We show that the theoretical expressions presented in Khusnutdinoff et al. [Phys. Rev. B 101, 214312 (2020)] for the longitudinal current spectral function $C^L(k,\omega )$ and the dispersion of collective excitations are not correct. Indeed, they are not compatible with the continuum limit and $C^L(k,\omega \to 0)$ contradicts the continuity equation.

I determine the intrinsic Hamiltonian of the relative motion of the constituent fermions of bosonic composites in superfluid fermion systems, assuming the effective fermion-fermion potential to be a sum of separable terms. The derivation is based on an expansion that treats composites, quasiparticles, and their interactions on the same footing. The intrinsic Hamiltonian of the composites is expressed in terms of the solution of the gap equation and of the form factors of the potential. It has the Brillouin-Wigner form, namely it is Hermitian and energy-dependent. In such a context, a solid justification is given for discarding negative energies of the composites. As a check, I rederive the dispersion law of the Bogoliubov-Anderson mode and the spectrum of the Bardasis and Schrieffer Hamiltonian. The possible occurrence of gapped modes depending on the form of the fermion-fermion interaction is discussed. The intrinsic wave functions are derived explicitly in the long wave limit, while their full determination requires numerical calculations.

We study the effects of electron-electron interaction in a three-dimensional crystalline topological insulator by adding on-site repulsion to the single-particle Hamiltonian and solving the many-body problem within cluster perturbation theory. The goal is to clarify how many-body effects modify the topological phase that stems from the crystal symmetries. Tuning the strength of the on-site interaction, we show that band inversion disappears in the bulk and surface states lose their metallic character.

We study the quantum entanglement structure of integer quantum Hall states via the reduced density matrix of spatial subregions. In particular, we examine the eigenstates, spectrum, and entanglement entropy (EE) of the density matrix for various ground and excited states, with or without mass anisotropy. We focus on an important class of regions that contain sharp corners or cusps, leading to a geometric angle-dependent contribution to the EE. We unravel surprising relations by comparing this corner term at different fillings. We further find that the corner term, when properly normalized, has nearly the same angle dependence as numerous conformal field theories (CFTs) in two spatial dimensions, which hints at a broader structure. In fact, the Hall corner term is found to obey bounds that were previously obtained for CFTs. In addition, the low-lying entanglement spectrum and the corresponding eigenfunctions reveal “excitations” localized near corners. Finally, we present an outlook for fractional quantum Hall states.

There is immense interest in how the local environment influences the electronic structure of materials at the single-layer limit. We characterize moiré induced spatial variations in the electronic structure of in situ grown monolayer ${\mathrm{V}}_2{\mathrm{S}}_3$ on Au(111) by means of low-temperature scanning tunneling microscopy and spectroscopy. We observe a long-range modulation of the integrated local density of states (LDOS), and quantify this modulation with respect to the moiré superstructure for multiple orientations of the monolayer with respect to the substrate. Scanning tunneling spectroscopy reveals a prominent peak in the LDOS, which is shifted in energy at different points of the moiré superstructure. Comparing ab initio calculations with angle-resolved photoemission, we are able to attribute this peak to bands that exhibit a large out of plane $d$-orbital character. This suggests that the moiré driven variations in the measured density of states are driven by a periodic modulation of the monolayer-substrate hybridization.

We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin that is motivated and relevant also for the description of real two-dimensional materials such as the two-dimensional trimer magnet ${\mathrm{Ba}}_4{\mathrm{Ir}}_3{\mathrm{O}}_{10}$. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. Overall, we find in the regime of strong rung coupling a gapped dimerized phase related to competing symmetry sectors in Hilbert space that ultimately results in frustration-driven spin-Peierls transition. In the weak rung-coupling regime, the system is uniform yet shows a gapped spinon continuum together with a sharp coherent low-energy branch that renders the system critical overall. In either case, the additional rung spin quickly gets sidelined and nearly decouples once its bare coupling to the ladder drops somewhat below the direct Heisenberg coupling of the legs.

The layered perovskite ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5+x}$ demonstrates a strong negative thermal expansion (NTE) which holds potential for being fabricated into composites with zero thermal expansion. The NTE was found to be intimately associated with the spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we report with compelling evidence that the continuouslike MVE in ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5+x}$ is intrinsically of discontinuous character, originating from a magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic less-insulating small-volume (FLISV) phase. Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple external stimuli such as temperature, carrier doping, hydrostatic pressure, magnetic field, etc. In contrast to the well-known ME such as colossal magnetoresistance and multiferroic effect which involve symmetry breaking of crystal structure, the ME in the cobaltite is purely isostructural. Our discovery provides a pathway to realizing the ME as well as the NTE, which may find applications in new techniques.

