We present experimental investigations of transport properties in the insulating phase of two-dimensional Josephson-junction arrays (JJAs) by systematically changing the ratio of Josephson energy $E_{\mathrm{J}}$ and charging energy $E_{\mathrm{C}}$. The observed temperature dependence of resistance indicates that the JJAs do not show a sharp phase transition but exhibit a gradual crossover to the insulating phase. At low temperatures, the current-voltage ($I\text{−}V$) characteristics become nonlinear as described by $I=cV+b{V}^a$ ($a, b$, and $c$ are temperature-dependent coefficients). This nonlinear behavior is understood in terms of the Berezinskii-Kosterlitz-Thouless mechanism by taking into account the influence of a finite-range cutoff of the logarithmic interaction between Cooper pairs. From the analysis of the nonlinearity, we deduce the crossover temperature to the insulating phase and determine the phase diagram in the insulating side as a function of $E_{\mathrm{J}}/E_{\mathrm{C}}$. We also show that, at very low temperatures, the $I\text{−}V$ characteristics continuously develop into the negative differential conductance caused by coherent single-Cooper-pair tunneling.

We have combined ultrafast time-resolved techniques, available at European infrared free-electron laser facilities (HFML-FELIX and ELBE), for the analysis of intracenter relaxation processes in an atomiclike energy spectrum of the double donor magnesium in silicon. We show that complementary techniques enable the accurate analysis of complex transients inherent to strongly absorbing multilevel media. From the combined analysis of degenerate pump-probe, photon echo, and transient grating spectroscopy, we find short decay times of $\left(10\pm 1\right)\mathrm{ps}$ and $(30\pm 3)\mathrm{ps}$ for the $2p_0$ and $2p_{\pm }$ states, respectively. These high decay rates exceed those obtained earlier for the shallow, one-electron donors in silicon, where intracenter relaxation profits from interaction with a single phonon of a host lattice.

Disorder-free localization in translation-invariant gauge theories presents a counterintuitive yet powerful framework of ergodicity breaking in quantum many-body physics. The fragility of this phenomenon in the presence of gauge-breaking errors has recently been addressed, but no scheme has been able to reliably stabilize disorder-free localization through all accessible evolution times while preserving the disorder-free property. Here, we introduce the concept of Stark gauge protection, which entails a linear sum in gauge-symmetry local (pseudo)generators weighted by a Stark potential. Using exact diagonalization and Krylov-based methods, we show how this scheme can stabilize or even enhance disorder-free localization against gauge-breaking errors in $\mathrm{U}\left(1\right)$ and ${\mathbb{Z}}_2$ gauge theories up to all accessible evolution times, without inducing bona fide Stark many-body localization. We show through a Magnus expansion that the dynamics under Stark gauge protection is described by an effective Hamiltonian where gauge-breaking terms are suppressed locally by the protection strength and additionally by the matter site index, which we argue is the main reason behind stabilizing the localization up to all accessible times. Our scheme is readily feasible in modern ultracold-atom experiments and Rydberg-atom setups with optical tweezers.

Neutron spectroscopy measurements have been performed on single crystals of the antiferromagnetic van der Waals compound $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$. Linear spin-wave theory using a Heisenberg Hamiltonian with single-ion anisotropies has been applied to determine the magnetic exchange parameters and the nature of the anisotropy. The analysis reveals that $\mathrm{Ni}\mathrm{P}{\mathrm{S}}_3$ is less two-dimensional than its sister compounds, with a relatively large ferromagnetic exchange of $J^{\prime }=-0.3$ meV between the layered $\mathit{ab}$ planes. In-plane magnetic exchange interactions up to the third-nearest neighbor were required to fit the data. The nearest-neighbor exchange was ferromagnetic with $J_1=-2.6$ meV, the second neighbor was antiferromagnetic and small with $J_2=0.2$ meV, and the dominant antiferromagnetic third-neighbor exchange was $J_3=13.5$ meV. The anisotropy was shown to be largely $\mathit{XY}$-like with a small uniaxial component, leading to the appearance of two low-energy spin wave modes in the spin-wave spectrum at the Brillouin zone center. The analysis could reproduce the spin-wave energies, however, there are discrepancies with the calculated neutron intensities, hinting at more exotic phenomena.

