Despite extensive work on high-temperature superconductors, the critical behavior of an incipient condensate has so far been studied exclusively under equilibrium conditions. Here, we excite ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ with a femtosecond laser pulse and monitor the subsequent nonequilibrium dynamics of the midinfrared conductivity. Our data allow us to discriminate temperature regimes where superconductivity is either coherent, fluctuating or vanishingly small. Above the transition temperature $T_{\mathrm{c}}$, we make the striking observation that the relaxation to equilibrium exhibits power-law dynamics and scaling behavior, both for optimally and underdoped superconductors. Our findings can in part be modeled using time-dependent Ginzburg-Landau theory, and they provide strong indication of universality in systems far from equilibrium.

The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane, where only the molecular reorientational degrees of freedom freeze without long-range order, is investigated by nonlinear dielectric spectroscopy. Marked differences to canonical glass formers show up: While molecular cooperativity governs the glassy freezing, it leads to a much weaker slowing down of molecular dynamics than in supercooled liquids. Moreover, the observed nonlinear effects cannot be explained with the same heterogeneity scenario recently applied to canonical glass formers. This supports ideas that molecular relaxation in plastic crystals may be intrinsically nonexponential. Finally, no nonlinear effects were detected for the secondary processes in cyclo-octanol.

We show that the flocking transition in the Vicsek model is best understood as a liquid-gas transition, rather than an order-disorder one. The full phase separation observed in flocking models with $Z_2$ rotational symmetry is, however, replaced by a microphase separation leading to a smectic arrangement of traveling ordered bands. Remarkably, continuous deterministic descriptions do not account for this difference, which is only recovered at the fluctuating hydrodynamics level. Scalar and vectorial order parameters indeed produce different types of number fluctuations, which we show to be essential in selecting the inhomogeneous patterns. This highlights an unexpected role of fluctuations in the selection of flock shapes.

We show that for classical and quantum observables, the integrability-to-ergodicity transition leaves constant the sum of (a) the ensemble variance of the temporal average and (b) the ensemble average of temporal variance. The induced Frobenius (Hilbert-Schmidt) geometry of quantum observables encodes how eigenstate thermalization appears, the inverse participation ratio decreases, and the integrals of motion disappear during the transition. We use it to optimize the set of conserved quantities entering the generalized Gibbs ensemble for integrable, near-integrable, or mesoscopic systems.

Considering the problem of sampling from the output photon-counting probability distribution of a linear-optical network for input Gaussian states, we obtain results that are of interest from both quantum theory and the computational complexity theory point of view. We derive a general formula for calculating the output probabilities, and by considering input thermal states, we show that the output probabilities are proportional to permanents of positive-semidefinite Hermitian matrices. It is believed that approximating permanents of complex matrices in general is a #P-hard problem. However, we show that these permanents can be approximated with an algorithm in the ${\mathrm{BPP}}^{\mathrm{NP}}$ complexity class, as there exists an efficient classical algorithm for sampling from the output probability distribution. We further consider input squeezed-vacuum states and discuss the complexity of sampling from the probability distribution at the output.

We investigate the motion of a colloidal particle driven out of equilibrium by a time-varying stiffness of the optical trap that produces persistent nonequilibrium work. Measurements of work production for repeated cycles composed of the compression and expansion processes for the optical potential show huge fluctuations due to thermal motion. Using a precise technique to modulate the stiffness in time, we accurately estimate the probability distributions of work produced for the compression and expansion processes. We confirm the fluctuation theorem from the ratio of the two distributions. We also show that the average values of work for the two processes comply with the Jarzynski equality. This system has an analogy with a gas in a breathing soft wall. We discuss about its applicability to a heat engine and an information engine operated by feedback control.

We extract the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $V_{\mathrm{cb}}$ and the most relevant parameters of the heavy quark expansion from data of inclusive semileptonic $B$ decays. Our calculation includes the recently computed $O({\alpha }_s{\mathrm{\Lambda }}_{\mathrm{QCD}}^2/m_b^2)$ corrections and a careful estimate of the residual theoretical uncertainty. Using a recent determination of the charm quark mass, we obtain $|V_{\mathrm{cb}}|=(42.21\pm 0.78)\times 10^{-3}$ and $m_b^{\text{kin}}(1\mathrm{GeV})=(4.553\pm 0.020)\mathrm{GeV}$.

We analyze the temperature and doping dependence of the specific heat $C(T)$ in ${\mathrm{Na}}_x{\mathrm{CoO}}_2$. This material was conjectured to undergo a Lifshitz-type topological transition at $x=x_c=0.62$, in which a new electron Fermi pocket emerges at the $\mathrm{\Gamma }$ point, in addition to the existing hole pocket with large $k_F$. The data show that near $x=x_c$, the temperature dependence of $C(T)/T$ at low $T$ gets stronger as $x$ approaches $x_c$ from below and then reverses the trend and changes sign at $x\ge x_c$. We argue that this behavior can be quantitatively explained within the spin-fluctuation theory. We show that magnetic fluctuations are enhanced near $x_c$ at momenta around $k_F$, and their dynamics changes between $x\le x_c$ and $x>x_c$, when the new pocket forms. We demonstrate that this explains the temperature dependence of $C(T)/T$. We show that at larger $x$ ($x>0.65$) the system enters a magnetic quantum critical regime where $C(T)/T$ roughly scales as $\log T$. This behavior extends to progressively lower $T$ as $x$ increases towards a magnetic instability at $x\approx 0.75$.

We demonstrate optical manipulation of the position of a domain wall in a dilute magnetic semiconductor, GaMnAsP. Two main contributions are identified. First, photocarrier spin exerts a spin-transfer torque on the magnetization via the exchange interaction. The direction of the domain-wall motion can be controlled using the helicity of the laser. Second, the domain wall is attracted to the hot spot generated by the focused laser. Unlike magnetic-field-driven domain-wall depinning, these mechanisms directly drive domain-wall motion, providing an optical tweezerlike ability to position and locally probe domain walls.

