We report the STAR measurements of dielectron ($e^{+}{e}^{-}$) production at midrapidity ($|y_{ee}|<1$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurements are evaluated in different invariant mass regions with a focus on 0.30–0.76 ($\rho$-like), 0.76–0.80 ($\omega$-like), and 0.98–1.05 ($\varphi$-like) $\mathrm{GeV}/{\mathrm{c}}^2$. The spectrum in the $\omega$-like and $\varphi$-like regions can be well described by the hadronic cocktail simulation. In the $\rho$-like region, however, the vacuum $\rho$ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of $1.77\pm 0.11(\text{stat})\pm 0.24(\text{syst})\pm 0.33(\text{cocktail})$ is determined with respect to the hadronic cocktail simulation that excludes the $\rho$ meson. The excess yield in the $\rho$-like region increases with the number of collision participants faster than the $\omega$ and $\varphi$ yields. Theoretical models with broadened $\rho$ contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.

We studied simultaneously the ${}^4\mathrm{He}(e,e^{\prime }p)$, ${}^4\mathrm{He}(e,e^{\prime }pp)$, and ${}^4\mathrm{He}(e,e^{\prime }pn)$ reactions at $Q^2=2(\mathrm{GeV}/c{)}^2$ and $x_B>1$, for an $(e,e^{\prime }p)$ missing-momentum range of 400 to $830\mathrm{MeV}/c$. The knocked-out proton was detected in coincidence with a proton or neutron recoiling almost back to back to the missing momentum, leaving the residual $A=2$ system at low excitation energy. These data were used to identify two-nucleon short-range correlated pairs and to deduce their isospin structure as a function of missing momentum, in a region where the nucleon-nucleon ($NN$) force is expected to change from predominantly tensor to repulsive. The abundance of neutron-proton pairs is reduced as the nucleon momentum increases beyond $\sim 500\mathrm{MeV}/c$. The extracted fraction of proton-proton pairs is small and almost independent of the missing momentum. Our data are compared with calculations of two-nucleon momentum distributions in ${}^4\mathrm{He}$ and discussed in the context of probing the elusive repulsive component of the $NN$ force.

Contrary to the belief that ignition of a combustible mixture by a high-energy kernel is more difficult in turbulence than in quiescence because of the increased dissipation rate of the deposited energy, we experimentally demonstrate that it can actually be facilitated by turbulence for mixtures whose thermal diffusivity sufficiently exceeds its mass diffusivity. In such cases, turbulence breaks the otherwise single spherical flame of positive curvature, and hence positive aerodynamics stretch, into a multitude of wrinkled flamelets subjected to either positive or negative stretch, such that the intensified burning of the latter constitutes local sources to facilitate ignition.

The law of the wall and the log law rule the near-wall mean velocity profile of three-dimensional turbulent flows. These well-known laws, which are validated by legions of experiments and simulations, may be universal. Here, using a soap-film channel, we report the first experimental test of these laws in quasi-two-dimensional turbulent channel flows under two disparate turbulent spectra. We find that despite the differences with three-dimensional flows, the laws prevail, albeit with notable distinctions: the two parameters of the log law are markedly distinct from their three-dimensional counterpart; further, one parameter (the von Kármán constant) is independent of the spectrum whereas the other (the offset of the log law) depends on the spectrum. Our results suggest that the classical theory of scaling in wall-bounded turbulence is incomplete wherein a key missing element is the link with the turbulent spectrum.

We report complex plasma experiments, assisted by numerical simulations, providing an alternative qualitative link between the macroscopic response of polycrystalline solid matter to small shearing forces and the possible underlying microscopic processes. In the stationary creep regime we have determined the exponents of the shear rate dependence of the shear stress and defect density, being $\alpha =1.15\pm 0.1$ and $\beta =2.4\pm 0.4$, respectively. We show that the formation and rapid glide motion of dislocation pairs in the lattice are dominant processes.

