We present the first measurements of the ratios of cross sections $\sigma (p\overline{p}\to Z+c\text{jet})/\sigma (p\overline{p}\to Z+\text{jet})$ and $\sigma (p\overline{p}\to Z+c\text{jet})/\sigma (p\overline{p}\to Z+b\text{jet})$ for the associated production of a $Z$ boson with at least one charm or bottom quark jet. Jets have transverse momentum $\text{}{p}_T^{\text{jet}}\text{}>20\mathrm{GeV}$ and pseudorapidity $|\text{}{\eta }^{\text{jet}}\text{}|<2.5$. These cross section ratios are measured differentially as a function of jet and $Z$ boson transverse momenta, based on $9.7\mathrm{fb}{}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at the Fermilab Tevatron Collider at $\sqrt{s}=1.96\mathrm{TeV}$. The measurements show significant deviations from perturbative QCD calculations and predictions from various event generators.

We investigate phase shifts in the strong coupling regime of single-atom cavity quantum electrodynamics. On the light transmitted through the system, we observe a phase shift associated with an antiresonance and show that both its frequency and width depend solely on the atom, despite the strong coupling to the cavity. This shift is optically controllable and reaches 140°–the largest ever reported for a single emitter. Our result offers a new technique for the characterization of complex integrated quantum circuits.

Transformative technologies for desalination and chemical separations call for understanding molecular transport through man-made and biological nanochannels. Using numerical simulation of single-file flow of water through carbon nanotubes, we find that flow is due to fast-moving density variations (solitons) that are additive so flow rate is proportional to number of solitons. Simulation results match predictions from a theoretical model for soliton propagation. From 1–300 K flow rates increase as temperature decreases. Our results build a fundamentally new understanding of nanochannel flows and suggest new principles for the design of nanoscale devices.

Using an ultracold gas of atoms, we have realized a quasi-two-dimensional Fermi system with widely tunable $s$-wave interactions nearly in a ground state. Pressure and density are measured. The experiment covers physically different regimes: weakly and strongly attractive Fermi gases and a Bose gas of tightly bound pairs of fermions. In the Fermi regime of weak interactions, the pressure is systematically above a Fermi-liquid-theory prediction, maybe due to mesoscopic effects. In the opposite Bose regime, the pressure agrees with a bosonic mean-field scaling in a range beyond simplest expectations. In the strongly interacting regime, measurements disagree with a purely 2D model. Reported data may serve for sensitive testing of theoretical methods applicable across different quantum physics disciplines.

We theoretically predict that thin-film topological crystalline insulators can host various quantum anomalous Hall phases when doped by ferromagnetically ordered dopants. Any Chern number between $\pm 4$ can, in principle, be reached as a result of the interplay between (a) the induced Zeeman field, depending on the magnetic doping concentration, (b) the structural distortion, either intrinsic or induced by a piezoelectric material through the proximity effect, and (c) the thickness of the thin film. We propose a heterostructure to realize quantum anomalous Hall phases with Chern numbers that can be tuned by electric fields.

We consider an electronic analog of the Hong-Ou-Mandel (HOM) interferometer, where two single electrons travel along opposite chiral edge states and collide at a quantum point contact. Studying the current noise, we show that because of interactions between copropagating edge states, the degree of indistinguishability between the two electron wave packets is dramatically reduced, leading to reduced contrast for the HOM signal. This decoherence phenomenon strongly depends on the energy resolution of the packets. Insofar as interactions cause charge fractionalization, we show that charge and neutral modes interfere with each other, leading to satellite dips or peaks in the current noise. Our calculations explain recent experimental results [, $\mathrm{et}\mathrm{al}\mathrm{.}$, Science 339, 1054 (2013)] where an electronic HOM signal with reduced contrast was observed.

In monolayer ${\mathrm{MoS}}_2$, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300 K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4 ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300 K, close to the maximum emission polarization for this sample at 4 K.

