Motivated by the question of whether all fast scramblers are holographically dual to quantum gravity, we study the dynamics of a nonintegrable spin chain model composed of two ingredients: a nearest neighbor Ising coupling, and an infinite range $XX$ interaction. Unlike other fast scrambling many-body systems, this model is not known to be dual to a black hole. We quantify the spreading of quantum information using an out-of time-ordered correlator (OTOC), and demonstrate that our model exhibits fast scrambling for a wide parameter regime. Simulation of its quench dynamics finds that the rapid decline of the OTOC is accompanied by a fast growth of the entanglement entropy, as well as a swift change in the magnetization. Finally, potential realizations of our model are proposed in current experimental setups. Our work establishes a promising route to create fast scramblers.

We present a general framework, based on Lagrange duality, for computing physical bounds on a wide array of electromagnetic scattering problems. Namely, we show that, via projections into increasingly localized spatial clusters, the central equality of scattering theory—the definition of the $\mathbb{T}$ operator—can be used to generate a hierarchy of increasingly accurate mean-field approximations (enforcing local power conservation) that naturally complement the standard design problem of optimizing some objective with respect to structural degrees of freedom. Utilizing the systematic control over the spatial extent of local violations of physics offered by the approach, proof-of-concept application to maximizing radiative Purcell enhancement for a dipolar current source in the vicinity of a structured medium, an effect central to many sensing and quantum technologies, yields bounds that are often more than an order of magnitude tighter than past results, highlighting the need for a theory capable of accurately handling differing domain and field-localization length scales. Similar to related domain decomposition and multigrid notions, analogous constructions are possible in any branch of wave physics, providing a unified approach for investigating fundamental limits.

By means of time- and angle-resolved photoelectron spectroscopy, we give evidence of a remarkable reduction of the Fermi velocity of out-of-equilibrium Dirac bands in the quasi-two-dimensional semimetal ${\mathrm{BaNiS}}_2$. This effect is accompanied by a nonrigid shift of the bands at the center of the Brillouin zone. Analytical and first-principles calculations indicate that this band renormalization is ascribed to a change in nonlocal electron correlations driven by a photoinduced enhancement of screening properties. Our results are explained by a scaling relation between inverse screening length and electronic temperature that is of general relevance for the description of out-of-equilibrium dynamics and screening effects in all layered Dirac materials.

The behavior of electrons in condensed matter systems is mostly determined by the repulsive Coulomb interaction. However, under special circumstances, the Coulomb interaction can be effectively attractive, giving rise to electron pairing in unconventional superconductors and specifically designed mesoscopic setups. In quantum Hall systems, electron interactions can play a particularly important role due to the huge degeneracy of Landau levels, leading, for instance, to the emergence of quasiparticles with fractional charge and anyonic statistics. Quantum Hall Fabry-Pérot interferometers (FPI) have attracted increasing attention due to their ability to probe such exotic physics. In addition, such interferometers are affected by electron interactions themselves in interesting ways. Recently, experimental evidence for electron pairing in a quantum Hall FPI was found [Choi et al., Nat. Commun. 6, 7435 (2015)]. Theoretically describing an FPI in the limit of strong backscattering and under the influence of a screened Coulomb interaction, we compute electron shot noise and indeed find a two-fold enhanced Fano factor for some parameters, indicative of electron pairing. This result is explained in terms of an electron interaction due to exchange of neutral interedge plasmons, so-called neutralons.

We systematically study the effect of disorder and interactions on a quasi-one-dimensional diamond chain possessing flat bands. Disorder localizes all the single-particle eigenstates, while at low disorder strengths, we obtain weak flat-band based localization (FBL), at high disorder strengths, we see conventional Anderson localization (AL). The compactly localized (CL) eigenstates of flat bands show a persisting oscillatory recurrence in the study of single-particle wave-packet dynamics. For low disorder, a damped oscillatory recurrence behavior is observed which is absent for high disorder. Noninteracting many-particle fermion states also follow the same trend except showing a delocalizing tendency at intermediate disorder due to the fermionic statistics in the system. As interactions are switched on, for the finite sizes that we are able to study, a nonergodic “mixed phase” is observed at low disorder which is separated from the MBL phase at high disorder by a thermal phase at intermediate disorder. A study of many-body nonequilibrium dynamics reinforces these findings.

We report a combined theoretical and experimental investigation of magnetic proximity and Hall transport in Pt/Cr bilayers. Density-functional theory indicates that interfacial magnetic moments can be induced in the Pt layer and a strong magnetocrystalline anisotropy with an out-of-plane easy axis arises in the antiferromagnet. A signal ascribed to the anomalous Hall effect is detected and associated to the interface between Pt and Cr layers. We discuss how this effect originates from the Berry curvature of the electronic structure at the interface and the correlation with the character of the proximity-induced magnetic moments.

