Using $4.479\times 10^8\psi (3686)$ events collected with the BESIII detector, we search for the decays $\psi (3686)\to e^{+}{e}^{-}{\chi }_{cJ}$ and ${\chi }_{cJ}\to e^{+}{e}^{-}J/\psi$, where $J=0$, 1, 2. The decays $\psi (3686)\to e^{+}{e}^{-}{\chi }_{cJ}$ and ${\chi }_{cJ}\to e^{+}{e}^{-}J/\psi$ are observed for the first time. The measured branching fractions are $\mathcal{B}\mathbf{(}\psi (3686)\to e^{+}{e}^{-}{\chi }_{cJ}\mathbf{)}=(11.7\pm 2.5\pm 1.0)\times {10}^{-4}$, $(8.6\pm 0.3\pm 0.6)\times {10}^{-4}$, $(6.9\pm 0.5\pm 0.6)\times {10}^{-4}$ for $J=0$, 1, 2, and $\mathcal{B}({\chi }_{cJ}\to e^{+}{e}^{-}J/\psi )=(1.51\pm 0.30\pm 0.13)\times {10}^{-4}$, $(3.73\pm 0.09\pm 0.25)\times {10}^{-3}$, $(2.48\pm 0.08\pm 0.16)\times {10}^{-3}$ for $J=0$, 1, 2, respectively. The ratios of the branching fractions $\mathcal{B}\mathbf{(}\psi (3686)\to e^{+}{e}^{-}{\chi }_{cJ}\mathbf{)}/\mathcal{B}\mathbf{(}\psi (3686)\to \gamma {\chi }_{cJ}\mathbf{)}$ and $\mathcal{B}\mathbf{(}{\chi }_{cJ}\to e^{+}{e}^{-}J/\psi \mathbf{)}/\mathcal{B}\mathbf{(}{\chi }_{cJ}\to \gamma J/\psi \mathbf{)}$ are also reported. Also, the $\alpha$ values of helicity angular distributions of the $e^{+}{e}^{-}$ pair are determined for $\psi (3686)\to e^{+}{e}^{-}{\chi }_{c1,2}$ and ${\chi }_{c1,2}\to e^{+}{e}^{-}J/\psi$.

Photostriction is predicted for group-IV monochalcogenide monolayers, two-dimensional ferroelectrics with rectangular unit cells (the lattice vector ${\mathbf{a}}_1$ is larger than ${\mathbf{a}}_2$) and an intrinsic dipole moment parallel to ${\mathbf{a}}_1$. Photostriction is found to be related to the structural change induced by a screened electric polarization (i.e., a converse piezoelectric effect) in photoexcited electronic states with either $p_x$ or $p_y$ (in-plane) orbital symmetry that leads to a compression of $a_1$ and a comparatively smaller increase of $a_2$ for a reduced unit cell area. The structural change documented here is 10 times larger than that observed in ${\mathrm{BiFeO}}_3$, making monochalcogenide monolayers an ultimate platform for this effect. This structural modification should be observable under experimentally feasible densities of photexcited carriers on samples that have been grown already, having a potential usefulness for light-induced, remote mechano-optoelectronic applications.

We report the first measurement of the $\tau$ lepton polarization $P_{\tau }(D^{*})$ in the decay $\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$ as well as a new measurement of the ratio of the branching fractions $R(D^{*})=\mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell })$, where ${\ell }^{-}$ denotes an electron or a muon, and the $\tau$ is reconstructed in the modes ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$ and ${\tau }^{-}\to {\rho }^{-}{\nu }_{\tau }$. We use the full data sample of $772\times 10^{6}B\overline{B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our results, $P_{\tau }(D^{*})=-0.38\pm 0.51{(\text{stat})}_{-0.16}^{+0.21}(\text{syst})$ and $R(D^{*})=0.270\pm 0.035{(\text{stat})}_{-0.025}^{+0.028}(\text{syst})$, are consistent with the theoretical predictions of the standard model.

The reactions $\gamma p\to \eta p$ and $\gamma p\to {\eta }^{\prime }p$ are measured from their thresholds up to the center-of-mass energy $W=1.96\mathrm{GeV}$ with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for $\eta$ photoproduction at the energies in the vicinity of the ${\eta }^{\prime }$ threshold, $W=1896\mathrm{MeV}$ ($E_{\gamma }=1447\mathrm{MeV}$). Within the framework of a revised $\eta \mathrm{MAID}$ isobar model, the cusp, in connection with a steep rise of the ${\eta }^{\prime }$ total cross section from its threshold, can only be explained by a strong coupling of the poorly known $N(1895)1/2^{-}$ state to both $\eta p$ and ${\eta }^{\prime }p$. Including the new high-accuracy results in the $\eta \mathrm{MAID}$ fit to available $\eta$ and ${\eta }^{\prime }$ photoproduction data allows the determination of the $N(1895)1/2^{-}$ properties.

We report the first measurement of the elliptic anisotropy ($v_2$) of the charm meson $D^0$ at midrapidity ($|y|<1$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured $D^0$ $v_2$ in 0%–80% centrality $\mathrm{Au}+\mathrm{Au}$ collisions can be described by a viscous hydrodynamic calculation for a transverse momentum ($p_T$) of less than $4\mathrm{GeV}/c$. The $D^0$ $v_2$ as a function of transverse kinetic energy ($m_T-m_0$, where $m_T=\sqrt{p_T^2+m_0^2}$) is consistent with that of light mesons in 10%–40% centrality $\mathrm{Au}+\mathrm{Au}$ collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature-dependent, dimensionless charm spatial diffusion coefficient ($2\pi T{D}_s$) in the range of $\sim 2–12$, are able to simultaneously reproduce our $D^0$ $v_2$ result and our previously published results for the $D^0$ nuclear modification factor.

