Electron relaxation is studied in endofullerene $\mathrm{Mg}@{\mathrm{C}}_{60}$ after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i) an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii) a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to ${\mathrm{C}}_{60}$ within tens of femtoseconds. Method (i) further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

We report on the formation of a dispersive shock wave in a nonlinear optical medium. We monitor the evolution of the shock by tuning the incoming beam power. The experimental observations for the position and intensity of the solitonic edge of the shock, as well as the location of the nonlinear oscillations are well described by recent developments of Whitham modulation theory. Our work constitutes a detailed and accurate benchmark for this approach. It opens exciting possibilities to engineer specific configurations of optical shock wave for studying wave-mean flow interaction.

We present the first observation of instability in weakly magnetized, pressure dominated plasma Couette flow firmly in the Hall regime. Strong Hall currents couple to a low frequency electromagnetic mode that is driven by high-$\beta$ ($>1$) pressure profiles. Spectroscopic measurements show heating (factor of 3) of the cold, unmagnetized ions via a resonant Landau damping process. A linear theory of this instability is derived that predicts positive growth rates at finite $\beta$ and shows the stabilizing effect of very large $\beta$, in line with observations.

Dirac semimetals associated with bulk Dirac fermions are well known in topological electronic systems. In sharp contrast, three-dimensional (3D) Dirac phonons in crystalline solids are still unavailable. Here we perform symmetry arguments and first-principles calculations to systematically investigate 3D Dirac phonons in all space groups with inversion symmetry. The results show that there are two categories of 3D Dirac phonons depending on their protection mechanisms and positions in momentum space. The first category originates from the four-dimensional irreducible representations at the high symmetry points. The second category arises from the phonon branch inversion, and the symmetry guarantees Dirac points to be located along the high symmetry lines. Furthermore, we reveal that nonsymmorphic symmetries and the combination of inversion and time-reversal symmetries play essential roles in the emergence of 3D Dirac phonons. Our work not only offers a comprehensive understanding of 3D Dirac phonons but also provides significant guidance for exploring Dirac bosons in both phononic and photonic systems.

High density carbon (HDC) ablator is one of the promising candidates toward thermonuclear ignition in inertial confinement fusion (ICF), but it shows the largest ablation front instability growth as compared to other traditional ablator materials. In this Letter, we propose a novel HDC-CH capsule design, opening the way to mitigate the hydrodynamic instabilities by using CH as the outermost ablator layer, while keeping HDC as the main ablator for maintaining the advantage of short laser pulses. The CH layer is completely ablated during the shock transit phase. In the HDC-CH design, it is the first shock reflected from the HDC/CH interface that meets the ablation front first, which reduces the ablation front growth factor by about one order of magnitude at peak implosion velocity due to the Richtmyer-Meshkov and the Rayleigh-Taylor instabilities. Our 2D simulation studies demonstrate convincingly that the ablation front growth factor of the HDC-CH capsule can be significantly reduced at both the end of shock transit phase and the time at peak implosion velocity, as compared to a HDC capsule. This novel HDC-CH capsule not only keeps the main advantage of the HDC ablator, but also has the advantage of low hydrodynamic instabilities, which can provide a larger margin toward ICF ignition. It can be applicable to both indirect-drive and direct-drive targets.

The energy spectrum of positronium atoms generated at a solid surface reflects the electron density of states (DOS) associated solely with the first surface layer. Using spin-polarized positrons, the spin-dependent surface DOS can be studied. For this purpose, we have developed a spin-polarized positronium time-of-flight spectroscopy apparatus based on a ${}^{22}\mathrm{Na}$ positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the $\mathrm{\Gamma }$ point and near the Fermi level. We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni(111), Co(0001) and graphene on them, ${\mathrm{Co}}_2{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) and ${\mathrm{Co}}_2\mathrm{MnSi}$ (CMS). The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement. The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.

We report the observation of an antipolar phase in cubic ${\mathrm{GaNb}}_4{\mathrm{S}}_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of ${\mathrm{Nb}}_4{\mathrm{S}}_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_{\mathrm{JT}}=31\mathrm{K}$, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_{\mathrm{JT}}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that ${\mathrm{GaNb}}_4{\mathrm{S}}_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.

Degeneracies in the energy spectra of physical systems are commonly considered to be either of accidental character or induced by symmetries of the Hamiltonian. We develop an approach to explain degeneracies by tracing them back to symmetries of an isospectral effective Hamiltonian derived by subsystem partitioning. We provide an intuitive interpretation of such latent symmetries by relating them to corresponding local symmetries in the powers of the underlying Hamiltonian matrix. As an application, we relate the degeneracies induced by the rotation symmetry of a real Hamiltonian to a non-Abelian latent symmetry group. It is demonstrated that the rotational symmetries can be broken in a controlled manner while maintaining the underlying more fundamental latent symmetry. This opens up the perspective of investigating accidental degeneracies in terms of latent symmetries.

