Recent observations of planar defects in boron carbide have been shown to deviate from perfect mirror symmetry and are referred to as “asymmetric twins.” Here, we demonstrate that these asymmetric twins are really phase boundaries that form in stoichiometric ${\mathrm{B}}_4\mathrm{C}$ (i.e., ${\mathrm{B}}_{12}{\mathrm{C}}_3$) but not in ${\mathrm{B}}_{13}{\mathrm{C}}_2$. TEM observations and ab initio simulations have been coupled to show that these planar defects result from an interplay of stoichiometry, atomic positioning, icosahedral twinning, and structural hierarchy. The composition of icosahedra in ${\mathrm{B}}_4\mathrm{C}$ is ${\mathrm{B}}_{11}\mathrm{C}$ and translation of the carbon atom from a polar to equatorial site leads to a shift in bonding and a slight distortion of the lattice. No such distortion is observed in boron-rich ${\mathrm{B}}_{13}{\mathrm{C}}_2$ because the icosahedra do not contain carbon. Implications for tailoring boron carbide with stoichiometry and extrapolations to other hierarchical crystalline materials are discussed.

We present spin-noise spectroscopy measurements on an ensemble of donor-bound electrons in ultrapure GaAs:Si covering temporal dynamics over 6 orders of magnitude from milliseconds to nanoseconds. The spin-noise spectra detected at the donor-bound exciton transition show the multifaceted dynamical regime of the ubiquitous mutual electron and nuclear spin interaction typical for III-V-based semiconductor systems. The experiment distinctly reveals the finite Overhauser shift of an electron spin precession at zero external magnetic field and a second contribution around zero frequency stemming from the electron spin components parallel to the nuclear spin fluctuations. Moreover, at very low frequencies, features related with time-dependent nuclear spin fluctuations are clearly resolved making it possible to study the intricate nuclear spin dynamics at zero and low magnetic fields. The findings are in agreement with the developed model of electron and nuclear spin noise.

We show that, as happens in graphene, elastic deformations couple to the electronic degrees of freedom as pseudogauge fields in Weyl semimetals. We derive the form of the elastic gauge fields in a tight-binding model hosting Weyl nodes and see that this vector electron-phonon coupling is chiral, providing an example of axial gauge fields in three dimensions. As an example of the new response functions that arise associated with these elastic gauge fields, we derive a nonzero phonon Hall viscosity for the neutral system at zero temperature. The axial nature of the fields provides a test of the chiral anomaly in high energy with three axial vector couplings.

We demonstrate excitation of a single trapped cold ${{}^{40}\mathrm{Ca}}^{+}$ ion to Rydberg levels by laser radiation in the vacuum ultraviolet at a wavelength of 122 nm. Observed resonances are identified as $3d{{}^{2}D}_{3/2}$ to $51F$, $52F$ and $3d{{}^{2}D}_{5/2}$ to $64F$. We model the line shape and our results imply a large state-dependent coupling to the trapping potential. Rydberg ions are of great interest for future applications in quantum computing and simulation, in which large dipolar interactions are combined with the superb experimental control offered by Paul traps.

Nonrotating (“locked”) magnetic islands often lead to complete losses of confinement in tokamak plasmas, called major disruptions. Here locked islands were suppressed for the first time, by a combination of applied three-dimensional magnetic fields and injected millimeter waves. The applied fields were used to control the phase of locking and so align the island $O$ point with the region where the injected waves generated noninductive currents. This resulted in stabilization of the locked island, disruption avoidance, recovery of high confinement, and high pressure, in accordance with the expected dependencies upon wave power and relative phase between the $O$ point and driven current.

In an extensive computational experiment, we test Polyakov’s conjecture that under certain circumstances an isotropic Heisenberg model can develop algebraic spin correlations. We demonstrate the emergence of a multispin U(1) order parameter in a Heisenberg antiferromagnet on interpenetrating honeycomb and triangular lattices. The correlations of this relative phase angle are observed to decay algebraically at intermediate temperatures in an extended critical phase. Using finite-size scaling we show that both phase transitions are of the Berezinskii-Kosterlitz-Thouless type, and at lower temperatures we find long-range ${\mathbb{Z}}_6$ order.

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling Monte Carlo scheme, and we obtain temperature-dependent line shapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.

We experimentally show how classical correlations can be turned into quantum entanglement, via the presence of dissipation and the action of a cnot gate. We first implement a simple two-qubit protocol in which entanglement production is not possible in the absence of such kind of noise, while it arises with its introduction, and is proportional to its amount. We then perform a more elaborate four-qubit experiment, by employing two hyperentangled photons initially carrying only classical correlations. We demonstrate a scheme where the entanglement is generated via local dissipation, with the advantage of being robust against local unitaries performed by an adversary.

