It has been claimed recently that massive sterile neutrinos could bring about a new concordance between observations of the cosmic microwave background, the large-scale structure of the Universe, and local measurements of the Hubble constant, $H_0$. We demonstrate that this apparent concordance results from combining data sets which are in significant tension, even within this extended model, possibly indicating remaining systematic biases in the measurements. We further show that this tension remains when the cosmological model is further extended to include significant tensor modes, as suggested by the recent BICEP2 results. Using the Bayesian evidence, we show that the cold dark matter model with a cosmological constant is strongly favored over its neutrino extensions by various combinations of data sets. Robust data combinations yield stringent limits of $\sum m_{\nu }\lesssim 0.3\mathrm{eV}$ and $m_{\nu ,\text{sterile}}^{\text{eff}}\lesssim 0.3\mathrm{eV}$ at 95% C.L. for the sum of active and sterile neutrinos, respectively.

We present transport experiments performed in high-quality quantum point contacts embedded in a GaAs two-dimensional hole gas. The strong spin-orbit interaction results in peculiar transport phenomena, including the previously observed anisotropic Zeeman splitting and level-dependent effective $g$ factors. Here we find additional effects, namely, the crossing and the anticrossing of spin-split levels depending on subband index and magnetic field direction. Our experimental observations are reconciled in a heavy-hole effective spin-orbit Hamiltonian where cubic- and quadratic-in-momentum terms appear. The spin-orbit components, being of great importance for quantum computing applications, are characterized in terms of magnitude and spin structure. In light of our results, we explain the level-dependent effective $g$ factor in an in-plane field. Through a tilted magnetic field analysis, we show that the quantum point contact out-of-plane $g$ factor saturates around the predicted 7.2 bulk value.

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of $1\mu \mathrm{m}$. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample.

We study extreme-value statistics of Brownian trajectories in one dimension. We define the maximum as the largest position to date and compare maxima of two particles undergoing independent Brownian motion. We focus on the probability $P(t)$ that the two maxima remain ordered up to time $t$ and find the algebraic decay $P\sim t^{-\beta }$ with exponent $\beta =1/4$. When the two particles have diffusion constants $D_1$ and $D_2$, the exponent depends on the mobilities, $\beta =(1/\pi )\arctan \sqrt{D_2/D_1}$. We also use numerical simulations to investigate maxima of multiple particles in one dimension and the largest extension of particles in higher dimensions.

We measure the mass of the top quark in $\text{lepton}+\text{jets}$ final states using the full sample of $p\overline{p}$ collision data collected by the D0 experiment in Run II of the Fermilab Tevatron Collider at $\sqrt{s}=1.96\mathrm{TeV}$, corresponding to $9.7{\mathrm{fb}}^{-1}$ of integrated luminosity. We use a matrix element technique that calculates the probabilities for each event to result from $t\overline{t}$ production or background. The overall jet energy scale is constrained in situ by the mass of the $W$ boson. We measure $m_t=174.98\pm 0.76\mathrm{GeV}$. This constitutes the most precise single measurement of the top-quark mass.

We propose a feasible experimental scheme to realize a three-dimensional chiral topological insulator with cold fermionic atoms in an optical lattice, which is characterized by an integer topological invariant distinct from the conventional ${\mathbb{Z}}_2$ topological insulators and has a remarkable macroscopic zero-energy flat band. To probe its property, we show that its characteristic surface states—the Dirac cones—can be probed through time-of-flight imaging or Bragg spectroscopy and the flat band can be detected via measurement of the atomic density profile in a weak global trap. The realization of this novel topological phase with a flat band in an optical lattice will provide a unique experimental platform to study the interplay between interaction and topology and open new avenues for application of topological states.

We study a voltage biased InAs double quantum dot (DQD) that is coupled to a superconducting transmission line resonator. Inelastic tunneling in the DQD is mediated by electron phonon coupling and coupling to the cavity mode. We show that electronic transport through the DQD leads to photon emission from the cavity at a rate of 10 MHz. With a small cavity drive field, we observe a gain of up to 15 in the cavity transmission. Our results are analyzed in the context of existing theoretical models and suggest that it may be necessary to account for inelastic tunneling processes that proceed via simultaneous emission of a phonon and a photon.

X-ray diffraction, Raman spectroscopy, and electrical conductivity measurements of molybdenum disulfide ${\mathrm{MoS}}_2$ are performed at pressures up to 81 GPa in diamond anvil cells. Above 20 GPa, we find discontinuous changes in Raman spectra and x-ray diffraction patterns which provide evidence for isostructural phase transition from $2H_c$ to $2H_a$ modification through layer sliding previously predicted theoretically. This first-order transition, which is completed around 40 GPa, is characterized by a collapse in the $c$-lattice parameter and volume and also by changes in interlayer bonding. After the phase transition completion, ${\mathrm{MoS}}_2$ becomes metallic. The reversibility of the phase transition is identified from all these techniques.