We investigate the charge-density wave (CDW) transition for one-dimensional spinless fermions at half band filling with nearest-neighbor electron transfer amplitude $t$ and interaction $V$. The model is equivalent to the anisotropic XXZ Heisenberg model for which the Bethe Ansatz provides an exact solution. For $V>V_{\mathrm{c}}=2t$, the CDW order parameter and the single-particle gap are finite but exponentially small, as is characteristic for a Kosterlitz-Thouless transition. It is notoriously difficult to locate such infinite-order phase transitions in the phase diagram using approximate analytical and numerical approaches. Second-order Hartree-Fock theory is qualitatively applicable for all interaction strengths, and predicts the CDW transition to occur at $V_{\mathrm{c},2}^{\left(2\right)}\approx 1.5t$. Second-order Hartree Fock theory is almost variational because the density of quasiparticle excitations is small. We apply the density-matrix renormalization group (DMRG) for periodic boundary conditions for system sizes up to 514 sites, which permits a reliable extrapolation of all physical quantities to the thermodynamic limit, apart from the critical region. We investigate the ground-state energy, the gap, the order parameter, the momentum distribution, the quasiparticle density, and the density-density correlation function to locate $V_{\mathrm{c}}$ from the DMRG data. Tracing the breakdown of the Luttinger liquid and the peak in the quasiparticle density at the band edge permits us to reproduce $V_{\mathrm{c}}$ with an accuracy of one percent.

We study Rabi rotations arising in intensity-dependent photon echoes from an ensemble of self-assembled InGaAs quantum dots. To achieve a uniform distribution of intensities within the excited ensemble, we introduce flattop intensity profiles of picosecond laser pulses. This allows us to overcome the damping of Rabi rotations imposed by the spatial inhomogeneity of Rabi frequencies by a Gaussian laser profile. Using photon echo polarimetry, we distinguish between the coherent optical responses from exciton and trion ensembles. Here, we demonstrate that a photo-induced charging of the quantum dots leads to a significant reduction of the number of neutral quantum dots under resonant excitation with intensive optical pulses with areas exceeding $\pi /2$. The trion ensemble shows robust Rabi rotations when the area of the refocussing pulse is increased up to 5.5$\pi$. We analyze the remaining attenuation of Rabi rotations by theoretical modeling of excitation induced dephasing, inhomogeneity of dipole moments, and coupling to acoustic phonons. The latter is identified as the dominating mechanism resulting in a loss of optical coherence during the action of the involved optical pulses.

Magnetic diffraction in combination with x-ray detected ferromagnetic resonance (DFMR) is a powerful technique for performing time-resolved measurements on individual spin textures. Here, we study the ferromagnetic resonance (FMR) modes of both the conical and field-polarized phases in the chiral magnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. Following the identification of the FMR modes at different temperatures using broadband vector network analyzer FMR, we use DFMR on the crystalline (001) Bragg peak to reveal the time-dependent spin configurations of the selected FMR modes. By being able to measure both the amplitude and phase response of the spin system across the resonance, a continuous phase advance (of ${180}^{\circ }$) in the conical mode and a phase lag (of ${180}^{\circ }$) in the field-polarized mode is found. By performing dynamic measurements in the conical phase as a function of the linear polarization angle of the x rays, i.e., successively probing the dynamics of the moments, we find an inversion of the dynamics along the conical axis upon inverting the applied field direction. By allowing for time-resolved measurements of the phase and amplitude of individual magnetic phases, DFMR opens up new opportunities for obtaining a deeper understanding of the complex dynamics of chiral magnets.

The recent observation of cyclotron resonance in optimally doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. One can measure the cyclotron mass in the more disordered cuprates possessing short scattering times, therefore expanding the study to materials and dopings in which quantum oscillations have not been observed. Here we present the measurement of the carrier mass of the hole-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ across a range of dopings spanning from the slightly underdoped ($p=0.13$) to highly overdoped ($p=0.26$), near the termination of the superconducting dome. These results reveal a systematic increase of $m_c$ with doping, up to values greater than 13 times the bare electron mass. This is in contrast with the masses extracted from the heat capacity, which show a peak near the pseudogap critical point $p^{*}$ and/or Lifshitz transition. The cyclotron frequency is linear in field up to 31 T for all dopings, giving no evidence for field-induced Fermi surface reconstructions. The cyclotron mass is found to be positive for all dopings, but with a magnitude systematically below the heat-capacity mass for under and optimally doped samples, while exceeding it for overdoped samples. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite-frequency resonance is, in conventional theories, a topological quantity only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near $p^{*}$ nor near the Lifshitz transition, showing that any singularity, if it exists, is not strong enough to affect the cyclotron mass.

Unlike conventional two-dimensional (2D) semiconductor superlattices, moiré patterns in 2D materials are flexible and their electronic, magnetic, optical, and mechanical properties depend on their topography. Within a $\text{continuous}+\text{atomistic}$ theory treating 2D materials as crystalline elastic membranes, we abandon the flat-membrane scenario usually assumed for these materials and address out-of-plane deformations. We confront our predictions to experimental analyses on model systems, epitaxial graphene, and ${\mathrm{MoS}}_2$ on metals and reveal that compression/expansion and bending energies stored in the membrane can compete with adhesion energy, leading to a subtle moiré wavelength selection and the formation of wrinkles.

