$1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ shows various electronic phases including complex charge density waves (CDWs), Mott insulator, superconductor, and spin liquid. In addition to such thermodynamically stable states, thermally inaccessible metastable metallic states can be induced by means of current. We investigated the nonlinear, nonvolatile current-induced phase transition to the metastable state by performing noise spectroscopy measurements. We found a broadband noise emerging above the threshold current for the transition. This indicates the sliding motion of the current-induced metastable CDWs, which are stabilized after CDW sheets slide to form another stacking structure with metallic characters. Nonvolatility is likely attributed to the commensurability of CDWs.

Signatures of the Kondo effect in the electrical conductance of strongly correlated quantum dots are well understood both experimentally and theoretically, while those in the thermopower have been the subject of recent interest, both theoretically and experimentally. Here, we extend theoretical work [T. A. Costi, Phys. Rev. B 100, 161106(R) (2019)] on the field-dependent thermopower of such systems to the mixed valence and empty orbital regimes, and carry out calculations in order to address a recent experiment on the field-dependent thermoelectric response of Kondo-correlated quantum dots [A. Svilans et al., Phys. Rev. Lett. 121, 206801 (2018)]. In addition to the sign changes in the thermopower at temperatures $T_1\left(B\right)$ and $T_2\left(B\right)$ (present also for $B=0$) in the Kondo regime, an additional sign change was found [T. A. Costi, Phys. Rev. B 100, 161106(R) (2019)] at a temperature $T_0\left(B\right)<T_1\left(B\right)<T_2\left(B\right)$ for fields exceeding a gate-voltage-dependent value $B_0$, where $B_0$ is comparable to, but larger than, the field $B_c$ at which the Kondo resonance splits. We describe the evolution of the Kondo-induced sign changes in the thermopower at temperatures $T_0\left(B\right),T_1\left(B\right)$, and $T_2\left(B\right)$ with magnetic field and gate voltage from the Kondo regime to the mixed valence and empty orbital regimes and show that these temperatures merge to the single temperature $T_0\left(B\right)$ upon entry into the mixed valence regime. By carrying out detailed numerical renormalization group calculations for the above quantities, using appropriate experimental parameters, we address a recent experiment which measures the field-dependent thermoelectric response of InAs quantum dots exhibiting the Kondo effect [A. Svilans et al., Phys. Rev. Lett. 121, 206801 (2018)]. This allows us to understand the overall trends in the measured field- and temperature-dependent thermoelectric response as a function of gate voltage. In addition, we determine which signatures of the Kondo effect [sign changes at $T_0\left(B\right),T_1\left(B\right)$, and $T_2\left(B\right)$] have been observed in this experiment, and find that while the Kondo-induced signature at $T_1\left(B\right)$ is indeed measured in the data, the signature at $T_0\left(B\right)$ can only be observed by carrying out further measurements at a lower temperature. In addition, the less interesting (high-temperature) signature at $T_2\left(B\right)\gtrsim \mathrm{\Gamma }$, where $\mathrm{\Gamma }$ is the electron tunneling rate onto the dot, is found to lie above the highest temperature in the experiment, and was therefore not accessed. Our calculations provide a useful framework for interpreting future experiments on direct measurements of the thermopower of Kondo-correlated quantum dots in the presence of finite magnetic fields, e.g., by extending zero-field measurements of the thermopower [B. Dutta et al., Nano Lett. 19, 506 (2019)] to finite magnetic fields.

We apply high-resolution scanning tunneling microscopy to study intrinsic defect states of bulk FeSe. Four types of intrinsic defects, including the type-I dumbbell, type-II dumbbell, top-layer Se vacancy, and inner-layer Se-site defect, are extensively analyzed by scanning tunneling spectroscopy. Through characterized depression and enhancement of density of states measured in a large energy range, the type-I dumbbell and type-II dumbbell are determined to be the Fe vacancy and ${\mathrm{Se}}_{\mathrm{Fe}}$ defect, respectively. The top-layer Se vacancy and possible inner-layer Se-site vacancy are also determined by spectroscopy analysis. The determination of defects is compared and largely confirmed in the annular dark-field scanning transmission electron microscopy measurement of the exfoliated FeSe. The detailed mapping of the defect states in our experiment lays a foundation for its comparison with the result of complex theoretical calculations in the future.

We study chaos in a classical limit of the Sachdev-Ye-Kitaev (SYK) model obtained in a suitably defined large-$S$ limit. The low-temperature Lyapunov exponent is found to depend linearly on temperature, with a slope that is parametrically different than in the quantum case: It is proportional to $N/S$. The classical dynamics can be understood as the rotation of an $N$-dimensional body with a random inertia tensor, corresponding to the random couplings of the SYK Hamiltonian. This allows us to find an extensive number of fixed points, corresponding to the body's principal axes of rotation. The thermodynamics is mapped to the $p$-spin model with $p=2$, which exhibits a spin glass phase at low temperature whose presence does not preclude the existence of chaos.

