Using high-resolution in situ small angle x-ray scattering in conjunction with oscillatory shear on highly monodisperse silica suspensions, we demonstrate that an order-to-disorder transition leads to a dynamic shear thickening in a lower stress regime than the standard steady shear thickening. We show that the order-to-disorder transition is controlled by strain, which is distinguishably different from steady shear thickening, which is a stress-related phenomenon. The appearance of this two-step shear thinning and thickening transition is also influenced by the particle size, monodispersity, and measurement conditions (i.e., oscillatory shear versus steady shear). Our results show definitively that the order-to-disorder transition-induced thickening is completely unrelated to the mechanism that drives steady shear thickening.

Boundary conformal field theories have several additional terms in the trace anomaly of the stress tensor associated purely with the boundary. We constrain the corresponding boundary central charges in three- and four-dimensional conformal field theories in terms of two- and three-point correlation functions of the displacement operator. We provide a general derivation by comparing the trace anomaly with scale dependent contact terms in the correlation functions. We conjecture a relation between the $a$-type boundary charge in three dimensions and the stress tensor two-point function near the boundary. We check our results for several free theories.

We consider a class of models in which thermal dark matter is lighter than a MeV. If dark matter thermalizes with the standard model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cool and heat the standard model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. We demonstrate this mechanism in a model consisting of fermionic dark matter coupled to a light scalar mediator. Thermal dark matter can be as light as a few keV, while remaining compatible with existing cosmological and astrophysical observations. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.

During 2015, the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized $p+p$ collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in $p+p$ collisions predicts only a moderate atomic-mass-number ($A$) dependence. In contrast, the asymmetries observed at RHIC in $p+A$ collisions showed a surprisingly strong $A$ dependence in inclusive forward neutron production. The observed asymmetry in $p+\mathrm{Al}$ collisions is much smaller, while the asymmetry in $p+\mathrm{Au}$ collisions is a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed $A$ dependence.

Taking advantage of the high acceptance and axial symmetry of the WASA-at-COSY detector, and the high polarization degree of the proton beam of COSY, the reaction $\overrightarrow{p}p\to pp\eta$ has been measured close to threshold to explore the analyzing power $A_y$. The angular distribution of $A_y$ is determined with the precision improved by more than 1 order of magnitude with respect to previous results, allowing a first accurate comparison with theoretical predictions. The determined analyzing power is consistent with zero for an excess energy of $Q=15\mathrm{MeV}$, signaling $s$-wave production with no evidence for higher partial waves. At $Q=72\mathrm{MeV}$ the data reveal strong interference of $Ps$ and $Pp$ partial waves and cancellation of $(Pp{)}^2$ and $S{s}^{*}Sd$ contributions. These results rule out the presently available theoretical predictions for the production mechanism of the $\eta$ meson.

Directional amplifiers are an important resource in quantum-information processing, as they protect sensitive quantum systems from excess noise. Here, we propose an implementation of phase-preserving and phase-sensitive directional amplifiers for microwave signals in an electromechanical setup comprising two microwave cavities and two mechanical resonators. We show that both can reach their respective quantum limits on added noise. In the reverse direction, they emit thermal noise stemming from the mechanical resonators; we discuss how this noise can be suppressed, a crucial aspect for technological applications. The isolation bandwidth in both is of the order of the mechanical linewidth divided by the amplitude gain. We derive the bandwidth and gain-bandwidth product for both and find that the phase-sensitive amplifier has an unlimited gain-bandwidth product. Our study represents an important step toward flexible, on-chip integrated nonreciprocal amplifiers of microwave signals.

Contact hysteresis between sliding interfaces is a widely observed phenomenon from macro- to nanoscale sliding interfaces. Most such studies are done using an atomic force microscope (AFM) where the sliding speed is a few $\mu \mathrm{m}/\mathrm{s}$. Here, we present a unique study on stiction between the head-disk interface of commercially available hard disk drives, wherein the vertical clearance between the head and the disk is of the same order as in various AFM-based fundamental studies but with a sliding speed that is nearly 6 orders of magnitude higher. We demonstrate that, although the electrostatic force (dc or ac voltage) is an attractive force, the ac-voltage-induced out-of-plane oscillation of the head with respect to the disk is able to completely suppress the contact hysteresis.

