The dissociation energy of ${\mathrm{H}}_2$ represents a benchmark quantity to test the accuracy of first-principles calculations. We present a new measurement of the energy interval between the $EF$ ${{}^1\mathrm{\Sigma }}_g^{+}(v=0,N=1)$ state and the $54p{1}_1$ Rydberg state of ${\mathrm{H}}_2$. When combined with previously determined intervals, this new measurement leads to an improved value of the dissociation energy $D_0^{N=1}$ of ortho-${\mathrm{H}}_2$ that has, for the first time, reached a level of uncertainty that is 3 times smaller than the contribution of about 1 MHz resulting from the finite size of the proton. The new result of $35999.582834(11){\mathrm{cm}}^{-1}$ is in remarkable agreement with the theoretical result of $35999.582820(26){\mathrm{cm}}^{-1}$ obtained in calculations including high-order relativistic and quantum-electrodynamics corrections, as reported in the following Letter [M. Puchalski, J. Komasa, P. Czachorowski, and K. Pachucki, Phys. Rev. Lett. 122, 103003 (2019)]. This agreement resolves a recent discrepancy between experiment and theory that had hindered a possible use of the dissociation energy of ${\mathrm{H}}_2$ in the context of the current controversy on the charge radius of the proton.

Nodal line semimetals exhibiting line degeneracies in three-dimensional momentum space have been demonstrated recently. In general, the presence of nodal line semimetals is protected by special symmetries, such as mirror symmetry. However, these symmetries are usually necessary but not sufficient conditions, as nodal lines can be annihilated even without breaking them. Very recently, nodal line semimetal possessing an hourglass-shaped band structure emerges as a more robust candidate, where line degeneracies cannot be annihilated while preserving all underlying spatial symmetries. Here, for the first time, we experimentally demonstrate the presence of an hourglass nodal line (HNL) in photonic metacrystal at microwave frequency. We observe the HNL through near-field scanning of the spatial fields, followed by subsequent Fourier transformations. The observed photonic HNL resides in a clean and large frequency interval and is immune to symmetry preserving perturbation, which provides an ideal robust platform for photonic applications, such as anomalous quantum oscillation, spontaneous emission and resonant scattering.

The GRAPES-3 muon telescope located in Ooty, India records rapid ($\sim 10\min$) variations in the muon intensity during major thunderstorms. Out of a total of 184 thunderstorms recorded during the interval of April 2011–December 2014, the one on December 1, 2014 produced a massive potential of 1.3 GV. The electric field measured by four well-separated (up to 6 km) monitors on the ground was used to help estimate some of the properties of this thundercloud, including its altitude and area that were found to be 11.4 km above mean sea level and $\ge 380{\mathrm{km}}^2$, respectively. A charging time of 6 min to reach 1.3 GV implied the delivery of a power of $\ge 2\mathrm{GW}$ by this thundercloud that was moving at a speed of $\sim 60\mathrm{km}{\mathrm{h}}^{-1}$. This work possibly provides the first direct evidence for the generation of gigavolt potentials in thunderclouds that could also possibly explain the production of highest-energy (100 MeV) gamma rays in the terrestrial gamma-ray flashes.

Twisted bilayer graphene (TBG) was recently shown to host superconductivity when tuned to special “magic angles” at which isolated and relatively flat bands appear. However, until now the origin of the magic angles and their irregular pattern have remained a mystery. Here we report on a fundamental continuum model for TBG which features not just the vanishing of the Fermi velocity, but also the perfect flattening of the entire lowest band. When parametrized in terms of $\alpha \sim 1/\theta$, the magic angles recur with a remarkable periodicity of $\mathrm{\Delta }\alpha \simeq 3/2$. We show analytically that the exactly flat band wave functions can be constructed from the doubly periodic functions composed of ratios of theta functions—reminiscent of quantum Hall wave functions on the torus. We further report on the unusual robustness of the experimentally relevant first magic angle, address its properties analytically, and discuss how lattice relaxation effects help justify our model parameters.

During its orbit around the four million solar mass black hole Sagittarius A* the star S2 experiences significant changes in gravitational potential. We use this change of potential to test one part of the Einstein equivalence principle: the local position invariance (LPI). We study the dependency of different atomic transitions on the gravitational potential to give an upper limit on violations of the LPI. This is done by separately measuring the redshift from hydrogen and helium absorption lines in the stellar spectrum during its closest approach to the black hole. For this measurement we use radial velocity data from 2015 to 2018 and combine it with the gravitational potential at the position of S2, which is calculated from the precisely known orbit of S2 around the black hole. This results in a limit on a violation of the LPI of $|{\beta }_{\mathrm{He}}-{\beta }_{\mathrm{H}}|=(2.4\pm 5.1)\times {10}^{-2}$. The variation in potential that we probe with this measurement is six magnitudes larger than possible for measurements on Earth, and a factor of 10 larger than in experiments using white dwarfs. We are therefore testing the LPI in a regime where it has not been tested before.

