Gold-capped Janus particles immersed in a near-critical binary mixture can be propelled using illumination. We employ a nonisothermal diffuse interface approach to investigate the self-propulsion mechanism of a single colloid. We attribute the motion to body forces at the edges of a micronsized droplet that nucleates around the particle. Thus, the often-used concept of a surface velocity cannot account for the self-propulsion. The particle’s swimming velocity is related to the droplet shape and size, which is determined by a so-called critical isotherm. Two distinct swimming regimes exist, depending on whether the droplet partially or completely covers the particle. Interestingly, the dependence of the swimming velocity on temperature is nonmonotonic in both regimes.

The statistical significance of network properties is conditioned on null models which satisfy specified properties but that are otherwise random. Exponential random graph models are a principled theoretical framework to generate such constrained ensembles, but which often fail in practice, either due to model inconsistency or due to the impossibility to sample networks from them. These problems affect the important case of networks with prescribed clustering coefficient or number of small connected subgraphs (motifs). In this Letter we use the Wang-Landau method to obtain a multicanonical sampling that overcomes both these problems. We sample, in polynomial time, networks with arbitrary degree sequences from ensembles with imposed motifs counts. Applying this method to social networks, we investigate the relation between transitivity and homophily, and we quantify the correlation between different types of motifs, finding that single motifs can explain up to 60% of the variation of motif profiles.

In the standard model, the weak interaction is formulated with a purely vector-axial-vector ($V\text{−}A$) structure. Without restriction on the chirality of the neutrino, the most general limits on tensor currents from nuclear $\beta$ decay are dominated by a single measurement of the $\beta \text{−}\overline{\nu }$ correlation in ${}^6\mathrm{He}$ $\beta$ decay dating back over a half century. In the present work, the $\beta \text{−}\overline{\nu }\text{−}\alpha$ correlation in the $\beta$ decay of ${}^8\mathrm{Li}$ and subsequent $\alpha$-particle breakup of the ${{}^8\mathrm{Be}}^{*}$ daughter was measured. The results are consistent with a purely $V\text{−}A$ interaction and in the case of couplings to right-handed neutrinos ($C_T=-C_T^{\prime }$) limits the tensor fraction to $|C_T/C_A{|}^2<0.011$ (95.5% C.L.). The measurement confirms the ${}^6\mathrm{He}$ result using a different nuclear system and employing modern ion-trapping techniques subject to different systematic uncertainties.

We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies $\hslash {\omega }_{\mathrm{ctr}}$ near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential ($\hslash \omega >54.4\mathrm{eV}$) and nonsequential ($\hslash \omega <54.4\mathrm{eV}$) double-ionization regimes, the sequential and nonsequential double-ionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for $\hslash {\omega }_{\mathrm{ctr}}=50\mathrm{eV}$ pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations.

It is shown numerically and analytically that when an optical pulse approaches a moving temporal boundary across which the refractive index changes, it undergoes a temporal equivalent of reflection and refraction of optical beams at a spatial boundary. The main difference is that the role of angles is played by changes in the frequency. The frequency dependence of the dispersion of the material in which the pulse is propagating plays a fundamental role in determining the frequency shifts experienced by the reflected and refracted pulses. Our analytic expressions for these frequency shifts allow us to find the condition under which an analog of total internal reflection may occur at the temporal boundary.

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of $3000\mathrm{T}/\mathrm{m}$, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

By combining molecular dynamics simulations and quantum mechanics calculations, we show the formation of a composite structure composed of embedded water molecules and the COOH matrix on carboxyl-terminated self-assembled monolayers (COOH SAMs) with appropriate packing densities. This composite structure with an integrated hydrogen bond network inside reduces the hydrogen bonds with the water above. This explains the seeming contradiction on the stability of the surface water on COOH SAMs observed in experiments. The existence of the composite structure at appropriate packing densities results in the two-step distribution of contact angles of water droplets on COOH SAMs, around 0° and 35°, which compares favorably to the experimental measurements of contact angles collected from forty research articles over the past 25 years. These findings provide a molecular-level understanding of water on surfaces (including surfaces on biomolecules) with hydrophilic functional groups.