A fundamental goal of quantum technologies concerns the exploitation of quantum coherent dynamics for the realization of novel quantum applications such as quantum computing, quantum simulation, and quantum metrology. A key challenge on the way towards these goals remains the protection of quantum coherent dynamics from environmental noise. Here, we propose a concept of a hybrid dressed state from a pair of continuously driven systems. It allows sufficiently strong driving fields to suppress the effect of environmental noise while at the same time being insusceptible to both the amplitude and phase noise in the continuous driving fields. This combination of robust features significantly enhances coherence times under realistic conditions and at the same time provides new flexibility in Hamiltonian engineering that otherwise is not achievable. We demonstrate theoretically applications of our scheme for a noise-resistant analog quantum simulation in the well-studied physical systems of nitrogen-vacancy centers in diamond and of trapped ions. The scheme may also be exploited for quantum computation and quantum metrology.

Recent progress in the determination of both masses and radii of neutron stars is starting to place stringent constraints on the dense matter equation of state. In particular, new theoretical developments together with improved statistical tools seem to favor stellar radii that are significantly smaller than those predicted by models using purely nucleonic equations of state. Given that the underlying equation of state must also account for the observation of $2M_{\odot }$ neutron stars, theoretical approaches to the study of the dense matter equation of state are facing serious challenges. In response to this challenge, we compute the underlying equation of state associated with an assumed mass-radius template similar to the “common radius” assumption used in recent studies. Once such a mass-radius template is adopted, the equation of state follows directly from the implementation of Lindblom’s algorithm; assumptions on the nature or composition of the dense stellar core are not required. By analyzing mass-radius profiles with a maximum mass consistent with observation and common radii in the 8–11 km range, a lower limit on the stellar radius of a $1.4M_{\odot }$ neutron star of $R_{\mathrm{NS}}\gtrsim 10.7\mathrm{km}$ is required to prevent the equation of state from violating causality.

Three-dimensional Euclidean pure gravity with a negative cosmological constant can be formulated in terms of the Chern-Simons theory, classically. This theory can be written in a supersymmetric way by introducing auxiliary gauginos and scalars. We calculate the exact partition function of this Chern-Simons theory by using the localization technique. Thus, we obtain the quantum gravity partition function, assuming that it can be obtained nonperturbatively by summing over partition functions of the Chern-Simons theory on topologically different manifolds. The resultant partition function is modular invariant, and, in the case in which the central charge is expected to be 24, it is the $J$ function, predicted by Witten.

A search is presented for hidden-sector bosons, $\chi$, produced in the decay $B^0\to K^{*}(892{)}^0\chi$, with $K^{*}(892{)}^0\to K^{+}{\pi }^{-}$ and $\chi \to {\mu }^{+}{\mu }^{-}$. The search is performed using $pp$-collision data corresponding to $3.0{\mathrm{fb}}^{-1}$ collected with the LHCb detector. No significant signal is observed in the accessible mass range $214\le m(\chi )\le 4350\mathrm{MeV}$, and upper limits are placed on the branching fraction product $\mathcal{B}\mathbf{(}{B}^0\to K^{*}(892{)}^0\chi \mathbf{)}\times \mathcal{B}(\chi \to {\mu }^{+}{\mu }^{-})$ as a function of the mass and lifetime of the $\chi$ boson. These limits are of the order of $10^{-9}$ for $\chi$ lifetimes less than 100 ps over most of the $m(\chi )$ range, and place the most stringent constraints to date on many theories that predict the existence of additional low-mass bosons.

We present the mass excesses of ${}^{52–57}\mathrm{Sc}$, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of ${}^{56}\mathrm{Sc}$ and ${}^{57}\mathrm{Sc}$ were determined for the first time with atomic mass excesses of $-24.85(59){(}_{-54}^{+0})\mathrm{MeV}$ and $-21.0(1.3)\mathrm{MeV}$, respectively, where the asymmetric uncertainty for ${}^{56}\mathrm{Sc}$ was included due to possible contamination from a long-lived isomer. The ${}^{56}\mathrm{Sc}$ mass indicates a small odd-even mass staggering in the $A=56$ mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of ${}^{56}\mathrm{Sc}$, our results strongly reduce uncertainties in model calculations of the heating and cooling at the ${}^{56}\mathrm{Ti}$ electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in $A\approx 56$ nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-$A$ nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.

Unstable spinor Bose-Einstein condensates are ideal candidates to create nonlinear three-mode interferometers. Our analysis goes beyond the standard SU(1,1) parametric approach and therefore provides the regime of parameters where sub-shot-noise sensitivities can be reached with respect to the input total average number of particles. Decoherence due to particle losses and finite detection efficiency are also considered.

In ponderomotive spectroscopy an amplitude-modulated optical standing wave is employed to probe Rydberg-atom transitions, utilizing a ponderomotive rather than a dipole-field interaction. Here, we engage nonlinearities in the modulation to drive dipole-forbidden transitions up to the fifth order. We reach transition frequencies approaching the sub-THz regime. We also demonstrate magic-wavelength conditions, which result in symmetric spectral lines with a Fourier-limited peak at the line center. Applicability to precision measurement is discussed.