Quantum Hall (QH) edge channels appear not only along the edge of the electron gas but also along an interface between two QH regions with different filling factors. However, the fundamental transport characteristics of such interface channels are not well understood, particularly in the high-frequency regime. In this study, we investigate the interface plasmon mode along the edge of a metal gate electrode with ungated and gated QH regions in both integer and fractional QH regimes using a time-resolved measurement scheme. The observed plasmon wave form was delayed and broadened due to the influence of the charge puddles formed around the channel. The charge velocity and diffusion constant of the plasmon mode were evaluated by analyzing the wave form using a distributed circuit model. We found that the conductive puddles in the gated region induce significant dissipation in plasmon transport. For instance, a fractional interface channel with a reasonably fast velocity was obtained by preparing a fractional state in the ungated region and an integer state in the gated region, whereas a channel in the swapped configuration was quite dissipative. This reveals a high-quality interface channel that provides a clean path to transport fractional charges for studying various fractional QH phenomena.

We study untwisted heterobilayers of ferromagnetic and antiferromagnetic van der Waals materials, with in particular a Dzyaloshinskii-Moriya interaction in the ferromagnetic layer. A continuum low-energy field theory is utilized to study such systems. We develop a phase diagram as a function of the strength of interlayer exchange and Dzyaloshinskii-Moriya interactions, combining perturbative and strong coupling analyses with numerical simulations using Landau-Lifshitz-Gilbert equations. Various moiré-periodic commensurate phases are found, and the commensurate-incommensurate transition is discussed. Among the commensurate phases, we observe an interesting skyrmion lattice phase wherein each moiré unit-cell hosts one skyrmion.

A threefold symmetric kagome lattice that has negative spin chirality can give a nonzero x-ray magnetic circular dichroism (XMCD) signal, despite the total spin moment amounting to zero. In order to explain this, I present here a rule for the rotational symmetry invariance of the XMCD signal. A necessary condition is the existence of an anisotropic XMCD signal for the local magnetic atom, which can arise from a spin anisotropy either in the ground state or the final state. The angular dependence of the XMCD as a function of the beam direction has an unusual behavior. The maximum dichroism is not aligned along the spin direction, but depends on the relative orientation of the spin with respect to the atomic orientation. Therefore, different geometries can result in the same angular dependence, and the spin direction can only be determined if the atomic orientation is known. The consequences for the x-ray magneto-optical sum rules are given. The integrated XMCD signals are proportional to the anisotropy in the orbital moment and the magnetic dipole term, where the isotropic spin moment drops out.

In symmetry-protected topological (SPT) phases, the combination of symmetries and a bulk gap stabilizes protected modes at surfaces or at topological defects. Understanding the fate of these modes at a quantum critical point, when the protecting symmetries are on the verge of being broken, is an outstanding problem. This interplay of topology and criticality must incorporate both the bulk dynamics of critical points, often described by nontrivial conformal field theories, and SPT physics. Here, we study the simplest nontrivial setting, that of a $(0+1)$-dimensional topological mode, a quantum spin, coupled to a $(2+1)$-dimensional critical bulk. Using the large-$N$ technique we solve a series of models which, as a consequence of topology, demonstrate intermediate coupling fixed points. We compare our results to previous numerical simulations and find good agreement. We also point out intriguing connections to generalized Kondo problems and Sachdev-Ye-Kitaev (SYK) models. In particular, we show that a Luttinger theorem derived for the complex SYK models, that relates the charge density to particle-hole asymmetry, also holds in our setting. These results should help stimulate further analytical study of the interplay between SPT physics and quantum criticality.

