The Zeeman effect lowers the energy of electrons with spin states which are antiparallel to the applied magnetic field but lifts that of spin parallel states. In quantum spin liquids where the spin and charge degrees of freedom are fractionalized, an anomalous Zeeman response may be expected. In the case of spin liquids with a spinon Fermi surface, the threshold energy to excite an electronic state is found to exhibit no Zeeman shift. This is specific to the spinon Fermi surface case. In contrast, other gapped spin liquids are expected to exhibit the standard Zeeman shift at the band edge even though they also exhibit spin-charge fractionalization. When gauge-field fluctuations are included, we find that the Zeeman shift of the electronic states gets affected by the gauge field induced binding. In the electronic density of states spectra, weak gauge binding induces band-edge resonance peaks which exhibit the Zeeman shift in the same direction as that in the standard Zeeman effect, but the shift is reduced as the binding potential increases. With further increase in the binding potential, the resonance becomes true in-gap bound states and eventually the Zeeman shift direction reverses so it is opposite to the standard Zeeman effect. We propose that one can perform spin-polarized scanning tunneling microscope measurements as a test of the spinon Fermi sea ground state in quantum spin liquid candidate materials.

Nematic systems are two component superconductors that break rotational symmetry, but exhibit a mixed symmetry that couples spatial rotations and phase difference rotations. We show that a consequence of this induced spatial anisotropy is mixed normal modes, that is the linear response to a small perturbation of the system about its ground state, generally couples magnetic and condensate degrees of freedom. We will study the effect of mode mixing on the magnetic response of a nematic system as the strength of applied field is increased. In general we show that the coupled modes generate magnetic field perpendicular to the applied field, causing the magnetic response to spontaneously twist direction. We will study this for the Meissner effect with weak fields and also for stronger applied fields, which produce a mixture of skyrmions and composite vortices, forming orientation dependent bound states. We will also calculate the anisotropies of the resulting first and second critical fields $H_{c_1}$ and $H_{c_2}$. The skyrmion lattices for $H_{c_1}\le H\le H_{c_2}$ in nematic superconductors are shown to be structurally complicated, in contrast to the triangular or square vortex lattices in conventional superconductors. For low fields the magnetic response of the system involves a loosely bound collection of parallel skyrmion chains. As the external field is increased the chains attract one another, causing a transition where the unit cell becomes triangular for high applied fields. This unique skyrmion lattice and the magnetic twisting are clear indicators that could be used experimentally to identify materials that exhibit nematic superconductivity. To obtain these results we develop and present a method to find the unit cell of a vortex lattice that can be applied to other kinds of superconducting systems.

The study of entanglement in the symmetry sectors of a theory has recently attracted a lot of attention since it provides better understanding of some aspects of quantum many-body systems. In this paper, we extend this analysis to the case of non-Hermitian models, in which the reduced density matrix ${\rho }_A$ may be nonpositive definite and the entanglement entropy negative or even complex. Here we examine in detail the symmetry-resolved entanglement in the ground state of the non-Hermitian Su-Schrieffer-Heeger chain at the critical point, a model that preserves particle number and whose scaling limit is a $bc$-ghost nonunitary conformal field theory (CFT). By combining bosonization techniques in the field theory and exact lattice numerical calculations, we analytically derive the charged moments of ${\rho }_A$ and $|{\rho }_A|$. From them, we can understand the origin of the nonpositiveness of ${\rho }_A$ and naturally define a positive-definite reduced density matrix in each charge sector, which gives a well-defined symmetry-resolved entanglement entropy. As a by-product, we also obtain the analytical distribution of the critical entanglement spectrum.

Ideal magnetic frustration forms the basis for the emergence of exotic quantum spin states that are entirely nonmagnetic. Such singlet spin states are the defining feature of the Shastry-Sutherland model, and of its faithful materials realization in the quantum antiferromagnet ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$. To address these states on ultrafast timescales, despite their lack of any microscopic order parameter, we introduce a nonlinear magnetophononic mechanism to alter the quantum spin dynamics by driving multiple optical phonon modes coherently and simultaneously. We apply intense terahertz pulses to create a nonequilibrium modulation of the magnetic interactions that breaks the ideal frustration of ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$, such that previously forbidden physics can be driven in a coherent manner. Specifically, this driving populates a purely magnetic excitation, the singlet branch of the two-triplon bound state, by resonance with the difference frequency of two pumped phonons. Our results demonstrate how light-driven phonons can be used for the ultrafast and selective manipulation of interactions in condensed matter, even at frequencies far from those of the pump spectrum, offering valuable additional capabilities for the dynamical control of quantum many-body phenomena.

