Quantum-to-classical crossover is a fundamental question in dynamics of quantum many-body systems. In frustrated magnets, for example, it is highly nontrivial to describe the crossover from the classical spin liquid with a macroscopically degenerate ground-state manifold to the quantum spin liquid phase with fractionalized excitations. This is an important issue, as we often encounter the demand for a sharp distinction between the classical and quantum spin liquid behaviors in real materials. Here we take the example of the classical spin liquid in a frustrated magnet with novel bond-dependent interactions to investigate the classical dynamics, and critically compare it with quantum dynamics in the same system. In particular, we focus on signatures in the dynamical spin structure factor. Combining Landau-Lifshitz dynamics simulations and the analytical Martin-Siggia-Rose approach, we show that the low-energy spectra are described by relaxational dynamics and highly constrained by the zero mode structure of the underlying degenerate classical manifold. Further, the higher energy spectra can be explained by precessional dynamics. Surprisingly, many of these features can also be seen in the dynamical structure factor in the quantum model studied by finite-temperature exact diagonalization. We discuss the implications of these results and their connection to recent experiments on frustrated magnets with strong spin-orbit coupling.

The recently investigated $1T$ polymorph of monolayer ${\mathrm{NbSe}}_2$ revealed an insulating behavior suggesting a star-of-David phase with $\sqrt{13}\times \sqrt{13}$ periodicity associated with a Mott insulator, reminiscent of $1T-{\mathrm{TaS}}_2$. We examine this novel two-dimensional material from first principles. We find an instability towards the formation of an incommensurate charge density wave (CDW) and establish the star-of-David phase as the most stable commensurate CDW. The Mottness in the star-of-David phase is confirmed and studied at various levels of theory: the spin-polarized generalized gradient approximation (GGA) and its extension involving the on-site Coulomb repulsion (GGA+$U$), as well as the dynamical mean-field theory. Finally, we estimate Heisenberg exchange couplings in this material and find a weak nearest-neighbor ferromagnetic coupling, at odds with most Mott insulators. We point out the close resemblance between this star-of-David phase and flat-band ferromagnetism models.

Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including bright and optically forbidden dark excitonic states. Here, we show how dark excitons can be experimentally revealed by probing the intraexcitonic $1s-2p$ transition. Distinguishing the optical response shortly after the excitation and after the exciton thermalization allows us to reveal the relative position of bright and dark excitons. We find both in theory and experiment a clear blueshift in the optical response demonstrating the transition of bright exciton populations into lower-lying dark excitonic states.

Here we present measurements on $p\text{−}n$ junctions in encapsulated graphene revealing several sets of magnetoconductance oscillations originating from quasiclassical snake states and edge state Aharonov-Bohm interferences. Even though some of these oscillations have already been observed in suspended and encapsulated devices including different geometries, their identification remained challenging as they were observed in separate measurements, and only a limited amount of data was available. Moreover, these effects have similar experimental signatures, therefore for their proper assignment their simultaneous observation and their detailed characterization is needed. The investigation of the charge carrier density, magnetic field, temperature, and bias dependence of the oscillations enabled us to properly identify their origin. Surprisingly we have found that snake states and Aharonov-Bohm interferences can coexist within a limited parameter range. We explain this using a unified picture of magneto-oscillations and confirm our findings using tight binding simulations. Since $p\text{−}n$ junctions are the most important building blocks of graphene based electron-optical elements and edge state interferometers, our findings will be crucial for the design and understanding of future devices.

Although numerical studies modeling the quantum Hall effect at filling fraction $5/2$ predict either the Pfaffian (Pf) or its particle-hole conjugate, the anti-Pfaffian (aPf) state, recent experiments appear to favor a quantized thermal Hall conductivity with quantum number $K=5/2$, rather than the value $K=7/2$ or $3/2$ expected for the Pf or aPF state, respectively. While a particle-hole symmetric topological order (the PH-Pfaffian) would be consistent with the experiments, this state is believed to be energetically unfavorable in a homogenous system. Here we study the effects of disorder that are assumed to locally nucleate domains of Pf and aPf. When the disorder is relatively weak and the size of domains is relatively large, we find that when the electrical Hall conductance is on the quantized plateau with ${\sigma }_{xy}=(5/2)(e^2/h)$, the value of $K$ can be only 7/2 or 3/2, with a possible first-order-like transition between them as the magnetic field is varied. However, for sufficiently strong disorder, an intermediate state might appear, which we analyze within a network model of the domain walls. Predominantly, we find a thermal metal phase, where $K$ varies continuously and the longitudinal thermal conductivity is nonzero, while the electrical Hall conductivity remains quantized at $(5/2)e^2/h$. However, in a restricted parameter range we find a thermal insulator with $K=5/2$, a disorder stabilized phase which is adiabatically connected to the PH-Pfaffian. We discuss a possible scenario to rationalize these special values of parameters.

