In experimentally realistic situations, quantum systems are never perfectly isolated and the coupling to their environment needs to be taken into account. Often, the effect of the environment can be well approximated by a Markovian master equation. However, solving this master equation for quantum many-body systems becomes exceedingly hard due to the high dimension of the Hilbert space. Here we present an approach to the effective simulation of the dynamics of open quantum many-body systems based on machine-learning techniques. We represent the mixed many-body quantum states with neural networks in the form of restricted Boltzmann machines and derive a variational Monte Carlo algorithm for their time evolution and stationary states. We document the accuracy of the approach with numerical examples for a dissipative spin lattice system.

We present a general variational approach to determine the steady state of open quantum lattice systems via a neural-network approach. The steady-state density matrix of the lattice system is constructed via a purified neural-network Ansatz in an extended Hilbert space with ancillary degrees of freedom. The variational minimization of cost functions associated to the master equation can be performed using a Markov chain Monte Carlo sampling. As a first application and proof of principle, we apply the method to the dissipative quantum transverse Ising model.

We show numerically that a three-dimensional model for structural glass displays aging, rejuvenation, and memory effects when subjected to a temperature cycle. These effects indicate that the free energy landscape of structural glasses may possess the complex hierarchical structure that characterizes materials such as spin and polymer glasses. We use the theoretical concept of marginal stability to interpret our results, and explain in which physical conditions a complex aging dynamics can emerge in dense supercooled liquids, paving the way for future experimental studies of complex aging dynamics in colloidal and granular glasses.

Energy-efficient and environment-friendly elastocaloric refrigeration, which is a promising replacement of the conventional vapor-compression refrigeration, requires extraordinary elastocaloric properties. Hitherto the largest elastocaloric effect is obtained in small-size films and wires of the prototype NiTi system. Here, we report a colossal elastocaloric effect, well exceeding that of NiTi alloys, in a class of bulk polycrystalline NiMn-based materials designed with the criterion of simultaneously having large volume change across phase transition and good mechanical properties. The reversible adiabatic temperature change reaches a strikingly high value of 31.5 K and the isothermal entropy change is as large as $45\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$. The achievement of such a colossal elastocaloric effect in bulk polycrystalline materials should push a significant step forward towards large-scale elastocaloric refrigeration applications. Moreover, our design strategy may inspire the discovery of giant caloric effects in a broad range of ferroelastic materials.

Thin film flexoelectricity is attracting more attention because of its enhanced effect and potential application in electronic devices. Here we find that a mechanical bending induced flexoelectricity significantly modulates the electrical transport properties of the interfacial two-dimensional electron gas (2DEG) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ ($\mathrm{LAO}/\mathrm{STO}$) heterostructure. Under variant bending states, both the carrier density and mobility of the 2DEG are changed according to the flexoelectric polarization direction, showing an electric field effect modulation. By measuring the flexoelectric response of LAO, it is found that the effective flexoelectricity in the LAO thin film is enhanced by 3 orders compared to its bulk. These results broaden the horizon of study on the flexoelectricity effect in the hetero-oxide interface and more research on the oxide interfacial flexoelectricity may be stimulated.

We investigate the dynamics of quantum entanglement after a global quench and uncover a qualitative difference between the behavior of the von Neumann entropy and higher Rényi entropies. We argue that the latter generically grow sub-ballistically, as $\propto \sqrt{t}$, in systems with diffusive transport. We provide strong evidence for this in both a U(1) symmetric random circuit model and in a paradigmatic nonintegrable spin chain, where energy is the sole conserved quantity. We interpret our results as a consequence of local quantum fluctuations in conserved densities, whose behavior is controlled by diffusion, and use the random circuit model to derive an effective description. We also discuss the late-time behavior of the second Rényi entropy and show that it exhibits hydrodynamic tails with three distinct power laws occurring for different classes of initial states.

Stimulated Raman adiabatic passage (stirap) allows efficiently transferring the populations between two discrete quantum states and has been used to prepare molecules in their rovibrational ground state. In realistic molecules, a well-resolved intermediate state is usually selected to implement the resonant stirap. Because of the complex molecular level structures, the detuned stirap always coexists with the resonant stirap and may cause unexpected interference phenomenon. However, it is generally accepted that the detuned stirap can be neglected if compared with the resonant stirap. Here we report on the first observation of interference between the resonant and detuned stirap in the adiabatic creation of ${}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ ground-state molecules. The interference is identified by observing that the number of Feshbach molecules after a round-trip stirap oscillates as a function of the hold time, with a visibility of about 90%. This occurs even if the intermediate excited states are well resolved, and the single-photon detuning of the detuned stirap is about 1 order of magnitude larger than the linewidth of the excited state and the Rabi frequencies of the stirap lasers. Moreover, the observed interference indicates that if more than one hyperfine level of the ground state is populated, the stirap prepares a coherent superposition state among them, but not an incoherent mixed state. Further, the purity of the hyperfine levels of the created ground state can be quantitatively determined by the visibility of the oscillation.