The Purcell effect has been the basis for several decades in understanding enhancement of photonic efficiency and decay rates of emitters through their coupling to cavity modes and metal nanostructures. However, it is not clear whether this regime of radiative enhancements can be extended to ultrasmall nanoparticle sizes or interparticle distances. Here we report large radiative enhancements of quantum dot assemblies with extremely small metal nanoparticles and emitter-particle separations $R$ of a few nanometers, where Purcell effect would lead to either no enhancements or quenching. We invoke a new regime of radiative enhancements to explain the experimental data and also correctly predict the emergence of strong coupling below certain $R$, as observed in experiments. In addition, we show that the widely used point emitter approximations diverge from actual observations in the case of finite size emitters at such small separations.

We theoretically investigate gate-defined graphene superlattices with broken inversion symmetry as a platform for realizing tunable valley-dependent transport. Our analysis is motivated by recent experiments [C. Forsythe et al., Nat. Nanotechnol. 13, 566 (2018)] wherein gate-tunable superlattice potentials have been induced on graphene by nanostructuring a dielectric in the graphene/patterned-dielectric/gate structure. We demonstrate how the electronic tight-binding structure of the superlattice system resembles a gapped Dirac model with associated valley-dependent transport using an unfolding procedure. In this manner we obtain the valley Hall conductivities from the Berry curvature distribution in the superlattice Brillouin zone, and demonstrate the tunability of this conductivity by the superlattice potential. Finally, we calculate the valley Hall angle relating the transverse valley current and longitudinal charge current and demonstrate the robustness of the valley currents against irregularities in the patterned dielectric.

Recent experiments have measured the signatures of the Kondo effect in the zero-field thermopower of strongly correlated quantum dots [Svilans et al., Phys. Rev. Lett. 121, 206801 (2018); Dutta et al., Nano Lett. 19, 506 (2019)]. They confirm the predicted Kondo-induced sign change in the thermopower, upon increasing the temperature through a gate-voltage dependent value $T_1\gtrsim T_{\mathrm{K}}$, where $T_{\mathrm{K}}$ is the Kondo temperature. Here, we use the numerical renormalization group (NRG) method to investigate the effect of a finite magnetic field $B$ on the thermopower of such quantum dots. We show that, for fields $B$ exceeding a gate-voltage dependent value $B_0$, an additional sign change takes place in the Kondo regime at a temperature $T_0(B\ge B_0)>0$ with $T_0<T_1$. The field $B_0$ is comparable to, but larger than, the field $B_c$ at which the zero-temperature spectral function splits in a magnetic field. The validity of the NRG results for $B_0$ are checked by comparison with asymptotically exact higher-order Fermi-liquid calculations [Oguri and Hewson, Phys. Rev. B 97, 035435 (2018)]. Our calculations clarify the field-dependent signatures of the Kondo effect in the thermopower of Kondo-correlated quantum dots and explain the recently measured trends in the $B$-field dependence of the thermoelectric response of such systems [Svilans et al., Phys. Rev. Lett. 121, 206801 (2018)].

Recently considerable excitement has arisen due to the experimental observation of a field-induced spin liquid phase in the compound $\alpha \text{−}{\mathrm{RuCl}}_3$. However, the nature of this putative spin liquid phase and the relevant microscopic model Hamiltonian remain still unclear. In this work, we address these questions by performing large-scale numerical simulations of a generalized Kitaev-Heisenberg model proposed to describe the physics of $\alpha \text{−}{\mathrm{RuCl}}_3$. While an intermediate phase does not appear for in-plane magnetic fields, our results strongly suggest that a stable intermediate spin liquid phase, sandwiched between a magnetically ordered phase at low fields and a high-field polarized phase, can be induced by out-of-plane magnetic fields. Moreover, we show that this field-induced spin liquid phase can be smoothly connected to a spin liquid possessing a spinon Fermi surface as proposed recently for the Kitaev model. The relevance of our results to $\alpha \text{−}{\mathrm{RuCl}}_3$ is also discussed.