The quantum Hall physics of bilayer graphene is extremely rich due to the interplay between a layer degree of freedom and delicate fractional states. Recent experiments show that when an electric field perpendicular to the bilayer causes Landau levels of opposing layers to cross in energy, an even-denominator Hall plateau can coexist with a finite density of interlayer excitons. We present theoretical and numerical evidence that this observation is due to a new phase of matter: a Fermi sea of topological excitons.

The spreading of correlations after a quantum quench is studied in a wide class of lattice systems, with short- and long-range interactions. Using a unifying quasiparticle framework, we unveil a rich structure of the correlation cone, which encodes the footprints of several microscopic properties of the system. When the quasiparticle excitations propagate with a bounded group velocity, we show that the correlation edge and correlation maxima move with different velocities that we derive. For systems with a divergent group velocity, especially relevant for long-range interacting systems, the correlation edge propagates slower than ballistic. In contrast, the correlation maxima propagate faster than ballistic in gapless systems but ballistic in gapped systems. Our results shed light on existing experimental and numerical observations and pave the way to the next generation of experiments. For instance, we argue that the dynamics of correlation maxima can be used as a witness of the elementary excitations of the system.

Rare-earth ions in crystals are a proven solid-state platform for quantum technologies in the ensemble regime and attractive for new opportunities at the single-ion level. Among the trivalent rare earths, ${}^{171}\mathrm{Yb}^{3+}$ is unique in that it possesses a single $4f$ excited-state manifold and is the only paramagnetic isotope with a nuclear spin of 1/2. In this work, we present measurements of the optical and spin properties of ${}^{171}\mathrm{Yb}^{3+}:{\mathrm{YVO}}_4$ to assess whether this distinct energy-level structure can be harnessed for quantum interfaces. The material was found to possess large optical absorption compared to other rare-earth-doped crystals owing to the combination of narrow inhomogeneous broadening and a large transition oscillator strength. In moderate magnetic fields, we measure optical linewidths less than 3 kHz and nuclear spin linewidths less than 50 Hz. We characterize the excited-state hyperfine and Zeeman interactions in this system, which enables the engineering of a $\mathrm{\Lambda }$ system and demonstration of all-optical coherent control over the nuclear-spin ensemble. Given these properties, ${}^{171}\mathrm{Yb}^{3+}:{\mathrm{YVO}}_4$ has significant potential for building quantum interfaces such as ensemble-based memories, microwave-to-optical transducers, and optically addressable single rare-earth-ion spin qubits.

We propose a two-orbital Hubbard model on an emergent honeycomb lattice to describe the low-energy physics of twisted bilayer graphene. Our model provides a theoretical basis for studying metal-insulator transition, Landau level degeneracy lifting, and unconventional superconductivity that are recently observed.

We report a persistent low-energy phonon broadening around $q_B\sim 0.28$ reciprocal lattice unit (r.l.u.) along the Cu-O bond direction in the high- ${\text{T}}_{\mathrm{c}}$ cuprate ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi-2212). We show that such broadening exists both inside and outside the conventional charge-density wave (CDW) phase, via temperature-dependent measurements in both underdoped and heavily overdoped samples. Combining inelastic hard x-ray scattering, diffuse scattering, angle-resolved photoemission spectroscopy, and resonant soft x-ray scattering at the Cu ${\mathrm{L}}_3$ -edge, we exclude the presence of a CDW in the heavily overdoped Bi-2212 similar to that observed in the underdoped systems. Finally, we discuss the origin of such anisotropic low-energy phonon broadening and its potential precursory role to the CDW phase in the underdoped region.

