It is now well established that structural glasses possess disorder- and frustration-induced soft quasilocalized excitations, which play key roles in various glassy phenomena. Recent work has established that in model glass formers in three dimensions, these nonphononic soft excitations may assume the form of quasilocalized, harmonic vibrational modes whose frequency follows a universal density of states $D(\omega )\sim {\omega }^4$, independently of microscopic details, and for a broad range of glass preparation protocols. Here, we further establish the universality of the nonphononic density of vibrational modes by direct measurements in model structural glasses in two dimensions and four dimensions. We also investigate their degree of localization, which is generally weaker in lower spatial dimensions, giving rise to a pronounced system-size dependence of the nonphononic density of states in two dimensions, but not in higher dimensions. Finally, we identify a fundamental glassy frequency scale ${\omega }_c$ above which the universal ${\omega }^4$ law breaks down.

Motivated by recent experiments in ultracold gases, we focus on the properties of the center-of-mass coordinate of an interacting one-dimensional Fermi gas, displaying several distinct phases. While the variance of the center of mass vanishes in insulating phases such as phase-separated and charge density wave phases, it remains finite in the metallic phase, which realizes a Luttinger liquid. By combining numerics with bosonization, we demonstrate that the autocorrelation function of the center-of-mass coordinate is universal throughout the metallic phase. It exhibits persistent oscillations, and its short time dynamics reveal important features of the quantum liquid, such as the Luttinger liquid parameter and the renormalized velocity. The full counting statistics of the center of mass follows a normal distribution already for small systems. Our results apply to nonintegrable systems as well and are within experimental reach for, e.g., carbon nanotubes and cold atomic gases.

Employing large-scale quantum Monte Carlo simulations, we study the extended $XXZ$ model on the kagome lattice. A ${\mathbb{Z}}_2$ quantum spin liquid phase with effective even Ising gauge field structure emerges from the delicate balance among three symmetry-breaking phases including stripe solid, staggered solid, and ferromagnet. This ${\mathbb{Z}}_2$ spin liquid is stabilized by an extended interaction related to the Rokhsar-Kivelson potential in the quantum dimer model limit. The phase transitions from the staggered solid to a spin liquid or ferromagnet are found to be first order and so is the transition between the stripe solid and ferromagnet. However, the transition between a spin liquid and ferromagnet is found to be continuous and belongs to the 3D $X{Y}^{*}$ universality class associated with the condensation of spinons. The transition between a spin liquid and stripe solid appears to be continuous and associated with the condensation of visons.

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs) embedded in thin films. The nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358–371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab initio calculations of the positron wave function and lifetime employing a recent formulation of the weighted density approximation demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the long-standing question regarding the nature of the positron state in semiconductor QDs and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

Atomically thin semiconductors provide an ideal testbed to investigate the physics of Coulomb-bound many-body states. We shed light on the intricate structure of such complexes by studying the magnetic-field-induced splitting of biexcitons in monolayer ${\mathrm{WS}}_2$ using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30 T. The observed $g$ factor of the biexciton amounts to about $-3.9$, closely matching the $g$ factor of the neutral exciton. The biexciton emission shows an inverted circular field-induced polarization upon linearly polarized excitation; i.e., it exhibits preferential emission from the high-energy peak in a magnetic field. This phenomenon is explained by taking into account the hybrid configuration of the biexciton constituents in momentum space and their respective energetic behavior in magnetic fields. Our findings reveal the critical role of dark excitons in the composition of this many-body state.

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer ${\mathrm{WSe}}_2$ reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than $1\mu \mathrm{s}$. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

Nanoscale photothermal sources find important applications in theranostics, imaging, and catalysis. In this context, graphene offers a unique suite of optical, electrical, and thermal properties, which we exploit to show self-consistent active photothermal modulation of its nanoscale response. In particular, we predict the existence of plasmons confined to the optical landscape tailored by continuous-wave external-light pumping of homogeneous graphene. This result relies on the high electron temperatures achievable in optically pumped clean graphene while its lattice remains near ambient temperature. Our study opens a new avenue toward the active optical control of the nanophotonic response in graphene with potential application in photothermal devices.

