Topological magnonics has attracted intense interest for application in energy efficient computational devices. Here, we show reconfigurable magnonic band structure and band gap by a bias-field controlled spin texture in chains of connected ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ submicron dots. Particularly for an identical field value, we achieve both “$S$” and shifted-core vortex states based on magnetic history leading to a drastic change in magnonic band structure. A first-order phase transition from the saturation to vortex state drives this change, as opposed to a continuous change from the saturation to $S$ state.

Here we present a neutron scattering-based study of magnetic excitations and magnetic order in ${\mathrm{NaYbO}}_2$ under the application of an external magnetic field. The crystalline electric field-split $J=7/2$ multiplet structure is determined, revealing a mixed $|m_z\rangle$ ground-state doublet, and is consistent with a recent report by Ding et al. [L. Ding, P. Manuel, S. Bachus, F. Grußler, P. Gegenwart, J. Singleton, R. D. Johnson, H. C. Walker, D. T. Adroja, A. D. Hillier, and A. A. Tsirlin, Phys. Rev. B 100, 144432 (2019)]. Our measurements further suggest signatures of exchange effects in the crystal-field spectrum, manifested by a small splitting in energy of the transition into the first excited doublet. The field dependence of the low-energy magnetic excitations across the transition from the quantum disordered ground state into the fluctuation-driven ordered regime is analyzed. Signs of a first-order phase transition into a noncollinear ordered state are revealed at the upper-field phase boundary of the ordered regime, and higher-order magnon scattering, suggestive of strong magnon-magnon interactions, is resolved within the previously reported up-up-down phase. Our results reveal a complex phase diagram of field-induced order and spin excitations within ${\mathrm{NaYbO}}_2$ and demonstrate the dominant role of quantum fluctuations across a broad range of fields within its interlayer frustrated triangular lattice.

Small amplitude dipolar oscillations are considered in artificial spin ice on a square lattice in two dimensions. The net magnetic moment of each elongated magnetic island in the spin ice is assumed to have Heisenberg-like dynamics and be influenced by shape anisotropies and nearest neighbor dipole interactions. The magnetic dynamics linearized around a ground state with four sublattices leads to an eigenvalue problem with four branches of magnetic spin wave modes. An energy analysis of the dipole motions enables an analytic solution while showing that the lowest frequency modes are antisymmetric with regard to their in-plane dynamic fluctuations in each four-island vertex. Although only the leading dipolar interactions are included, modes similar to these may be observable experimentally in artificial square spin ice.

Rare-earth hydrides can exhibit high-temperature superconductivity under high pressure. Here, we apply a crystal structure prediction method to the current record-holding $T_c$ material, ${\mathrm{LaH}}_{10}$. We find a pressure-induced phase transition from the experimentally observed cubic phase to a hexagonal phase at around 420 GPa. This phase is metastable down to low pressures and could explain experimental observations of hcp impurities in fcc samples. We go on to find that ${\mathrm{YH}}_{10}$ adopts similar structures and discuss the sensitivity of superconductivity calculations to certain computational parameters.

We develop a 16-band $\mathbf{k}·\mathbf{p}$ model for the description of wurtzite GaAs, together with a scheme to determine electronic structure parameters for multiband $\mathbf{k}·\mathbf{p}$ models. Our approach uses low-discrepancy sequences to fit $\mathbf{k}·\mathbf{p}$ band structures beyond the eight-band scheme to most recent ab initio data, obtained within the framework for hybrid-functional density functional theory with a screened-exchange hybrid functional. We report structural parameters, elastic constants, band structures along high-symmetry lines, and deformation potentials at the $\mathrm{\Gamma }$ point. Based on this, we compute the bulk electronic properties ($\mathrm{\Gamma }$ point energies, effective masses, Luttinger-like parameters, and optical matrix parameters) for a ten-band and a sixteen-band $\mathbf{k}·\mathbf{p}$ model for wurtzite GaAs. Our fitting scheme can assign priorities to both selected bands and $\mathbf{k}$ points that are of particular interest for specific applications. Finally, ellipticity conditions can be taken into account within our fitting scheme in order to make the resulting parameter sets robust against spurious solutions.

We reveal an ultrafast purely electronic phase transition in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photoexcited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective masses in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.

The recent discovery of materials featuring strong Rashba spin-orbit coupling (RSOC) and strong electronic correlation raises questions about the interplay of Mott and Rashba physics. In this work, we employ cluster perturbation theory to investigate the spectral properties of the two-dimensional Hubbard model in the presence of a significant or large RSOC. We show that RSOC strongly favors metallic phases and competes with Mott localization, leading to an unconventional scenario for the Mott transition, which is no longer controlled by the ratio between the Hubbard $U$ and an effective bandwidth. The results show a strong sensitivity to the value of the RSOC.

