We study resonant dipole-dipole coupling and the associated van der Waals energy shifts in Rydberg excited atomic rubidium and potassium and investigate Förster resonances between interspecies pair states. A comprehensive survey over experimentally accessible pair state combinations reveals multiple candidates with small Förster defects. We crucially identify the existence of an ultrastrong, “low” electric field K-Rb Förster resonance with an extremely large zero-field crossover distance exceeding $100\mu \mathrm{m}$ between the van der Waals regime and the resonant regime. This resonance allows for a strong interaction over a wide range of distances and by investigating its dependence on the strength and orientation of external fields we show this to be largely isotropic. As a result, the resonance offers a highly favorable setting for studying long-range resonant excitation transfer and entanglement generation between atomic ensembles in a flexible geometry. The two-species K-Rb system establishes a unique way of realizing a Rydberg single-photon optical transistor with a high input photon rate and we specifically investigate an experimental scheme with two separate ensembles.

The formation of nanosized spin misalignment can be observed in pure Fe processed via high-pressure torsion (HPT) straining. The magnetic field dependence of the small-angle neutron-scattering profiles indicates that spin misalignment is conserved in magnetic fields up to 10 T. This result demonstrates that HPT straining provides anomalous magnetic anisotropy in pure Fe due to the high densities of the grain boundaries and lattice defects.

The dynamics of low-energy charge carriers in a graphene quantum dot subjected to a time-dependent local field is investigated numerically. In particular, we study a configuration where a Coulomb electric field is provided by an ion traversing the graphene sample. A Galerkin-type numerical scheme is introduced to solve the massless Dirac equation describing charge carriers subjected to space- and time-dependent electromagnetic potentials and is used to evaluate the field-induced interband transitions. It is demonstrated that as the ion goes through graphene, electron-hole pairs are generated dynamically via the adiabatic pair creation mechanism around avoided crossings, similar to electron-positron pair generation in low-energy heavy-ion collisions.

We report the observation of strongly ferromagnetic $F=1$ spinor Bose-Einstein condensates of ${}^7\mathrm{Li}$ atoms. The condensates are generated in an optical dipole trap without using magnetic Feshbach resonances, so that the condensates have internal spin degrees of freedom. Studying the nonequilibrium spin dynamics, we have measured the ferromagnetic spin interaction energy and determined the $s$-wave scattering length difference among total spin $f$ channels to be $a_{f=2}-a_{f=0}=-18\left(3\right)$ Bohr radius. This strong collision-channel dependence leads to a large variation in the condensate size with different spin composition. We were able to excite a radial monopole mode after a spin-flip transition between the $|m_F=0\rangle$ and $|m_F=1\rangle$ spin states. From the experiments, we estimated the scattering length ratio $a_{f=2}/a_{f=0}=0.27\left(6\right)$, and determined $a_{f=2}=7\left(2\right)$ and $a_{f=0}=25\left(5\right)$ Bohr radii, respectively. The results indicate the spin-dependent interaction energy of our system is as large as $46\%$ of the condensate chemical potential.

Efficient heat dissipation to the substrate is crucial for optimal device performance in nanoelectronics. We develop a theory of electronic thermal boundary conductance (TBC) mediated by remote phonon scattering for the single-layer transition metal dichalcogenide (TMD) semiconductors $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{S}}_2$, and model their electronic TBC with different dielectric substrates ($\mathrm{Si}{\mathrm{O}}_2$, $\mathrm{Hf}{\mathrm{O}}_2$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$). Our results indicate that the electronic TBC is strongly dependent on the electron density, suggesting that it can be modulated by the gate electrode in field-effect transistors, and this effect is most pronounced with ${\mathrm{Al}}_2{\mathrm{O}}_3$. Our work paves the way for the design of novel thermal devices with gate-tunable cross-plane heat-dissipative properties.

Particle flows injected as beams and scattered by an intruder are numerically studied. We find a crossover of the drag force from Epstein's law to Newton's law, depending on the ratio of the speed to the thermal speed. These laws can be reproduced by a simple analysis of a collision model between the intruder and particle flows. The crossover from Epstein's law to Stokes' law is also found for the low-speed regime as the time evolution of the drag force caused by beam particles. We also show the existence of turbulentlike behavior of the particle flows behind the intruder with the aid of the second invariant of the velocity gradient tensor and the relative mean-square displacement for the high-speed regime and a large intruder.