We experimentally demonstrate that the spin-orbit interaction can be utilized for direct electric-field tuning of the propagation of spin waves in a single-crystal yttrium iron garnet magnonic waveguide. Magnetoelectric coupling not due to the spin-orbit interaction and, hence, an order of magnitude weaker leads to electric-field modification of the spin-wave velocity for waveguide geometries where the spin-orbit interaction will not contribute. A theory of the phase shift, validated by the experiment data, shows that, in the exchange spin wave regime, this electric tuning can have high efficiency. Our findings point to an important avenue for manipulating spin waves and developing electrically tunable magnonic devices.

We study mode competition in a multimode “phonon laser” comprised of an optical cavity employing a highly reflective membrane as the output coupler. Mechanical gain is provided by the intracavity radiation pressure, to which many mechanical modes are coupled. We calculate the gain and find that strong oscillation in one mode suppresses the gain in other modes. For sufficiently strong oscillation, the gain of the other modes actually switches sign and becomes damping, a process we call “anomalous cooling.” We demonstrate that mode competition leads to single-mode operation and find excellent agreement with our theory, including anomalous cooling.

The recent BICEP2 measurement of $B$-mode polarization in the cosmic microwave background ($r={0.2}_{-0.05}^{+0.07}$), a possible indication of primordial gravity waves, appears to be in tension with the upper limit from WMAP ($r<0.13$ at 95% C.L.) and Planck ($r<0.11$ at 95% C.L.). We carefully quantify the level of tension and show that it is very significant (around 0.1% unlikely) when the observed deficit of large-scale temperature power is taken into account. We show that measurements of $TE$ and $EE$ power spectra in the near future will discriminate between the hypotheses that this tension is either a statistical fluke or a sign of new physics. We also discuss extensions of the standard cosmological model that relieve the tension and some novel ways to constrain them.

An approach is presented to experimentally constrain previously unreachable ($p$, $\gamma$) reaction rates on nuclei far from stability in the astrophysical $rp$ process. Energies of all critical resonances in the ${}^{57}\mathrm{Cu}(p,\gamma ){}^{58}\mathrm{Zn}$ reaction are deduced by populating states in ${}^{58}\mathrm{Zn}$ with a ($d$, $n$) reaction in inverse kinematics at $75\mathrm{MeV}/u$, and detecting $\gamma$-ray-recoil coincidences with the state-of-the-art $\gamma$-ray tracking array GRETINA and the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The results reduce the uncertainty in the ${}^{57}\mathrm{Cu}(p,\gamma )$ reaction rate by several orders of magnitude. The effective lifetime of ${}^{56}\mathrm{Ni}$, an important waiting point in the $rp$ process in x-ray bursts, can now be determined entirely from experimentally constrained reaction rates.

A dense system of vortices can be treated as a fluid and itself could be described in terms of hydrodynamics. We develop the hydrodynamics of the vortex fluid. This hydrodynamics captures characteristics of fluid flows averaged over fast circulations in the intervortex space. The hydrodynamics of the vortex fluid features the anomalous stress absent in Euler’s hydrodynamics. The anomalous stress yields a number of interesting effects. Some of them are a deflection of streamlines, a correction to the Bernoulli law, and an accumulation of vortices in regions with high curvature in the curved space. The origin of the anomalous stresses is a divergence of intervortex interactions at the microscale which manifest at the macroscale. We obtain the hydrodynamics of the vortex fluid from the Kirchhoff equations for dynamics of pointlike vortices.

The mechanisms for accelerating electrons from thermal to relativistic energies in the terrestrial magnetosphere, on the sun, and in many astrophysical environments have never been verified. We present the first direct observation of two processes that, in a chain, cause this acceleration in Earth’s outer radiation belt. The two processes are parallel acceleration from electron-volt to kilovolt energies by parallel electric fields in time-domain structures (TDS), after which the parallel electron velocity becomes sufficiently large for Doppler-shifted upper band whistler frequencies to be in resonance with the electron gyration frequency, even though the electron energies are kilovolts and not hundreds of kilovolts. The electrons are then accelerated by the whistler perpendicular electric field to relativistic energies in several resonant interactions. TDS are packets of electric field spikes, each spike having duration of a few hundred microseconds and containing a local parallel electric field. The TDS of interest resulted from nonlinearity of the parallel electric field component in oblique whistlers and consisted of $\sim 0.1\mathrm{msec}$ pulses superposed on the whistler waveform with each such spike containing a net parallel potential the order of 50 V. Local magnetic field compression from remote activity provided the free energy to drive the two processes. The expected temporal correlations between the compressed magnetic field, the nonlinear whistlers with their parallel electric field spikes, the electron flux and the electron pitch angle distributions were all observed.