We investigate the dynamics of a deterministic self-propelled particle endowed with coherent memory. We evidence experimentally and numerically that it exhibits several stable free states. The system is composed of a self-propelled drop bouncing on a vibrated liquid driven by the waves it emits at each bounce. This object possesses a propulsion memory resulting from the coherent interference of the waves accumulated along its path. We investigate here the transitory regime of the buildup of the dynamics which leads to velocity modulations. Experiments and numerical simulations enable us to explore unchartered areas of the phase space and reveal the existence of a self-sustained oscillatory regime. Finally, we show the coexistence of several free states. This feature emerges both from the spatiotemporal nonlocality of this path memory dynamics as well as the wave nature of the driving mechanism.

The classical notion of the coalescence of two droplets of the same radius $R$ is that surface tension drives an initially singular flow. In this Letter we show, using molecular dynamics simulations of coalescing water nanodroplets, that after single or multiple bridges form due to the presence of thermal capillary waves, the bridge growth commences in a thermal regime. Here, the bridges expand linearly in time much faster than the viscous-capillary speed due to collective molecular jumps near the bridge fronts. Transition to the classical hydrodynamic regime only occurs once the bridge radius exceeds a thermal length scale $l_T\sim \sqrt{R}$.

We demonstrate a laser-driven, tunable electron lens fabricated in monolithic silicon. The lens consists of an array of silicon pillars pumped symmetrically by two 300 fs, $1.95\mu \mathrm{m}$ wavelength, nJ-class laser pulses from an optical parametric amplifier. The optical near field of the pillar structure focuses electrons in the plane perpendicular to the pillar axes. With $100\pm 10\mathrm{MV}$/m incident laser fields, the lens focal length is measured to be $50\pm 4\mu \mathrm{m}$, which corresponds to an equivalent quadrupole focusing gradient $B^{\prime }$ of $1.4\pm 0.1\mathrm{MT}/\mathrm{m}$. By varying the incident laser field strength, the lens can be tuned from a $21\pm 2\mu \mathrm{m}$ focal length ($B^{\prime }>3.3\mathrm{MT}/\mathrm{m}$) to focal lengths on the centimeter scale.

Amorphous materials have a rich relaxation spectrum, which is usually described in terms of a hierarchy of relaxation mechanisms. In this work, we investigate the local dynamic modulus spectra in a model glass just above the glass transition temperature by performing a mechanical spectroscopy analysis with molecular dynamics simulations. We find that the spectra, at the local as well as on the global scale, can be well described by the Cole-Davidson formula in the frequency range explored with simulations. Surprisingly, the Cole-Davidson stretching exponent does not change with the size of the local region that is probed. The local relaxation time displays a broad distribution, as expected based on dynamic heterogeneity concepts, but the stretching is obtained independently of this distribution. We find that the size dependence of the local relaxation time and moduli can be well explained by the elastic shoving model.

We developed a microscopy technique that can measure the local refractive index without sampling the optical phase delay of the electromagnetic radiation. To do this, we designed and experimentally demonstrated a setup with two colocalized Brillouin scattering interactions that couple to a common acoustic phonon axis; in this scenario, the ratio of Brillouin frequency shifts depends on the refractive index, but not on any other mechanical and/or optical properties of the sample. Integrating the spectral measurement within a confocal microscope, the refractive index is mapped at micron-scale three-dimensional resolution. As the refractive index is probed in epidetection and without assumptions on the geometrical dimensions of the sample, this method may prove useful to characterize biological cells and tissues.

Gravity-driven drainage of small volumes of condensates, such as natural dew, is a challenge because small drops usually remain pinned to inclined surfaces. We report that submillimetric grooves substantially reduce dew retention by modifying the repartition of liquid: Because of a long-range coalescence mechanism mediated by grooves imbibition, the growth and shedding of large drops are accelerated. Such findings can be applied to increase the passive harvesting of dew as well as to accelerate the drainage of other condensates.

Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g., pressure). Most of the electrides under high pressure are nonmetallic, and superconducting electrides are very rare. Here we report that a pressure-induced stable ${\mathrm{Li}}_6\mathrm{P}$ electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature $T_c$ of 39.3 K, which is the highest among the already known electrides. The interstitial electrons in ${\mathrm{Li}}_6\mathrm{P}$, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, ${\mathrm{Li}}_5\mathrm{P}$, ${\mathrm{Li}}_{11}{\mathrm{P}}_2$, ${\mathrm{Li}}_{15}{\mathrm{P}}_2$, and ${\mathrm{Li}}_8\mathrm{P}$, are also predicted to be superconducting electrides, but with a lower $T_c$. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of phosphorus (P) with a strong electropositivity of lithium (Li), and opens up the interest to explore high-temperature superconductivity in similar binary compounds.