Non-Hermitian quantum many-body systems are a fascinating subject to be explored. Using the generalized density matrix renormalization group method and complementary exact diagonalization, we elucidate the many-body ground states and dynamics of a 1D interacting non-Hermitian Aubry-André-Harper model for bosons. We find stable ground states in the superfluid and Mott-insulating regimes under wide range of conditions in this model. We reveal a skin superfluid state induced by the non-Hermiticity from the nonreciprocal hopping. We investigate the topology of the Mott-insulating phase and find its independence of the non-Hermiticity. The topological Mott insulators in this non-Hermitian system are characterized by four equal Chern numbers and a quantized shift of biorthogonal many-body polarizations. Furthermore, we show generic asymmetric expansion and correlation dynamics in the system.

The dependence of quasiparticle Fermi energy on electron density is investigated by analyzing radiative recombination spectra of two-dimensional electrons with photoexcited holes bound to remote acceptors. This method enables us to measure the dependence of renormalized quasiparticle mass on the concentration of two-dimensional electrons. It is established that with decreasing electron density (increasing parameter $r_s$ up to 4.5) the density-of-states effective mass of quasiparticles increases by 35% compared to the cyclotron electron mass. It is shown that in a perpendicular magnetic field the concept of quasiparticles in a two-dimensional Fermi liquid is applicable not only near the Fermi level but also deep below the Fermi surface, down to the bottom of the size-quantization band, since the broadening of excitations appears to be much less than their energy. The effective mass and broadening of quasiparticles were found to be significantly dependent on their energy measured from the Fermi surface down to the very bottom of the size-quantization band.

The method of many-body Green's functions is developed for arbitrary systems of electrons and nuclei starting from the full (beyond Born-Oppenheimer) Hamiltonian of Coulomb interactions and kinetic energies. The theory presented here resolves the problems arising from the translational and rotational invariance of this Hamiltonian that afflict the existing many-body Green's function theories. We derive a coupled set of exact equations for the electronic and nuclear Green's functions and provide a systematic way to approximately compute the properties of arbitrary many-body systems of electrons and nuclei beyond the Born-Oppenheimer approximation. The case of crystalline solids is discussed in detail.

We present the $G_0{W}_0$ band structure, core levels, and deformation potential of monolayer FeSe in the paramagnetic phase based on a starting mean field of the Kohn-Sham density functional theory (DFT) with the Perdew, Burke, and Ernzerhof functional. We find the GW correction increases the bandwidth of the states forming the $M$ pocket near the Fermi energy, while leaving the $\mathrm{\Gamma }$ pocket roughly unchanged. We then compare the $G_0{W}_0$ quasiparticle band energies with the band structure from a simple empirical $+\mathrm{A}$ approach, which was recently proposed to capture the renormalization of the electron-phonon interaction going beyond DFT in FeSe, when used as a starting point in density functional perturbation theory. We show that this empirical correction succeeds in approximating the GW nonlocal and dynamical self-energy in monolayer FeSe and reproduces the GW band structure near the Fermi surface, the core energy levels, and the deformation potential (electron-phonon coupling).

We introduce a characterization of topological order based on bulk oscillations of the entanglement entropy and the definition of an “entanglement gap,” showing that it is generally applicable to pure and disordered quantum systems. Using exact diagonalization and the strong disorder renormalization group method, we demonstrate that this approach gives results in agreement with the use of traditional topological invariants, especially in cases where topological order is known to persist in the presence of off-diagonal bond disorder. The entanglement gap is then used to analyze classes of quantum systems with alternating bond types, allowing us to construct their topological phase diagrams. The validity of these phase diagrams is verified in limiting cases of dominant bond types, where the solution is known.

The temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe, is calculated using first-principles methods. We explicitly include the effect of thermal-expansion-induced modification of electronic states and their band inversion on electron-phonon interaction. We show that the direct gap decreases with temperature, as both thermal expansion and electron-phonon interaction drive SnTe towards the phase transition to a topologically trivial phase as temperature increases. The band gap renormalization due to electron-phonon interaction exhibits a nonlinear dependence on temperature as the material approaches the phase transition, while the lifetimes of the conduction band states near the band edge show a nonmonotonic behavior with temperature. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.