An interplay between electron correlation and reduced dimensionality due to the Landau quantization gives rise to exotic electronic phases in three-dimensional semimetals under high magnetic field. Using an unbiased theoretical method, we clarify for the first time comprehensive ground-state phase diagrams of a three-dimensional semimetal with a pair of electron and hole pockets in the quantum limit. For the electron interaction, we consider either screened Coulomb repulsive interaction or an attractive electron-electron interaction mediated by a screened electron-phonon coupling, where a screening length is generally given by a dimensionless constant times magnetic length $l$. By solving the parquet RG equation numerically and employing a mean-field argument, we construct comprehensive ground-state phase diagrams of the semimetal in the quantum limit for these two cases, as a function of the Fermi wave length and the screening length (both normalized by $l)$. In the repulsive interaction case, the ground state is either excitonic insulator (EI) in strong screening regime or Ising-type spin density wave in weak screening regime. In the attractive interaction case, the ground state is either EI that breaks the translational symmetries (strong screening regime), topological EI, charge Wigner crystal (intermediate screening regime), plain charge density wave or possible non-Fermi liquid (weak screening regime). We show that the topological EI supports a single copy of massless Dirac fermion at its side surface, and thereby exhibit a $\sqrt{H_{\perp }}$-type surface Shubnikov-de Haas (SdH) oscillation in in-plane surface transports as a function of a canted magnetic field $H_{\perp }$. Armed with these theoretical knowledge, we discuss implications of recent transport experiments on graphite under the high field.

We propose a scheme for the spatial exciton energy control and exciton routing in a transition-metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide ${\mathrm{SrTiO}}_3$, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle band gap and exciton binding energy, Bohr radius, and nonlinearity (an exciton-exciton interaction). The combination of particle band gap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow toward the energetically lower region of the sample.

We consider the prospects of locating and characterizing individual defect centers in bulk materials using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). We simulate STEM-EELS maps for two important defect centers in diamond, namely, the negatively charged nitrogen-vacancy defect and the neutral silicon-vacancy defect. We use density-functional theory to compute the defect electronic structure and a Møller potential formalism to compute the inelastic electron scattering. Our results indicate that it should be possible to use STEM-EELS to obtain the transverse locations of these defects to within about 1 nm. We calculate the plane-wave scattering cross sections for these individual defects to be of the order of ${10}^{-4}$ Å${}^2$, which indicates that the EELS signals should be within detectable limits. Calculated spectral maps and scattering cross sections are given as a function of the defect orientation, and we show that the results can be interpreted using a tight-binding description of the defect electronic structure.

Quantum many-body physics simulations with matrix product states can often be accelerated if the quantum symmetries present in the system are explicitly taken into account. Conventionally, quantum symmetries have to be determined beforehand when constructing the tensors for the matrix product state algorithm. In this work, we present a matrix product state algorithm with an adaptive $U\left(1\right)$ symmetry. This algorithm can take into account or benefit from $U\left(1\right)$ or $Z_2$ symmetries when they are present or analyze the nonsymmetric scenario when the symmetries are broken without any external alteration of the code. To give some concrete examples we consider an XYZ model and show the insight that can be gained by (i) searching the ground state and (ii) evolving in time after a symmetry-changing quench. To show the generality of the method, we also consider an interacting bosonic system under the effect of a symmetry-breaking dissipation.

Magnetic rare-earth/nonmagnetic metal superlattices are well-known to display chiral spin helices in the rare-earth layers that propagate coherently across the nonmagnetic layers. However, the underlying mechanism that preserves the magnetic phase and chirality coherence across the nonmagnetic layers has remained elusive. In this paper, we use resonant and element-specific x-ray scattering to evidence directly the formation of two fundamentally different long-range modulations in a holmium/yttrium (Ho/Y) multilayer: the known Ho chiral spin helix with periodicity 25 Å and a newly observed charge density wave with periodicity 16 Å that propagates through both the Ho and nonmagnetic Y layer. With x-ray circular magnetic dichroism measurements ruling out the existence of a magnetic proximity effect induced moment in the nonmagnetic Y layers, we propose that the charge density wave is also chiral, thus providing the means for the transmittance of magnetic chirality coherence between Ho layers.

In this work, the electric modulation of magnetic anisotropy is investigated on a granule/matrix interface of a Py/PZT composite. Outside the Py/PZT interface, the electric polarization is derived through the Thomas-Fermi model and mean field approximation. Inside the Py/PZT interface, the magnetizing energy is derived through the spin-split model and spherical shell approximation. As the Py volume fraction varies from 0.0% to 0.8%, the electric polarization evolves from radiationlike to vortexlike distribution, while the magnetizing energy evolves from symmetric-type to asymmetric-type distribution. Under downward and upward polarization, parallel and perpendicular magnetization are preferred, respectively. As the granule diameter or matrix thickness is varied, the downward polarization induces unstable parallel magnetic anisotropy, while the upward polarization induces stable perpendicular magnetic anisotropy. This work is beneficial for achieving electrostatic manipulation of a magnetic vortex.