${\mathrm{SmB}}_6$ is a strongly correlated material that has been attributed as a topological insulator and a Kondo insulator. Recent studies have found the topological surface states and low temperature insulating character to be profoundly robust against magnetic and nonmagnetic impurities. Here, we use angle resolved photoemission spectroscopy to chart the evolution of topologically linked electronic structure features versus magnetic doping and temperature in ${\mathrm{Sm}}_{1-x}{M}_x{\mathrm{B}}_6(M=\mathrm{Eu},\mathrm{Ce})$. Topological coherence phenomena are observed out to $\sim 30\%$ Eu and 50% Ce concentrations that represent extreme nominal hole and electron doping, respectively. Theoretical analysis reveals that a recent redesignation of the topologically inverted band symmetries provides a natural route to reconciling the persistence of topological surface state emergence even as the insulating gap is lost through decoherence.

We study the interplay between Mott physics, driven by Coulomb repulsion $U$, and Hund physics, driven by Hund's coupling $J$, for a minimal model for Hund metals, the orbital-symmetric three-band Hubbard-Hund model (3HHM) for a lattice filling of $1/3$. Hund-correlated metals are characterized by spin-orbital separation (SOS), a Hund's-rule-induced two-stage Kondo-type screening process, in which spin screening occurs at much lower energy scales than orbital screening. By contrast, in Mott-correlated metals, lying close to the phase boundary of a metal-insulator transition, the SOS window becomes negligibly small and the Hubbard bands are well separated. Using dynamical mean-field theory and the numerical renormalization group as real-frequency impurity solver, we identify numerous fingerprints distinguishing Hundness from Mottness in the temperature dependence of various physical quantities. These include ARPES-type spectra, the local self-energy, static local orbital and spin susceptibilities, resistivity, thermopower, and lattice and impurity entropies. Our detailed description of the behavior of these quantities within the context of a simple model Hamiltonian will be helpful for distinguishing Hundness from Mottness in experimental and theoretical studies of real materials.

The concepts of deconfinement and topological order are of great current interest for quantum information science and for our understanding of quantum materials. Here, we introduce a simple model of three antiferromagnetically coupled Kondo impurities, a Kondo triangle, which can be used to further extend the application of these concepts to electronic systems. We show that, by tuning the magnetic frustration, the Kondo triangle undergoes a quantum phase transition between two phases of unbroken symmetry, signaling a phase transition beyond the Landau paradigm. We demonstrate that the frustrated spin liquid phase is described by a three-channel Kondo (3CK) fixed point and thus displays an irrational ground state degeneracy. Using an Abrikosov pseudofermion representation, this quantum state is categorized by an emergent $U$(1) gauge field and its projective symmetry group. The gauge theory is deconfining in the sense that a miniature Wilson loop orders and topological defects (instantons in the gauge field) are expelled. This phase persists in the presence of moderate Kondo screening until proliferation of topological defects leads to a quantum phase transition to an unfrustrated Fermi liquid phase. Based on this evidence, we propose that the 3CK phase displays topological order in a similar sense as gapless spin liquids.

$\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is an $S=5/2$ antiferromagnet exhibiting two magnetic orders that shows regions of coexistence at some temperatures. Using a Green's function formalism, we model neutron scattering data of the spin wave excitations in this material, elucidating the microscopic spin Hamiltonian. In doing so, we suggest that the low-temperature A phase order $(\uparrow \uparrow \downarrow \downarrow )$ finds its origins in the freezing of antiphase boundaries created by thermal fluctuations in a parent B phase order $(\uparrow \downarrow \uparrow \downarrow )$. The low-temperature magnetic order observed in $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is thus the result of a competition between the exchange coupling along $c$, which favors the B phase, and the single-ion anisotropy, which stabilizes thermally generated antiphase boundaries, leading to static metastable A phase order at low temperatures.

Silicon-vacancy and tin-vacancy defects in diamond are of interest as alternative qubits to the nitrogen vacancy (NV) center due to their superior optical properties. While the availability of optical transitions in these defects is one of their assets, high-fidelity optical coherent control has not been demonstrated. Here, we design optical control schemes tailored to these defects. We evaluate the performance of arbitrary single-qubit rotations of the electron spin qubit both in the absence and presence of an external magnetic field, by taking into account both coherent and incoherent errors. We find that rotations in excess of $98.0\%$ ($T=4$ K) and $99.71\%$ ($T=6$ K) can be achieved for SiV and SnV, respectively, in the presence of realistic relaxation and leakage errors.