We present a combined experimental and theoretical study of the charge transport properties of $\mathrm{Ti}{\mathrm{H}}_x\left(x=1.6\text{–}2.0\right)$ epitaxial thin films. We found that the Hall coefficient of $\mathrm{Ti}{\mathrm{H}}_x$ strongly depends on hydrogen content and unit-cell volume: Nearly stoichiometric $\mathrm{Ti}{\mathrm{H}}_x\left(x\approx 2.0\right)$ films with large unit-cell volumes showed positive Hall coefficients at 4 K, whereas $\mathrm{Ti}{\mathrm{H}}_x$ samples with $x<\sim 1.7$ and small unit-cell volumes showed negative Hall coefficients at 4 K. Our density functional theory calculations reveal that the volume change leads to the change in the aspect ratio of the tetragonal lattice, thereby lifting the degeneracy of Ti $t_{2g}$ states, and alters the contributions of electrons and holes at the Fermi surface and the sign of the Hall coefficient. The present study clarifies the important role of the lattice symmetry in determining the charge carrier polarity, and we suggest that electronic properties of metal hydrides can be tuned by the lattice parameters via the hydrogen contents.

Quantum spin liquids are strongly correlated phases of matter displaying a highly entangled ground state. Because of their unconventional nature, finding experimental signatures of these states has proven to be a remarkable challenge. Here we show that the effects of local impurities can provide strong signatures of a Dirac quantum spin-liquid state. Focusing on a gapless Dirac quantum spin-liquid state as realized in $\mathrm{Na}\mathrm{Yb}{\mathrm{O}}_2$, we show that a single magnetic impurity coupled to the quantum spin-liquid state creates a resonant spinon peak at zero frequency, coexisting with the original Dirac spinons. We explore the spatial dependence of this zero-bias resonance and show how different zero modes stemming from several impurities interfere. We finally address how such spinon zero-mode resonances can be experimentally probed with inelastic spectroscopy and electrically driven paramagnetic resonance with scanning tunnel microscopy. Our results put forward impurity engineering as a means of identifying Dirac quantum spin liquids with scanning probe techniques, highlighting the dramatic impact of magnetic impurities in a macroscopically entangled many-body ground state.

We develop the framework of probability estimation for certifying randomness with respect to classical side information from a sequence of Bell-test or other randomness-generating trials. The framework is based on directly estimating the probability of measurement outcomes conditional on settings choices and classical side information with adaptive test supermartingales. Accordingly, the number of trials needs not to be predetermined, and one can stop performing trials early, as soon as the desired amount of randomness is extractable. It can be used with arbitrary, partially known and time-varying probabilities for the random settings choices. It can also adapt to other time-varying experimental parameters. Furthermore, it is suitable for application to experiments with low Bell violation per trial, such as current optical loophole-free Bell tests. Compared with our previous work [Phys. Rev. A 98 040304(R) (2018)], here we formulate the framework for the general situation where the randomness can be extracted from a sequence of private data determined in an arbitrary way by the measurement outcomes of the trials. Trial-wise probability estimators can be adapted using all accessible, private information in addition to the results of previous trials. We prove that probability estimation achieves the asymptotically optimal rate for certified randomness generation and makes possible the exponential expansion of settings entropy. We implement probability estimation numerically and apply it to a representative settings-conditional distribution of the measurement outcomes from an atomic loophole-free Bell test [W. Rosenfeld et al., Phys. Rev. Lett. 119, 010402 (2017)] to illustrate trade-offs between the amount of randomness, error, settings entropy, adversarial settings bias, and number of trials. We then show that probability estimation yields more randomness from the optical loophole-free Bell-test data analyzed in [P. Bierhorst et al., arXiv:1702.05178v1] and tolerates adversarial settings biases.