In a semimetal, both electrons and holes contribute to the density of states at the Fermi level. The small band overlaps and multiband effects engender novel electronic properties. We show that a moderate hydrostatic pressure effectively suppresses the band gap in the elemental semiconductor black phosphorus. An electronic topological transition takes place at approximately 1.2 GPa, above which black phosphorus evolves into a semimetal state that is characterized by a colossal positive magnetoresistance and a nonlinear field dependence of Hall resistivity. The Shubnikov–de Haas oscillations detected in magnetic field reveal the complex Fermi surface topology of the semimetallic phase. In particular, we find a nontrivial Berry phase in one Fermi surface that emerges in the semimetal state, as evidence of a Dirac-like dispersion. The observed semimetallic behavior greatly enriches the material property of black phosphorus and sets the stage for the exploration of novel electronic states in this material.

We investigate many-body localization in the presence of a single-particle mobility edge. By considering an interacting deterministic model with an incommensurate potential in one dimension we find that the single-particle mobility edge in the noninteracting system leads to a many-body mobility edge in the corresponding interacting system for certain parameter regimes. Using exact diagonalization, we probe the mobility edge via energy resolved entanglement entropy (EE) and study the energy resolved applicability (or failure) of the eigenstate thermalization hypothesis (ETH). Our numerical results indicate that the transition separating area and volume law scaling of the EE does not coincide with the nonthermal to thermal transition. Consequently, there exists an extended nonergodic phase for an intermediate energy window where the many-body eigenstates violate the ETH while manifesting volume law EE scaling. We also establish that the model possesses an infinite temperature many-body localization transition despite the existence of a single-particle mobility edge. We propose a practical scheme to test our predictions in atomic optical lattice experiments which can directly probe the effects of the mobility edge.

A longitudinal magnetoresistance asymmetry (LMA) between a positive and negative magnetic field is known to occur in both the extreme quantum limit and the classical Drude limit in samples with a nonuniform doping density. By analyzing the current stream function in van der Pauw measurement geometry, it is shown that the electron density gradient can be quantitatively deduced from this LMA in the Drude regime. Results agree with gradients interpolated from local densities calibrated across an entire wafer, establishing a generalization of the van der Pauw method to quantify density gradients.

We introduce the numerical linked cluster expansion as a controlled numerical tool for the study of the many-body localization transition in a disordered system with continuous nonperturbative disorder. Our approach works directly in the thermodynamic limit, in any spatial dimension, and does not rely on any finite size scaling procedure. We study the onset of many-body delocalization through the breakdown of area-law entanglement in a generic many-body eigenstate. By looking for initial signs of an instability of the localized phase, we obtain a value for the critical disorder, which we believe should be a lower bound for the true value, that is higher than current best estimates from finite size studies. This implies that most current methods tend to overestimate the extent of the localized phase due to finite size effects making the localized phase appear stable at small length scales. We also study the mobility edge in these systems as a function of energy density, and we find that our conclusion is the same at all examined energies.

We present a continuum theory of self-propelled particles, without alignment interactions, in a momentum-conserving solvent. To address phase separation, we introduce a dimensionless scalar concentration field $\varphi$ with advective-diffusive dynamics. Activity creates a contribution ${\mathrm{\Sigma }}_{ij}=-\widehat{\kappa }[({\partial }_i\varphi )({\partial }_j\varphi )-(\nabla \varphi {)}^2{\delta }_{ij}/d]$ to the deviatoric stress, where $\widehat{\kappa }$ is odd under time reversal and $d$ is the number of spatial dimensions; this causes an effective interfacial tension contribution that is negative for contractile swimmers. We predict that domain growth then ceases at a length scale where diffusive coarsening is balanced by active stretching of interfaces, and confirm this numerically. Thus, there is a subtle interplay of activity and hydrodynamics, even without alignment interactions.

The dynamics of wave packets in the fractional Schrödinger equation is still an open problem. The difficulty stems from the fact that the fractional Laplacian derivative is essentially a nonlocal operator. We investigate analytically and numerically the propagation of optical beams in the fractional Schrödinger equation with a harmonic potential. We find that the propagation of one- and two-dimensional input chirped Gaussian beams is not harmonic. In one dimension, the beam propagates along a zigzag trajectory in real space, which corresponds to a modulated anharmonic oscillation in momentum space. In two dimensions, the input Gaussian beam evolves into a breathing ring structure in both real and momentum spaces, which forms a filamented funnel-like aperiodic structure. The beams remain localized in propagation, but with increasing distance display an increasingly irregular behavior, unless both the linear chirp and the transverse displacement of the incident beam are zero.

