We argue that static nonlinear Hall conductivity can always be represented as a vector in two dimensions and as a pseudotensor in three dimensions independent of its microscopic origin. In a single-band model with a constant relaxation rate, this vector or tensor is proportional to the Berry curvature dipole I. Sodemann and L. Fu, Phys. Rev. Lett 115, 216806 (2015). Here, we develop a quantum Boltzmann formalism to second order in electric fields. We find that in addition to the Berry curvature dipole term, there exist additional disorder-mediated corrections to the nonlinear Hall tensor that have the same scaling in the impurity scattering rate. These can be thought of as the nonlinear counterparts to the side-jump and skew-scattering corrections to the Hall conductivity in the linear regime. We illustrate our formalism by computing the different contributions to the nonlinear Hall conductivity of two-dimensional tilted Dirac fermions.

We present a combined experimental and theoretical investigation of the low-frequency ESR dynamics in the ordered phases of magnetic mineral linarite. This material consists of weakly coupled spin-1/2 chains of copper ions with frustrated ferro- and antiferromagnetic interactions. In zero magnetic field, linarite orders into a spiral structure and exhibits a peculiar magnetic phase diagram sensitive to the field orientation. The resonance frequencies and their field dependence are analyzed combining microscopic and macroscopic theoretical approaches and precise values of magnetic anisotropy constants are obtained. We conclude that possible realization of exotic multipolar quantum states in this material is greatly influenced by the biaxial anisotropy.

In this paper, we theoretically investigate a mesoscopic electron spectrometer that allows for the probing of relaxation processes in quantum Hall edge channels, which has recently been experimentally realized. The device is composed of an emitter quantum dot that injects energy-resolved electrons into the channel closest to the sample edge, to be subsequently probed downstream by a detector quantum dot of the same type. In addition to inelastic processes in the sample that stem from interactions inside the region between the quantum dot energy filters (inner region), anomalous signals are measured when the detector energy exceeds the emitter energy. Considering finite-range Coulomb interactions in the sample, we find that energy exchange between electrons in the current-inducing source channel and the inner region, similar to Auger recombination processes, is responsible for such anomalous currents. In addition, our perturbative treatment of interactions shows that electrons emitted from the source, which dissipate energy to the inner region before entering the detector, contribute to the current most strongly when emitter-detector energies are comparable. Charge transfer in which the emitted electron is exchanged for a charge carrier from the Fermi sea, on the other hand, preferentially occurs close to the Fermi level.

$\mathrm{ZrSiS}$, an intriguing candidate of topological nodal line semimetals, was discovered to have exotic surface floating two-dimensional (2D) electrons [Phys. Rev. X 7, 041073 (2017)], which are likely to interact with surface phonons. Here, we reveal a prominent Kohn anomaly in a surface phonon branch by mapping out the surface phonon dispersions of $\mathrm{ZrSiS}$ using high-resolution electron energy loss spectroscopy. Theoretical analysis via an electron-phonon coupling (EPC) model attributes the strong renormalization of the surface phonon branch to the interactions with the surface floating 2D electrons. With the random phase approximation, we calculate the phonon self-energy and evaluate the mode-specific EPC constant by fitting the experimental data. The EPC picture provided here may be important for potential applications of topological nodal line semimetals.

The element specificity of soft x-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using ${[\mathrm{Fe}/\mathrm{MgO}]}_n$ as a model system, we perform ultraviolet pump/soft x-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe ${\mathrm{L}}_3$ absorption edge and relax within 1 ps by electron-phonon coupling, soft x-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe ${\mathrm{L}}_3$ edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics.

Time- and angle-resolved photoemission spectroscopy (trARPES) is a powerful spectroscopic method to measure the ultrafast electron dynamics directly in momentum space. However, band gap materials with exceptionally strong Coulomb interaction such as monolayer transition-metal dichalcogenides exhibit tightly bound excitons, which dominate their optical properties. This raises the question of whether excitons, in particular their formation and relaxation dynamics, can be detected in photoemissions. Here, we develop a fully microscopic theory of the temporal dynamics of excitonic time- and angle-resolved photoemission with a particular focus on the phonon-mediated thermalization of optically excited excitons to momentum-forbidden dark exciton states. We find that trARPES is able to probe the ultrafast exciton formation and relaxation throughout the Brillouin zone.

We show that the spatially homogeneous and isotropic evolution of a macroscopic Coulomb system of identical particles obeys equations that have the structure of the cosmological equations of the general theory of relativity. There is a Hubble law, and the background charge (if present) mimics the effect of a negative cosmological constant. Specifically, Coulomb explosions mimic the nonsingular open cosmologies in negatively curved spaces, while breathing modes in conductors model oscillatory universes including the anti–de Sitter space.