First-principles calculations enable accurate predictions of electronic interactions and dynamics. However, computing the electron spin dynamics remains challenging. The spin-orbit interaction causes various dynamical phenomena that couple with phonons, such as spin precession and spin-flip $e$-ph scattering, which are difficult to describe with current first-principles calculations. In this work, we show a rigorous framework to study phonon-induced spin relaxation and decoherence, by computing the spin-spin correlation function and its vertex corrections due to $e$-ph interactions. We apply this approach to a model system and develop corresponding first-principles calculations of spin relaxation in GaAs. Our vertex-correction formalism is shown to capture the Elliott-Yafet, Dyakonov-Perel, and strong-precession mechanisms—three independent spin decoherence regimes with distinct physical origins—thereby unifying their theoretical treatment and calculation. Our method is general and enables quantitative studies of spin relaxation, decoherence, and transport in a wide range of materials and devices.

Despite being the oldest known superconductor, solid mercury is mysteriously absent from all current computational databases of superconductors. In this Research Letter, we present a critical study of its superconducting properties based on state-of-the-art superconducting density functional theory. Our calculations reveal numerous anomalies in electronic and lattice properties, which can mostly be handled, with due care, by modern ab initio techniques. In particular, we highlight an anomalous role of spin-orbit coupling in the dynamical stability and of semicore $d$ levels in the effective Coulomb interaction and, ultimately, the critical temperature.

We study the interplay of superconductivity and a wide spectrum of critical (multifractal) wave functions (“spectrum-wide quantum criticality,” SWQC) in the one-dimensional Aubry-André and power-law random-banded matrix models with attractive interactions, using self-consistent BCS theory. We find that SWQC survives the incorporation of attractive interactions at the Anderson localization transition, whereas the pairing amplitude is maximized near this transition in both models. Our results suggest that SWQC, recently discovered in two-dimensional topological surface-state and nodal superconductor models, can robustly enhance superconductivity.

Interlayer excitons (IXs) in two-dimensional semiconductors have long lifetimes and spin-valley coupled physics, with a long-standing goal of single-exciton trapping for valleytronic applications. In this work, we use a nanopatterned graphene gate to create an electrostatic IX trap. We measure a unique power-dependent blueshift of IX energy, where narrow linewidth emission exhibits discrete energy jumps. We attribute these jumps to quantized increases of the number occupancy of IXs within the trap and compare to a theoretical model to assign the lowest energy emission line to single-IX recombination.

Broken Galilean invariance in a spin-orbit coupled system can amplify many-body effects on its different responses. We study the anomalous Hall and spin Hall conductivities of a magnetic two-dimensional electron gas with Rashba spin-orbit coupling. We show that both of these conductivities in the intrinsic limit are fully specified in terms of the longitudinal and transverse spin-spin response functions. We include the effect of electron-electron interaction in the spin-spin linear-response functions, going beyond the random-phase approximation. We do this by incorporating the local-field correction in the response functions, which takes into account the many-body exchange-correlation effects. We observe a significant enhancement of the static anomalous Hall conductivity due to the electron-electron interaction. The many-body correction on the spin Hall effect is more nontrivial, and strong electron-electron interaction can even reverse the sign of the static spin Hall conductivity.

The ferrimagnetic insulator ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, which features a Curie temperature $T_c$ at 78 K and a delicate yet consequential magnetic frustration, exhibits colossal magnetoresistance (CMR) when the magnetic field is applied along the magnetic hard axis, surprisingly inconsistent with existing precedents [Y. Ni, H. Zhao, Y. Zhang, B. Hu, I. Kimchi, and G. Cao, Phys. Rev. B 103, L161105 (2021)]. This discovery motivates a thorough single-crystal neutron diffraction study in order to gain insights into the magnetic structure and its hidden correlation with the new type of CMR. Here we report a noncollinear magnetic structure below the $T_c$ where the moments lie predominantly within the basal plane but tilt toward the $c$ axis by $\approx {10}^{\circ }$ at ambient conditions. A substantial magnetic diffuse scattering decays slowly and persists well above the $T_c$. The evolution of the spin correlation lengths agrees well with the electrical resistivity, underscoring the role of spin fluctuation contributing to the magnetoresistivity near the transition. Application of magnetic field along the $c$ axis renders a swift occurrence of CMR but only a slow tilting of the magnetic moments toward the $c$ axis. The unparalleled changes indicate a nonconsequential role of magnetic polarization.

