The electronic states at the direct band gap of monolayer transition metal dichalcogenides such as ${\mathrm{WSe}}_2$ at the $K^{+}$ and $K^{-}$ valleys are related by time reversal and may be viewed as pseudospins. The corresponding optical interband transitions are governed by robust excitons. Excitation with linearly polarized light yields the coherent superposition of exciton pseudospin states, referred to as coherent valley states. Here, we uncover how and why valley coherence can be generated efficiently. In double resonant Raman spectroscopy, we show that the optically generated $2s$ exciton state differs from the $1s$ state by exactly the energy of the combination of several prominent phonons. Superimposed on the exciton photoluminescence (PL), we observe the double resonant Raman signal. This spectrally narrow peak shifts with the excitation laser energy as incoming photons match the $2s$ and outgoing photons the $1s$ exciton transition. The multiphonon resonance has important consequences: following linearly polarized excitation of the $2s$ exciton, a superposition of valley states is efficiently transferred from the $2s$ to $1s$ state. This explains the high degree of valley coherence measured for the $1s$ exciton PL.

Several emergent phenomena and phases in solids arise from configurations of the electronic Berry phase in momentum space that are similar to gauge field configurations in real space such as magnetic monopoles. We show that the momentum-space analogue of the “axion electrodynamics” term $\mathbf{E}·\mathbf{B}$ plays a fundamental role in a unified theory of Berry-phase contributions to optical gyrotropy in time-reversal invariant materials and the chiral magnetic effect. The Berry-phase mechanism predicts that the rotatory power along the optic axes of a crystal must sum to zero, a constraint beyond that stipulated by point-group symmetry, but observed to high accuracy in classic experimental observations on alpha quartz. Furthermore, the Berry mechanism provides a microscopic basis for the surface conductance at the interface between gyrotropic and nongyrotropic media.

Time is a valuable resource and it is expected that a longer time period should lead to better precision in Hamiltonian parameter estimation. However, recent studies in quantum metrology have shown that in certain cases more time may even lead to worse estimations, which puts this intuition into question. In this Letter we show that by including feedback controls this intuition can be restored. By deriving asymptotically optimal feedback controls we quantify the maximal improvement feedback controls can provide in Hamiltonian parameter estimation and show a universal time scaling for the precision limit under the optimal feedback scheme. Our study reveals an intriguing connection between noncommutativity in the dynamics and the gain of feedback controls in Hamiltonian parameter estimation.

We show that the entropy of a message can be tested in a device-independent way. Specifically, we consider a prepare-and-measure scenario with classical or quantum communication, and develop two different methods for placing lower bounds on the communication entropy, given observable data. The first method is based on the framework of causal inference networks. The second technique, based on convex optimization, shows that quantum communication provides an advantage over classical communication, in the sense of requiring a lower entropy to reproduce given data. These ideas may serve as a basis for novel applications in device-independent quantum information processing.

Many transport processes in nature exhibit anomalous diffusive properties with nontrivial scaling of the mean square displacement, e.g., diffusion of cells or of biomolecules inside the cell nucleus, where typically a crossover between different scaling regimes appears over time. Here, we investigate a class of anomalous diffusion processes that is able to capture such complex dynamics by virtue of a general waiting time distribution. We obtain a complete characterization of such generalized anomalous processes, including their functionals and multipoint structure, using a representation in terms of a normal diffusive process plus a stochastic time change. In particular, we derive analytical closed form expressions for the two-point correlation functions, which can be readily compared with experimental data.

In this work we present results of a direct search for strange quark matter (SQM) in cosmic rays with the PAMELA space spectrometer. If this state of matter exists it may be present in cosmic rays as particles, called strangelets, having a high density and an anomalously high mass-to-charge ($A/Z$) ratio. A direct search in space is complementary to those from ground-based spectrometers. Furthermore, it has the advantage of being potentially capable of directly identifying these particles, without any assumption on their interaction model with Earth’s atmosphere and the long-term stability in terrestrial and lunar rocks. In the rigidity range from 1.0 to $\sim 1.0\times {10}^3\mathrm{GV}$, no such particles were found in the data collected by PAMELA between 2006 and 2009. An upper limit on the strangelet flux in cosmic rays was therefore set for particles with charge $1\le Z\le 8$ and mass $4\le A\le 1.2\times {10}^5$. This limit as a function of mass and as a function of magnetic rigidity allows us to constrain models of SQM production and propagation in the Galaxy.