We performed neutron resonance spin echo measurements on MnSi to investigate how the magnetic skyrmions crystalize into a lattice on cooling in a magnetic field. We found that the skyrmions aggregate into small skyrmion-lattice (SkL) domains with orientational disorder upon the transition from the paramagnetic phase to the SkL phase. These domains are nearly static in time, and coexist with chiral spin fluctuations with a characteristic relaxation time of ${10}^{-9}\mathrm{s}$ near the transition temperature. As the temperature is further decreased, the SkL domains merge into the long-range-ordered SkL. These observations provide a microscopic picture of the crystallization of the magnetic skyrmions.

Superconductivity is often found in a dome around quantum critical points, i.e., second-order quantum phase transitions. Here, we show that an enhancement of superconductivity is avoided at the critical pressure of the charge-density-wave (CDW) state in $2H\text{−}{\mathrm{NbSe}}_2$. We present comprehensive high-pressure Hall effect and magnetic susceptibility measurements of the CDW and superconducting state in $2H\text{−}{\mathrm{NbSe}}_2$. Initially, the second-order CDW transition is suppressed smoothly but it drops to zero abruptly at $P_{\text{CDW}}=4.4\mathrm{GPa}$ thus indicating a change to first order, while the superconducting transition temperature $T_{\text{c}}$ rises continuously up to $P_{\text{CDW}}$ but is constant above. The putative first-order nature of the CDW transition is suggested as the cause for the absence of a superconducting dome at $P_{\text{CDW}}$. Indeed, we show that the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase, while the initial suppression of the CDW state is accounted for by the stiffening of the underlying bare phonon mode.

We investigate mucosalivary dispersal and deposition on horizontal surfaces corresponding to human exhalations with physical experiments under still-air conditions. Synthetic fluorescence tagged sprays with size and speed distributions comparable to human sneezes are observed with high-speed imaging. We show that while some larger droplets follow parabolic trajectories, smaller droplets stay aloft for several seconds and settle slowly with speeds consistent with a buoyant cloud dynamics model. The net deposition distribution is observed to become correspondingly broader as the source height $H$ is increased, ranging from sitting at a table to standing upright. We find that the deposited mucosaliva decays exponentially in front of the source, after peaking at distance $x=0.71\mathrm{m}$ when $H=0.5\mathrm{m}$, and $x=0.56\mathrm{m}$ when $H=1.5\mathrm{m}$, with standard deviations $\approx 0.5\mathrm{m}$. Greater than 99% of the mucosaliva is deposited within $x=2\mathrm{m}$, with faster landing times further from the source. We then demonstrate that a standard nose and mouth mask reduces the mucosaliva dispersed by a factor of at least a hundred compared to the peaks recorded when unmasked.

Many-body nonequilibrium steady states can be described by a Landau-Ginzburg theory if one allows nonanalytic terms in the potential. We substantiate this claim by working out the case of the Ising magnet in contact with a thermal bath and undergoing stochastic reheating: It is reset to a paramagnet at random times. By a combination of stochastic field theory and Monte Carlo simulations, we unveil how the usual ${\phi }^4$ potential is deformed by nonanalytic operators of intrinsic nonequilibrium nature. We demonstrate their infrared relevance at low temperatures by a renormalization-group analysis of the nonequilibrium steady state. The equilibrium ferromagnetic fixed point is thus destabilized by stochastic reheating and we identify the new nonequilibrium fixed point.

Films produced by assembling bare gold clusters well beyond the electrical percolation threshold show a resistive switching behavior whose investigation has started only recently. Here we address the challenge to charaterize the resistance of a nanogranular film starting from limited information on the structure at the microscopic scale by the means of Bruggeman's approach to multicomponent media, within the framework of effective medium approximations. The approach is used to build a model that proves that the observed resistive switching can be explained by thermally regulated local structural rearrangements.

We theoretically describe a thermoelectric effect that is entirely due to Andreev processes involving the formation of Cooper pairs through the coupling of electrons and holes. The Andreev thermoelectric effect can occur in ballistic ferromagnet-superconductor junctions with a dominant superconducting proximity effect on the ferromagnet, and it is very sensitive to surface states emerging in unconventional superconductors. We consider hybrid junctions in two and three dimensions to demonstrate that the thermoelectric current is always reversed in the presence of low-energy Andreev bound states at the superconductor surface. A microscopic analysis of the proximity-induced pairing reveals that the thermoelectric effect only arises if even- and odd-frequency Cooper pairs coexist in mixed singlet and triplet states. Our results are an example of the richness of emergent phenomena in systems that combine magnetism and superconductivity, and they open a pathway for exploring exotic surface states in unconventional superconductors.