The magnetic proximity effect in Pt and W thin films grown on ${\mathrm{Dy}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (DyIG) is examined at temperatures above and below the magnetic compensation temperature of the ferrimagnetic insulator. Polarized neutron reflectometry indicates that the proximity effect is positive in Pt/DyIG both above and below the compensation temperature, and x-ray magnetic circular dichroism shows a weak positive W magnetization below compensation in W/DyIG. This demonstrates a qualitative difference compared to heavy metal/ferrimagnetic rare earth–transition metal alloys, where the proximity-induced magnetism of the heavy metal changes sign at the compensation temperature. Reflectometry, structural, and spin transport measurements show that depositing the heavy metal film on the ferrimagnetic insulator in situ without breaking vacuum avoids the formation of a low density interfacial layer between the Pt and DyIG.

It has been shown that many seemingly contradictory experimental findings concerning the superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ can be accounted for as resulting from the existence of an assumed tetracritical point at near-ambient pressure at which $d_{x^2-y^2}$ and $g_{xy(x^2-y^2)}$ superconducting states are degenerate. We perform both a Landau-Ginzburg and a microscopic mean-field analysis of the effect of spatially varying strain on such a state. In the presence of finite $xy$ shear strain, the superconducting state consists of two possible symmetry-related time-reversal symmetry (TRS) preserving states $d\pm g$. However, at domain walls between two such regions, TRS can be broken, resulting in a $d+ig$ state. More generally, we find that various natural patterns of spatially varying strain induce a rich variety of superconducting textures, including half-quantum fluxoids. These results may resolve some of the apparent inconsistencies between the theoretical proposal and various experimental observations, including the suggestive evidence of half-quantum vortices.

Twisted bilayer transition metal dichalcogenides have emerged as important model systems for the investigation of correlated electron physics because their interaction strength, carrier concentration, band structure, and inversion symmetry breaking are controllable by device fabrication, twist angle, and, most importantly, gate voltage, which can be varied in situ. The low-energy physics of some of these materials has been shown to be described by a “moiré Hubbard model” generalized from the usual Hubbard model by the addition of strong, tunable spin-orbit coupling and inversion symmetry breaking. In this work, we use a Hartree-Fock approximation to reach a comprehensive understanding of the moiré Hubbard model on the mean-field level. We determine the magnetic and metal-insulator phase diagrams, and assess the effects of spin-orbit coupling, inversion symmetry breaking, and the tunable van Hove singularity. We also consider the spin and orbital effects of applied magnetic fields. This work provides guidance for experiments and sets the stage for beyond-mean-field calculations.

Kagome superconductors with $T_{\mathrm{C}}$ up to 7 K have been discovered for over 40 y. Recently, unconventional chiral charge order has been reported in kagome superconductor $\mathrm{K}{\mathrm{V}}_3{\mathrm{Sb}}_5$, with an ordering temperature of one order of magnitude higher than the $T_{\mathrm{C}}$. However, the chirality of the charge order has not been reported in the cousin kagome superconductor $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$, and the electronic nature of the chirality remains elusive. In this paper, we report the observation of electronic chiral charge order in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ via scanning tunneling microscopy (STM). We observe a 2 × 2 charge modulation and a 1 × 4 superlattice in both topographic data and tunneling spectroscopy. 2 × 2 charge modulation is highly anticipated as a charge order by fundamental kagome lattice models at van Hove filling, and is shown to exhibit intrinsic chirality. We find that the 1 × 4 superlattices form various small domain walls, and can be a surface effect as supported by our first-principles calculations. Crucially, we find that the amplitude of the energy gap opened by the charge order exhibits real-space modulations, and features 2 × 2 wave vectors with chirality, highlighting the electronic nature of the chiral charge order. STM study at 0.4 K reveals a superconducting energy gap with a gap size $2\mathrm{\Delta }=0.85\mathrm{meV}$, which estimates a moderate superconductivity coupling strength with $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{C}}=3.9$. When further applying a $c$-axis magnetic field, vortex core bound states are observed within this gap, indicative of clean-limit superconductivity.