Top quark production in the forward region in proton-proton collisions is observed for the first time. The $W+b$ final state with $W\to \mu \nu$ is reconstructed using muons with a transverse momentum, $p_T$, larger than 25 GeV in the pseudorapidity range $2.0<\eta <4.5$. The $b$ jets are required to have $50<p_T<100\mathrm{GeV}$ and $2.2<\eta <4.2$, while the transverse component of the sum of the muon and $b$-jet momenta must satisfy $p_T>20\mathrm{GeV}$. The results are based on data corresponding to integrated luminosities of 1.0 and $2.0{\mathrm{fb}}^{-1}$ collected at center-of-mass energies of 7 and 8 TeV by LHCb. The inclusive top quark production cross sections in the fiducial region are $$\begin{gathered} \sigma (\text{top})[7\mathrm{TeV}]=239\pm 53(\text{stat})\pm 33(\text{syst})\pm 24(\text{theory})\mathrm{fb}, \\ \sigma (\text{top})[8\mathrm{TeV}]=289\pm 43(\text{stat})\pm 40(\text{syst})\pm 29(\text{theory})\mathrm{fb}. \end{gathered}$$These results, along with the observed differential yields and charge asymmetries, are in agreement with next-to-leading order standard model predictions.

The bound–bound transition from the $5d^{2}6s^2$ ${}^3{F}_2^e$ ground state to the $5d6s^{2}6p$ ${}^3{D}_1^o$ excited state in negative lanthanum has been proposed as a candidate for laser cooling, which has not yet been achieved for negative ions. Anion laser cooling holds the potential to allow the production of ultracold ensembles of any negatively charged species. We have studied the aforementioned transition in a beam of negative La ions by high-resolution laser spectroscopy. The center-of-gravity frequency was measured to be 96.592 80(10) THz. Seven of the nine expected hyperfine structure transitions were resolved. The observed peaks were unambiguously assigned to the predicted hyperfine transitions by a fit, confirmed by multiconfigurational self-consistent field calculations. From the determined hyperfine structure we conclude that ${\mathrm{La}}^{-}$ is a promising laser cooling candidate. Using this transition, only three laser beams would be required to repump all hyperfine levels of the ground state.

Gravity- or buoyancy-driven bodies moving in a slightly viscous fluid frequently follow fluttering or helical paths. Current models of such systems are largely empirical and fail to predict several of the key features of their evolution, especially close to the onset of path instability. Here, using a weakly nonlinear expansion of the full set of governing equations, we present a new generic reduced-order model based on a pair of amplitude equations with exact coefficients that drive the evolution of the first pair of unstable modes. We show that the predictions of this model for the style (e.g., fluttering or spiraling) and characteristics (e.g., frequency and maximum inclination angle) of path oscillations compare well with various recent data for both solid disks and air bubbles.

Using ab initio lattice methods, we calculate the finite temperature thermodynamics of homogeneous two-dimensional spin-$1/2$ fermions with attractive short-range interactions. We present results for the density, pressure, compressibility, and quantum anomaly (i.e., Tan’s contact) for a wide range of temperatures (mostly above the superfluid phase, including the pseudogap regime) and coupling strengths, focusing on the unpolarized case. Within our statistical and systematic uncertainties, our prediction for the density equation of state differs quantitatively from the prediction by Luttinger-Ward theory in the strongly coupled region of parameter space, but otherwise agrees well with it. We also compare our calculations with the second- and third-order virial expansion, with which they are in excellent agreement in the low-fugacity regime.