We study hidden-sector particles at past (CERN-Hamburg-Amsterdam-Rome-Moscow Collaboration and NuCal), present (NA62, SeaQuest, and DarkQuest), and future (LongQuest) experiments at the high-energy intensity frontier. We focus on exploring the minimal vector portal and the next-to-minimal models in which the productions and decays are decoupled. These next-to-minimal models have mostly been devised to explain experimental anomalies while avoiding existing constraints. We demonstrate that proton fixed-target experiments provide one of the most powerful probes for the MeV to few GeV mass range of these models, using inelastic dark matter (iDM) as an example. We consider an iDM model with a small mass splitting that yields the observed dark matter relic abundance, and a scenario with a sizable mass splitting that can also explain the muon $g-2$ anomaly. We set strong limits based on the CERN-Hamburg-Amsterdam-Rome-Moscow Collaboration and NuCal experiments, which come close to excluding iDM as a full-abundance thermal dark matter candidate in the MeV to GeV mass range. We also make projections based on NA62, SeaQuest, and DarkQuest and update the constraints of the minimal dark photon parameter space. We find that NuCal sets the only existing constraint in $\epsilon \sim {10}^{-8}–{10}^{-4}$ regime, reaching $\sim 800\mathrm{MeV}$ in dark photon mass due to the resonant enhancement of proton bremsstrahlung production. These studies also motivate LongQuest, a three-stage retooling of the SeaQuest experiment with short ($\lesssim 5\mathrm{m}$), medium ($\sim 5\mathrm{m}$), and long ($\gtrsim 35\mathrm{m}$) baseline tracking stations and detectors as a multipurpose machine to explore new physics.

The CUPID-Mo experiment at the Laboratoire Souterrain de Modane (France) is a demonstrator for CUPID, the next-generation ton-scale bolometric $0\nu \beta \beta$ experiment. It consists of a 4.2 kg array of 20 enriched ${\mathrm{Li}}_2$${}^{100}\mathrm{Mo}{\mathrm{O}}_4$ scintillating bolometers to search for the lepton-number-violating process of $0\nu \beta \beta$ decay in ${}^{100}\mathrm{Mo}$. With more than one year of operation (${}^{100}\mathrm{Mo}$ exposure of $1.17\mathrm{kg}\times \mathrm{yr}$ for physics data), no event in the region of interest and, hence, no evidence for $0\nu \beta \beta$ is observed. We report a new limit on the half-life of $0\nu \beta \beta$ decay in ${}^{100}\mathrm{Mo}$ of $T_{1/2}>1.5\times {10}^{24}\mathrm{yr}$ at 90% C.I. The limit corresponds to an effective Majorana neutrino mass $⟨m_{\beta \beta }⟩<(0.31–0.54)\mathrm{eV}$, dependent on the nuclear matrix element in the light Majorana neutrino exchange interpretation.

In two recent papers by Pore et al. and Khuyagbaatar et al., discovery of the new isotope ${}^{244}\mathrm{Md}$ was reported. The decay data, however, are conflicting. While Pore et al. report two isomeric states decaying by $\alpha$ emission with $E_{\alpha }(1)=8.66(2)\mathrm{MeV}$, $T_{1/2}(1)={0.4}_{-0.1}^{+0.4}\mathrm{s}$ and $E_{\alpha }(2)=8.31(2)\mathrm{MeV}$, $T_{1/2}(2)\approx 6\mathrm{s}$, Khuyagbaatar et al. [Phys. Rev. Lett. 125, 142504 (2020).] report only a single transition with a broad energy distribution of $E_{\alpha }=(8.73–8.86)\mathrm{MeV}$ and $T_{1/2}={0.30}_{-0.09}^{+0.19}\mathrm{s}$. The data published in Pore et al. are very similar to those published for ${}^{245m}\mathrm{Md}$ [$E_{\alpha }=8.64(2)$, 8.68(2) MeV, $T_{1/2}={0.35}_{-0.16}^{+0.23}\mathrm{s}$ [V. Ninov, F. P. Heßberger, S. Hofmann, H. Folger, G. Münzenberg, P. Armbruster, A. V. Yeremin, A. G. Popeko, M. Leino, and S. Saro, Z. Phys. A 356, 11 (1996).] ]. Therefore, we compare the data presented for ${}^{244}\mathrm{Md}$ in Pore et al. with those reported for ${}^{245}\mathrm{Md}$ in Ninov et al. and also in Khuyagbaatar et al. We conclude that the data presented in Pore et al. shall be attributed to ${}^{245}\mathrm{Md}$ with small contributions (one event each) from ${}^{245}\mathrm{Fm}$ and probably ${}^{246}\mathrm{Md}$.