We introduce a new class of electrides with nontrivial band topology by coupling materials database searches and first-principles-calculations-based analysis. ${\mathrm{Cs}}_3\mathrm{O}$ and ${\mathrm{Ba}}_3\mathrm{N}$ are for the first time identified as a new class of electrides, consisting of one-dimensional (1D) nanorod building blocks. Their crystal structures mimic $\beta \text{−}{\mathrm{TiCl}}_3$ with the position of anions and cations exchanged. Unlike the weakly coupled nanorods of $\beta \text{−}{\mathrm{TiCl}}_3$, ${\mathrm{Cs}}_3\mathrm{O}$ and ${\mathrm{Ba}}_3\mathrm{N}$ retain 1D anionic electrons along the hollow interrod sites; additionally, a strong interrod interaction in ${\mathrm{C}}_3\mathrm{O}$ and ${\mathrm{Ba}}_3\mathrm{N}$ induces band inversion in a 2D superatomic triangular lattice, resulting in Dirac-node lines. The new class of electrides can serve as a prototype for new electrides with a large cavity space that can be utilized for various applications such as gas storage, ion transport, and metal intercalation.

We investigate electromagnetic corrections to the rare $B$-meson leptonic decay $B_{s,d}\to {\mu }^{+}{\mu }^{-}$ from scales below the bottom-quark mass $m_b$. Contrary to QCD effects, which are entirely contained in the $B$-meson decay constant, we find that virtual photon exchange can probe the $B$-meson structure, resulting in a “nonlocal annihilation” effect. We find that this effect gives rise to a dynamical enhancement by a power of $m_b/{\mathrm{\Lambda }}_{\mathrm{QCD}}$ and by large logarithms. The impact of this novel effect on the branching ratio of $B_{s,d}\to {\mu }^{+}{\mu }^{-}$ is about 1%, of the order of the previously estimated nonparametric theoretical uncertainty, and four times the size of previous estimates of next-to-leading order QED effects due to residual scale dependence. We update the standard model (SM) prediction to $\overline{\mathcal{B}}(B_s\to {\mu }^{+}{\mu }^{-}{)}_{\mathrm{SM}}=(3.57\pm 0.17)\times {10}^{-9}$.

Fluid dynamics is traditionally thought to apply only to systems near local equilibrium. In this case, the effective theory of fluid dynamics can be constructed as a gradient series. Recent applications of resurgence suggest that this gradient series diverges, but can be Borel resummed, giving rise to a hydrodynamic attractor solution which is well defined even for large gradients. Arbitrary initial data quickly approaches this attractor via nonhydrodynamic mode decay. This suggests the existence of a new theory of far-from-equilibrium fluid dynamics. In this Letter, the framework of fluid dynamics far from local equilibrium for a conformal system is introduced, and the hydrodynamic attractor solutions for resummed Baier-Romatschke-Son-Starinets-Stephanov theory, kinetic theory in the relaxation time approximation, and strongly coupled $N=4$ super Yang-Mills theory are identified for a system undergoing Bjorken flow.

We study the polarization of particles in relativistic heavy-ion collisions at very high energy along the beam direction within a relativistic hydrodynamic framework. We show that this component of the polarization decreases much slower with center-of-mass energy compared to the transverse component, even in the ideal longitudinal boost-invariant scenario with nonfluctuating initial state, and that it can be measured by taking advantage of its quadrupole structure in the transverse momentum plane. In the ideal longitudinal boost-invariant scenario, the polarization is proportional to the gradient of temperature at the hadronization and its measurement can provide important information about the cooling rate of the quark-gluon plasma around the critical temperature.