A material under compression often forms creases. When the material is elastic and soft, the nucleation of creases depends on both elasticity and capillarity. Here we introduce a model of elastocapillary creases. The model assumes that the surface tension remains constant on the free surface, but may change upon self-contact. In particular, surface tension vanishes upon self-contact for a pristine surface of elastomers and gels. The model predicts that the nucleation of creases depends on the sizes of surface defects relative to the elastocapillary length, and happens over a well-defined range of strains, instead of a specific strain. The loss of surface tension upon self-contact lowers the energy barrier for nucleation, and widens the range of nucleation strains for materials of any thickness relative to the elastocapillary length. We test this model by conducting experiments with materials of various elastocapillary lengths, along with the data available in the literature.

A search for mixing between active neutrinos and light sterile neutrinos has been performed by looking for muon neutrino disappearance in two detectors at baselines of 1.04 and 735 km, using a combined MINOS and MINOS+ exposure of $16.36\times {10}^{20}$ protons on target. A simultaneous fit to the charged-current muon neutrino and neutral-current neutrino energy spectra in the two detectors yields no evidence for sterile neutrino mixing using a $3+1$ model. The most stringent limit to date is set on the mixing parameter ${\sin }^2{\theta }_{24}$ for most values of the sterile neutrino mass splitting $\mathrm{\Delta }{m}_{41}^2>{10}^{-4}{\mathrm{eV}}^2$.

We study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime, and short-range correlations of Fermi polarons at low temperatures $T$. In this regime, we observe a characteristic $T^2$ dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high $T$, the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, $\propto T^{-1/2}$. In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins and yield the compressibility along with the effective mass of Fermi polarons.

We demonstrate the use of shortcuts to adiabaticity protocols for initialization, read-out, and coherent control of dressed states generated by closed-contour, coherent driving of a single spin. Such dressed states have recently been shown to exhibit efficient coherence protection, beyond what their two-level counterparts can offer. Our state transfer protocols yield a transfer fidelity of $\sim 99.4(2)\%$ while accelerating the transfer speed by a factor of 2.6 compared to the adiabatic approach. We show bidirectionality of the accelerated state transfer, which we employ for direct dressed state population read-out after coherent manipulation in the dressed state manifold. Our results enable direct and efficient access to coherence-protected dressed states of individual spins and thereby offer attractive avenues for applications in quantum information processing or quantum sensing.

The emergence of a special type of fluidlike behavior at large scales in one-dimensional (1D) quantum integrable systems, theoretically predicted in O. A. Castro-Alvaredo et al., Emergent Hydrodynamics in Integrable Quantum Systems Out of Equilibrium, Phys. Rev. X 6, 041065 (2016) and B. Bertini et al., Transport in Out-of-Equilibrium XXZ Chains: Exact Profiles of Charges and Currents, Phys. Rev. Lett. 117, 207201 (2016), is established experimentally, by monitoring the time evolution of the in situ density profile of a single 1D cloud of ${}^{87}\mathrm{Rb}$ atoms trapped on an atom chip after a quench of the longitudinal trapping potential. The theory can be viewed as a dynamical extension of the thermodynamics of Yang and Yang, and applies to the whole range of repulsive interaction strength and temperature of the gas. The measurements, performed on weakly interacting atomic clouds that lie at the crossover between the quasicondensate and the ideal Bose gas regimes, are in very good agreement with the theory. This contrasts with the previously existing “conventional” hydrodynamic approach—that relies on the assumption of local thermal equilibrium—which is unable to reproduce the experimental data.

We show that the commonly accepted statement that sound waves do not transport mass is only true at linear order. Using effective field theory techniques, we confirm the result found by Nicolis and Penco [Phys. Rev. B 97, 134516 (2018)] for zero-temperature superfluids, and extend it to the case of solids and ordinary fluids. We show that, in fact, sound waves do carry mass—in particular, gravitational mass. This implies that a sound wave not only is affected by gravity but also generates a tiny gravitational field, an aspect not appreciated thus far. Our findings are valid for nonrelativistic media as well, and could have intriguing experimental implications.

We propose a general framework for solving statistical mechanics of systems with finite size. The approach extends the celebrated variational mean-field approaches using autoregressive neural networks, which support direct sampling and exact calculation of normalized probability of configurations. It computes variational free energy, estimates physical quantities such as entropy, magnetizations and correlations, and generates uncorrelated samples all at once. Training of the network employs the policy gradient approach in reinforcement learning, which unbiasedly estimates the gradient of variational parameters. We apply our approach to several classic systems, including 2D Ising models, the Hopfield model, the Sherrington-Kirkpatrick model, and the inverse Ising model, for demonstrating its advantages over existing variational mean-field methods. Our approach sheds light on solving statistical physics problems using modern deep generative neural networks.

The Laser Interferometer Space Antenna (LISA) and its metrology chain have to fulfill stringent performance requirements to enable the space-based detection of gravitational waves. This implies the necessity of performance verification methods. In particular, the extraction of the interferometric phase, implemented by a phasemeter, needs to be probed for linearity and phase noise contributions. This Letter reports on a hexagonal quasimonolithic optical bench implementing a three-signal test for this purpose. Its characterization as sufficiently stable down to picometer levels is presented as well as its usage for a benchmark phasemeter performance measurement under LISA conditions. These results make it a candidate for the core of a LISA metrology verification facility.