The $E2/M1$ multipole mixing ratio ${\delta }_{1\to 2}$ of the $1_{\mathrm{sc}}^{+}\to 2_1^{+}$ $\gamma$-ray decay in ${}^{156}\mathrm{Gd}$ and hence the isovector $E2$ transition rate of the scissors mode of a well-deformed rotational nucleus has been measured for the first time. It has been obtained from the angular distribution of an artificial quasimonochromatic linearly polarized $\gamma$-ray beam of energy 3.07(6) MeV scattered inelastically off an isotopically highly enriched ${}^{156}\mathrm{Gd}$ target. The data yield first direct support for the deformation dependence of effective proton and neutron quadrupole boson charges in the framework of algebraic nuclear models. First evidence for a low-lying $J^{\pi }=2^{+}$ member of the rotational band of states on top of the $1^{+}$ band head is obtained, too, indicating a significant signature splitting in the $K=1$ scissors mode rotational band.

The lack of analytical solutions for the exit momentum in the laser-driven tunneling theory is a well-recognized problem in strong field physics. Theoretical studies of electron momentum distributions in the neighborhood of the tunneling exit depend heavily on ad hoc assumptions. In this Letter, we apply a new numerical method to study the exiting electron’s longitudinal momentum distribution under intense short-pulse laser excitation. We present the first realizations of the dynamic behavior of an electron near the so-called tunneling exit region without adopting a tunneling approximation.

We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state $\mathrm{Ti}(a{{}^{3}F}_2)\text{−}\mathrm{He}$ binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.

The appearance of nonlinear effects in elastic wave propagation is one of the most reliable and sensitive indicators of the onset of material damage. However, these effects are usually very small and can be detected only using cumbersome digital signal processing techniques. Here, we propose and experimentally validate an alternative approach, using the filtering and focusing properties of phononic crystals to naturally select and reflect the higher harmonics generated by nonlinear effects, enabling the realization of time-reversal procedures for nonlinear elastic source detection. The proposed device demonstrates its potential as an efficient, compact, portable, passive apparatus for nonlinear elastic wave sensing and damage detection.

We report a record-size, two-dimensional polariton condensate of a fraction of a millimeter radius free from the presence of an exciton reservoir. This macroscopically occupied state is formed by the ballistically expanding polariton flow that relaxes and condenses over a large area outside of the excitation spot. The density of this trap-free condensate is $<1\mathrm{polariton}/\mu {\mathrm{m}}^2$, reducing the phase noise induced by the interaction energy. Moreover, the backflow effect, recently predicted for the nonparabolic polariton dispersion, is observed here for the first time in the fast-expanding wave packet.

Geometric frustration describes the inability of a local molecular arrangement, such as icosahedra found in metallic glasses and in model atomic glass formers, to tile space. Local icosahedral order, however, is strongly frustrated in Euclidean space, which obscures any causal relationship with the observed dynamical slowdown. Here we relieve frustration in a model glass-forming liquid by curving three-dimensional space onto the surface of a 4-dimensional hypersphere. For sufficient curvature, frustration vanishes and the liquid “freezes” in a fully icosahedral structure via a sharp “transition.” Frustration increases upon reducing the curvature, and the transition to the icosahedral state smoothens while glassy dynamics emerge. Decreasing the curvature leads to decoupling between dynamical and structural length scales and the decrease of kinetic fragility. This sheds light on the observed glass-forming behavior in Euclidean space.

Fundamental understanding of strongly interacting systems necessarily involves collective modes, but their nature and evolution is not generally understood in dynamically disordered and strongly interacting systems such as liquids and supercritical fluids. We report the results of extensive molecular dynamics simulations and provide direct evidence that liquids develop a gap in a solidlike transverse spectrum in the reciprocal space, with no propagating modes between zero and a threshold value. In addition to the liquid state, this result importantly applies to the supercritical state of matter. We show that the emerging gap increases with the inverse of liquid relaxation time and discuss how the gap affects properties of liquid and supercritical states.

The transformation of the free-energy landscape from smooth to hierarchical is one of the richest features of mean-field disordered systems. A well-studied example is the de Almeida–Thouless transition for spin glasses in a magnetic field, and a similar phenomenon—the Gardner transition—has recently been predicted for structural glasses. The existence of these replica-symmetry-breaking phase transitions has, however, long been questioned below their upper critical dimension, $d_u=6$. Here, we obtain evidence for the existence of these transitions in $d<d_u$ using a two-loop calculation. Because the critical fixed point is found in the strong-coupling regime, we corroborate the result by resumming the perturbative series with inputs from a three-loop calculation and an analysis of its large-order behavior. Our study offers a resolution of the long-lasting controversy surrounding phase transitions in finite-dimensional disordered systems.

Decoherence due to charge noise is one of the central challenges in using spin qubits in semiconductor quantum dots as a platform for quantum information processing. Recently, it has been experimentally demonstrated in both Si and GaAs singlet-triplet qubits that the effects of charge noise can be suppressed if qubit operations are implemented using symmetric barrier control instead of the standard tilt control. Here, we investigate the key issue of whether the benefits of barrier control persist over the entire set of single-qubit gates by performing randomized benchmarking simulations. We find the surprising result that the improvement afforded by barrier control depends sensitively on the amount of spin noise: for the minimal nuclear spin noise levels present in Si, the coherence time improves by more than 2 orders of magnitude whereas in GaAs, by contrast the coherence time is essentially the same for barrier and tilt control. However, we establish that barrier control becomes beneficial if qubit operations are performed using a new family of composite pulses that reduce gate times by up to 90%. With these optimized pulses, barrier control is the best way to achieve high-fidelity quantum gates in singlet-triplet qubits.