In nonlinear dynamical systems with highly nonorthogonal linear eigenvectors, linear nonmodal analysis is more useful than normal mode analysis in predicting turbulent properties. However, the nontrivial time evolution of nonmodal structures makes quantitative understanding and prediction difficult. We present a technique to overcome this difficulty by modeling the effect that the advective nonlinearities have on spatial turbulent structures. The nonlinearities are taken as a periodic randomizing force with time scale consistent with critical balance arguments. We apply this technique to a model of drift wave turbulence in the Large Plasma Device [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)], where nonmodal effects dominate the turbulence. We compare the resulting growth rate spectra to the spectra obtained from a nonlinear simulation, showing good qualitative agreement, especially in comparison to the eigenmode growth rate spectra.

Electron supercollimation, in which a wave packet is guided to move undistorted along a selected direction, is a highly desirable property that has yet to be realized experimentally. Disorder in general is expected to inhibit supercollimation. Here we report a counterintuitive phenomenon of electron supercollimation by disorder in graphene and related Dirac fermion materials. We show that one can use one-dimensional disorder potentials to control electron wave packet transport. This is distinct from known systems where an electron wave packet would be further spread by disorder and hindered in the potential fluctuating direction. The predicted phenomenon has significant implications in the understanding and applications of electron transport in Dirac fermion materials.

It is shown that strong driving of a quantum system substantially enhances the Lamb shift induced by broadband reservoirs, which are typical for solid-state devices. By varying drive parameters the impact of environmental vacuum fluctuations with continuous spectral distribution onto system observables can be tuned in a distinctive way. This provides experimentally feasible measurement schemes for the Lamb shift in superconducting circuits based on Cooper pair boxes, where it can be detected either in shifted dressed transition frequencies or in pumped charge currents.

We investigate the effect of the Berry phase on quadrupoles that occur, for example, in the low-energy description of spin models. Specifically, we study here the one-dimensional bilinear-biquadratic spin-one model. An open question for many years about this model is whether it has a nondimerized fluctuating nematic phase. The dimerization has recently been proposed to be related to Berry phases of the quantum fluctuations. We use an effective low-energy description to calculate the scaling of the dimerization according to this theory and then verify the predictions using large scale density-matrix renormalization group simulations, giving good evidence that the state is dimerized all the way up to its transition into the ferromagnetic phase. We furthermore discuss the multiplet structure found in the entanglement spectrum of the ground state wave functions.

In a solid-state platform for quantum information science, the biexciton cascade is an important source of entangled photons. However, the entanglement is usually reduced considerably by the fine-structure splitting of the exciton levels. We show how to counteract this loss of entanglement by applying optical feedback. Substantial control and enhancement of photon entanglement can be achieved by coherently feeding back a part of the emitted signal, e.g., by a mirror, and by tuning the feedback phase and delay time. We present full quantum-mechanical calculations, which include the external photon mode continuum, and discuss the mechanisms leading to the above effects.

Sensitive nanoscale magnetic resonance imaging of target spins using nitrogen-vacancy (NV) centers in diamond requires a quantitative understanding of dominant noise at the surface. We probe this noise by applying dynamical decoupling to shallow NVs at calibrated depths. Results support a model of NV dephasing by a surface bath of electronic spins having a correlation rate of 200 kHz, much faster than that of the bulk N spin bath. Our method of combining nitrogen delta-doping growth and nanoscale depth imaging paves a way for studying spin noise present in diverse material surfaces.

We show that the number of distinct topological states associated with a given knotted defect, $L$, in a nematic liquid crystal is equal to the determinant of the link $L$. We give an interpretation of these states, demonstrate how they may be identified in experiments, and describe the consequences for material behavior and interactions between multiple knots. We show that stable knots can be created in a bulk cholesteric and illustrate the topology by classifying a simulated Hopf link. In addition, we give a topological heuristic for the resolution of strand crossings in defect coarsening processes which allows us to distinguish topological classes of a given link and to make predictions about defect crossings in nematic liquid crystals.