Spin-dependent transport phenomena due to relativistic spin-orbit coupling and broken space-inversion symmetry are often difficult to interpret microscopically, in particular when occurring at surfaces or interfaces. Here we present a theoretical and experimental study of spin-orbit torque and unidirectional magnetoresistance in a model room-temperature ferromagnet NiMnSb with inversion asymmetry in the bulk of this half-Heusler crystal. Aside from the angular dependence on magnetization, the competition of Rashba- and Dresselhaus-type spin-orbit couplings results in the dependence of these effects on the crystal direction of the applied electric field. The phenomenology that we observe highlights potential inapplicability of commonly considered approaches for interpreting experiments. We point out that, in general, there is no direct link between the current-induced nonequilibrium spin polarization inferred from the measured spin-orbit torque and the unidirectional magnetoresistance. We also emphasize that the unidirectional magnetoresistance has not only longitudinal but also transverse components in the electric field: current indices which complicate its separation from the thermoelectric contributions to the detected signals in common experimental techniques. We use the theoretical results to analyze our measurements of the on-resonance and off-resonance mixing signals in microbar devices fabricated from an epitaxial NiMnSb film along different crystal directions. Based on the analysis we extract an experimental estimate of the unidirectional magnetoresistance in NiMnSb.

Few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with optical 500–1000-nm supercontinuum and broadband XUV pulses (30–50 eV), simultaneously probes dynamics of photoexcited carriers in $\mathrm{W}{\mathrm{S}}_2$ at the W ${\mathrm{O}}_3$ edge (37–45 eV) and carrier-induced modifications of core-exciton absorption at the $\mathrm{W}{\mathrm{N}}_{6,7}$ edge (32–37 eV). Access to continuous core-to-conduction-band absorption features and discrete core-exciton transitions in the same XUV spectral region in a semiconductor provides a means to investigate the effect of carrier excitation on core-exciton dynamics. The core-level transient absorption spectra, measured with either pulse arriving first to explore both core-level and valence carrier dynamics, reveal that core-exciton transitions are strongly influenced by the photoexcited carriers. A $1.2\pm 0.3$ ps hole-phonon relaxation time and a $3.1\pm 0.4$ ps carrier recombination time are extracted from the XUV transient absorption spectra from the core-to-conduction-band transitions at the W ${\mathrm{O}}_3$ edge. Global fitting of the transient absorption signal at the W ${\mathrm{N}}_{6,7}$ edge yields $\sim 10$ fs coherence lifetimes of core-exciton states and reveals that the photoexcited carriers, which alter the electronic screening and band filling, are the dominant contributor to the spectral modifications of core excitons and direct field-induced changes play a minor role. This work provides a first look at the modulations of core-exciton states by photoexcited carriers and advances our understanding of carrier dynamics in metal dichalcogenides.

We study electric polarization and nonlinear optical effects in spin systems with broken inversion symmetry. We apply strong-coupling expansion to the underlying electronic Hamiltonians and systematically derive expressions for electric polarization in spin systems that are represented in terms of spin operators. The magnon representation of the obtained electric polarization operator allows us to compute linear and nonlinear optical responses by the standard diagrammatic method. We apply our formalism to the Heisenberg model with alternating coupling constants and the $J_1\text{−}{J}_2$ model with inversion symmetry breaking. We demonstrate that these inversion-broken spin systems support dc current flow upon magnon excitations which arises from the shift current mechanism.