Using Floquet dynamical mean-field theory, we study the high-harmonic generation in the time-periodic steady states of wide-gap Mott insulators under ac driving. In the strong-field regime, the harmonic intensity exhibits multiple plateaus, whose cutoff energies ${\epsilon }_{\mathrm{cut}}=U+m{E}_0$ scale with the Coulomb interaction $U$ and the maximum field strength $E_0$. In this regime, the created doublons and holons are localized because of the strong field and the $m$th plateau originates from the recombination of $m$th nearest-neighbor doublon-holon pairs. In the weak-field regime, there is only a single plateau in the intensity, which originates from the recombination of itinerant doublons and holons. Here, ${\epsilon }_{\mathrm{cut}}={\mathrm{\Delta }}_{\mathrm{gap}}+\alpha E_0$, with ${\mathrm{\Delta }}_{\mathrm{gap}}$ the band gap and $\alpha >1$. We demonstrate that the Mott insulator shows a stronger high-harmonic intensity than a semiconductor model with the same dispersion as the Mott insulator, even if the semiconductor bands are broadened by impurity scattering to mimic the incoherent scattering in the Mott insulator.

While nearly all theoretical and computational studies of entangled polymer melts have focused on uniform samples, polymer synthesis routes always result in some dispersity, albeit narrow, of distribution of molecular weights (${Đ}_M=M_w/M_n\sim 1.02–1.04$). Here, the effects of dispersity on chain mobility are studied for entangled, disperse melts using a coarse-grained model for polyethylene. Polymer melts with chain lengths set to follow a Schulz-Zimm distribution for the same average $M_w=36\mathrm{kg}/\mathrm{mol}$ with ${Đ}_M=1.0$ to 1.16, were studied for times of $600–800\mu \mathrm{s}$ using molecular dynamics simulations. This time frame is longer than the time required to reach the diffusive regime. We find that dispersity in this range does not affect the entanglement time or tube diameter. However, while there is negligible difference in the average mobility of chains for the uniform distribution ${Đ}_M=1.0$ and ${Đ}_M=1.02$, the shortest chains move significantly faster than the longest ones offering a constraint release pathway for the melts for larger ${Đ}_M$.

We present two complex scalar gauge invariants for perturbations of the Kerr spacetime defined covariantly in terms of the Killing vectors and the conformal Killing-Yano tensor of the background together with the linearized curvature and its first derivatives. These invariants are, in particular, sensitive to variations of the Kerr parameters. Together with the Teukolsky scalars and the linearized Ricci tensor, they form a minimal set that generates all local gauge invariants. We also present curvature invariants that reduce to the gauge invariants in the linearized theory.

Exclusive measurements of the quasifree $pp\to pp{\pi }^{+}{\pi }^{-}$ reaction have been carried out at WASA@COSY by means of $pd$ collisions at $T_p=1.2\mathrm{GeV}$. Total and differential cross sections have been extracted covering the energy region $T_p=1.08–1.36\mathrm{GeV}$, which is the region of $N^{*}(1440)$ and $\mathrm{\Delta }(1232)\mathrm{\Delta }(1232)$ resonance excitations. Calculations describing these excitations by $t$-channel meson exchange are at variance with the measured differential cross sections and underpredict substantially the experimental total cross section. An isotensor $\mathrm{\Delta }N$ dibaryon resonance with $I(J^P)=2(1^{+})$ produced associatedly with a pion is able to overcome these deficiencies.

We report on the observation of incoherent Cherenkov radiation emitted by a 5.3 GeV positron beam circulating in the Cornell electron-positron storage ring as the beam passes in the close vicinity of the surface of a fused silica radiator (i.e., at a distance larger than 0.8 mm). The shape of the radiator was designed in order to send the Cherenkov photons towards the detector, consisting of a compact optical system equipped with an intensified camera. The optical system allows both the measurements of 2D images and angular distribution including polarization study. The corresponding light intensity has been measured as a function of the distance between the beam and the surface of the radiator and has shown a good agreement with theoretical predictions. For highly relativistic particles, a large amount of incoherent radiation is produced in a wide spectral range. A light yield of $0.8\times {10}^{-3}$ photon per particle per turn has been measured at a wavelength of $600\pm 10\mathrm{nm}$ in a 2 cm long radiator and for an impact parameter of 1 mm. This will find applications in accelerators as noninvasive beam diagnostics for both leptons and hadrons.