Motivated by the properties of the iron chalcogenides, we study the phase diagram of a generalized Heisenberg model with frustrated bilinear-biquadratic interactions on a square lattice. We identify zero-temperature phases with antiferroquadrupolar and Ising-nematic orders. The effects of quantum fluctuations and interlayer couplings are analyzed. We propose the Ising-nematic order as underlying the structural phase transition observed in the normal state of FeSe, and discuss the role of the Goldstone modes of the antiferroquadrupolar order for the dipolar magnetic fluctuations in this system. Our results provide a considerably broadened perspective on the overall magnetic phase diagram of the iron chalcogenides and pnictides, and are amenable to tests by new experiments.

Ingenious suggestions continue to be made for separation of racemic mixtures according to the inert structural chirality of the constituents. Recently discovered self-motile micro- or nanoparticles express dynamical chirality, i.e., that which originates in motion, not structure. Here, we predict how dynamically chiral objects, with overdamped dynamics in a soft periodic two-dimensional potential, can display not only separation into well-defined dynamical subclasses defined by motility characteristics, but also the ability to be steered to arbitrary locations in the plane by simply changing the amplitude of the external potential. Orientational and translational diffusion produce new types of drift absent in the noise-free case. As practical implementation seems feasible with acoustic or optical fields, these phenomena can be useful for laboratory microscales manipulations, possibly including reconfigurable microfluidic circuits with complex networks of unidirectional channels.

Living systems need to be highly responsive, and also to keep fluctuations low. These goals are incompatible in equilibrium systems due to the fluctuation dissipation theorem (FDT). Here, we show that biological sensory systems, driven far from equilibrium by free energy consumption, can reduce their intrinsic fluctuations while maintaining high responsiveness. By developing a continuum theory of the E. coli chemotaxis pathway, we demonstrate that adaptation can be understood as a nonequilibrium phase transition controlled by free energy dissipation, and it is characterized by a breaking of the FDT. We show that the maximum response at short time is enhanced by free energy dissipation. At the same time, the low frequency fluctuations and the adaptation error decrease with the free energy dissipation algebraically and exponentially, respectively.

The antineutrino spectra measured in recent experiments at reactors are inconsistent with calculations based on the conversion of integral beta spectra recorded at the ILL reactor. ${}^{92}\mathrm{Rb}$ makes the dominant contribution to the reactor antineutrino spectrum in the 5–8 MeV range but its decay properties are in question. We have studied ${}^{92}\mathrm{Rb}$ decay with total absorption spectroscopy. Previously unobserved beta feeding was seen in the 4.5–5.5 region and the GS to GS feeding was found to be 87.5(25)%. The impact on the reactor antineutrino spectra calculated with the summation method is shown and discussed.

We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.

A gap in understanding the link between continuum theories of ion transport in ionic liquids and the underlying microscopic dynamics has hindered the development of frameworks for transport phenomena in these concentrated electrolytes. Here, we construct a continuum theory for ion transport in ionic liquids by coarse graining a simple exclusion process of interacting particles on a lattice. The resulting dynamical equations can be written as a gradient flow with a mobility matrix that vanishes at high densities. This form of the mobility matrix gives rise to a charging behavior that is different to the one known for electrolytic solutions, but which agrees qualitatively with the phenomenology observed in experiments and simulations.

Here we provide a picture of transport in quantum well heterostructures with a periodic driving field in terms of a probabilistic occupation of the topologically protected edge states in the system. This is done by generalizing methods from the field of photon-assisted tunneling. We show that the time dependent field dresses the underlying Hamiltonian of the heterostructure and splits the system into sidebands. Each of these sidebands is occupied with a certain probability which depends on the drive frequency and strength. This leads to a reduction in the topological transport signatures of the system because of the probability to absorb or emit a photon. Therefore when the voltage is tuned to the bulk gap the conductance is smaller than the expected $2e^2/h$. We refer to this as photon-inhibited topological transport. Nevertheless, the edge modes reveal their topological origin in the robustness of the edge conductance to disorder and changes in model parameters. In this work the analogy with photon-assisted tunneling allows us to interpret the calculated conductivity and explain the sum rule observed by Kundu and Seradjeh.