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction. Two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni $L_3$ x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test. This rigorous statistical approach provides an unambiguous proof of the existence of interlayer coupling mediated by spin pumping.

We demonstrate that the one-dimensional helical Majorana edges of two-dimensional time-reversal symmetric topological superconductors (class DIII) can become gapless and insulating by a combination of random edge velocity and interaction. Such a gapless insulating edge breaks time-reversal symmetry inhomogeneously, and the local symmetry broken regions can be regarded as static mass potentials or dynamical Ising spins. In both limits, we find that such gapless insulating Majorana edges are generically exponentially localized and trap Majorana zero modes. Interestingly, for a statistically time-reversal symmetric edge (symmetry is broken locally, but the symmetry breaking order parameter is zero on average), the low-energy theory can be mapped to a Dyson model at zero energy, manifesting a diverging density of states and exhibiting marginal localization (i.e., a diverging localization length). Although the ballistic edge state transport is absent, the localized Majorana zero modes reflect the nontrivial topology in the bulk. Experimental signatures are also discussed.

The recently found crystalline silicon-amorphous boron ($c$-Si/$a$-B) heterojunction has been successfully applied in the detection of short-wave UV photons. These detectors play a decisive role in the progress of nanoelectronics fabrication. The $c$-Si/$a$-B heterojunction could not be explained using the existing ‘instrumentarium’ in semiconductor physics. We investigated the $c$-Si/$a$-B interfaces using ab initio molecular dynamics simulations. The simulations reveal atomic ordering of the $a$-B atoms adjacent to both the Si{0 0 1} and Si{1 1 1} substrates. Charge transfer occurs from the interfacial Si to B, thereby forming ${\mathrm{Si}}^{+}/{\mathrm{B}}^{-}$ charge barriers, which induce an electric field in the nearby regions. The obtained information here is helpful in furthering our understanding of the physics behind the $c$-Si/$a$-B junctions, as well as driving the development of a new ‘instrumentatrium’ in solid state physics.

We have calculated the exciton fine structure splittings (FSS) of asymmetric GaAs/AlGaAs quantum dots (QDs) obtained after Al droplet epitaxy and subsequent nanoholes formation followed by annealing and GaAs filling of nanoholes. We used a $\mathbf{k}·\mathbf{p}$ model and considered the heavy-hole and light-hole mixing to calculate the electron-hole exchange interaction (EI). The two components, long-range (LR) and short-range (SR) of the EI, were deduced. The exciton fine structure is organized, as usual in zinc-blende compounds, into two groups of states: bright (optically active) and dark states. The bright-dark and bright-bright splittings contain LR and SR contributions, the LR part representing 5 to 68% of the total bright-dark splitting and 69 to 76% of the total bright-bright splitting for sizes experimentally explored. In QDs having ${\mathrm{C}}_{2v}$ symmetry, LR and SR contributions to dark-dark splitting have to be calculated at the second order of perturbation theory. A good agreement between the theory and experiment is obtained for QDs with different degrees of asymmetry, from QD having an isotropic shape to QD with a very anisotropic shape.

We demonstrate a concept for a large enhancement of the transverse magneto-optical Kerr effect (TMOKE) in the transmission mode. The concept exploits diffracted waves from dielectric-MO grating ribs to excite the guided modes in a high-refractive-index planar waveguide, thus opening different routes for efficient, integrated, and low-loss magneto-optical devices. The high $Q$ factors from very narrow peaks may permit applications in high-resolution sensing/biosensing.

We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlens-enhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to $0.95\pm 0.01$ and postselected photon indistinguishabilities of up to $0.93\pm 0.01$, the suitability for quantum repeater based long range quantum entanglement distribution schemes is shown. Comprehensive investigations of a complete set of polarization selective two-photon correlations facilitate an innovative method to determine the extraction and excitation efficiencies directly, as opposed to commonly employed indirect techniques. Additionally, time-resolved analysis of Hong-Ou-Mandel interference traces reveal an alternative approach to the investigation of pure photon dephasing.

Achieving a population imbalance between the two inequivalent valleys is a critical first step for any valleytronic device. A valley polarization can be induced in biased bilayer graphene using circularly polarized light. In this paper, we present a detailed theoretical study of valley polarization in biased bilayer graphene. We show that a nearly perfect valley polarization can be achieved with the proper choices of external bias and pulse frequency. We find that the optimal pulse frequency $\omega$ is given by $\hslash \omega =2a,$ where $2a$ is the potential energy difference between the graphene layers. We also find that the valley polarization originates not from the Dirac points themselves, but rather from a ring of states surrounding each. Intervalley scattering is found to greatly reduce the valley polarization for high-frequency pulses. Thermal populations are found to significantly reduce the valley polarization for small biases. This work provides insight into the origin of valley polarization in bilayer graphene and will aid experimentalists seeking to study valley polarization in the laboratory.

Quantum physics in flat-band (FB) systems embodies a variety of exotic phenomenon and even counterintuitive features. The quantum transport in several graphene based compounds that exhibit a flat band and a tunable gap is investigated. Despite the localized nature of the FB states and a zero group velocity, a supermetallic (SM) phase at the FB energy is revealed. The SM phase is robust against the inelastic scattering strength and controlled only by the interband transitions between the FB and the dispersive bands. The SM phase appears insensitive and quasi-independent of the gap amplitude and nature of the lattice (disordered or nanopatterned). The universal nature of the unconventional FB transport is illustrated with the case of electrons in the Lieb lattice.