A scroll wave in a very thin layer of excitable medium is similar to a spiral wave, but its behavior is affected by the layer geometry. We identify the effect of sharp variations of the layer thickness, which is separate from filament tension and curvature-induced drifts described earlier. We outline a two-step asymptotic theory describing this effect, including asymptotics in the layer thickness and calculation of the drift of so-perturbed spiral waves using response functions. As specific examples, we consider drift of scrolls along thickness steps, ridges, ditches, and disk-shaped thickness variations. Asymptotic predictions agree with numerical simulations.

The COMPASS collaboration at CERN has investigated pion Compton scattering, ${\pi }^{-}\gamma \to {\pi }^{-}\gamma$, at center-of-mass energy below 3.5 pion masses. The process is embedded in the reaction ${\pi }^{-}\mathrm{Ni}\to {\pi }^{-}\gamma \mathrm{Ni}$, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015(\mathrm{GeV}/c{)}^2$. From a sample of 63 000 events, the pion electric polarizability is determined to be ${\alpha }_{\pi }=(2.0\pm 0.6_{\mathrm{stat}}\pm 0.7_{\mathrm{syst}})\times 10^{-4}{\mathrm{fm}}^3$ under the assumption ${\alpha }_{\pi }=-{\beta }_{\pi }$, which relates the electric and magnetic dipole polarizabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambiguously known, was performed for an independent estimate of the systematic uncertainty.

There is a significant discrepancy between the values of the proton electric form factor, $G_E^p$, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of $G_E^p$ from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization ($ϵ$) and momentum transfer ($Q^2$) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing $ϵ$ at $Q^2=1.45{\mathrm{GeV}}^2$. This measurement is consistent with the size of the form factor discrepancy at $Q^2\approx 1.75{\mathrm{GeV}}^2$ and with hadronic calculations including nucleon and $\mathrm{\Delta }$ intermediate states, which have been shown to resolve the discrepancy up to $2–3{\mathrm{GeV}}^2$.

Several experimental investigations have observed parity violation (PV) in nuclear systems—a consequence of the weak force between quarks. We apply the $1/N_c$ expansion of QCD to the $P$-violating $T$-conserving component of the nucleon-nucleon (NN) potential. We show there are two leading-order operators, both of which affect $\overrightarrow{p}p$ scattering at order $N_c$. We find an additional four operators at order $N_c^0{\sin }^2{\theta }_W$ and six at $\mathcal{O}(1/N_c)$. Pion exchange in the PV NN force is suppressed by $1/N_c$ and ${\sin }^2{\theta }_W$, providing a quantitative explanation for its nonobservation up to this time. The large-$N_c$ hierarchy of other PV NN force mechanisms is consistent with estimates of the couplings in phenomenological models. The PV observed in $\overrightarrow{p}p$ scattering data is compatible with natural values for the strong and weak coupling constants: there is no evidence of fine-tuning.

In reduced-dimensional semiconductors, doping-induced carrier plasmons can strongly couple with quasiparticle excitations, leading to a significant band gap renormalization. However, the physical origin of this generic effect remains obscure. We develop a new plasmon-pole theory that efficiently and accurately captures this coupling. Using monolayer ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$ as prototype two-dimensional (2D) semiconductors, we reveal a striking band gap renormalization above 400 meV and an unusual nonlinear evolution of their band gaps with doping. This prediction significantly differs from the linear behavior that is observed in one-dimensional structures. Notably, our predicted band gap renormalization for ${\mathrm{MoSe}}_2$ is in excellent agreement with recent experimental results. Our developed approach allows for a quantitative understanding of many-body interactions in general doped 2D semiconductors and paves the way for novel band gap engineering techniques.

Nanoscale corrugations are of great importance in determining the physical properties of two-dimensional crystals. However, the mechanical behavior of atomically thin films under strain is not fully understood. In this Letter, we show a layer-dependent mechanical response of molybdenum disulfide (${\mathrm{MoS}}_2$) subject to atomistic-precision strain induced by $2H$-bilayer island epitaxy. Dimensional crossover in the mechanical properties is evidenced by the formation of star-shaped nanoripple arrays in the first monolayer, while rippling instability is completely suppressed in the bilayer. Microscopic-level quantum mechanical simulations reveal that the nanoscale rippling is realized by the twisting of neighboring Mo—S bonds without modifying the chemical bond length, and thus invalidates the classical continuum mechanics. The formation of nanoripple arrays significantly changes the electronic and nanotribological properties of monolayer ${\mathrm{MoS}}_2$. Our results suggest that quantum mechanical behavior is not unique for $s{p}^2$ bonding but general for atomic membranes under strain.

The surface state of a ${\mathbb{Z}}_2$ topological insulator connects with the conduction and valence band continua of the bulk, thereby bridging the band gap of the volume. We investigate this connection of the surface and bulk electronic structure for ${\mathrm{Sb}}_2{\mathrm{Te}}_3(0001)$ by photoemission experiments and calculations. Upon crossing the topmost valence band the topological surface state (TSS) maintains a coherent spectral signature, a two-dimensional character, and a linear dispersion relation. Surface-bulk coupling manifests itself in the spectra through (i) a characteristic kink in the TSS dispersion as it crosses the topmost valence band and (ii) the appearance of hybridization gaps between the TSS and bulk-derived surface resonance states at higher binding energies. The findings provide a natural explanation for the unexpectedly weak surface-bulk mixing indicated by recent transport experiments on ${\mathrm{Sb}}_2{\mathrm{Te}}_3$.