Based on the nonrelativistic QCD factorization scheme, we present the first complete next-to-leading-order study on the yield and polarization of ${\rm Y}(1S,2S,3S)$ hadroproduction. By using the color-octet long-distance matrix elements obtained from fits of the experimental measurements on ${\rm Y}$ yield and polarization at the Tevatron and LHC, our results can explain the measurements on the yield very well, and for the polarizations of ${\rm Y}(1S,2S,3S)$, they are in (good, good, bad) agreement with recent CMS measurement, but still have some distance from the CDF measurement.

The ability to control wave propagation in highly deformable layered media with elastic instability-induced wrinkling of interfacial layers is presented. The onset of a wrinkling instability in initially straight interfacial layers occurs when a critical compressive strain is achieved. Further compression beyond the critical strain leads to an increase in the wrinkle amplitude of the interfacial layer. This, in turn, gives rise to the formation of a system of periodic scatterers, which reflect and interfere with wave propagation. We demonstrate that the topology of wrinkling interfacial layers can be controlled by deformation and used to produce band gaps in wave propagation and, hence, to selectively filter frequencies. Remarkably, the mechanism of frequency filtering is effective even for composites with similar or identical densities, such as polymer-polymer composites. Since the microstructure change is reversible, the mechanism can be used for tuning and controlling wave propagation by deformation.

A novel scheme for the focusing of high-energy leptons in future linear colliders was proposed in 2001 [P. Raimondi and A. Seryi, Phys. Rev. Lett. 86, 3779 (2001)]. This scheme has many advantageous properties over previously studied focusing schemes, including being significantly shorter for a given energy and having a significantly better energy bandwidth. Experimental results from the ATF2 accelerator at KEK are presented that validate the operating principle of such a scheme by demonstrating the demagnification of a 1.3 GeV electron beam down to below 65 nm in height using an energy-scaled version of the compact focusing optics designed for the ILC collider.

The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar responses of single centers and a SiV ensemble in a low strain reference sample prove our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect’s electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects.

We study theoretically the dynamical Casimir effect in an exciton-polariton condensate that is suddenly created by an ultrashort laser pulse at normal incidence. As a consequence of the abrupt change of the quantum vacuum, Bogoliubov excitations are generated. The subsequent evolution, governed by polariton interactions and losses, is studied within a linearized truncated Wigner approximation. We focus in particular on the momentum distribution and spatial coherence. The limiting behavior at large and small momenta is determined analytically. A simple scaling relation for the final condensate depletion as a function of the system parameters is found and the correlation length is shown to depend linearly on the condensate depletion.

We show that the isotropic conductivity in the normal state of rare-earth tritelluride $R{\mathrm{Te}}_3$ compounds is broken by the occurrence of the unidirectional charge density wave (CDW) in the ($a$, $c$) plane below the Peierls transition temperature. In contrast with quasi-one-dimensional systems, the resistivity anomaly associated with the CDW transition is strong in the direction perpendicular to the CDW wave vector $\mathbf{Q}$ ($a$ axis) and very weak in the CDW wave vector $\mathbf{Q}$ direction ($c$ axis). We qualitatively explain this result by calculating the electrical conductivity for the electron dispersion with momentum-dependent CDW gap as determined by angle-resolved photoemission spectroscopy. Similar measurements of in-plane conductivity may uncover the gap anisotropy in other compounds for which angle-resolved photoemission spectroscopy is not available.

We demonstrate spin injection into superconducting Nb by employing a spin absorption technique in lateral spin valve structures. Spin currents flowing in a nonmagnetic Cu channel are preferably absorbed into Nb due to its strong spin-orbit interaction, the amount of which dramatically changes below or above the superconducting critical temperature ($T_C$). The charge imbalance effect observed in the $\mathrm{Cu}/\mathrm{Nb}$ interface ensures that superconducting Nb absorbs pure spin currents even below $T_C$. Our analyses based on the density of states calculated using the Usadel equation can well reproduce the experimental results, implying that the strong spin-orbit interaction of Nb is still effective for the spin absorption even below $T_C$. Most importantly, our method allows us to determine the intrinsic spin relaxation time in the superconducting Nb, which reaches more than 4 times greater than that in the normal state.