Because of the one-dimensional confinement of electron momentum in narrow semiconductor wires, spin relaxation is suppressed irrespective of the presence of spin-orbit (SO) interaction. In quantum transport, weak localization corrections to conductivity are reflected as suppressed spin relaxation, which makes quantification of the SO strength difficult because quantum correction theory requires weak antilocalization when evaluating SO coefficients. Narrow wires with strong SO interaction are potential platform for Majorana particles and parafermions for topological electronics and quantum computation, so revealing the SO strength in semiconductor wire structures is beneficial. Herein, we present quantification of the full SO coefficient under weak localization in InGaAs-based narrow wires. Using anisotropic weak localization observed under an in-plane external magnetic field with various orientations, one can ascertain the relative ratio between Rashba ($\alpha$) and linear Dresselhaus (${\beta }_1$) SO coefficients with no fitting. Furthermore, we find that widely tuning the potential profile of the quantum well through the top gate can expose a Rashba-predominant region in magnetoconductance, where the $\alpha$ value can be extracted reliably from two-dimensional quantum correction theory. Finally, we quantify the full SO coefficients including Rashba, linear Dresselhaus, and cubic Dresselhaus terms in the wire.

Applying a magnetic field in the hexagonal plane of $\mathrm{Y}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ leads to a complex magnetic phase diagram of commensurate and incommensurate phases, one of which coexists with the topological Hall effect (THE) generated by a unique fluctuation-driven mechanism. Using unpolarized neutron diffraction, we report on the solved magnetic structure for two previously identified, but unknown, commensurate phases. These include a low-temperature, high-field fanlike phase and a room-temperature, low-field canted antiferromagnetic phase. An intermediate incommensurate phase between the fanlike and forced ferromagnetic phases is also identified as the last known phase of the in-plane field-temperature diagram. Additional characterization using synchrotron powder diffraction reveals extremely high-quality, single-phase crystals, which suggests that the presence of two incommensurate magnetic structures throughout much of the phase diagram is an intrinsic property of the system. Interestingly, polarized neutron diffraction shows that the centrosymmetric system hosts preferential chirality in the zero-field double-flat-spiral phase, which, along with the THE, is a topologically nontrivial characteristic.

We carry out a theoretical study of the nonlinear behavior of cavity magnon polaritons (CMPs) under strong microwave driving, with two kinds of nonlinearity being involved: either (i) the photon Kerr effect (PKE) and the magnon saturation effect (MSE) or (ii) the magnon Kerr effect (MKE) and the MSE. The effects arising in these two cases are similar for the lower polariton mode of CMPs but opposite for the upper polariton mode. In case (i), the dominant nonlinearity for the upper polariton mode of CMPs changes from the MSE to the PKE as the driving intensity increases, which can lead to two hysteresis loops in opposite directions and hence to tristability. In case (ii), however, the upper polariton mode exhibits only the usual bistability rather than tristability since the MKE is dominated by the MSE. In particular, there is no bistable behavior in the upper polariton mode when the two nonlinearities balance each other. The results of this study provide a possible basis for the practical manipulation and control of information via the double nonlinearities in a cavity-magnon system.

We present the ultralow-temperature specific-heat and thermal-conductivity measurements on single crystals of the triangular rare-earth lattice material $\mathrm{KBaYb}{\left({\mathrm{BO}}_3\right)}_2$. Our experiments show the absence of long-range magnetic order down to 50 mK, giving the possibility of a quantum spin liquid ground state in this frustrated compound. Furthermore, no signature of spin freezing is observed down to 50 mK in the ac susceptibility measurements. However, no notable contributions from gapless magnetic excitations, but only a Schottky anomaly is observed in the specific-heat data. The ultralow-temperature thermal-conductivity data show that the residual linear term at zero field is negligible, and the thermal conductivity is insensitive to the magnetic field up to 9 T. The ground state of this frustrated triangular rare-earth lattice material is discussed.