We present a method for the exact construction of the fully particle-hole symmetric Pfaffian (PH-Pfaffian) ground state and its charged excitations on a sphere. We adopt the Moore-Read state, but with a nonholomorphic pairing component as in previous studies, and project it to the lowest Landau level. We study the energetics as well as other properties of these states and find that in a pure system interacting with the Coulomb forces the PH-Pfaffian cannot compete with either the Moore-Read state or its particle-hole conjugate, the anti-Pfaffian state, as an explanation for the $\frac{5}{2}$ effect.

The symmetry of the superconducting condensate in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ remains controversial after recent experiments overturned the dominant chiral $p$-wave paradigm. We perform a theoretical study of pairing by spin- and charge-fluctuations in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, including the effects of spin-orbit coupling, and both local and longer-range Coulomb repulsion. The latter has important consequences for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ due to the near-degeneracy of symmetry-distinct pairing states in this material. We find leading nodal $s^{\prime }, d_{xy}$, and helical ($p$) solutions, while both the $g$- and $d_{x^2-y^2}$-wave channels remain noncompetitive. This suggests nodal time-reversal symmetry broken $s^{\prime }+i{d}_{xy}$ or $s^{\prime }+ip$ phases, promoted by longer-range Coulomb repulsion, as the most favorable candidates for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We analyze the properties of these states and show that the $s^{\prime }+i{d}_{xy}$ solution agrees with the bulk of available experimental data.

Topology in quantum matter is typically associated with gapped phases. For example, in symmetry protected topological (SPT) phases, the bulk energy gap localizes edge modes near the boundary. In this work we identify a new mechanism that leads to topological phases which are not only gapless but where the absence of a gap is essential. These “intrinsically gapless SPT phases” have no gapped counterpart and are hence also distinct from recently discovered examples of gapless SPT phases. The essential ingredient of these phases is that on-site symmetries act in an anomalous fashion at low energies. Intrinsically gapless SPT phases are found to display several unique properties including (i) protected edge modes that are impossible to realize in a gapped system with the same symmetries, (ii) string order parameters that are likewise forbidden in gapped phases, and (iii) constraints on the phase diagram obtained upon perturbing the phase. We verify predictions of the general theory in a specific realization protected by ${\mathbb{Z}}_4$ symmetry, the one dimensional Ising-Hubbard chain, using both numerical simulations and effective field theory. We also discuss extensions to higher dimensions and possible experimental realizations.

YbAgGe contains a magnetic geometrically frustrated kagome-like lattice that also features significant local single-ion anisotropy. The electronic state is established by hybridization of $4f$ and conduction electrons leading to heavy electronic masses. The competition between these various interactions leads to nontrivial behavior under external magnetic field, including a sequence of magnetic phase transitions, non-Fermi-liquid states, and possibly a quantum critical point. We present a series of neutron diffraction experiments performed in the mK temperature range and under magnetic fields up to 8 T in the hexagonal plane, revealing the microscopic nature of the first four subsequent magnetic states of this phase diagram. The magnetic phases are associated with the propagation vectors ${\mathbf{k}}_{\mathbf{1}}$ = ($\frac{1}{3}$ 0 $\frac{1}{3}$) for $H<2$ T, ${\mathbf{k}}_{\mathbf{2}}$ = (0 0 0.32) for 2 T $<$ $H<3$ T, ${\mathbf{k}}_{\mathbf{1}}=(\frac{1}{3}$ 0 $\frac{1}{3}$) for 3 T $<$ $H<4.5$ T, and ${\mathbf{k}}_{\mathbf{3}}=\left(0.1950.1950.38\right)$ for 4.5 T $<$ $H<7$ T. Our structural refinements reveal a strong modulation of the magnetic moment amplitude in all phases. We observe that the ordered moments of the three magnetically different Yb sites become increasingly different in field, which complies with the principle local anisotropy directions relative to the field direction. While the ordered moments are aligned predominantly in the hexagonal plane, we also find a significant out-of-plane component and a ferromagnetic contribution above 2 T. We discuss possible scenarios that may evolve around the phase boundary at 4.5 T, which is associated with putative quantum criticality as identified by various bulk probes. We propose further steps that are required to better understand the microscopic interactions in this material.