Vanadium dioxide ($\mathrm{V}{\mathrm{O}}_2$), an archetypal correlated-electron material, undergoes an insulator-metal transition near room temperature that exhibits electron-correlation-driven and structurally driven physics. Using ultrafast temperature- and fluence-dependent optical spectroscopy and x-ray scattering, we show that multiple interrelated electronic and structural processes in the nonequilibrium dynamics in $\mathrm{V}{\mathrm{O}}_2$ can be disentangled in the time domain. Specifically, following intense subpicosecond terahertz (THz) electric-field excitation, a partial collapse of the insulating gap occurs within the first picosecond. At temperatures sufficiently close to the transition temperature and for THz peak fields above a threshold of approximately 1 MV/cm, this electronic reconfiguration initiates a change in lattice symmetry taking place on a slower timescale. We identify the kinetic energy increase of electrons tunneling in the strong electric field as the driving force, illustrating a promising method to control electronic and structural interactions in correlated materials on an ultrafast timescale.

We propose and analyze magnetic traps and lattices for electrons in semiconductors. We provide a general theoretical framework and show that thermally stable traps can be generated by magnetically driving the particle's internal spin transition, akin to optical dipole traps for ultracold atoms. Next we discuss in detail periodic arrays of magnetic traps, i.e., magnetic lattices, as a platform for quantum simulation of exotic Hubbard models, with lattice parameters that can be tuned in real time. Our scheme can be readily implemented in state-of-the-art experiments, as we particularize for two specific setups, one based on a superconducting circuit and another one based on surface acoustic waves.

Here we report the observation of pressure-induced melting of antiferromagnetic (AFM) order and the emergence of a new state in the honeycomb-lattice halide $\alpha \text{−}\mathrm{RuC}{\mathrm{l}}_3$, a candidate compound in the proximity of quantum spin liquid state. Our high-pressure heat capacity measurements demonstrate that the AFM order smoothly melts away at a critical pressure ($P_C$) below 1 GPa. Intriguingly, the AFM transition temperature displays an increase upon applying pressure below the $P_C$, in stark contrast to usual phase diagrams, for example, in pressurized parent compounds of unconventional superconductors. Furthermore, a stable magnetoresistance is observed in the high-pressure phase above $P_C$ which is robust against pressure up to $\sim 140$ GPa.

We investigate the tunneling anisotropic magnetoresistance (TAMR) in thick hcp Co films at cryogenic temperatures using scanning tunneling microscopy. At around $-350\mathrm{mV}$, a strong TAMR up to 30% is found with a characteristic voltage dependence and a reversal of sign. With the help of ab initio calculations, the TAMR can be traced back to a spin-polarized occupied surface state that experiences a strong spin-orbit interaction leading to a magnetization direction depending on hybridization with bulk states.

The ability to induce, observe, and control quantum coherent interactions in room-temperature, electrically driven optoelectronic devices is of utmost significance for advancing quantum science and engineering towards practical applications. We demonstrate here a coherent interference phenomena, Ramsey fringes, in an inhomogeneously broadened InAs/InP quantum-dot (QD) ensemble in the form of a 1.5-mm-long optical amplifier operating at room temperature. The observation of Ramsey fringes in semiconductor QDs was previously achieved only at cryogenic temperatures and only in isolated single-dot systems. A high-resolution pump-probe scheme where both pulses are characterized by cross-frequency-resolved optical gating reveals a clear oscillatory behavior both in the amplitude and the instantaneous frequency of the probe pulse with a period that equals one optical cycle at the operational wavelength. Using nominal input delays of 600–900 fs and scanning the separation around each delay in 1-fs steps, we map the evolution of the material decoherence and extract a coherence time of 340 fs. Moreover, we observed a unique phenomenon, which cannot be observed in single-dot systems, that the temporal position of the output probe pulse also oscillates with the same periodicity but with a quarter cycle delay relative to the intensity variations. The modulation of the pulse separation results from coupling between the real and imaginary parts of the susceptibility and the quadrature delay is the time domain manifestation of its complex nature.