We demonstrate several explicit duality mappings between elasticity of two-dimensional crystals and fracton tensor gauge theories, expanding on recent works by two of the present authors. We begin by dualizing the quantum elasticity theory of an ordinary commensurate crystal, which maps directly onto a fracton tensor gauge theory, in a natural tensor analog of the conventional particle-vortex duality transformation of a superfluid. The transverse and longitudinal phonons of a crystal map onto the two gapless gauge modes of the tensor gauge theory, while the topological lattice defects map onto the gauge charges, with disclinations corresponding to isolated fractons and dislocations corresponding to dipoles of fractons. We use the classical limit of this duality to make new predictions for the finite-temperature phase diagram of fracton models, and provide a simpler derivation of the Halperin-Nelson-Young theory of thermal melting of two-dimensional solids. We extend this duality to incorporate bosonic statistics, which is necessary for a description of the quantum melting transitions. We thereby derive a hybrid vector-tensor gauge theory which describes a supersolid phase, hosting both crystalline and superfluid orders. The structure of this gauge theory puts constraints on the quantum phase diagram of bosons, and also leads to the concept of symmetry enriched fracton order. We formulate the extension of these dualities to systems breaking time-reversal symmetry. We also discuss the broader implications of these dualities, such as a possible connection between fracton phases and the study of interacting topological crystalline insulators.

We realize an elementary quantum network node consisting of a silicon-vacancy (SiV) color center inside a diamond nanocavity coupled to a nearby nuclear spin with 100-ms-long coherence times. Specifically, we describe experimental techniques and discuss effects of strain, magnetic field, microwave driving, and spin bath on the properties of this two-qubit register. We then employ these techniques to generate Bell states between the SiV spin and an incident photon as well as between the SiV spin and a nearby nuclear spin. We also discuss control techniques and parameter regimes for utilizing the SiV-nanocavity system as an integrated quantum network node.

We report a combined ${}^{115}\mathrm{In}$ nuclear quadrupole resonance, ${}^{51}\mathrm{V}$ nuclear magnetic resonance, and muon spin-relaxation spectroscopic study of the low-temperature magnetic properties of ${\mathrm{InCu}}_{2/3}{\mathrm{V}}_{1/3}{\mathrm{O}}_3$, a quasi-two-dimensional (2D) compound comprising in the spin sector a honeycomb lattice of antiferromagnetically coupled spins $S=1/2$ associated with ${\mathrm{Cu}}^{2+}$ ions. Despite substantial experimental and theoretical efforts, the ground state of this material has not been ultimately identified. In particular, two characteristic temperatures of about 40 and 20 K manifesting themselves as anomalies in different magnetic measurements are discussed controversially. A combined analysis of the experimental data complemented with theoretical calculations of exchange constants enabled us to identify, below 39 K, an “intermediate” quasi-2D static spin state. This spin state is characterized by a staggered magnetization with a temperature evolution that agrees with the predictions for the 2D $\mathit{XY}$ model. We observe that this state gradually transforms at 15 K into a fully developed 3D antiferromagnetic Néel state. We ascribe such an extended quasi-2D static regime to an effective magnetic decoupling of the honeycomb planes due to a strong frustration of the interlayer exchange interactions, which inhibits long-range spin-spin correlations across the planes. Interestingly, we find indications of the topological Berezinsky-Kosterlitz-Thouless transition in the quasi-2D static state of the honeycomb spin-1/2 planes of ${\mathrm{InCu}}_{2/3}{\mathrm{V}}_{1/3}{\mathrm{O}}_3$.

The study of gapped quantum many-body systems in three spatial dimensions has uncovered the existence of quantum states hosting quasiparticles that are confined, not by energetics but by the structure of local operators, to move along lower dimensional submanifolds. These so-called “fracton” phases are beyond the usual topological quantum field theory description, and thus require new theoretical frameworks to describe them. Here we consider coupling fracton models to topological quantum field theories in ($3+1$) dimensions by starting with two copies of a known fracton model and gauging the ${\mathbb{Z}}_2$ symmetry that exchanges the two copies. This yields a class of exactly solvable lattice models that we study in detail for the case of the X-cube model and Haah's cubic code. The resulting phases host finite-energy non-Abelian immobile quasiparticles with robust degeneracies that depend on their relative positions. The phases also host non-Abelian string excitations with robust degeneracies that depend on the string geometry. Applying the construction to Haah's cubic code in particular provides an exactly solvable model with finite energy yet immobile non-Abelian quasiparticles that can only be created at the corners of operators with fractal support.