The ability to identify and characterize homogeneous and inhomogeneous dephasing processes is crucial in solid-state quantum optics. In particular, spectral diffusion leading to line broadening is difficult to evidence when the associated time scale is shorter than the inverse of the photon detection rate. Here, we show that a combination of resonant laser excitation and second-order photon correlations allows us to access such fast dynamics. The resonant laser drive converts spectral diffusion into intensity fluctuations, leaving a signature in the second-order correlation function $g^{\left(2\right)}\left(\tau \right)$ of the scattered light that can be characterized using two-photon coincidences, which simultaneously provides the homogeneous dephasing time. We experimentally implement this method to investigate the fast spectral diffusion of a color center generated by an electron beam in the two-dimensional material hexagonal boron nitride. The $g^{\left(2\right)}\left(\tau \right)$ function of the quantum emitter measured over more than ten orders of magnitude of delay times, at various laser powers, establishes that the color center experiences spectral diffusion at a characteristic time scale of a few tens of microseconds, while emitting Fourier-limited single photons ($T_2/2T_1\sim 1$) between spectral jumps.

The magnetic excitations in ferromagnetic ${\mathrm{SrRuO}}_3$ were studied by inelastic neutron scattering combining experiments on triple-axis and time-of-flight spectrometers with and without polarization analysis. A quadratic spin-wave dispersion with an anisotropy gap describes the low-energy low-temperature response. The magnon dispersion extends to at least 35 meV and there is no direct evidence for a continuum of Stoner excitations below this energy. However, the magnon response is weakened at higher energy. In addition to the anomalous softening of the spin-wave stiffness and of the gap, which is induced by the topology of the Bloch states, the magnon excitations are broadened in energy and this effect increases upon heating.

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report nonresonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3 to 7 K and in magnetic fields up to 35 T.

Quantum pathways inducing coherent lattice dynamics are studied in bismuth (Bi). A crystalline Bi film is excited by femtosecond midinfrared pulses and transient intensity changes on the (111) Bragg reflection are probed by hard x-ray pulses. A fast decrease and coherent oscillations of the diffracted intensity display up to 50% and 10% intensity change, respectively. The oscillation frequency of 2.6 THz is independent of pump intensity. Midinfrared excitation opens different quantum pathways for electron-hole generation, such as field-driven carrier tunneling at the $L$ points, which reduces the crystal symmetry and leads to optical phonon excitation at the $X$ point with the strongest electron-phonon coupling.

High-order sideband generation (HSG), as an analog of the interband processes in high-harmonic generation (HHG) in solids, is a nonperturbative nonlinear optical phenomenon in semiconductors that are simultaneously driven by a relatively weak near-infrared (NIR) laser and a sufficiently strong terahertz (THz) field. We derive an explicit formula for sideband polarization vectors in a prototypical two-band model based on the saddle-point method. Our formula connects the sideband amplitudes with the laser-field parameters, electronic structures, and nonequilibrium dephasing rates in a highly nontrivial manner. Our results indicate the possibility of extracting information on band structures and dephasing rates from high-order sideband generation experiments with simple algebraic calculations. We also expect our approach to be useful on the quantitative understanding of the interband HHG.

We consider a model of a Bloch band subjected to an oscillating electric field and coupled to a featureless fermionic heat bath, which can be solved exactly. We demonstrate rigorously that in the limit of vanishing coupling to this bath (so that it acts as an ideal thermodynamic bath) the occupation of the Floquet band is not a simple Fermi-Dirac distribution function of the Floquet energy, but instead it becomes a “staircase” version of this distribution. We show that this distribution generically leads to a finite rectified electric current within the optical gap of a metal even in the limit of vanishing carrier relaxation rates, providing a rigorous demonstration that such rectification is generically possible and clarifying previous statements in the optoelectronics literature. We show that this current remains nonzero even up to the leading perturbative second order in the amplitude of electric fields, and that it approaches the standard perturbative expression of the Jerk current obtained from a simpler Boltzmann description within a relaxation time approximation when the frequencies are small compared to the bandwidth.