The coexistence of superconductivity (SC) and charge density waves (CDWs) was investigated for pure and Se-doped $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ via electrical resistivity under hydrostatic pressure. A universal superconducting phase diagram was revealed on the border of CDWs. Unlike isovalent S/Se doping and uniaxial pressure, the application of high hydrostatic pressure suppresses CDW more sensitively and thoroughly with the critical pressures $P_{\mathrm{c}}\left(x\right)\sim 4.70–6.55$ GPa. A pressure-induced superconducting state coexists with various CDWs, then bulk SC emerges along with the complete collapse of various CDWs. The superconducting transition temperature increases monotonously up to $\sim 7.3\mathrm{K}$ at 15 GPa without a domelike shape. The results clarify that the superconducting Cooper pairing is associated with the CDWs’ instability near $P_{\mathrm{c}}\left(x\right)$. Above $P_{\mathrm{c}}\left(x\right)$, the monotonous increase of $T_{\mathrm{c}}$ is attributed to different evolutions of the electronic structures and phonon vibration spectra under hydrostatic and uniaxial pressures and a further inhibition of undetected CDWs.

Electron pairing due to a repulsive Coulomb interaction in a triple quantum dot (TQD) is experimentally studied. It is found that electron pairing in two dots of a TQD is mediated by the third dot, when the third dot strongly couples with the other two via Coulomb repulsion so that the TQD is in the twofold degenerate ground states of (1,0,0) and (0,1,1) charge configurations. Using the transport spectroscopy that monitors electron transport through each individual dot of a TQD, we analyze how to achieve the degeneracy in experiments, how the degeneracy is related to electron pairing, and the resulting nontrivial behavior of electron transport. Our findings may be used to design a system with nontrivial electron correlations and functionalities.

Understanding the entanglement structure of out-of-equilibrium many-body systems is a challenging yet revealing task. Here, we investigate the entanglement dynamics after a quench from a piecewise homogeneous initial state in integrable systems. This is the prototypical setup for studying quantum transport, and it consists in the sudden junction of two macroscopically different and homogeneous states. By exploiting the recently developed integrable hydrodynamic approach and the quasiparticle picture for the entanglement dynamics, we conjecture a formula for the entanglement production rate after joining two semi-infinite reservoirs, as well as for the steady-state entanglement entropy of a finite subregion. We show that both quantities are determined by the quasiparticles created in the nonequilibrium steady state (NESS) appearing at large times at the interface between the two reservoirs. Specifically, the steady-state entropy coincides with the thermodynamic entropy of the NESS, whereas the entropy production rate reflects its spreading into the bulk of the two reservoirs. Our results are numerically corroborated using time-dependent density matrix renormalization group (tDMRG) simulations in the paradigmatic XXZ spin-$1/2$ chain.

We identify experimentally the magnetostatic modes active for Brillouin light scattering in the optical whispering gallery modes of a yttrium iron garnet sphere. Each mode is identified by magnetic-field dispersion of ferromagnetic-resonance spectroscopy and coupling strength to the known field distribution of the microwave drive antenna. Our optical measurements confirm recent predictions that higher-order magnetostatic modes can also generate optical scattering, according to the selection rules derived from the axial symmetry. From this we summarize the selection rules for Brillouin light scattering. We give experimental evidence that the optomagnonic coupling to nonuniform magnons can be higher than that of the uniform Kittel mode.

We performed resonant microwave measurements on superconducting titanium (Ti) down to temperatures of 40 mK, well below its critical temperature $T_{\mathrm{c}}\approx 0.5$ K. Our wide frequency range 3.3–40 GHz contains the zero-temperature energy gap $2{\mathrm{\Delta }}_0$ and allows us to probe the full electrodynamics of the superconducting state, including excitations across the gap and the low-frequency responses of superfluid condensate and thermal quasiparticles. The observed behavior follows the predictions of the BCS-based Mattis-Bardeen formalism, which implies that superconducting Ti is in the dirty limit, in agreement with our determination of the scattering rate. We directly determine the temperature dependence of the energy gap, which is in accordance with BCS predictions, and $2{\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_{\mathrm{c}}\approx 3.5$ with ${\mathrm{\Delta }}_0\approx 72\mu \mathrm{eV}$. We also evaluate the penetration depth, and we characterize the behavior of superconducting Ti in external magnetic field.