The response of a particle in a periodic potential to an applied force is commonly described by an effective mass, which accounts for the detailed interaction between the particle and the surrounding potential. Using a Bose-Einstein condensate of ${}^{87}\mathrm{Rb}$ atoms initially in the ground band of an optical lattice, we experimentally show that the initial response of a particle to an applied force is in fact characterized by the bare mass. Subsequently, the particle response undergoes rapid oscillations and only over time scales that are long compared to those of the interband dynamics is the effective mass observed to be an appropriate description. Our results elucidate the role of the effective mass on short time scales, which is relevant for example in the interaction of few-cycle laser pulses with dielectric and semiconductor materials.

In disordered systems, our present understanding of the Anderson transition is hampered by the possible presence of interactions between particles. We demonstrate that in boson gases, even weak interactions deeply alter the very nature of the Anderson transition. While there still exists a critical point in the system, below that point a novel phase appears, displaying a new critical exponent, subdiffusive transport, and a breakdown of the one-parameter scaling description of Anderson localization.

A search for flavor-changing neutral currents in top-quark decays $t\to Zq$ is performed in events produced from the decay chain $t\overline{t}\to Zq+Wb$, where both vector bosons decay leptonically, producing a final state with three leptons (electrons or muons). A data set collected with the CMS detector at the LHC is used, corresponding to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. No excess is seen in the observed number of events relative to the standard model prediction; thus, no evidence for flavor-changing neutral currents in top-quark decays is found. A combination with a previous search at 7 TeV excludes a $t\to Zq$ branching fraction greater than 0.05% at the 95% confidence level.

Dynamical systems with complex delayed interactions arise commonly when propagation times are significant, yielding complicated oscillatory instabilities. In this Letter, we introduce a class of systems with multiple, hierarchically long time delays, and using a suitable space-time representation we uncover features otherwise hidden in their temporal dynamics. The behavior in the case of two delays is shown to “encode” two-dimensional spiral defects and defects turbulence. A multiple scale analysis sets the equivalence to a complex Ginzburg-Landau equation, and a novel criterium for the attainment of the long-delay regime is introduced. We also demonstrate this phenomenon for a semiconductor laser with two delayed optical feedbacks.

We report measurements of the temperature variance ${\sigma }^2(z,r)$ and frequency power spectrum $P(f,z,r)$ ($z$ is the distance from the sample bottom and $r$ the radial coordinate) in turbulent Rayleigh-Bénard convection (RBC) for Rayleigh numbers $\mathrm{Ra}=1.6\times {10}^{13}$ and $1.1\times {10}^{15}$ and for a Prandtl number $\mathrm{Pr}\simeq 0.8$ for a sample with a height $L=224\mathrm{cm}$ and aspect ratio $D/L=0.50$ ($D$ is the diameter). For $z/L\lesssim 0.1$ ${\sigma }^2(z,r)$ was consistent with a logarithmic dependence on $z$, and there was a universal (independent of Ra, $r$, and $z$) normalized spectrum which, for $0.02\lesssim f{\tau }_0\lesssim 0.2$, had the form $P(f{\tau }_0)=P_0(f{\tau }_0{)}^{-1}$ with $P_0=0.208\pm 0.008$ a universal constant. Here ${\tau }_0=\sqrt{2R}$ where $R$ is the radius of curvature of the temperature autocorrelation function $C(\tau )$ at $\tau =0$. For $z/L\simeq 0.5$ the measurements yielded $P(f{\tau }_0)\sim (f{\tau }_0{)}^{-\alpha }$ with $\alpha$ in the range from $3/2$ to $5/3$. All the results are similar to those for velocity fluctuations in shear flows at sufficiently large Reynolds numbers, suggesting the possibility of an analogy between the flows that is yet to be determined in detail.

We employ an exact solution of the simplest model for pump-probe time-resolved photoemission spectroscopy in charge-density-wave systems to show how, in nonequilibrium, the gap in the density of states disappears while the charge density remains modulated, and then the gap reforms after the pulse has passed. This nonequilibrium scenario qualitatively describes the common short-time experimental features in ${\mathrm{TaS}}_2$ and ${\mathrm{TbTe}}_3$, indicating a quasiuniversality for nonequilibrium “melting” with qualitative features that can be easily understood within a simple picture.

