All known proper ferroelectrics are unable to polarize normal to a surface or interface if the resulting depolarization field is unscreened, but there is no fundamental principle that enforces this behavior. In this work, we introduce hyperferroelectrics, a new class of proper ferroelectrics which polarize even when the depolarization field is unscreened, this condition being equivalent to instability of a longitudinal optic mode in addition to the transverse-optic-mode instability characteristic of proper ferroelectrics. We use first-principles calculations to show that several recently discovered hexagonal ferroelectric semiconductors have this property, and we examine its consequences both in the bulk and in a superlattice geometry.

We study the dynamical response of a fluid membrane to the sudden conformation change of active inclusions linearly coupled to the membrane curvature. The mutual force between two inclusions triggered simultaneously is shown to exhibit a transient maximum much larger than the equilibrium force. Even in the presence of tension, this dynamical interaction is long range over distances much larger than the correlation length. We derive the scaling laws describing these phenomena analytically, and we stress the importance of the damping due to intermonolayer friction.

We describe a light-pulse atom interferometer that is suitable for any species of atom and even for electrons and protons as well as their antiparticles, in particular, for testing the Einstein equivalence principle with antihydrogen. The design obviates the need for resonant lasers through far-off resonant Bragg beam splitters and makes efficient use of scarce atoms by magnetic confinement and atom recycling. We expect to reach an initial accuracy of better than 1% for the acceleration of the free fall of antihydrogen, which can be improved to the part-per million level.

The constraint imposed by magnetic helicity conservation on the $\alpha$ effect is considered for both magnetically and flow dominated self-organizing plasmas. Direct numerical simulations are presented for a dominant contribution to the $\alpha$ effect, which can be cast in the functional form of a total divergence of an averaged helicity flux, called the helicity-flux-driven $\alpha$ ($\mathrm{H}\alpha$) effect. Direct numerical simulations of the $\mathrm{H}\alpha$ effect are presented for two examples—the magnetically dominated toroidal plasma unstable to tearing modes, and the flow-dominated accretion disk.

We report the observation, by means of high-resolution inelastic x-ray scattering, of an unusually large temperature dependence of the sound attenuation of a network glass at terahertz frequency, an unprecedentedly observed phenomenon. The anharmonicity can be ascribed to the interaction between the propagating acoustic wave and the bath of thermal vibrations. At low temperatures the sound attenuation follows a Rayleigh-Gans scattering law. As the temperature is increased the anharmonic process sets in, resulting in an almost quadratic frequency dependence of the damping in the entire frequency range. We show that the temperature variation of the sound damping accounts quantitatively for the temperature dependence of the density of vibrational states.

We consider time-reversal-symmetric two-channel semiconducting quantum wires proximity coupled to a conventional $s$-wave superconductor. We analyze the requirements for a nontrivial topological phase and find that the necessary conditions are (1) the determinant of the pairing matrix in channel space must be negative, (2) inversion symmetry must be broken, and (3) the two channels must have different spin-orbit couplings. The first condition can be implemented in semiconducting nanowire systems where interactions suppress intra-channel pairing, while the inversion symmetry can be broken by tuning the chemical potentials of the channels. For the case of collinear spin-orbit directions, we find a general expression for the topological invariant by block diagonalization into two blocks with chiral symmetry only. By projection to the low-energy sector, we solve for the zero modes explicitly and study the details of the gap closing, which in the general case happens at finite momenta.

Silicon microdisks are optical resonators that can exhibit surprising nonlinear behavior. We present a new analysis of the dynamics of these resonators elucidating the mathematical origin of spontaneous oscillations and deriving predictions for observed phenomena such as a frequency comb spectrum with MHz-scale repetition rate. We test predictions through laboratory experiment and numerical simulation.

The formation of extreme localizations in nonlinear dispersive media can be explained and described within the framework of nonlinear evolution equations, such as the nonlinear Schrödinger equation (NLS). Within the class of exact NLS breather solutions on a finite background, which describe the modulational instability of monochromatic wave trains, the hierarchy of rational solutions localized in both time and space is considered to provide appropriate prototypes to model rogue wave dynamics. Here, we use the time-reversal invariance of the NLS to propose and experimentally demonstrate a new approach to constructing strongly nonlinear localized waves focused in both time and space. The potential applications of this time-reversal approach include remote sensing and motivated analogous experimental analysis in other nonlinear dispersive media, such as optics, Bose-Einstein condensates, and plasma, where the wave motion dynamics is governed by the NLS.