The stratum corneum, the outer layer of mammalian skin, provides a remarkable barrier to the external environment, yet it has highly variable permeability properties where it actively mediates between inside and out. On prolonged exposure to water, swelling of the corneocytes (skin cells composed of keratin intermediate filaments) is the key process by which the stratum corneum controls permeability and mechanics. As for many biological systems with intricate function, the mesoscale geometry is optimized to provide functionality from basic physical principles. Here we show that a key mechanism of corneocyte swelling is the interplay of mesoscale geometry and thermodynamics: given helical tubes with woven geometry equivalent to the keratin intermediate filament arrangement, the balance of solvation free energy and elasticity induces swelling of the system, importantly with complete reversibility. Our result remarkably replicates macroscopic experimental data of native through to fully hydrated corneocytes. This finding not only highlights the importance of patterns and morphology in nature but also gives valuable insight into the functionality of skin.

We compute the universal conductivity of the ($2+1$)-dimensional $XY$ universality class, which is realized for a superfluid-to-Mott insulator quantum phase transition at constant density. Based on large-scale Monte Carlo simulations of the classical ($2+1$)-dimensional $J$-current model and the two-dimensional Bose-Hubbard model, we can precisely determine the conductivity on the quantum critical plateau, $\sigma (\infty )=0.359(4){\sigma }_Q$ with ${\sigma }_Q$ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottoms-up AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the $\sigma (i{\omega }_n)-\sigma (\infty )$ function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gauge-gravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation.

We develop a general theory for a quantum-walk search on a star graph. A star graph has $N$ edges each of which is attached to a central vertex. A graph $G$ is attached to one of these edges, and we would like to find out to which edge it is attached. This is done by means of a quantum walk, a quantum version of a random walk. This walk contains $O(\sqrt{N})$ steps, which represents a speedup over a classical search, which would require $O(N)$ steps. The overall graph, star plus $G$, is divided into two parts, and we find that for a quantum speedup to occur, the eigenvalues associated with these two parts in the $N\to \infty$ limit must be the same. Our theory tells us how the initial state of the walk should be chosen, and how many steps the walk must make in order to find $G$.

We show that, in conserved-mass transport processes, the steady-state distribution of mass in a subsystem is uniquely determined from the functional dependence of variance of the subsystem mass on its mean, provided that the joint mass distribution of subsystems is factorized in the thermodynamic limit. The factorization condition is not too restrictive as it would hold in systems with short-ranged spatial correlations. To demonstrate the result, we revisit a broad class of mass transport models and its generic variants, and show that the variance of the subsystem mass in these models is proportional to the square of its mean. This particular functional form of the variance constrains the subsystem mass distribution to be a gamma distribution irrespective of the dynamical rules.

We discuss the current carrying nonequilibrium steady state of an open fermionic Hubbard chain that is strongly driven by Markovian incoherent processes localized at the chain ends. An explicit form of an exact many-body density operator for any value of the coupling parameter is presented. The structure of a matrix product form of the solution is encoded in terms of a novel diagrammatic technique that should allow for generalization to other integrable nonequilibrium models.

The lifetime of the $\tau$ lepton is measured using the process $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$, where both $\tau$ leptons decay to $3\pi {\nu }_{\tau }$. The result for the mean lifetime, based on $711{\mathrm{fb}}^{-1}$ of data collected with the Belle detector at the $\mathrm{\Upsilon }(4S)$ resonance and $60\mathrm{MeV}$ below, is $\tau =\mathbf{(}290.17\pm 0.53(\text{stat})\pm 0.33(\text{syst})\mathbf{)}\times {10}^{-15}\mathrm{s}$. The first measurement of the lifetime difference between ${\tau }^{+}$ and ${\tau }^{-}$ is performed. The upper limit on the relative lifetime difference between positive and negative $\tau$ leptons is $|\mathrm{\Delta }\tau |/\tau <7.0\times {10}^{-3}$ at 90% C.L.