The strong mixing of close levels with two valence electrons in Be-like xenon greatly complicates ab initio QED calculations beyond the first-order approximation. Because of a strong interplay between the electron-electron correlation and QED effects, the standard single-level perturbative QED approach may fail, even if it takes into account the second-order screened QED diagrams. In the present Letter, the corresponding obstacles are overcome by working out the QED perturbation theory for quasidegenerate states. The contributions of all the Feynman diagrams up to the second order are taken into account. The many-electron QED effects are rigorously evaluated in the framework of the extended Furry picture to all orders in the nuclear-strength parameter $\alpha Z$. The higher-order electron-correlation effects are considered within the Breit approximation. The nuclear recoil effect is accounted for as well. The developed approach is applied to high-precision QED calculations of the ground and singly excited energy levels in Be-like xenon. The most accurate theoretical predictions for the binding and excitation energies are obtained. These results deviate from the most precise experimental value by $3\sigma$ but perfectly agree with a more recent measurement.

We provide strong evidence that the effective spin-spin interaction in a multimodal confocal optical cavity gives rise to a self-induced glassy phase, which emerges exclusively from the peculiar Euclidean correlations and is not related to the presence of disorder as in standard spin glasses. As recently shown, this spin-spin effective interaction is both nonlocal and nontranslational invariant, and randomness in the atoms’ positions produces a spin glass phase. Here we consider the simplest feasible disorder-free setting, where atoms form a one-dimensional regular chain and we study the thermodynamics of the resulting effective Ising model. We present extensive results showing that the system has a low-temperature glassy phase. The model depends on the adimensional parameter $\alpha =(a/w_0{)}^2$, $a$ being a lattice spacing and $w_0$ an interaction length scale. Notably, for rational values of $\alpha =p/q$, the number of metastable states at low temperature grows exponentially with $q$ and the problem of finding the ground state rapidly becomes computationally intractable, suggesting that the system develops high-energy barriers and ergodicity breaking occurs.

Starting from Shannon’s definition of dynamic entropy, we propose a theory to describe the rare-event-determined dynamic states in condensed matter and their transitions and apply it to high-pressure ice VII. A dynamic intensive quantity named dynamic field, rather than the conventional thermodynamic intensive quantities such as temperature and pressure, is taken as the controlling variable. The dynamic entropy versus dynamic field curve demonstrates two dynamic states in the stability region of ice VII and dynamic ice VII. Their microscopic differences were assigned to the dynamic patterns of proton transfer. This study puts a similar dynamical theory used in earlier studies of glass models on a simpler and more fundamental basis, which could be applied to describe the dynamic states of more realistic condensed matter systems.

We theoretically study an impulsively excited quantum bouncer (QB)—a particle bouncing off a surface in the presence of gravity. A pair of time-delayed pulsed excitations is shown to induce a wave-packet echo effect—a partial rephasing of the QB wave function appearing at twice the delay between pulses. In addition, an appropriately chosen observable [here, the population of the ground gravitational quantum state (GQS)] recorded as a function of the delay is shown to contain the transition frequencies between the GQSs, their populations, and partial phase information about the wave-packet quantum amplitudes. The wave-packet echo effect is a promising candidate method for precision studies of GQSs of ultracold neutrons, atoms, and antiatoms confined in closed gravitational traps.

Dissimilar notions of quantum correlations have been established, each being motivated through particular applications in quantum information science and each competing for being recognized as the most relevant measure of quantumness. In this contribution, we experimentally realize a form of quantum correlation that exists even in the absence of entanglement and discord. We certify the presence of such quantum correlations via negativities in the regularized two-mode Glauber-Sudarshan function. Our data show compatibility with an incoherent mixture of orthonormal photon-number states, ruling out quantum coherence and other kinds of quantum resources. By construction, the quantumness of our state is robust against dephasing, thus requiring fewer experimental resources to ensure stability. In addition, we theoretically show how multimode entanglement can be activated based on the generated, nonentangled state. Therefore, we implement a robust kind of nonclassical photon-photon correlated state with useful applications in quantum information processing.

A quantum system subject to continuous measurement and postselection evolves according to a non-Hermitian Hamiltonian. We show that, as one increases the strength of postselection, this non-Hermitian Hamiltonian can undergo a spectral phase transition. On one side of this phase transition (for weak postselection), an initially mixed density matrix remains mixed at all times, and an initially unentangled state develops volume-law entanglement; on the other side, an arbitrary initial state approaches a unique pure state with low entanglement. We identify this transition with an exceptional point in the spectrum of the non-Hermitian Hamiltonian, at which $PT$ symmetry is spontaneously broken. We characterize the transition as well as the nontrivial steady state that emerges at late times in the mixed phase using exact diagonalization and an approximate, analytically tractable mean-field theory; these methods yield consistent conclusions.