We investigate the resources needed for secure teleportation of coherent states. We extend continuous variable teleportation to include quantum teleamplification protocols that allow nonunity classical gains and a preamplification or postattenuation of the coherent state. We show that, for arbitrary Gaussian protocols and a significant class of Gaussian resources, two-way steering is required to achieve a teleportation fidelity beyond the no-cloning threshold. This provides an operational connection between Gaussian steerability and secure teleportation. We present practical recipes suggesting that heralded noiseless preamplification may enable high-fidelity heralded teleportation, using minimally entangled yet steerable resources.

As second-generation gravitational-wave detectors prepare to analyze data at unprecedented sensitivity, there is great interest in searches for unmodeled transients, commonly called bursts. Significant effort has yielded a variety of techniques to identify and characterize such transient signals, and many of these methods have been applied to produce astrophysical results using data from first-generation detectors. However, the computational cost of background estimation remains a challenging problem; it is difficult to claim a $5\sigma$ detection with reasonable computational resources without paying for efficiency with reduced sensitivity. We demonstrate a hierarchical approach to gravitational-wave transient detection, focusing on long-lived signals, which can be used to detect transients with significance in excess of $5\sigma$ using modest computational resources. In particular, we show how previously developed seedless clustering techniques can be applied to large data sets to identify high-significance candidates without having to trade sensitivity for speed.

We point out that kinetic and Stückelberg mixings that are generically present in the low energy effective action of axions can significantly widen the window of axion decay constants. We show that an effective super-Planckian decay constant can be obtained even when the axion kinetic matrix has only sub-Planckian entries. Our minimal model involves only two axions, a Stückelberg $U(1)$ and a modest rank instanton generating non-Abelian group. Below the mass of the Stückelberg $U(1)$, there is only a single axion with a nonperturbatively generated potential. In contrast to previous approaches, the enhancement of the axion decay constant is not tied to the number of degrees of freedom introduced. We also discuss how kinetic mixings can lower the decay constant to the desired axion dark matter window. String theory embeddings of this scenario and their phenomenological features are briefly discussed.

The LHCb Collaboration reported anomalies in $B\to K^{*}{\mu }^{+}{\mu }^{-}$, $B_s\to \varphi {\mu }^{+}{\mu }^{-}$, and $R(K)=B\to K{\mu }^{+}{\mu }^{-}/B\to K{e}^{+}{e}^{-}$. Furthermore, BABAR, BELLE, and LHCb Collaborations found hints for the violation of lepton-flavor universality violation in $R(D^{(*)})=B\to D^{(*)}\tau \nu /B\to D^{(*)}\ell \nu$. In this Letter we reexamine these decays and their correlations to $B\to K^{(*)}\nu \overline{\nu }$ using gauge invariant dim-6 operators. For the numerical analysis we focus on scenarios in which new physics couples, in the interaction eigenbasis, to third generation quarks and lepton only. We conclude that such a setup can explain the $b\to s{\mu }^{+}{\mu }^{-}$ data simultaneously with $R(D^{(*)})$ for small mixing angles in the lepton sector (of the order of $\pi /16$) and very small mixing angles in the quark sector (smaller than $V_{cb}$). In these regions of parameter space, $B\to K^{(*)}\tau \mu$ and $B_s\to \tau \mu$ can be order $10^{-6}$. Possible UV completions are briefly discussed.

We numerically solve the ($2+1$)-dimensional effective kinetic theory of weak coupling QCD under longitudinal expansion, relevant for early stages of heavy-ion collisions. We find agreement with viscous hydrodynamics and classical Yang-Mills simulations in the regimes where they are applicable. By choosing initial conditions that are motivated by a color-glass-condensate framework, we find that for $Q_s=2\mathrm{GeV}$ and ${\alpha }_s=0.3$ the system is approximately described by viscous hydrodynamics well before $\tau \lesssim 1.0\mathrm{fm}/c$.

We report on the measurement of the contribution of the magnetic-dipole hyperfine interaction to the tensor polarizaility of the electronic ground state in ${}^{87}\mathrm{Rb}$. This contribution was isolated by measuring the differential shift of the clock transition frequency in ${}^{87}\mathrm{Rb}$ atoms that were optically trapped in the focus of an intense ${\mathrm{CO}}_2$ laser beam. By comparing to previous tensor polarizability measurements in ${}^{87}\mathrm{Rb}$, the contribution of the interaction with the nuclear electric-quadrupole moment was isolated as well. Our measurement will enable better estimation of blackbody shifts in Rb atomic clocks. The methods reported here are applicable for future spectroscopic studies of atoms and molecules under strong quasistatic fields.