Pseudoelasticity in metals is typically associated with phase transformations (e.g., shape memory alloys) but has recently been observed in sub-10 nm Ag nanocrystals that rapidly recovered their original shape after deformation to large strains. The discovery of pseudoelasticity in nanoscale metals dramatically changes the current understanding of the properties of solids at the smallest length scales, and the motion of atoms at surfaces. Yet, it remains unclear whether pseudoelasticity exists in different metals and nanocrystal sizes. The challenge of observing deformation at atomistic to nanometer length scales has prevented a clear mechanistic understanding of nanoscale pseudoelasticity, although surface diffusion and dislocation-mediated processes have been proposed. We further the understanding of pseudoelasticity in nanoscale metals by using a diamond anvil cell to compress colloidal Au nanocrystals under quasihydrostatic and nonhydrostatic pressure conditions. Nanocrystal structural changes are measured using optical spectroscopy and transmission electron microscopy and modeled using electrodynamic theory. We find that 3.9 nm Au nanocrystals exhibit pseudoelastic shape recovery after deformation to large uniaxial strains of up to 20%, which is equivalent to an ellipsoid with an aspect ratio of 2. Nanocrystal absorbance efficiency does not recover after deformation, which indicates that crystalline defects may be trapped in the nanocrystals after deformation.

We demonstrate the presence of ferromagnetic (FM) fluctuations in the superconducting and nonsuperconducting heavily overdoped regimes of high-temperature superconducting copper oxides, using $(\mathrm{Bi},\mathrm{Pb}{)}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_{6+\delta }$ (Bi-2201) single crystals. Magnetization curves exhibit a tendency to be saturated in high magnetic fields at low temperatures in the heavily overdoped crystals, which is probably a precursor phenomenon of a FM transition at a lower temperature. Muon spin relaxation detects the enhancement of spin fluctuations at high temperatures below 200 K. Correspondingly, the $ab$-plane resistivity follows a $4/3$ power law in a wide temperature range, which is characteristic of metals with two-dimensional FM fluctuations due to itinerant electrons. As the Wilson ratio evidences the enhancement of spin fluctuations with hole doping in the heavily overdoped regime, it is concluded that two-dimensional FM fluctuations reside in the heavily overdoped Bi-2201 cuprates, which is probably related to the decrease in the superconducting transition temperature in the heavily overdoped cuprates.

The ultimate goal of multiferroic research is the development of a new-generation nonvolatile memory devices, where magnetic bits are controlled via electric fields with low energy consumption. Here, we demonstrate the optical identification of magnetoelectric (ME) antiferromagnetic (AFM) domains in the ${\mathrm{LiCoPO}}_4$ exploiting the strong absorption difference between the domains. This unusual contrast, also present in zero magnetic field, is attributed to the dynamic ME effect of the spin-wave excitations, as confirmed by our microscopic model, which also captures the characteristics of the observed static ME effect. The control and the optical readout of AFM/ME domains, demonstrated here, will likely promote the development of ME and spintronic devices based on AFM insulators.

We investigate the kinetics of a polymer collapse due to the formation of irreversible cross-links between its monomers. Using the contact probability $P(s)$ as a scale-dependent order parameter depending on the chemical distance $s$, our simulations show the emergence of a cooperative pearling instability. Namely, the polymer undergoes a sharp conformational transition to a set of absorbing states characterized by a length scale $\xi$ corresponding to the mean pearl size. This length and the transition time depend on the polymer equilibrium dynamics and the cross-linking rate. We confirm experimentally this transition using a DNA conformation capture experiment in yeast.

A deep understanding of the dynamical properties of natural time-varying images is essential for interpreting how they are efficiently processed in the brain. Here we examine natural time-varying images from the perspective of their spatiotemporal patterns and find evidence of dynamical thermodynamic criticality. We further demonstrate that these spatiotemporal patterns ubiquitously organize as localized, propagating patterns. By studying these propagating patterns and their spreading dynamics over time, we demonstrate that the critical dynamics of natural time-varying images belong to the universality class of directed percolation. These critical dynamics have important implications for understanding neural processing of time-varying stimuli.

We present the precision measurement from May 2011 to May 2017 (79 Bartels rotations) of the proton fluxes at rigidities from 1 to 60 GV and the helium fluxes from 1.9 to 60 GV based on a total of $1\times {10}^9$ events collected with the Alpha Magnetic Spectrometer aboard the International Space Station. This measurement is in solar cycle 24, which has the solar maximum in April 2014. We observed that, below 40 GV, the proton flux and the helium flux show nearly identical fine structures in both time and relative amplitude. The amplitudes of the flux structures decrease with increasing rigidity and vanish above 40 GV. The amplitudes of the structures are reduced during the time period, which started one year after solar maximum, when the proton and helium fluxes steadily increase. Above $\sim 3\mathrm{GV}$ the $p/\mathrm{He}$ flux ratio is time independent. We observed that below $\sim 3\mathrm{GV}$ the ratio has a long-term decrease coinciding with the period during which the fluxes start to rise.