We directly measure the flow-driven injection of DNA through nanopores at the level of single molecule and single pore using a modified zero-mode waveguide method. We observe a flow threshold independent of the pore radius, the DNA concentration, and length. We demonstrate that the flow injection of DNA in nanopores is controlled by an energy barrier as proposed in the de Gennes–Brochard suction model. Finally, we show that the height of the energy barrier is modulated by functionalizing the nanopores.

We discuss the existence of a nontrivial topological phase in one-dimensional interacting systems described by the extended Bose-Hubbard model with a mean filling of one boson per site. Performing large-scale density-matrix renormalization group calculations we show that the presence of nearest-neighbor repulsion enriches the ground-state phase diagram of the paradigmatic Bose-Hubbard model by stabilizing a novel gapped insulating state, the so-called Haldane insulator, which, embedded into superfluid, Mott insulator, and density wave phases, is protected by the lattice inversion symmetry. The quantum phase transitions between the different insulating phases were determined from the central charge via the von Neumann entropy. The Haldane phase reveals a characteristic fourfold degeneracy of the entanglement spectrum. We finally demonstrate that the intensity maximum of the dynamical charge structure factor, accessible by Bragg spectroscopy, features the gapped dispersion known from the spin-1 Heisenberg chain.

The quantum steering ellipsoid of a two-qubit state is the set of Bloch vectors that Bob can collapse Alice’s qubit to, considering all possible measurements on his qubit. We provide an elementary construction of the ellipsoid for arbitrary states, calculate its volume, and explain how this geometric representation can be made faithful. The representation provides a range of new results, and uncovers new features, such as the existence of “incomplete steering” in separable states. We show that entanglement can be analyzed in terms of three geometric features of the ellipsoid and prove that a state is separable if and only if it obeys a “nested tetrahedron” condition.

We show that Casimir-Polder forces between two relativistic uniformly accelerated atoms exhibit a transition from the short distance thermal-like behavior predicted by the Unruh effect to a long distance nonthermal behavior, associated with the breakdown of a local inertial description of the system. This phenomenology extends the Unruh thermal response detected by a single accelerated observer to an accelerated spatially extended system of two particles, and we identify the characteristic length scale for this crossover with the inverse of the proper acceleration of the two atoms. Our results are derived separating at fourth order in perturbation theory the contributions of vacuum fluctuations and radiation reaction field to the Casimir-Polder interaction between two atoms moving in two generic stationary trajectories separated by a constant distance and linearly coupled to a scalar field. The field can be assumed in its vacuum state or at finite temperature, resulting in a general method for the computation of Casimir-Polder forces in stationary regimes.

We present a general “fit-free” method for measuring the equation of state (EoS) of a scale-invariant gas. This method, which is inspired from the procedure introduced by Ku et al. [Science 335, 563 (2012)] for the unitary three-dimensional Fermi gas, provides a general formalism which can be readily applied to any quantum gas in a known trapping potential, in the frame of the local density approximation. We implement this method on a weakly interacting two-dimensional Bose gas across the Berezinskii-Kosterlitz-Thouless transition and determine its EoS with unprecedented accuracy in the critical region. Our measurements provide an important experimental benchmark for classical-field approaches which are believed to accurately describe quantum systems in the weakly interacting but nonperturbative regime.

We report on a conceptually new test of the equivalence principle performed by measuring the acceleration in Earth’s gravity field of two isotopes of strontium atoms, namely, the bosonic ${}^{88}\mathrm{Sr}$ isotope which has no spin versus the fermionic ${}^{87}\mathrm{Sr}$ isotope which has a half-integer spin. The effect of gravity on the two atomic species has been probed by means of a precision differential measurement of the Bloch frequency for the two atomic matter waves in a vertical optical lattice. We obtain the values $\eta =(0.2\pm 1.6)\times {10}^{-7}$ for the Eötvös parameter and $k=(0.5\pm 1.1)\times {10}^{-7}$ for the coupling between nuclear spin and gravity. This is the first reported experimental test of the equivalence principle for bosonic and fermionic particles and opens a new way to the search for the predicted spin-gravity coupling effects.