Electromagnetic anapoles are nonradiating oscillations of electric charge. They are known to be formed by a combination of an ordinary and a toroidal multipole. In this work, we introduce a different family of anapoles based on a Cartesian electric-current multipole expansion and propose a way to create the lowest-order, centrosymmetric member of this family. We also find a nontoroidal multipole structure that radiates as an electric dipole despite its zero total dipole moment. Radiation-free charge oscillations are interesting not only in view of their unique fundamental properties, but also in view of practical applications, e.g., in near-field-based optical sensing and detection.

A cat state is a superposition of two coherent states with amplitudes ${\alpha }_0$ and $-{\alpha }_0$. Recent experiments create cat states in a microwave cavity field using superconducting circuits. As with degenerate parametric oscillation (DPO) in an adiabatic and highly nonlinear limit, the states are formed in a signal cavity mode via a two-photon dissipative process induced by the down conversion of a pump field to generate pairs of signal photons. The damping of the signal and the presence of thermal fluctuations rapidly decoheres the state, and the effect on the dynamics is to either destroy the possibility of a cat state or else to sharply reduce the lifetime and size of the cat states that can be formed. In this paper, we study the effect on both the DPO and microwave systems of a squeezed reservoir coupled to the cavity. While the threshold nonlinearity is not altered, we show that the use of squeezed states significantly lengthens the lifetime of the cat states. This improves the feasibility of generating cat states of large amplitudes and with a greater degree of quantum macroscopic coherence, which is necessary for many quantum technology applications. Using current experimental parameters for the microwave setup, which requires a modified Hamiltonian, we further demonstrate how squeezed states enhance the quality of the cat states that could be formed in this regime. Squeezing also combats the significant decoherence due to thermal noise, which is relevant for microwave fields at finite temperature. By modeling a thermal squeezed reservoir, we show that the thermal decoherence of the dynamical cat states can be inhibited by a careful control of the squeezing of the reservoir. To signify the quality of the cat state, we consider different signatures including fringes and negativity, and the $C_{l_1}$ measure of quantum coherence.

We discuss distinctive features of spiral states in bulk chiral magnets such as MnSi and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ that stem from the effect of the cubic magnetocrystalline anisotropy. First of all, at both the helical-to-conical and the conical-to-ferromagnetic transitions, taking place at $H_{c1}$ and $H_{c2}$, respectively, the cubic anisotropy leads to reversible or irreversible jumplike reorientations of the spiral wave vectors. The subtle interplay between the easy and hard anisotropy axes gives rise to a phase transition between elliptically distorted conical states almost without any detectable change in the period. We show that the competition between on-site cubic and exchange anisotropy terms can also lead to oblique spiral states. Our work gives clear directions for further experimental studies to reveal theoretically predicted spiral states in cubic helimagnets beyond the aforementioned well-established states and thus it can help to understand the magnetic phase diagram of these archetypal skyrmion hosts. In addition, we show that properties of isolated skyrmions such as interskyrmion attraction, orientation, and/or nucleation are also rooted in the properties of host spiral states, in which skyrmions are stabilized.

We study unzipping of a complementary RNA-DNA helix applied to an external force, focusing on the force-force correlations. While at the microscopic level these are given by the sequence, the experiment measures effective, macroscopic correlations. The latter are sequence independent, i.e., universal, and constitute the central object of the underlying field theory of disordered systems. Comparing field-theory predictions and the exact solution of a one-dimensional toy model with the experiment, we find an excellent agreement, confirming fundamental theoretical concepts via biologically inspired experiments.

We have found an unusual competition of two frustration mechanisms in the 2D quantum antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$. The key actors are the alternation of single-ion planar anisotropy direction of the individual magnetic ${\mathrm{Co}}^{2+}$ ions, and their arrangement in a distorted triangular lattice structure. In particular, the uniquely oriented Ising-type anisotropy emerges from the competition of easy-plane ones, and for a magnetic field applied along this axis one finds a cascade of five ordered phases at low temperatures. Two of these phases feature magnetization plateaux. The low-field one is supposed to be a consequence of a collinear ground state stabilized by the anisotropy, while the other plateau bears characteristics of an “up-up-down” state exclusive for lattices with triangular exchange patterns. Such coexistence of the magnetization plateaux is a fingerprint of competition between the anisotropy and the geometric frustration in ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$.