Making use of experimental and theoretical considerations, in this Letter we deduce a criterion to determine the critical velocity for which a drop impacting a smooth dry surface either spreads over the substrate or disintegrates into smaller droplets. The derived equation, which expresses the splash threshold velocity as a function of the material properties of the two fluids involved, the drop radius, and the mean free path of the molecules composing the surrounding gaseous atmosphere, has been thoroughly validated experimentally at normal atmospheric conditions using eight different liquids with viscosities ranging from $\mu =3\times {10}^{-4}$ to $\mu ={10}^{-2}\mathrm{Pa}\mathrm{s}$, and interfacial tension coefficients varying between $\sigma =17$ and $\sigma =72\mathrm{mN}{\mathrm{m}}^{-1}$. Our predictions are also in fair agreement with the measured critical speed of drops impacting in different gases at reduced pressures given by Xu et al. [Phys. Rev. Lett. 94, 184505 (2005).].

Quantum experiments with nanomechanical oscillators are regarded as a test bed for hypothetical modifications of the Schrödinger equation, which predict a breakdown of the superposition principle and induce classical behavior at the macroscale. It is generally believed that the sensitivity to these unconventional effects grows with the mass of the mechanical quantum system. Here we show that the opposite is the case for optomechanical systems in the presence of generic noise sources, such as thermal and measurement noise. We determine conditions for distinguishing these decoherence processes from possible collapse-induced decoherence in continuous optomechanical force measurements.

A scheme of an intense coherent gamma-ray source based on the spontaneous radiation of positronium atoms in a Bose-Einstein condensate (BEC) due to two-photon collective annihilation decay is investigated analytically arising from the second quantized formalism. It is shown that because of the intrinsic instability of annihilation decay of BEC, the spontaneously emitted entangled photon pairs are amplified, leading to an exponential buildup of a macroscopic population into end-fire modes at a certain shape of the elongated condensate. The considered scheme may also be applied to a BEC of atoms or quasiparticles as a laser mechanism with double coherence to create entangled photonic beams with a macroscopic number of photons.

Gravitational lensing due to the large-scale distribution of matter in the cosmos distorts the primordial cosmic microwave background (CMB) and thereby induces new, small-scale $B$-mode polarization. This signal carries detailed information about the distribution of all the gravitating matter between the observer and CMB last scattering surface. We report the first direct evidence for polarization lensing based on purely CMB information, from using the four-point correlations of even- and odd-parity $E$- and $B$-mode polarization mapped over $\sim 30$ square degrees of the sky measured by the POLARBEAR experiment. These data were analyzed using a blind analysis framework and checked for spurious systematic contamination using null tests and simulations. Evidence for the signal of polarization lensing and lensing $B$ modes is found at $4.2\sigma$ ($\mathrm{stat}+\text{sys}$) significance. The amplitude of matter fluctuations is measured with a precision of 27%, and is found to be consistent with the Lambda cold dark matter cosmological model. This measurement demonstrates a new technique, capable of mapping all gravitating matter in the Universe, sensitive to the sum of neutrino masses, and essential for cleaning the lensing $B$-mode signal in searches for primordial gravitational waves.

Self-interacting dark matter (SIDM) models have been proposed to solve the small-scale issues with the collisionless cold dark matter paradigm. We derive equilibrium solutions in these SIDM models for the dark matter halo density profile including the gravitational potential of both baryons and dark matter. Self-interactions drive dark matter to be isothermal and this ties the core sizes and shapes of dark matter halos to the spatial distribution of the stars, a radical departure from previous expectations and from cold dark matter predictions. Compared to predictions of SIDM-only simulations, the core sizes are smaller and the core densities are higher, with the largest effects in baryon-dominated galaxies. As an example, we find a core size around 0.3 kpc for dark matter in the Milky Way, more than an order of magnitude smaller than the core size from SIDM-only simulations, which has important implications for indirect searches of SIDM candidates.

We demonstrate extreme superheating and single bubble nucleation in an electrolyte solution within a nanopore in a thin silicon nitride membrane. The high temperatures are achieved by Joule heating from a highly focused ionic current induced to flow through the pore by modest voltage biases. Conductance, nucleation, and bubble evolution are monitored electronically and optically. Temperatures near the thermodynamic limit of superheat are achieved just before bubble nucleation with the system at atmospheric pressure. Bubble nucleation is homogeneous and highly reproducible. This nanopore approach more generally suggests broad application to the excitation, detection, and characterization of highly metastable states of matter.