We study the possibility of realizing robust helical surface states in $Z_2=0$ systems. We find that the combination of anisotropy and finite-size confinement leads to the emergence of robust helical edge states in both two-dimensional and three-dimensional $Z_2=0$ systems. By investigating an anisotropic Bernevig-Hughes-Zhang model in a finite sample, we demonstrate that the transport manifestation of the surface states is robust against nonmagnetic disorder, resembling that of a $Z_2=1$ phase. Notably, the effective energy gap of the robust helical states can be efficiently engineered, allowing for potential applications as valley filters and valley valves. The realization of emerging robust helical surface states in realistic materials is also discussed.

Tunneling in a quantum coherent structure is not restricted to only nearest neighbors. Hopping between distant sites is possible via the virtual occupation of otherwise avoided intermediate states. Here we report the observation of long-range transitions in the transport through three quantum dots coupled in series. A single electron is delocalized between the left and right quantum dots, while the center one remains always empty. Superpositions are formed, and both charge and spin are exchanged between the outermost dots. The delocalized electron acts as a quantum bus transferring the spin state from one end to the other. Spin selection is enabled by spin correlations. The process is detected via the observation of narrow resonances which are insensitive to Pauli spin blockade.

We introduce a class of noisy quantum cellular automata on a qubit lattice that includes all classical Markov chains, as well as maps where quantum coherence between sites is allowed to build up over time. We apply such a construction to the problem of excitation transfer through 1D lattices, and compare the performance of classical and quantum dynamics with equal local transition probabilities. Our discrete approach has the merits of stripping down the complications of the open system dynamics, of clearly isolating coherent effects, and of allowing for an exact treatment of conditional dynamics, all while capturing a rich variety of dynamical behaviors.

This Letter details a measurement of the ionization yield ($Q_y$) of 6.7 keV ${}^{40}\mathrm{Ar}$ atoms stopping in a liquid argon detector. The $Q_y$ of 3.6–6.3 detected $e^{-}/\mathrm{keV}$, for applied electric fields in the range $240–2130\mathrm{V}/\mathrm{cm}$, is encouraging for the use of this detector medium to search for the signals from hypothetical dark matter particle interactions and from coherent elastic neutrino-nucleus scattering. A significant dependence of $Q_y$ on the applied electric field is observed and explained in the context of ion recombination.

We analyze the relationship between irrationality and quasiperiodicity in nonlinear driven systems. To that purpose, we consider a nonlinear system whose steady-state response is very sensitive to the periodic or quasiperiodic character of the input signal. In the infinite time limit, an input signal consisting of two incommensurate frequencies will be recognized by the system as quasiperiodic. We show that this is, in general, not true in the case of finite interaction times. An irrational ratio of the driving frequencies of the input signal is not sufficient for it to be recognized by the nonlinear system as quasiperiodic, resulting in observations which may differ by several orders of magnitude from the expected quasiperiodic behavior. Thus, the system response depends on the nature of the irrational ratio, as well as the observation time. We derive a condition for the input signal to be identified by the system as quasiperiodic. Such a condition also takes into account the sub-Fourier response of the nonlinear system.

The band structure of single-layer black phosphorus and the effect of strain are predicted using density functional theory and tight-binding models. Having determined the localized orbital composition of the individual bands from first principles, we use the system symmetry to write down the effective low-energy Hamiltonian at the $\mathrm{\Gamma }$ point. From numerical calculations and arguments based on the crystal structure of the material, we show that the deformation in the direction normal to the plane can be used to change the gap size and induce a semiconductor-metal transition.

We investigate a previously unknown phase of phosphorus that shares its layered structure and high stability with the black phosphorus allotrope. We find the in-plane hexagonal structure and bulk layer stacking of this structure, which we call “blue phosphorus,” to be related to graphite. Unlike graphite and black phosphorus, blue phosphorus displays a wide fundamental band gap. Still, it should exfoliate easily to form quasi-two-dimensional structures suitable for electronic applications. We study a likely transformation pathway from black to blue phosphorus and discuss possible ways to synthesize the new structure.