So far, the observation of the yellow $P$ exciton series in cuprous oxide has been limited to principal quantum numbers up to $n_{\text{max}}=25$ at a crystal temperature of $T=1.35\mathrm{K}$. Here, we address the origin of this limitation and whether an extension to higher quantum numbers is possible. To that end, absorption experiments are performed to study the variation of $n_{\text{max}}$ with the spot position on a particular sample and from sample to sample. In addition the temperature is varied. By reducing $T$ to below 1 K, we can extend $n_{\text{max}}$ to 28, not limited by the thermal energy exceeding the exciton binding energy. The data rather suggest that the ultimate limit is provided by residual charged impurities, despite their low density in the order of less than ${10}^9{\mathrm{cm}}^{-2}$, which exert an electric field on the highly excited excitons causing their ionization.

We develop a classical kinetic theory of magnetotransport of 2D electrons in narrow channels with partly diffusive boundary scattering and apply it to the description of magnetoresistance measured in the temperature interval 4.2–30 K in long mesoscopic bars fabricated from high-purity GaAs quantum well structures. Both experiment and theory demonstrate a number of characteristic features in the longitudinal and Hall resistances caused by the size effect in two dimensions owing to the high ballisticity of the transport. In addition to the features described previously, we also reveal a change in the slope of the first derivative of magnetoresistance when the cyclotron orbit diameter equals to half of the channel width. These features are suppressed with increasing temperature as a result of the electronic viscosity due to electron-electron interaction. By comparing theory and experiment, we determine the characteristic time of relaxation of angular distribution of electrons caused by electron-electron scattering.

Quantum dots are gate-defined within InSb nanowires, in proximity to NbTiN superconducting contacts. As the coupling between the dot and the superconductor is increased, the odd-parity occupation regions in transport become nondiscernible (erased) both above and below the induced superconducting gap. Above the gap, conductance in the odd Coulomb-blockade valleys increases until the valleys are lifted. Below the gap, Andreev bound states undergo quantum phase transitions to Kondo-screened singlet ground states at odd occupancy. We investigate to what degree the apparent erasure of odd-parity regimes coincides at low and high biases. We complement experiments with numerical renormalization group simulations. We interpret the results in terms of a competition between Kondo screening and superconductivity. In the erased odd-parity regime, the quantum dot exhibits transport features similar to a finite-size Majorana nanowire, drawing parallels between even-odd dot occupations and even-odd one-dimensional subband occupations.

Although van der Waals (vdW) layered $\mathrm{Mo}{\mathrm{S}}_2$ shows the phase transformation from the semiconducting $2H$ phase to the metallic $1T$ phase through chemical lithium intercalation, vdW $\mathrm{MoT}{\mathrm{e}}_2$ is thermodynamically reversible between the $2H$- and $1T^{\prime }$ phases, and can be further transformed by energetics, laser irradiation, strain or pressure, and electrical doping. Here, thickness- and temperature-dependent optical properties of $1T^{\prime }\text{−}\mathrm{MoT}{\mathrm{e}}_2$ thin films grown by chemical vapor depsition are investigated via Fourier-transformed infrared spectroscopy. An optical gap of $28\pm 2$ meV in a three-layer (or 2-nm)-thick $1T^{\prime }\text{−}\mathrm{MoT}{\mathrm{e}}_2$ is clearly observed at a low-temperature region below 50 K. No discernible optical band gap is observed in samples thicker than ∼4 nm. The observed thickness-dependent band-gap results agree with the measured dc resistivity data; the thickness-dependent $1T^{\prime }\text{−}\mathrm{MoT}{\mathrm{e}}_2$ clearly demonstrates the metal-semiconductor transition at a crossover below the 2-nm-thick sample.

Using the complex scaling and the stabilization method combined with the stochastic variational approach, we have shown that there are narrow resonance states in two-dimensional three particle systems of electrons and holes interacting via a screened Coulomb interaction. These resonances are loosely bound systems of excited state excitons with a third particle circling around them. Recent experimental studies of excited state trions might be explained and identified by these resonant states.

Experimental and theoretical investigations of excitons in cuprous oxide have revealed a significant fine-structure splitting of the excitonic Rydberg states caused by a strong impact of the valence band structure. We provide a semiclassical interpretation of that splitting by investigating the classical dynamics of the excitonic electron-hole pair beyond the hydrogenlike model. Considering the slow motion of Rydberg excitons in coordinate space compared to the fast dynamics of quasispin and hole spin we use an adiabatic approach and energy surfaces in momentum space for the computation of the exciton dynamics. We observe quasiperiodic motion on near-integrable tori. Semiclassical torus quantization yields the energy regions of the fine-structure splitting of $n$ manifolds in agreement with quantum-mechanical computations.