We present a precision measurement of the axial-vector coupling constant $g_A$ in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain $\lambda =g_A/g_V=-1.27641(45{)}_{\mathrm{stat}}(33{)}_{\mathrm{sys}}$, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of $A_0=-0.11985(17{)}_{\mathrm{stat}}(12{)}_{\mathrm{sys}}$. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element $V_{ud}$ and derive a limit on left-handed tensor interaction.

We present a new mechanism through which chemical oscillations and waves can be induced in batch conditions with a simple $A+B\to C$ reaction in the absence of any nonlinear chemical feedback or external trigger. Two reactants $A$ and $B$, initially separated in space, react upon diffusive contact and the product actively fuels in situ convective Marangoni flows by locally increasing the surface tension at the mixing interface. These flows combine in turn with the reaction-diffusion dynamics, inducing damped spatiotemporal oscillations of the chemical concentrations and the velocity field. By means of numerical simulations, we single out the detailed mechanism and minimal conditions for the onset of this periodic behavior. We show how the antagonistic coupling with buoyancy convection, due to concurrent chemically induced density changes, can control the oscillation properties, sustaining or suppressing this phenomenon depending on the relative strength of buoyancy- and surface-tension-driven forces. The oscillatory instability is characterized in the relevant parametric space spanned by the reactor height, the Marangoni (${\mathrm{Ma}}_i$) and the Rayleigh (${\mathrm{Ra}}_i$) numbers of the $i$th chemical species, the latter ruling the surface tension and buoyancy contributions to convection, respectively.

We develop a formalism and a computational method to study polarons in insulators and semiconductors from first principles. Unlike in standard calculations requiring large supercells, we solve a secular equation involving phonons and electron-phonon matrix elements from density-functional perturbation theory, in a spirit similar to the Bethe-Salpeter equation for excitons. We show that our approach describes seamlessly large and small polarons, and we illustrate its capability by calculating wave functions, formation energies, and spectral decomposition of polarons in LiF and ${\mathrm{Li}}_2{\mathrm{O}}_2$.

Understanding the interfacial properties of solids with their environment is a crucial problem in fundamental science and applications. Elastomers have challenged the scientific community in this respect, and a satisfying description is still missing. Here, we argue that the interfacial properties of elastomers, such as their wettability, can be understood with a nonlinear elastic model with the assumption of a strain-independent surface energy. We show that our model captures accurately available data on elastomer wettability and discuss its implications.

In many physical processes, including cloud electrification, electrospray, and demulsification, droplets and bubbles are exposed to electric fields and may either remain whole or burst in response to electrical stresses. Determining the stability limit of a droplet exposed to an external electric field has been a long-standing mathematical challenge, and the only analytical treatment to date is an approximate calculation for the particular case of a free-floating droplet. Here we demonstrate, experimentally and theoretically, that the stability limit of a conducting droplet or bubble exposed to an external electric field is described by a power law with broad generality that, in practice, applies to the cases in which the droplet or bubble is pinned or sliding on a conducting surface or free floating. This power law can facilitate the design of devices for liquid manipulation via a simple formula that captures the parameter range of bubbles and droplets that can be supported on electrified surfaces.

We show that supersymmetry is anomalous in $\mathcal{N}=1$ superconformal quantum field theories (SCFTs) with an anomalous $R$ symmetry. This anomaly was originally found in holographic SCFTs at strong coupling. Here we show that this anomaly is present, in general, and demonstrate it for the massless superconformal Wess-Zumino model via a one-loop computation. The anomaly appears first in four-point functions of two supercurrents either with two $R$ currents or with an $R$ current and an energy-momentum tensor. In fact, the Wess-Zumino consistency conditions together with the standard $R$-symmetry anomaly imply the existence of the anomaly. We outline the implications of this anomaly.

A famous example of gauge-gravity duality is the result that the entropy density of the strongly coupled $\mathcal{N}=4$ supersymmetric Yang-Mills theory in four dimensions for large $N$ is exactly $3/4$ of the Stefan-Boltzmann limit. In this work, I revisit the massless $\mathrm{O}(N)$ model in $2+1$ dimensions, which is analytically solvable at a finite-temperature $T$ for all couplings $\lambda$ in the large $N$ limit. I find that the entropy density monotonically decreases from the Stefan-Boltzmann limit at $\lambda =0$ to exactly $4/5$ of the Stefan-Boltzmann limit at $\lambda =\infty$. Calculating the retarded energy-momentum tensor correlator in the scalar channel at $\lambda =\infty$, I find that it has two logarithmic branch cuts originating at $\omega =\pm 4T\ln (1+\sqrt{5}/2)$ but no singularities in the whole complex frequency plane. I show that the ratio $4/5$ and the location of the branch points both are universal within a large class of bosonic conformal field theories in $2+1$ dimensions.