A number of problems in physics, mathematics, and philosophy involve observers in given situations which lead to debates about whether observer-specific information should affect the probability for some outcome or hypothesis. Our purpose is not to advocate for such observer selection effects but rather to show that any such effects depend greatly on the assumptions made. We focus on the debate about the existence of a “doomsday effect”—whether observer index information should cause one to favor possibilities with fewer observers, which has been argued to have implications for models of cosmology. Our central goal is to reconcile the apparent inconsistencies in the literature by introducing a formalism to lay bare assumptions made and address a key issue that has not been clearly articulated in such problems: whether the observer is selected by picking from or being in a set of worlds. In the former there generally are observer selection effects, and in the latter there generally are not. This leads us to differentiate what we call inclusive from exclusive selection and how they relate to the concept of a multiverse. Then we relax the assumption that all observers are equally typical and consider the problem of Boltzmann brains, showing that typicality can play a role in solving the problem. We then stress the need for scale-invariant questions, which causes us to analyze J. Richard Gott's approach to the problem. This all allows us to analyze the doomsday and universal doomsday arguments. We find that there is no doomsday effect, absent a set of assumptions we find somewhat unreasonable. Then we use our formalism to resolve a debate in the philosophy community called the “Sleeping Beauty problem.” Finally, we conclude with a heuristic summary, free from equations, and point to possible future directions of this line of research.

We establish a connection between quantum phase transitions (QPTs) and energy band theory in an extended Dicke-Hubbard lattice, where the periodical critical curves modulated by wave number $k$ leads to rich equilibrium dynamics. Interestingly, the chiral-symmetry-protected flat band and the localization that it engenders exclusively occurs in the normal phase and disappears in the superradiant phase. This originates from the QPT induced simultaneous breaking up of the on-site resonance condition and off-site chiral symmetry of the system, which prohibits the destructive interference for obtaining a flat band. Our work offers an approach to identify different phases of the lattice via detecting the flat bands or simply the related localizations in a single cell, and, in turn, to control the appearance of flat bands by QPT.

We report discovery of antiferromagnetic semimetal ${\mathrm{EuZnSb}}_2$, obtained and studied in the form of single crystals. Electric resistivity, magnetic susceptibility, and heat capacity indicate antiferromagnetic order of Eu with $T_N$ = 20 K. The effective moment of ${\mathrm{Eu}}^{2+}$ inferred from the magnetization and specific heat measurement is 3.5 ${\mu }_B$, smaller than the theoretical value of ${\mathrm{Eu}}^{2+}$ due to presence of both ${\mathrm{Eu}}^{3+}$ and ${\mathrm{Eu}}^{2+}$. Magnetic-field-dependent resistivity measurements suggest dominant quasi-two-dimensional Fermi surfaces whereas the first-principle calculations point to the presence of Dirac fermions. Therefore, ${\mathrm{EuZnSb}}_2$ could represent the first platform to study the interplay of dynamical charge fluctuations, localized magnetic $4f$ moments, and Dirac states with Sb orbital character.

We study the heating dynamics of a generic one-dimensional critical system when driven quasiperiodically. Specifically, we consider a Fibonacci drive sequence comprising the Hamiltonian of uniform conformal field theory (CFT) describing such critical systems and its sine-square deformed counterpart. The asymptotic dynamics is dictated by the Lyapunov exponent which has a fractal structure embedding Cantor lines where the exponent is exactly zero. Away from these Cantor lines, the system typically heats up fast to infinite energy in a nonergodic manner where the quasiparticle excitations congregate at a small number of select spatial locations resulting in a buildup of energy at these points. Periodic dynamics with no heating for physically relevant timescales is seen in the high-frequency regime. As we traverse the fractal region and approach the Cantor lines, the heating slows enormously and the quasiparticles completely delocalize at stroboscopic times. Our setup allows us to tune between fast and ultraslow heating regimes in integrable systems.