Early experiments on alkali-doped graphene demonstrated that the dopant adatoms modify the conductivity of graphene significantly, as extra carriers enhance conductivity while Coulomb scattering off the adatoms suppresses it. However, conductivity probes the overall scattering rate, so a dominant channel associated with long-range Coulomb scattering will mask weaker short-range channels. We present weak localization measurements of epitaxial graphene with lithium adatoms that separately quantify intra- and intervalley scattering rates, then compare the measurements to tight-binding calculations of expected rates for this system. The intravalley rate is strongly enhanced by Li deposition, consistent with Coulomb scattering off the Li adatoms. A simultaneous enhancement of intervalley scattering is partially explained by extra carriers in the graphene interacting with residual disorder. But differences between measured and calculated rates at high Li coverage may indicate adatom-induced modifications to the band structure that go beyond the applied model. Similar adatom-induced modifications of the graphene bands have recently been observed in angle-resolved photoemission spectroscopy, but a full theoretical understanding of these effects is still in development.

We consider the evolution of two-time correlations in the quantum XXZ spin chain in contact with an environment causing dephasing. Extending numerically exact, time-dependent matrix product state techniques to consider the dynamics of two-time correlations within dissipative systems, we uncover the full quantum behavior for these correlations along all spin directions. Together with insights from adiabatic elimination and kinetic Monte Carlo, we identify three dynamical regimes. For initial times, their evolution is dominated by the system unitary dynamics and depends on the initial state and the Hamiltonian parameters. For weak spin-spin interaction anisotropy, after this initial dynamical regime, two-time correlations enter an algebraic scaling regime signaling the breakdown of time-translation invariance and the emergence of aging. For stronger interaction anisotropy, these correlations first go through a stretched exponential regime before entering the algebraic one. Such complex relaxation arises due to the competition between the proliferation dynamics of energetically costly excitations and their motion. As a result, dissipative heating dynamics of spin systems can be used to probe the entire spectrum of the underlying Hamiltonian.

We study single-electron charging events in an Al/InAs nanowire hybrid system with deliberately introduced gapless regions. The occupancy of a Coulomb island is detected using a nearby radio-frequency quantum dot as a charge sensor. We demonstrate that a 1-$\mu \mathrm{m}$-long gapped segment of the wire can be used to efficiently suppress single electron poisoning of the gapless region and therefore protect the parity of the island while maintaining good electrical contact with a normal lead. In the absence of protection by charging energy, the $1e$ switching rate can be reduced below $\mathrm{\Gamma }=200{\mathrm{s}}^{-1}$. In the same configuration, we observe strong quantum charge fluctuations due to the exchange of electron pairs between the island and the lead. The magnetic-field dependence of the poisoning rate yields a zero-field superconducting coherence length of $\xi =90\pm 10$ nm.

While the nonperturbative interaction effects in the fractional quantum Hall regime can be readily simulated through exact diagonalization, it has been challenging to establish a suitable diagnostic that can label different phases in the presence of competing interactions and disorder. Here we introduce a multifaceted framework using a simple artificial neural network (ANN) to detect defining features of a fractional quantum Hall state, a charge-density-wave state and a localized state using the entanglement spectra and charge density as independent input. We consider the competing effects of a perturbing interaction $(l=1$ pseudopotential $\mathrm{\Delta }{V}_1)$, a disorder potential $W$, and the Coulomb interaction to the system at filling fraction $\nu =1/3$. Our phase diagram benchmarks well against previous estimates of the phase boundary along the axes of our phase diagram where other measures exist. Moreover, exploring the entire two-dimensional phase diagram, we establish the robustness of the fractional quantum Hall state and map out the charge-density-wave microemulsion phase wherein droplets of the charge-density-wave region appear before the charge density wave is completely disordered. Hence we establish that the ANN can access and learn the defining traits of topological as well as broken symmetry phases using multifaceted inputs of entanglement spectra and charge density.

We report a study of electronic transport in van der Waals heterostructures composed of flakes of the antiferromagnetic Mott insulator $\alpha \text{−}{\mathrm{RuCl}}_3$ placed on top of monolayer graphene Hall bars. While the zero-field transport shows a strong resemblance to that of isolated graphene, we find a consistently $p$-type Hall effect suggestive of multiband conduction, along with a nonmonotonic and gate-voltage-dependent excursion of the resistivity at low temperatures that is reminiscent of transport in the presence of a magnetic phase transition. We interpret these data as evidence for charge transfer from graphene to $\alpha \text{−}{\mathrm{RuCl}}_3$ in an inhomogeneous device yielding both highly and lightly doped regions of graphene, while the latter shows a particular sensitivity to magnetism in the $\alpha \text{−}{\mathrm{RuCl}}_3$. Thus proximity to graphene is a means to access magnetic properties of thin layers of magnetic insulators.