An adiabatic connection (AC) formula for the electron correlation energy is derived for a broad class of multireference wave functions. The AC expression recovers dynamic correlation energy and assures a balanced treatment of the correlation energy. Coupling the AC formalism with the extended random phase approximation allows one to find the correlation energy only from reference one- and two-electron reduced density matrices. If the generalized valence bond perfect pairing model is employed a simple closed-form expression for the approximate AC formula is obtained. This results in the overall $M^5$ scaling of the computation cost making the method one of the most efficient multireference approaches accounting for dynamic electron correlation also for the strongly correlated systems.

A new pathway of strong-laser-field-induced ionization of an atom is identified which is based on recollisions under the tunneling barrier. With an amended strong-field approximation, the interference of the direct and the under-the-barrier recolliding quantum orbits are shown to induce a measurable shift of the peak of the photoelectron momentum distribution. The scaling of the momentum shift is derived relating the momentum shift to the tunneling delay time according to the Wigner concept. This allows us to extend the Wigner concept for the quasistatic tunneling time delay into the nonadiabatic domain. The obtained corrections to photoelectron momentum distributions are also relevant for state-of-the-art accuracy of strong-field photoelectron spectrograms in general.

In the presence of $P$, $T$-violating interactions, the exchange of axionlike particles between electrons and nucleons in atoms and molecules induces electric dipole moments (EDMs) of atoms and molecules. We perform calculations of such axion-exchange-induced atomic EDMs using the relativistic Hartree-Fock-Dirac method including electron core polarization corrections. We present analytical estimates to explain the dependence of these induced atomic EDMs on the axion mass and atomic parameters. From the experimental bounds on the EDMs of atoms and molecules, including ${}^{133}\mathrm{Cs}$, ${}^{205}\mathrm{Tl}$, ${}^{129}\mathrm{Xe}$, ${}^{199}\mathrm{Hg}$, ${}^{171}\mathrm{Yb}{}^{19}\mathrm{F}$, ${}^{180}\mathrm{Hf}{}^{19}{\mathrm{F}}^{+}$, and ${}^{232}\mathrm{Th}{}^{16}\mathrm{O}$, we constrain the $P$, $T$-violating scalar-pseudoscalar nucleon-electron and electron-electron interactions mediated by a generic axionlike particle of arbitrary mass. Our limits improve on existing laboratory bounds from other experiments by many orders of magnitude for $m_a\gtrsim {10}^{-2}\mathrm{eV}$. We also place constraints on $CP$ violation in certain types of relaxion models.

We realize a spin-orbit interaction between the collective spin precession and center-of-mass motion of a trapped ultracold atomic gas, mediated by spin- and position-dependent dispersive coupling to a driven optical cavity. The collective spin, precessing near its highest-energy state in an applied magnetic field, can be approximated as a negative-mass harmonic oscillator. When the Larmor precession and mechanical motion are nearly resonant, cavity mediated coupling leads to a negative-mass instability, driving exponential growth of a correlated mode of the hybrid system. We observe this growth imprinted on modulations of the cavity field and estimate the full covariance of the resulting two-mode state by observing its transient decay during subsequent free evolution.

We employed ab initio calculations to identify a class of crystalline materials of $M\mathrm{Si}$ ($M=\mathrm{Fe}$, Co, Mn, Re, Ru) having double-Weyl points in both their acoustic and optical phonon spectra. They exhibit novel topological points termed “spin-1 Weyl point” at the Brillouin zone center and “charge-2 Dirac point” at the zone corner. The corresponding gapless surface phonon dispersions are two helicoidal sheets whose isofrequency contours form a single noncontractible loop in the surface Brillouin zone. In addition, the global structure of the surface bands can be analytically expressed as double-periodic Weierstrass elliptic functions.

We investigated the crystal-electric field ground state of the $4f$ manifold in the strongly correlated topological insulator ${\mathrm{SmB}}_6$ using core-level nonresonant inelastic x-ray scattering. The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the ${\mathrm{\Gamma }}_8$ quartet state of the Sm $f^{5}J=5/2$ configuration governs the ground-state symmetry and, hence, the topological properties of ${\mathrm{SmB}}_6$. Our findings contradict the results of density functional calculations reported so far.