Polarization-resolved Raman spectroscopy was performed and analyzed from large, high-quality, monodomain single crystal of $\alpha \text{−}{\mathrm{RuCl}}_3$, a proximate Kitaev quantum spin liquid. Spectra were collected with laser polarizations parallel and perpendicular to the honeycomb plane. Pairs of nearly degenerate phonons were discovered and show either a fourfold or twofold polarization angle dependence in their Raman intensity, thereby providing evidence to definitively assign the bulk crystal point group as $C_{2h}$. The low-frequency continuum that is often attributed to scattering from pairs of Majorana fermions was also examined and found to disappear when the laser excitation and scattered photon polarizations were perpendicular to the honeycomb plane. This disappearance, along with the behavior of the phonon spectrum in the same polarization configuration, strongly suggests that the scattering continuum is two-dimensional. We argue that this scattering continuum originates from the Kitaev magnetic interactions that survives up to room temperature, a scale larger than the bare Kitaev exchange energy of approximately 50 K.

${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ was recently reported to exhibit remarkable magnetic-field-induced magnetization plateaus and ferroelectricity [R. Chen et al., Phys. Rev. B 98, 184404 (2018)], but its magnetic ground state remains ambiguous. Magnetometry measurements and time-of-flight neutron powder diffraction (NPD) have been employed to study the structural and magnetic properties of ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$, which includes two nonequivalent Co sites. Upon cooling below the Néel temperature $T_{\text{N}}=6.0\left(2\right)$ K, we observe magnetic Bragg peaks at 2 K in NPD, which indicates the formation of long-range magnetic order of ${\mathrm{Co}}^{2+}$ moments. After symmetry analysis and magnetic structure refinement, we demonstrate that ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ possesses a complicated noncollinear magnetic ground state with Co moments mainly located in the b-c plane and forming a noncollinear spin-chain-like structure along the c-axis. The ab initio calculations demonstrate that the noncollinear magnetic structure is more stable than various ferromagnetic states at low temperature. The noncollinear magnetic structure with a canted $\uparrow \uparrow \downarrow \downarrow$ spin configuration is considered to be the origin of magnetoelectric coupling in ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ because the inequivalent exchange striction induced by the spin-exchange interaction between the neighboring spins could be the driving force of ferroelectricity. It is also found that the deviation of lattice parameters a and b is opposite below $T_{\text{N}}$, while the lattice parameter c and $\beta$ stay almost constant below $T_{\text{N}}$, evidencing the anisotropic magnetoelastic coupling in ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$.

The Abelian Higgs model is the textbook example for the superconducting transition and the Anderson-Higgs mechanism, and has become pivotal in the description of deconfined quantum criticality. We study the Abelian Higgs model with $n$ complex scalar fields at unprecedented four-loop order in the $4-\epsilon$ expansion and find that the annihilation of the critical and bicritical points occurs at a critical number of $n_c\approx 182.95\left(1-1.752\epsilon +0.798{\epsilon }^2+0.362{\epsilon }^3\right)+O\left({\epsilon }^4\right)$. Consequently, below $n_c$, the transition turns from second to first order. Resummation of the series to extract the result in three dimensions provides strong evidence for a critical $n_c(d=3)$ which is significantly below the leading-order value, but the estimates for $n_c$ are widely spread. Conjecturing the topology of the renormalization group flow between two and four dimensions, we obtain a smooth interpolation function for $n_c\left(d\right)$ and find $n_c\left(3\right)\approx 12.2\pm 3.9$ as our best estimate in three dimensions. Finally, we discuss Miransky scaling occurring below $n_c$ and comment on implications for weakly first-order behavior of deconfined quantum transitions. We predict an emergent hierarchy of length scales between deconfined quantum transitions corresponding to different $n$.

Motivated by the interest in topological quantum paramagnets in candidate spin-1 magnets, we investigate the diamond-lattice compound ${\mathrm{NiRh}}_2{\mathrm{O}}_4$ using ab initio theory and model Hamiltonian approaches. Our density functional study, taking into account the unquenched orbital degrees of freedom, shows stabilization of $S=1$, $L=1$ state. We highlight the importance of spin-orbit coupling, in addition to Coulomb correlations, in driving the insulating gap, and uncover frustrating large second-neighbor exchange mediated by Ni-Rh covalency. A single-site model Hamiltonian incorporating the large tetragonal distortion is shown to give rise to a spin-orbit entangled nonmagnetic ground state, largely accounting for the entropy, magnetic susceptibility, and inelastic neutron scattering results. Incorporating intersite exchange within a slave-boson theory, we show that exchange frustration can suppress exciton condensation. We capture the dispersive gapped magnetic modes, uncover “dark states” invisible to neutrons, and make predictions.