We present a theoretical study of the structure and functionality of ferroelastic domain walls in tungsten trioxide, ${\mathrm{WO}}_3$. ${\mathrm{WO}}_3$ has a rich structural phase diagram, with the stability and properties of the various structural phases strongly affected both by temperature and by electron doping. The existence of superconductivity is of particular interest, with the underlying mechanism as of now not well understood. In addition, reports of enhanced superconductivity at structural domain walls are particularly intriguing. Focusing specifically on the orthorhombic $\beta$ phase, we calculate the structure and properties of the domain walls both with and without electron doping. We use two theoretical approaches: Landau-Ginzburg theory, with free energies constructed from symmetry considerations and parameters extracted from our first-principles density functional calculations, and direct calculation using large-scale, GPU-enabled density functional theory. We find that the structure of the $\beta$-phase domain walls resembles that of the bulk tetragonal ${\alpha }_1$ phase, and that the electronic charge tends to accumulate at the walls. Motivated by this finding, we perform ab initio computations of electron-phonon coupling in the bulk ${\alpha }_1$ structure and extract the superconducting critical temperatures, $T_c$, within Bardeen-Cooper-Schrieffer theory. Our results provide insight into the experimentally observed unusual trend of decreasing $T_c$ with increasing electronic charge carrier concentration.

We introduce a quantum spin-1/2 model with many-body correlated Heisenberg-type interactions on the two-dimensional square lattice, designed so that the system can host a fourfold degenerate plaquette valence-bond solid (PVBS) ground state that spontaneously breaks ${\mathbb{Z}}_4$ symmetry. The system is sign-problem free and amenable to large-scale quantum Monte Carlo simulations, thus allowing us to carry out a detailed study of the quantum phase transition between the standard Néel antiferromagnetic (AFM) and PVBS states. We find a first-order transition, in contrast to previously studied continuous transitions from the AFM phase into a columnar valence-bond solid (CVBS) phase. The theory of deconfined quantum criticality predicts generic continuous AFM-CVBS and AFM-PVBS transitions, and, in one version of the theory, the two critical order parameters transform under SO(5) symmetry. Emergent SO(5) symmetry has indeed been observed in studies of the AFM-CVBS transition, and here we show that the first-order AFM-PVBS transition also exhibits SO(5) symmetry at the transition point. Such unexpected symmetry of the coexistence state, which implies a lack of energy barriers between the coexisting phases, has recently been observed at other first-order transitions, but the case presented here is the first example with SO(5) symmetry. The extended symmetry may indicate that the transition is connected to a deconfined critical point. We also discuss the first-order transition in the context of a recent proposal of spinons with fracton properties in the PVBS state, concluding that the fracton scenario is unlikely. Furthermore, we discover a novel type of eightfold degenerate VBS phase, arising when the PVBS state breaks a remaining ${\mathbb{Z}}_2$ symmetry. This second phase transition, which is continuous, implies that the PVBS phase can be regarded as an intermediate “vestigial” phase, a concept recently introduced to describe multistage phase transitions involving a continuous symmetry. Here we construct a six-dimensional order parameter and also introduce a general graph-theoretic approach to describe the two-stage discrete symmetry breaking. We discuss different ways of breaking the symmetries in one or two stages at zero and finite temperatures. In the latter case, we observe fluctuation-induced first-order transitions, which are hallmarks of vestigial phase transitions. We also mention possible connections of the AFM-PVBS transition to the SO(5) theory of high-$T_{\mathrm{c}}$ superconductivity.

We propose that quantum Hall bilayers in the presence of a periodic potential at the scale of the magnetic length can host examples of a deconfined metal-insulator transition (DMIT), where a Fermi-liquid (FL) metal with a generic electronic Fermi surface evolves into a gapped insulator (or an insulator with Goldstone modes) through a continuous quantum phase transition. The transition can be accessed by tuning a single parameter, and its universal critical properties can be understood using a controlled framework. At the transition, the two layers are effectively decoupled, where each layer undergoes a continuous transition from a FL to a generalized composite Fermi liquid (gCFL). The thermodynamic and transport properties of the gCFL are similar to the usual CFL, while its spectral properties are qualitatively different. The FL-gCFL quantum critical point hosts a sharply defined Fermi surface without long-lived electronic quasiparticles. Immediately across the transition, the two layers of gCFL are unstable to forming an insulating phase. We discuss the topological properties of the insulator and various observable signatures associated with the DMIT.