We introduce the concept of Floquet odd-frequency superconducting pairs and establish their emergence in time-periodic conventional superconductors. We show that these exotic Cooper pairs are possible because the Floquet modes in time-periodic systems provide an additional index (a Floquet index) that broadens the classification of superconducting pair symmetries, with no analog in the static regime. Our results thus put forward a different route for odd-frequency superconducting pairs, and pave the way for Floquet engineered dynamical superconducting states.

Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream $\mathrm{p}$-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in $\mathrm{n}$-type FMSs is theoretically nontrivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the mechanism of carrier-induced ferromagnetism in FMSs. Here our SX-ARPES study on the prototypical $\mathrm{n}$-type FMS (In,Fe)As reveals the entire band structure, including the Fe-$3d$ impurity bands (IBs) and the host InAs ones, and provides direct evidence for electron occupation of the InAs-derived conduction band (CB). A minority-spin Fe-$3d$ IB is found to be located just below the conduction-band minimum (CBM). The IB is formed by the hybridization of the unoccupied Fe $3d$ states with the occupied CBM of InAs in a spin-dependent way, resulting in the large spin polarization of CB. The band structure with the IB is varied with band filling, which cannot be explained by the rigid-band picture, suggesting a unified picture for realization of carrier-induced ferromagnetism in FMS materials.

Nodal lines, as one-dimensional band degeneracies in momentum space, usually feature a linear energy splitting. Here we propose the concept of magnetic higher-order nodal lines, which are nodal lines with higher-order energy splitting and realized in magnetic systems with broken time-reversal symmetry. We provide sufficient symmetry conditions for stabilizing magnetic quadratic and cubic nodal lines, based on which concrete lattice models are constructed to demonstrate their existence. Unlike its counterpart in nonmagnetic systems, the magnetic quadratic nodal line can exist as the only band degeneracy at the Fermi level. We show that these nodal lines can be accompanied by torus surface states, which form a surface band that span over the whole surface Brillouin zone. Under symmetry breaking, these magnetic nodal lines can be transformed into a variety of interesting topological states, such as three-dimensional quantum anomalous Hall insulator, multiple linear nodal lines, and magnetic triple-Weyl semimetal. The three-dimensional quantum anomalous Hall insulator features a Hall conductivity ${\sigma }_{xy}$ quantized in units of $e^2/\left(hd\right)$, where $d$ is the lattice constant normal to the $x\text{−}y$ plane. Our work reveals previously unknown topological states and offers guidance to search for them in realistic material systems.

For an isolated generic quantum system out of equilibrium, the long time average of observables is given by the diagonal ensemble, i.e., the mixed state with the same probability for energy eigenstates as the initial state but without coherences between different energies. In this work we present a method to approximate the diagonal ensemble using tensor networks. Instead of simulating the real time evolution, we adapt a filtering scheme introduced earlier [M. C. Bañuls, D. A. Huse, and J. I. Cirac, Phys. Rev. B 101, 144305 (2020)] to this problem. We analyze the performance of the method on a nonintegrable spin chain, for which we observe that local observables converge towards thermal values polynomially with the inverse width of the filter.

Hole spins in semiconductors are a potential qubit alternative to electron spins. In nuclear-spin-rich host crystals like GaAs, the hyperfine interaction of hole spins with nuclei is considerably weaker than that for electrons, leading to potentially longer dephasing times. Here we demonstrate optical pumping and coherent population trapping for acceptor-bound holes in a strained GaAs epitaxial layer. We find $\mu \text{s}$-scale longitudinal spin relaxation time $T_1$ and an inhomogeneous dephasing time $T_2^{*}$ of $\sim 7$ ns. We attribute the spin relaxation mechanism to the combined effect of a hole-phonon interaction through the deformation potentials, and heavy-hole–light-hole mixing in an in-plane magnetic field. We attribute the short $T_2^{*}$ to $g$-factor broadening due to strain inhomogeneity. $T_1$ and $T_2^{*}$ are calculated based on these mechanisms and compared with the experimental results. While the hyperfine-mediated decoherence is mitigated, our results highlight the important contribution of strain to relaxation and dephasing of acceptor-bound hole spins.

We show that for a quantum wire with a local asymmetric scattering potential the principal channels for charge and heat rectification decouple and renormalize differently under electron interactions, with heat rectification generally being more relevant. The polarization of the rectification results from quantum interference and is tunable through external gating. Furthermore, for spin-polarized or helical electrons and sufficiently strong interactions a regime can be obtained in which heat transport is strongly rectified but charge rectification is very weak.