Phonon-based approaches and molecular dynamics are widely established methods for gaining access to a temperature-dependent description of material properties. However, when a compound's phase space is vast, density-functional-theory-backed studies quickly reach prohibitive levels of computational expense. Here, we explore the complex phase structure of ${\mathrm{HfO}}_2$ using effective harmonic potentials based on a neural-network force field (NNFF) as a surrogate model. We detail the data acquisition and training strategy that enable the NNFF to provide almost ab-initio accuracy at a significantly reduced cost and present a recipe for automation. We demonstrate how the NNFF can generalize beyond its training data and that it is transferable between several phases of hafnia. We find that the thermal expansion coefficient of the low-symmetry phases agrees well with experimental results, and we determine the $P\overline{4}3m$ phase to be the favorable (stoichiometric) cubic phase over the established $Fm\overline{3}m$. In contrast, the experimental lattice constants of the cubic phases are substantially larger than what is calculated for the corresponding stoichiometric phases. Furthermore, we show that the stoichiometric cubic phases are unlikely to be thermodynamically stable compared to the tetragonal and monoclinic phases and hypothesize that they exist only in defect-stabilized forms.

We experimentally explore an easy-plane dominant magnetic tunnel junction's thermal stochastic fluctuation for possible applications in physical random number generation for computing. We use materials and fabrication methods similar to those in spin-transfer-torque based magnetic random access memory technology. This work provides an experiment-based view of the device properties that could be optimized for use as an entropy source. We illustrate some relevant device-level metrologies and show several materials and device-physics related factors one could further investigate. Those include the role of a combined strong easy-plane and a weak in-plane anisotropy on fluctuation, the combined stochastic dynamics of the free and reference layer of such magnetic tunnel junctions under bias-voltage induced spin-current drive, and the dynamics' dependence on bias voltage, on magnetic field, and on device and materials parameters in need of control. These observations provide a base-line view of such stochastic tunnel junctions for future applications-specific optimization.

It is well known that there are two types of magnetic bubbles in uniaxial magnets. Here, using Lorentz-transmission electronic microscopy magnetic imaging, we report the direct experimental observation of three-dimensional (3D) type-III hybrid bubbles, which comprise Néel-twisted skyrmion bubbles with topological charge $Q=-1$ in near-surface layers and type-II bubbles with $Q=0$ in interior layers, in ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ nanodisks. Using the tilted magnetic field, we further show the controlled topological magnetic transformations of three types of bubbles in a confined ferromagnetic nanodisk. Our observations are well reproduced using micromagnetic simulations based on measured magnetic parameters. Our results advance fundamental classification and understanding of magnetic bubbles, which could propel the applications of 3D magnetism.

We theoretically investigate the interaction between magnons and a skyrmion-textured domain wall in a two-dimensional antiferromagnet and elucidate the resultant properties of magnon transport. Using supersymmetric quantum mechanics, we solve the scattering problem of magnons on top of the domain wall and obtain the exact solutions of propagating and bound magnon modes. Then, we find their properties of reflection and refraction in the skyrmion-textured domain wall, where magnons experience an emergent magnetic field due to their nontrivial spin texture-induced effective gauge field. Based on the obtained scattering properties of magnons and the domain wall, we show that the thermal transport decreases as the domain wall's chirality increases. Our results suggest that the thermal transport of an antiferromagnet is tunable by modulating the skyrmion charge density of the domain wall, which might be useful for realizing electrically tunable spin caloritronic devices.

Quantum many-body systems with fracton constraints are widely conjectured to exhibit unconventional low-energy phases of matter. In this paper, we demonstrate the existence of a variety of such exotic quantum phases in the ground states of a dipole-moment conserving Bose-Hubbard model in one dimension. For integer boson fillings, we perform a mapping of the system to a model of microscopic local dipoles, which are composites of fractons. We apply a combination of low-energy field theory and large-scale tensor network simulations to demonstrate the emergence of a dipole Luttinger liquid phase. At noninteger fillings our numerical approach shows an intriguing compressible state described by a quantum Lifshitz model in which charge density-wave order coexists with dipole long-range order and superfluidity—a “dipole supersolid”. While this supersolid state may eventually be unstable against lattice effects in the thermodynamic limit, its numerical robustness is remarkable. We discuss potential experimental implications of our results.