We examine the hydrodynamic approach to dynamical correlations in one-dimensional superfluids near integrability and calculate the characteristic time scale, $\tau$, beyond which this approach is valid. For time scales shorter than $\tau$, hydrodynamics fails, and we develop an approach based on kinetics of fermionic quasiparticles described as mobile impurities. New universal results for the dynamical structure factor relevant to experiments in ultracold atomic gases are obtained.

We consider the generation of dark matter mass via radiative electroweak symmetry breaking in an extension of the conformal standard model containing a singlet scalar field with a Higgs portal interaction. Generating the mass from a sequential process of radiative electroweak symmetry breaking followed by a conventional Higgs mechanism can account for less than 35% of the cosmological dark matter abundance for dark matter mass $M_s>80\mathrm{GeV}$. However, in a dynamical approach where both Higgs and scalar singlet masses are generated via radiative electroweak symmetry breaking, we obtain much higher levels of dark matter abundance. At one-loop level we find abundances of 10%–100% with $106\mathrm{GeV}<M_s<120\mathrm{GeV}$. However, when the higher-order effects needed for consistency with a 125 GeV Higgs mass are estimated, the abundance becomes 10%–80% for $80\mathrm{GeV}<M_s<96\mathrm{GeV}$, representing a significant decrease in the dark matter mass. The dynamical approach also predicts a small scalar-singlet self-coupling, providing a natural explanation for the astrophysical observations that place upper bounds on dark matter self-interaction. The predictions in all three approaches are within the $M_s>80\mathrm{GeV}$ detection region of the next generation XENON experiment.

We consider single interval Rényi and entanglement entropies for a two dimensional conformal field theory on a circle at nonzero temperature. Assuming that the finite size of the system introduces a unique ground state with a nonzero mass gap, we calculate the leading corrections to the Rényi and entanglement entropy in a low temperature expansion. These corrections have a universal form for any two dimensional conformal field theory that depends only on the size of the mass gap and its degeneracy. We analyze the limits where the size of the interval becomes small and where it becomes close to the size of the spatial circle.

A search for the production of heavy partners of the top quark with charge $5/3$ is performed in events with a pair of same-sign leptons. The data sample corresponds to an integrated luminosity of $19.5{\mathrm{fb}}^{-1}$ and was collected at $\sqrt{s}=8\mathrm{TeV}$ by the CMS experiment. No significant excess is observed in the data above the expected background, and the existence of top-quark partners with masses below 800 GeV is excluded at a 95% confidence level, assuming they decay exclusively to $tW$. This is the first limit on these particles from the LHC, and it is significantly more restrictive than previous limits.

Single-neutron states in ${}^{133}\mathrm{Sn}$ and ${}^{209}\mathrm{Pb}$, which are analogous to single-electron states outside of closed atomic shells in alkali metals, were populated by the (${}^9\mathrm{Be}$, ${}^8\mathrm{Be}$) one-neutron transfer reaction in inverse kinematics using particle-$\gamma$ coincidence spectroscopy. In addition, the $s_{1/2}$ single-neutron hole-state candidate in ${}^{131}\mathrm{Sn}$ was populated by (${}^9\mathrm{Be}$, ${}^{10}\mathrm{Be}$). Doubly closed-shell ${}^{132}\mathrm{Sn}$ (radioactive) and ${}^{208}\mathrm{Pb}$ (stable) beams were used at sub-Coulomb barrier energies of 3 MeV per nucleon. Level energies, $\gamma$-ray transitions, absolute cross sections, spectroscopic factors, asymptotic normalization coefficients, and excited-state lifetimes are reported and compared with shell-model expectations. The results include a new transition and precise level energy for the $3p_{1/2}$ candidate in ${}^{133}\mathrm{Sn}$, new absolute cross sections for the $1h_{9/2}$ candidate in ${}^{133}\mathrm{Sn}$ and $3s_{1/2}$ candidate in ${}^{131}\mathrm{Sn}$, and new lifetimes for excited states in ${}^{133}\mathrm{Sn}$ and ${}^{209}\mathrm{Pb}$. This is the first report on excited-state lifetimes of ${}^{133}\mathrm{Sn}$, which allow for a unique test of the nuclear shell model and ${}^{132}\mathrm{Sn}$ double-shell closure.