Coulomb interaction has important consequences for the physics of quantum spin Hall edge states, weakening the topological protection via two-particle scattering and renormalizing both the velocity and the charge of collective plasmon modes compared to that of free electrons. Despite these effects, interactions remain difficult to quantify. We propose here simple and robust edge resonator geometries to characterize Coulomb interaction by means of high-frequency measurements. They rely on a transmission line approach and take advantage of the impedance mismatch between the edge states and their microwave environment.

The seemingly simple process of inhalation relies on a complex interplay between muscular contraction in the thorax, elastocapillary interactions in individual lung branches, propagation of air between different connected branches, and overall air flow into the lungs. These processes occur over considerably different length and timescales; consequently, linking them to the biomechanical properties of the lungs, and quantifying how they together control the spatiotemporal features of inhalation, remains a challenge. We address this challenge by developing a computational model of the lungs as a hierarchical, branched network of connected liquid-lined flexible cylinders coupled to a viscoelastic thoracic cavity. Each branch opens at a rate and a pressure that is determined by input biomechanical parameters, enabling us to test the influence of changes in the mechanical properties of lung tissues and secretions on inhalation dynamics. By summing the dynamics of all the branches, we quantify the evolution of overall lung pressure and volume during inhalation, reproducing the shape of measured breathing curves. Using this model, we demonstrate how changes in lung muscle contraction, mucus viscosity, and surface tension, and airway wall stiffness—characteristic of many respiratory diseases, including those arising from COVID-19, cystic fibrosis, chronic obstructive pulmonary disease, asthma, and emphysema—drastically alter inhaled lung capacity and breathing duration. Our work, therefore, helps to identify the key factors that control breathing dynamics and provides a way to quantify how disease-induced changes in these factors lead to respiratory distress.

This study provides a phenomenological method to describe the exponential growth, saturation, and decay of coronavirus disease 2019 (COVID-19) deaths and infections via a Monte Carlo approach. The calculations connect Gompertz-type trial distributions of infected people per day with the distribution of deaths adopting two gamma distributions to account for the elapsed time that encompass the incubation and symptom onset to death periods. The analyses include death data from the USA, Brazil, Mexico, the United Kingdom (UK), India, and Russia, which comprise the four countries with the highest number of deaths and the four countries with the highest number of confirmed cases, as of August 07, 2020, according to the World Health Organization webpage. The Gompertz functions were fitted to the data of weekly averaged confirmed deaths per day by mapping the ${\chi }^2$ values. The uncertainties, variances, and covariances of the model parameters were calculated by propagation, taking into account the standard errors of the data for each epidemiological week. The fitted functions for the average deaths per day for the USA and India have an upward trend, with the former having a higher growth rate and quite huge uncertainties. For Mexico, the UK, and Russia, the fits are consistent with a downward-sloping pattern. For Brazil we found a subtle trend down but with significant uncertainties. The USA, UK, and India data showed first peaks with higher growth rates compared with the second ones (4.2, 2.2, and 3.5 times higher, respectively), demonstrating the benefits of nonpharmacological interventions of sanitary measures and social distance flattening the secondary peaks of the pandemic. For the case of the USA, however, a third peak seems quite plausible, most likely related with the recent relaxation policies. Brazil's data are satisfactorily described by two highly overlapped Gompertz functions with similar growth rates, suggesting a two-step process for the pandemic spreading. For the cases of Mexico and Russia single peaks with smoother slopes fitted the data satisfactorily. The 95% confidence intervals for the total number of deaths $(\times {10}^3)$ predicted by the model for August 31, 2020, are 160 to 220, 110 to 130, 59 to 62, 41.3 to 41.4, 54 to 63, and 16.0 to 16.7 for the USA, Brazil, Mexico, the UK, India, and Russia, respectively. Our estimates for the point prevalences of infections are compared with some preliminary data from serological studies and/or model calculations focused on the USA, Brazil, and UK scenarios. The point prevalences and 95% confidence intervals for August 1, 2020, were estimated to be 5.7 (3.9–7.5)%, 8.9 (7.4–10.3)%, 9.3 (8.3–10.3)%, 5.7 (4.5–6.9)%, 0.9 (0.8–1.0%), and 1.2 (1.0–1.3)% for the USA, Brazil, Mexico, the UK, India, and Russia, respectively. The method represents an effective few-parameter framework to estimate the line shape of the infection curves and the uncertainties of the relevant parameters based on the actual death data of a pandemic.