We study the quantum phase diagram of a Bose-Hubbard chain whose dynamics conserves both a boson number and boson dipole moment, a situation which can arise in strongly tilted optical lattices. The conservation of the dipole moment has a dramatic effect on the phase diagram, which we analyze by combining a field theory analysis with DMRG simulations. In the thermodynamic limit, the phase diagram is dominated by various types of incompressible dipolar condensates. In finite-sized systems, however, it may be possible to stabilize a Bose-Einstein insulator: an exotic compressible phase which is insulating, despite the absence of a charge gap. We suggest several ways by which these exotic phases can be identified in near-term cold-atom experiments.

We present a first-principles theoretical study employing nonlinear response theory to investigate the dc photocurrent generated by linearly polarized light in the type-II Weyl semimetal ${\mathrm{TaIrTe}}_4$. We report the low-energy spectrum of several nonlinear optical effects. At second order, we consider the shift and injection currents. Assuming the presence of a built-in static electric field, at third order we study the current-induced shift and injection currents, as well as the jerk current. We discuss our results in the context of a recent experiment measuring an exceptionally large photoconductivity in this material [J. Ma et al., Nat. Mater. 18, 476 (2019)]. According to our results, the jerk current is the most likely origin of the large response. Finally, we propose means to discern the importance of the various mechanisms involved in a time-resolved experiment.

We consider the origin of superconductivity found recently in Bernal bilayer graphene at the onset of isospin-polarized order, trying to infer the pairing mechanism and superconducting order from the measurements available to date. The superconductivity is induced by a parallel magnetic field and persists well above the Pauli limit, indicating an unconventional scenario of quantum-critical pairing, where soft fluctuations of isospin give rise to spin-triplet superconductivity. We consider the scenario in which the pairing interaction is entirely repulsive, which stands in contrast to the typical quantum-critical pairing mechanisms. Superconductivity emerges through a “transformer” mechanism where, in the presence of an in-plane magnetic field, the incipient valley polarization converts a frequency-independent repulsion into one with a strong nonmonotonic frequency dependence. Such an interaction enables a nonzero solution for the pairing gap function that changes sign as a function of frequency. The same mechanism holds at zero field in the presence of spin-orbit coupling, providing a likely explanation for the recently observed superconductivity in bilayer graphene on the ${\mathrm{WSe}}_2$ monolayer.

We use piecewise-linear functionals to study the polaron energy landscape and hopping rates in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, which we adopt as an example of an anisotropic material hosting multiple polaronic states. We illustrate various functionals for polaron localization, including a hybrid functional and two types of semilocal functionals, and discuss how to ensure the piecewise-linearity condition. Then, we determine the formation energies of stable polarons, and show that single-site and multisite polaronic states can be found in close energetic competition. We calculate the hyperfine and superhyperfine parameters associated with each polaron, and discuss the comparison with experiment. Next, we perform nudged-elastic-band calculations to determine energy landscapes and hole transfer rates of all first-nearest-neighbor polaron hoppings. We show that when the piecewise-linearity condition is ensured polaron properties are robust upon variation of the functional adopted, including formation energies, energy barriers, and charge transfer rates. This supports the use of semilocal functionals for calculating polaron transport properties.

The electrocaloric effect (ECE) offers a promising alternative to the traditional gas compressing refrigeration due to its high efficiency and environmental friendliness. The unusual inverse (negative) electrocaloric effect refers to the adiabatic temperature drops due to the application of electric field, in contrast with the normal (positive) ECE, and provides ways to reach a larger temperature span in refrigeration cycles by combining the normal and inverse ECE. However, the inverse ECE is unusual and requires a clear understanding of microscopic mechanisms. Here, we found the unexpected and extensive inverse ECE in nonpolar orthorhombic, tetragonal, and cubic phases of halide and oxide perovskite at a wide temperature range by means of first-principle-based large-scale Monte Carlo methods. Such an unexpected inverse ECE originates from the octahedral tilting related entropy change rather than the polarization entropy change under the application of electric field. Furthermore, a giant inverse ECE with temperature change of 8.6 K is found at room temperature. This giant and extensive inverse ECE in perovskite opens up a new horizon in the research of caloric effects and broadens the electrocaloric refrigeration ways with high efficiency.