Many experiments in twisted bilayer graphene (TBG) differ from each other in terms of the details of their phase diagrams. Few controllable aspects aside, this discrepancy is largely believed to arise from the presence of a varying degree of twist angle inhomogeneity across different samples. Real-space maps indeed reveal TBG devices splitting into several large domains of different twist angles. Motivated by these observations, we study the quantum mechanical tunneling across a domain wall (DW) that separates two such regions. We show that the tunneling of the moiré particles can be understood by the formation of an effective step potential at the DW. The height of this step potential is simply a measure of the difference in twist angles. These computations lead us to identify the global transport signatures for detecting and quantifying the local twist angle variations. In particular, using Landauer-Büttiker formalism, we compute single-channel conductance ($dI/dV$) and Fano factor for shot noise (ratio of noise power and mean current). A reduction in the low-energy conductance and a zero-bias sub-meV gap in the conductance are observed which scale with the twist angle difference. One of the key findings of our work is that transport in presence of twist angle inhomogeneity is “noisy,” though sub-Poissonian. In particular, the differential Fano factor peaks near the van Hove energies corresponding to the domains in the sample. The location and the strength of the peak are simply a measure of the degree of twist angle inhomogeneity.

Medium- or large-amplitude oscillatory shear (MAOS and LAOS, respectively) is sensitive to polymer chain structure, yet poses unsolved challenges for a priori structural characterization. We present a MAOS protocol applied to near-monodisperse linear polymer melts, from which chain-stretch relaxation, a key structural feature, is discernible. The third harmonics of MAOS frequency sweeps are decomposed into real and imaginary components and found to obey time-temperature superposition. Significantly, these third harmonic features occur at low frequency and are readily accessible with standard rheometers. For materials where phase transitions restrict the use of time temperature superposition, this method has potential to greatly increase the scope of rotational rheometry for structural analysis of polymers. However, the relationship between MAOS data and characteristic relaxation times is complex, and to elucidate this, a modeling approach is required. The GLaMM molecular tube-based model of linear entangled melt rheology and structure, which has no free parameters, closely follows the form of our experimental results for the third harmonics and contains discriminatory features which depend only on the polymer's chain stretch relaxation time. However, we find fundamental differences in magnitude and the frequency dependence of the third harmonics which must be resolved in order to fully understand the molecular basis of the stress response and quantitatively study chain stretch.

We employ a recently developed quantization scheme for quasinormal modes (QNMs) to study a nonperturbative open cavity–QED system consisting of a hybrid metal-dielectric resonator coupled to a quantum emitter. This hybrid cavity system allows one to explore the complex coupling between a low-$Q$ (quality factor) resonance and a high-$Q$ resonance, manifesting in a striking Fano resonance, an effect that is not captured by traditional quantization schemes using normal modes or a Jaynes-Cummings (JC)–type model. The QNM quantization approach rigorously includes dissipative coupling between the QNMs and is supplemented with generalized input-output relations for the output electric field operator for multiple modes in the system and correlation functions outside the system. The role of the dissipation-induced mode coupling is explored in the strong coupling regime between the photons and emitter beyond the first rung of the JC dressed-state ladder. Important differences in the quantum master equation and input-output relations between the QNM quantum model and phenomenological dissipative JC models are found. For the hybridized high-$Q$ cavity mode, we show how the dissipation-induced coupling causes a significant reduction in the cavity-emitter coupling rate, and the cavity decay rate, compared to a simpler JC model. In a second step, numerical results for the Fock distributions and system as well as output correlation functions obtained from the quantized QNM model for the hybrid structure are compared with results from a phenomenological approach. We demonstrate explicitly how the quantized QNM model manifests in multiphoton quantum correlations beyond what is predicted by the usual JC models.

Given two pairs of quantum states, a fundamental question in the resource theory of asymmetric distinguishability is whether there exists a quantum channel converting one pair to the other. In this work, we reframe this question in such a way that a catalyst can be used to help perform the transformation, with the only constraint on the catalyst being that its reduced state is returned unchanged, so that it can be used again to assist a future transformation. What we find here, for the special case in which the states in a given pair are commuting, and thus quasiclassical, is that this catalytic transformation can be performed if and only if the relative entropy of one pair of states is larger than that of the other pair. This result endows the relative entropy with a fundamental operational meaning that goes beyond its traditional interpretation in the setting of independent and identical resources. Our finding thus has an immediate application and interpretation in the resource theory of asymmetric distinguishability, and we expect it to find application in other domains.