An acoustic topological insulator (TI) is synthesized using topology optimization, a free material inverse design method. The TI appears spontaneously from the optimization process without imposing explicit requirements on the existence of $\mathrm{pseudospin}\text{−}1/2$ states at the TI interface edge, or the Chern number of the topological phases. The resulting TI is passive, consisting of acoustically hard members placed in an air background and has an operational bandwidth of $\approx 12.5\%$ showing high transmission. Further analysis demonstrates confinement of more than 99% of the total field intensity in the TI within at most six lattice constants from the TI interface. The proposed design hereby outperforms a reference from recent literature regarding energy transmission, field confinement, and operational bandwidth.

We report the first observation of superconductivity in a heterostructure consisting of an insulating ferroelectric film (${\mathrm{Ba}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{TiO}}_3$) grown on an insulating parent compound of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ with [001] orientation. The heterostructure was prepared by magnetron sputtering on a nonatomically flat surface with inhomogeneities of the order of 1–2 nm. The measured superconducting transition temperature $T_c$ is about 30 K. We have shown that superconductivity is confined near the interface region. Application of a weak magnetic field perpendicular to the interface leads to the appearance of the finite resistance. That confirms the quasi-two-dimensional nature of the superconductive state. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices.

Planned cryogenic gravitational-wave detectors will require improved coatings with a strain thermal noise reduced by a factor of 25 compared to Advanced LIGO. We present investigations of ${\mathrm{HfO}}_2$ doped with ${\mathrm{SiO}}_2$ as a new coating material for future detectors. Our measurements show an extinction coefficient of $k=6\times {10}^{-6}$ and a mechanical loss of $\varphi =3.8\times {10}^{-4}$ at 10 K, which is a factor of 2 below that of ${\mathrm{SiO}}_2$, the currently used low refractive-index coating material. These properties make ${\mathrm{HfO}}_2$ doped with ${\mathrm{SiO}}_2$ ideally suited as a low-index partner material for use with $a$-Si in the lower part of a multimaterial coating. Based on these results, we present a multimaterial coating design which, for the first time, can simultaneously meet the strict requirements on optical absorption and thermal noise of the cryogenic Einstein Telescope.

Circularly polarized light with spin angular momentum is one of the most valuable probes of magnetism. We demonstrate that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Resonant optical absorption in a ferrimagnetic crystal depends strongly on both the handedness of the vortex and the direction of the beam propagation with respect to the sample magnetization. This effect exceeds the conventional dichroism for circularly polarized light. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter.

The first systematic study of opacity dependence on atomic number at stellar interior temperatures is used to evaluate discrepancies between measured and modeled iron opacity [J. E. Bailey et al., Nature (London) 517, 56 (2015)]. High-temperature ($>180\mathrm{eV}$) chromium and nickel opacities are measured with $\pm 6\%–10\%$ uncertainty, using the same methods employed in the previous iron experiments. The 10%–20% experiment reproducibility demonstrates experiment reliability. The overall model-data disagreements are smaller than for iron. However, the systematic study reveals shortcomings in models for density effects, excited states, and open $L$-shell configurations. The 30%–45% underestimate in the modeled quasicontinuum opacity at short wavelengths was observed only from iron and only at temperature above 180 eV. Thus, either opacity theories are missing physics that has nonmonotonic dependence on the number of bound electrons or there is an experimental flaw unique to the iron measurement at temperatures above 180 eV.

Motivated by recent experimental observations of coherent many-body revivals in a constrained Rydberg atom chain, we construct a weak quasilocal deformation of the Rydberg-blockaded Hamiltonian, which makes the revivals virtually perfect. Our analysis suggests the existence of an underlying nonintegrable Hamiltonian which supports an emergent SU(2)-spin dynamics within a small subspace of the many-body Hilbert space. We show that such perfect dynamics necessitates the existence of atypical, nonergodic energy eigenstates—quantum many-body scars. Furthermore, using these insights, we construct a toy model that hosts exact quantum many-body scars, providing an intuitive explanation of their origin. Our results offer specific routes to enhancing coherent many-body revivals and provide a step toward establishing the stability of quantum many-body scars in the thermodynamic limit.