A combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and molecular dynamics simulations employing a many-body interatomic potential model is used to investigate the structure of cold-compressed silica glass. The simulations give a good account of the neutron diffraction results and of existing x-ray diffraction results at pressures up to $\sim 60\mathrm{GPa}$. On the basis of the molecular dynamics results, an atomistic model for densification is proposed in which rings are “zipped” by a pairing of five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence of the mean primitive ring size $⟨n⟩$ on the mean Si-O coordination number, thereby linking a parameter that is sensitive to ordering on multiple length scales to a readily measurable parameter that describes the local coordination environment.

In the framework of the continuum shell theory, we analytically predict a new phenomenon: the weak localization of optical low-frequency oscillations in carbon nanotubes. We clarify the origin of the localization by means of the concept of the limiting phase trajectory and confirm the obtained analytical results by molecular dynamics simulations of simply supported carbon nanotubes. The performed analysis contributes to the new universal approach to the treatment of nonstationary resonant processes.

By atomistic modeling of moiré patterns of graphene on a substrate with a small lattice mismatch, we find qualitatively different strain distributions for small and large misorientation angles, corresponding to the commensurate-incommensurate transition recently observed in graphene on hexagonal BN. We find that the ratio of C-N and C-B interactions is the main parameter determining the different bond lengths in the center and edges of the moiré pattern. Agreement with experimental data is obtained only by assuming that the C-B interactions are at least twice weaker than the C-N interactions. The correspondence between the strain distribution in the nanoscale moiré pattern and the potential energy surface at the atomic scale found in our calculations makes the moiré pattern a tool to study details of dispersive forces in van der Waals heterostructures.

Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of $\{\mathrm{N},\mathrm{O},\mathrm{F}\}$ causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates.

A color center in $c$-BN which is isoelectronic to diamond ${\mathrm{NV}}^{-}$ is predicted based on first-principles electronic structure calculations using the Heyd-Scuseria-Ernzerhof hybrid functional. The defect consists of a substitutional oxygen and an adjacent boron vacancy (${\mathrm{O}}_{\mathrm{N}}-V_{\mathrm{B}}$). We find that the ${\mathrm{O}}_{\mathrm{N}}-V_{\mathrm{B}}$ center is optically accessible with a zero-phonon line of about 1.6 eV. The ${\mathrm{O}}_{\mathrm{N}}-V_{\mathrm{B}}$ center also shares much of the characteristics of the GC-2 center often observed in $c$-BN. A prominent vibronic coupling peak is predicted to be around 55 meV, which is in excellent agreement with the characteristic phonon frequency (56 meV) observed in the luminescence spectra of the GC-2 center.

We use polarization-dependent angle-resolved photoemission spectroscopy (ARPES) to study the high-energy anomaly (HEA) in the dispersion of ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_4$, $x=0.123$. We find that at particular photon energies the anomalous, waterfall-like dispersion gives way to a broad, continuous band. This suggests that the HEA is a matrix element effect: it arises due to a suppression of the intensity of the broadened quasiparticle band in a narrow momentum range. We confirm this interpretation experimentally, by showing that the HEA appears when the matrix element is suppressed deliberately by changing the light polarization. Calculations of the matrix element using atomic wave functions and simulation of the ARPES intensity with one-step model calculations provide further evidence for this scenario. The possibility to detect the full quasiparticle dispersion further allows us to extract the high-energy self-energy function near the center and at the edge of the Brillouin zone.

We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system ${\mathrm{GeV}}_4{\mathrm{S}}_8$. Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon.

Suspended, slender self-assembled domains of magnetically responsive colloids are observed to buckle in microgravity. Upon cessation of the magnetic field that drives their assembly, these columns expand axially and buckle laterally. This phenomenon resembles the buckling of long beams due to thermal expansion; however, linear stability analysis predicts that the colloidal columns are inherently susceptible to buckling because they are freely suspended in a Newtonian fluid. The dominant buckling wavelength increases linearly with column thickness and is quantitatively described using an elastohydrodynamic model and the suspension thermodynamic equation of state.

We obtain the all-loop worldsheet $S$ matrix for fundamental excitations on ${\mathrm{AdS}}_3\times S^3\times T^4$ by studying the off-shell symmetry algebra of the superspace action in light cone gauge. The massless modes, unaccounted for in earlier works, are automatically included in our treatment. Their exact dispersion relation is found to be nonrelativistic, of giant-magnon form, and their scattering is naturally well defined. This opens the way to a complete investigation of ${\mathrm{AdS}}_3/{\mathrm{CFT}}_2$ integrability.

