The quasiskutterudite superconductor ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ features a pronounced anomaly in electrical resistivity at $T^{*}\sim 138\mathrm{K}$. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of $T^{*}$ as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., $x_c=0.9$). This establishes the ${({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x})}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.

The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$, has been a long-standing puzzle. Here, we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Néel temperature $T_N$ in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.

Three-dimensional antiferromagnets with random magnetic anisotropy (RMA) that have been experimentally studied to date have competing two-dimensional and three-dimensional exchange interactions which can obscure the authentic effects of RMA. The magnetic phase diagram of ${\mathrm{Fe}}_x{\mathrm{Ni}}_{1-x}{\mathrm{F}}_2$ epitaxial thin films with true random single-ion anisotropy was deduced from magnetometry and neutron scattering measurements and analyzed using mean-field theory. Regions with uniaxial, oblique, and easy-plane anisotropies were identified. A RMA-induced glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs.

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

We introduce a variational approximation to the microscopic dynamics of rare conformational transitions of macromolecules. Within this framework it is possible to simulate on a small computer cluster reactions as complex as protein folding, using state of the art all-atom force fields in explicit solvent. We test this method against MD simulations of the folding of an $\alpha$ and a $\beta$ protein performed with the same all-atom force field on the Anton supercomputer. We find that our approach yields results consistent with those of MD simulations, at a computational cost orders of magnitude smaller.

We develop a percolation model motivated by recent experimental studies of gels with active network remodeling by molecular motors. This remodeling was found to lead to a critical state reminiscent of random percolation (RP), but with a cluster distribution inconsistent with RP. Our model not only can account for these experiments, but also exhibits an unusual type of mixed phase transition: We find that the transition is characterized by signatures of criticality, but with a discontinuity in the order parameter.

We describe a simple, efficient method for simulating Hamiltonian dynamics on a quantum computer by approximating the truncated Taylor series of the evolution operator. Our method can simulate the time evolution of a wide variety of physical systems. As in another recent algorithm, the cost of our method depends only logarithmically on the inverse of the desired precision, which is optimal. However, we simplify the algorithm and its analysis by using a method for implementing linear combinations of unitary operations together with a robust form of oblivious amplitude amplification.

For a wide class of stochastic athermal systems, we derive Langevin-like equations driven by non-Gaussian noise, starting from master equations and developing a new asymptotic expansion. We found an explicit condition whereby the non-Gaussian properties of the athermal noise become dominant for tracer particles associated with both thermal and athermal environments. Furthermore, we derive an inverse formula to infer microscopic properties of the athermal bath from the statistics of the tracer particle. We apply our formulation to a granular motor under viscous friction and analytically obtain the angular velocity distribution function. Our theory demonstrates that the non-Gaussian Langevin equation is the minimal model of athermal systems.

The LHCb Collaboration’s measurement of $R_K=\mathcal{B}(B^{+}\to K^{+}{\mu }^{+}{\mu }^{-})/\mathcal{B}(B^{+}\to K^{+}{e}^{+}{e}^{-})$ lies $2.6\sigma$ below the Standard Model prediction. Several groups suggest this deficit to result from new lepton nonuniversal interactions of muons. But nonuniversal leptonic interactions imply lepton flavor violation in $B$ decays at rates much larger than are expected in the Standard Model. A simple model shows that these rates could lie just below current limits. An interesting consequence of our model, that $\mathcal{B}(B_s\to {\mu }^{+}{\mu }^{-}{)}_{\text{exp}}/\mathcal{B}(B_s\to {\mu }^{+}{\mu }^{-}{)}_{\mathrm{SM}}\cong R_K\cong 0.75$, is compatible with recent measurements of these rates. We stress the importance of searches for lepton flavor violations, especially for $B\to K\mu e$, $K\mu \tau$, and $B_s\to \mu e$, $\mu \tau$.

The strange contribution to the electric and magnetic form factors of the nucleon is determined at a range of discrete values of $Q^2$ up to $1.4{\mathrm{GeV}}^2$. This is done by combining a recent analysis of lattice QCD results for the electromagnetic form factors of the octet baryons with experimental determinations of those quantities. The most precise result is a small negative value for the strange magnetic moment: $G_M^s(Q^2=0)=-0.07\pm 0.03{\mu }_N$. At larger values of $Q^2$ both the electric and magnetic form factors are consistent with zero to within 2 standard deviations.

We present the first application of a recently developed effective theory of jet propagation in matter, soft-collinear effective theory with Glauber gluons (${\mathrm{SCET}}_{\mathrm{G}}$), to inclusive hadron suppression in nucleus-nucleus collisions at RHIC and the LHC. ${\mathrm{SCET}}_{\mathrm{G}}$-based splitting kernels allow us to go beyond the traditional energy loss approximation and unify the treatment of vacuum and medium-induced parton showers. In the soft gluon emission limit, we establish a simple analytic relation between the QCD evolution and energy loss approaches to jet quenching. We quantify the uncertainties associated with the implementation of the in-medim modification of hadron production cross sections and show that the coupling between the jet and the medium can be constrained with better than 10% accuracy.

We observed optical precursors in four-wave mixing based on a cold-atom gas. Optical precursors appear at the edges of pulses of the generated optical field, and propagate through the atomic medium without absorption. Theoretical analysis suggests that these precursors correspond to high-frequency components of the signal pulse, which means the atoms cannot respond quickly to rapid changes in the electromagnetic field. In contrast, the low-frequency signal components are absorbed by the atoms during transmission. We also showed experimentally that the backward precursor can be stored using a Raman transition of the atomic ensemble and retrieved later.

Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the nonrelativistic to ultrarelativistic limit. In the antiparallel configuration, the inflow speed increases with the upstream magnetization parameter $\sigma$ and approaches the speed of light when $\sigma >O(100)$, leading to an enhanced reconnection rate. In all regimes, the divergence of the pressure tensor is the dominant term responsible for breaking the frozen-in condition at the $x$ line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region, modified by the compression factor of proper density, remains $\sim 0.1$ in both the nonrelativistic and relativistic limits.

We map out the critical velocity in the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer superfluidity with ultracold ${}^6\mathrm{Li}$ gases. A small attractive potential is dragged along lines of constant column density. The rate of the induced heating increases steeply above a critical velocity $v_c$. In the same samples, we measure the speed of sound $v_s$ by exciting density waves and compare the results to the measured values of $v_c$. We perform numerical simulations in the Bose-Einstein condensation regime and find very good agreement, validating the approach. In the strongly correlated regime our measurements of $v_c$ provide a testing ground for theoretical approaches.

High piezoelectric activity of many ferroelectrics has been the focus of numerous recent studies. The structural origin of this activity remains poorly understood due to a lack of appropriate experimental techniques and mixing of different mechanisms related to ferroelectricity and ferroelasticity. Our work reports on the study of a uniaxial ${\mathrm{Sr}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{Nb}}_2{\mathrm{O}}_6$ ferroelectric where the formation of regions with different spontaneous strains is ruled out by the symmetry and where the interrelation between piezoelectricity and ferroelectricity can be inspected in an isolated fashion. We performed x-ray diffraction experiments on a single crystalline sample under alternating electric field and observed an unknown hidden-in-the-bulk mechanism, which suggests that the highest piezoelectric activity is realized in the volumes where nucleation of small ferroelectric domains takes place. This new mechanism creates a novel roadmap for designing materials with enhanced piezoelectric properties.