We show that the recent magnetoresistance (MR) data on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) in strong magnetic fields $B$ [P. Giraldo-Gallo et al., Science 361, 479 (2018)] obeys single-parameter scaling of the form $\mathrm{MR}(B,T)=f({\mu }_H\left(T\right)B)$, where ${\mu }_H^{-1}\left(T\right)\sim T^{\alpha }$ ($1\le \alpha \le 2$), from $T=180$ K until $T\sim 20$ K, at which point the single-parameter scaling breaks down. The functional form of the MR is distinct from the simple quadratic-to-linear combination of temperature and magnetic field found in the optimally doped iron superconductor $\mathrm{BaFe}{}_2\left(\mathrm{As}{}_{1-x}\mathrm{P}{}_x\right){}_2$ [I. M. Hayes et al., Nat. Phys. 12, 916 (2016)]. Further, the low-temperature departure of the MR in LSCO from its high-temperature scaling law leads us to conclude that the MR curve collapse is not the result of quantum critical scaling. We examine the classical two-dimensional (2D) effective medium theory (2DEMT) previously [A. A. Patel et al., Phys. Rev. X 8, 021049 (2018)] used to obtain the quadratic-to-linear resistivity dependence on field and temperature for metals with a $T$-linear zero-field resistivity. It appears that this scaling form results only for a binary, random distribution of metallic components. More generally, we find a low-temperature, high-field region where the resistivity is simultaneously $T$ and $B$ linear when multiple metallic components are present. Our findings indicate that if mesoscopic disorder is relevant to the magnetoresistance in strange metal materials, the binary-distribution model which seems to be relevant to the iron pnictides is distinct from the more broad-continuous distributions relevant to the cuprates. Using the latter, we examine the applicability of 2DEMT to the MR in LSCO and compare calculated MR curves with the experimental data.

We have used Ramsey tomography to characterize charge noise in a weakly charge-sensitive superconducting qubit. We find a charge noise that scales with frequency as $1/f^{\alpha }$ over five decades with $\alpha =1.93$ and a magnitude $S_q\left(\text{1}\text{Hz}\right)=2.9\times {10}^{-4}{e}^2/\text{Hz}$. The noise exponent and magnitude of the low-frequency noise are much larger than those seen in prior work on single electron transistors, yet are consistent with reports of frequency noise in other superconducting qubits. Moreover, we observe frequent large-amplitude jumps in offset charge exceeding $0.1e$; these large discrete charge jumps are incompatible with a picture of localized dipolelike two-level fluctuators. The data reveal an unexpected dependence of charge noise on device scale and suggest models involving either charge drift or fluctuating patch potentials.

We present a candidate material, ${\mathrm{NaYbO}}_2$, that realizes the genuine spin-liquid state on the triangular lattice and benchmarks recent theoretical predictions on the relevant spin models. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness. Our thermodynamic measurements, neutron diffraction, and muon spectroscopy coincidentally prove the absence of magnetic order and persistent spin dynamics down to at least 70 mK. Continuous magnetic excitations first observed by inelastic neutron scattering show a gapless feature and the low-energy spectral weight accumulating at the $K$ point of the Brillouin zone, in agreement with theoretical predictions for the spin-liquid phase of triangular antiferromagnets. Such a gapless spin-liquid phase is further confirmed by our magnetic specific heat analysis that reveals a departure from simple power-law behavior. Our work demonstrates that ${\mathrm{NaYbO}}_2$ practically gives direct experimental access to the spin-liquid physics of triangular antiferromagnets.

We investigated the magnetic structure and dynamics of ${\mathrm{YbMnBi}}_2$, with elastic and inelastic neutron scattering, to shed light on the topological nature of the charge carriers in the antiferromagnetic phase. We confirm $C$-type antiferromagnetic ordering of the Mn spins below $T_{\mathrm{N}}=290\mathrm{K}$ and determine that the spins point along the $c$ axis to within about $3^{\circ }$. The observed magnon spectrum can be described very well by the same effective spin Hamiltonian that was used previously to model the magnon spectrum of ${\mathrm{CaMnBi}}_2$. Our results show conclusively that the creation of Weyl nodes in ${\mathrm{YbMnBi}}_2$ by the time-reversal symmetry-breaking mechanism can be excluded in the bulk.