Using full-scale 3D particle-in-cell simulations we show that the radiation reaction dominated regime can be reached in an all-optical configuration through the collision of a $\sim 1\mathrm{GeV}$ laser wakefield accelerated electron bunch with a counterpropagating laser pulse. In this configuration the radiation reaction significantly reduces the energy of the particle bunch, thus providing clear experimental signatures for the process with currently available lasers. We also show that the transition between the classical and quantum radiation reaction could be investigated in the same configuration with laser intensities of $10^{23}\mathrm{W}/{\mathrm{cm}}^2$.

We study the effect of longitudinal space charge on the correlated energy spread of a relativistic high-brightness electron beam that has been density modulated for the emission of coherent, high-harmonic radiation. We show that, in the case of electron bunching induced by a laser modulator followed by a dispersive chicane, longitudinal space charge forces can act to strongly reduce the induced energy modulation of the beam without a significant reduction in the harmonic bunching content. This effect may be optimized to enhance the output power and overall performance of free-electron lasers that produce coherent light through high-gain harmonic generation. It also increases the harmonic number achievable in these devices, which are otherwise gain-limited by the induced energy modulation from the laser.

Using fast electron spin resonance spectroscopy of a single nitrogen-vacancy defect in diamond, we demonstrate real-time readout of the Overhauser field produced by its nuclear spin environment under ambient conditions. These measurements enable narrowing the Overhauser field distribution by postselection, corresponding to a conditional preparation of the nuclear spin bath. Correlations of the Overhauser field fluctuations are quantitatively inferred by analyzing the Allan deviation over consecutive measurements. This method allows us to extract the dynamics of weakly coupled nuclear spins of the reservoir.

We study the effects of topological (connectivity) disorder on phase transitions. We identify a broad class of random lattices whose disorder fluctuations decay much faster with increasing length scale than those of generic random systems, yielding a wandering exponent of $\omega =(d-1)/(2d)$ in $d$ dimensions. The stability of clean critical points is thus governed by the criterion $(d+1)\nu >2$ rather than the usual Harris criterion $d\nu >2$, making topological disorder less relevant than generic randomness. The Imry-Ma criterion is also modified, allowing first-order transitions to survive in all dimensions $d>1$. These results explain a host of puzzling violations of the original criteria for equilibrium and nonequilibrium phase transitions on random lattices. We discuss applications, and we illustrate our theory by computer simulations of random Voronoi and other lattices.

To gain insight into the quantum nature of cosmological singularities, we study anisotropic Kasner solutions in gauge-gravity duality. The dual description of the bulk evolution towards the singularity involves $\mathcal{N}=4$ super Yang-Mills theory on the expanding branch of deformed de Sitter space and is well defined. We compute two-point correlators of Yang-Mills operators of large dimensions using spacelike geodesics anchored on the boundary. The correlators show a strong signature of the singularity around horizon scales and decay at large boundary separation at different rates in different directions. More generally, the boundary evolution exhibits a process of particle creation similar to that in inflation. This leads us to conjecture that information on the quantum nature of cosmological singularities is encoded in long-wavelength features of the boundary wave function.

We present the results of searches for nucleon decay via $n\to \overline{\nu }{\pi }^0$ and $p\to \overline{\nu }{\pi }^{+}$ using data from a combined $172.8\mathrm{kt}·\mathrm{yr}$ exposure of Super-Kamiokande-I,-II, and-III. We set lower limits on the partial lifetime for each of these modes: ${\tau }_{n\to \overline{\nu }{\pi }^0}>1.1\times {10}^{33}$ years and ${\tau }_{p\to \overline{\nu }{\pi }^{+}}>3.9\times {10}^{32}$ years at a 90% confidence level.

We demonstrate finite range binding potentials between pairs of Rydberg atoms interacting with each other via attractive and repulsive van der Waals potentials and driven by a microwave field. We show that, using destructive quantum interference to cancel single-atom Rydberg excitation, the Rydberg-dimer states can be selectively and coherently populated from the two-atom ground state. This can be used to realize a two-qubit interaction gate which is not susceptible to mechanical forces between the atoms and is therefore immune to motional decoherence.