We investigate the integer quantum Hall effect in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin films under conditions of strong to moderate quantum confinement (thicknesses of 10, 12, and 15 nm). In all the films, we observe the integer quantum Hall effect in the spin-polarized lowest Landau level (filling factor $\nu =1$) and at spin-degenerate higher index Landau levels with even filling factors ($\nu =2,4,6$). With increasing quantum confinement, we also observe a lifting of the Landau-level spin degeneracy at $\nu =3$, manifest as the emergence of an anomaly in the longitudinal and Hall resistivities. Tight-binding calculations show that the enhanced $g$ factor likely arises from a combination of quantum confinement and corrections from nearby subbands.

Variational Monte Carlo with neural network quantum states has proven to be a promising avenue for evaluating the ground-state energy of spin Hamiltonians. However, despite continuous efforts the performance of the method on frustrated Hamiltonians remains significantly worse than those on stoquastic Hamiltonians that are sign free. We present a detailed and systematic study of restricted Boltzmann machine (RBM) based variational Monte Carlo for quantum spin chains, resolving how relevant stoquasticity is in this setting. We show that in most cases, when the Hamiltonian is phase connected with a stoquastic point, the complex RBM state can faithfully represent the ground state, and local quantities can be evaluated efficiently by sampling. On the other hand, we identify several new phases that are challenging for the RBM Ansatz, including nontopological robust nonstoquastic phases as well as stoquastic phases where sampling is nevertheless inefficient. We further find that, in contrast to the common belief, an accurate neural network representation of ground states in nonstoquastic phases is hindered not only by the sign structure but also by their amplitudes.

We extend the scope of full configuration interaction quantum Monte Carlo (FCIQMC) to be applied to coupled fermion-boson Hamiltonians, alleviating the a priori truncation in boson occupation which is necessary for many other wave-function-based approaches to be tractable. Detailing the required algorithmic changes for efficient excitation generation, we apply FCIQMC in two contrasting settings. The first is a sign-problem-free Hubbard-Holstein model of local electron-phonon interactions, where we show that, with care to control for population bias via importance sampling and/or reweighting, the method can achieve unbiased energies extrapolated to the thermodynamic limit, without suffering additional computational overheads from relaxing boson occupation constraints. Second, we apply the method as a “solver” within a quantum embedding scheme which maps electronic systems to local electron-boson auxiliary models, with the bosons representing coupling to long-range plasmoniclike fluctuations. We are able to sample these general electron-boson Hamiltonians with ease despite a formal sign problem, including a faithful reconstruction of converged reduced density matrices of the system.

Recent experimental observations of unexpectedly large thermal Hall conductivities ${\kappa }_{\text{H}}$ in insulating materials, including the parent compounds of the high-temperature superconducting cuprates, likely reflect an extrinsic contribution from a yet-to-be-identified extrinsic source of skew scattering of acoustic phonons. We show that resonant scattering of phonons from a certain class of three-level systems produces strong skew scattering in the presence of a modest magnetic field. We interpret this as a first step toward understanding the experiments.

We describe and discuss the low-temperature resistivity (and the temperature-dependent inelastic scattering rate) of several different doped two-dimensional semiconductor systems from the perspective of the Planckian hypothesis asserting that $\hslash /\tau =k_{\mathrm{B}}T$ provides a scattering bound, where $\tau$ is the appropriate relaxation time. The regime of transport considered here is well below the Bloch-Grüneisen regime so that phonon scattering is negligible. The temperature-dependent part of the resistivity is almost linear-in-$T$ down to arbitrarily low temperatures, with the linearity arising from an interplay between screening and disorder, connected with carrier scattering from impurity-induced Friedel oscillations. The temperature dependence disappears if the Coulomb interaction between electrons is suppressed. The temperature coefficient of the resistivity is enhanced at lower densities, enabling a detailed study of the Planckian behavior as a function of both the materials system and the carrier density. Although the precise Planckian bound never holds, we find somewhat surprisingly that the bound seems to apply approximately with the scattering rate never exceeding $k_{\mathrm{B}}T$ by more than an order of magnitude either in the experiment or in the theory. In addition, we calculate the temperature-dependent electron-electron inelastic scattering rate by obtaining the temperature-dependent self-energy arising from Coulomb interaction, also finding it to obey the Planckian bound within an order of magnitude at all densities and temperatures. We introduce the concept of a generalized Planckian bound where $\hslash /\tau$ is bounded by $\alpha k_{\mathrm{B}}T$ with $\alpha \sim 10$ or so in the super-Planckian regime with the strict Planckian bound of $\alpha =1$ being a nongeneric fine-tuned situation.