A method is proposed to generate femtosecond, ultralow emittance ($\sim {10}^{-8}\mathrm{m}\mathrm{rad}$), electron beams in a laser-plasma accelerator using two lasers of different colors. A long-wavelength pump pulse, with a large ponderomotive force and small peak electric field, excites a wake without fully ionizing a high-$Z$ gas. A short-wavelength injection pulse, with a small ponderomotive force and large peak electric field, copropagating and delayed with respect to the pump laser, ionizes a fraction of the remaining bound electrons at a trapping wake phase, generating an electron beam that is accelerated in the wake.

Comprehensive 2D turbulence and eddy flow velocity measurements on DIII-D demonstrate a rapidly increasing turbulence-driven shear flow that develops $\sim 100\mu \mathrm{s}$ prior to the low-confinement ($L$ mode) to high-confinement ($H$ mode) transition and appears to trigger it. These changes are localized to a narrow layer 1–2 cm inside the magnetic boundary. Increasing heating power increases the Reynolds stress, the energy transfer from turbulence to the poloidal flow, and the edge flow shearing rate that then exceeds the decorrelation rate, suppressing turbulence and triggering the transition.

We have studied the mechanism of the partial dissociation of water on Ru(0001) by high resolution scanning tunneling microscopy (STM). The thermal evolution of water at submonolayer coverage has been tracked in the 110–145 K temperature range to identify the precursor structures for the partial dissociation. These were found to consist of hexagons arranged in thin stripes aligned along the close packed Ru [$2\overline{1}\overline{1}0$] directions. The partially dissociated phase, on the other hand, contains a mixture of ${\mathrm{H}}_2\mathrm{O}$ and OH hexagons arranged into wider stripes and rotated by 30° with respect to the intact water stripes. The atomic structure of both types of stripes is determined with the aid of density functional theory and STM simulations, providing insights into the partial dissociation reaction path. The reaction is found to be exothermic by around 0.4 eV and initiating at the edges of the intact water stripes. Hydrogen atoms, from water dissociation or already present at the surface, are found to play an important role in the kinetics of the reactions.

We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in ${\mathrm{NaFe}}_{1-\mathrm{x}}{\mathrm{Co}}_{\mathrm{x}}\mathrm{As}$ iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where fourfold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always fourfold symmetric. We argue this implies small nematic susceptibility and, hence, insignificant nematic fluctuation in optimally doped iron pnictides. Since $T_C$ is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.

We study the mechanical properties of stoichiometric SiN resonators through a combination of spectroscopic and interferometric imaging techniques. At room temperature, we demonstrate ultrahigh quality factors of $5\times {10}^7$ and a $f\times Q$ product of $1\times {10}^{14}\mathrm{Hz}$. To our knowledge, these correspond to the largest values yet reported for mesoscopic flexural resonators. Through a comprehensive study of the limiting dissipation mechanisms as a function of resonator and substrate geometry, we identify radiation loss through the supporting substrate as the dominant loss process. In addition to pointing the way towards higher quality factors through optimized substrate designs, our work realizes an enabling platform for the observation and control of quantum behavior in a macroscopic mechanical system.

We study the fingering instability of the interface between two miscible fluids, a colloidal suspension and its own solvent. The temporal evolution of the interface in a Hele-Shaw cell is found to be governed by the competition between the nonlinear viscosity of the suspension and an off-equilibrium, effective surface tension ${\mathrm{\Gamma }}_e$. By studying suspensions in a wide range of volume fractions, ${\mathrm{\Phi }}_C$, we show that ${\mathrm{\Gamma }}_e\sim {\mathrm{\Phi }}_C^2$, in agreement with Korteweg’s theory for miscible fluids. The surface tension exhibits an anomalous increase with particle size, which we account for using entropy arguments.

The standard model extension is an effective field theory introducing all possible Lorentz-violating (LV) operators to the standard model and general relativity (GR). In the pure-gravity sector of minimal standard model extension, nine coefficients describe dominant observable deviations from GR. We systematically implemented 27 tests from 13 pulsar systems to tightly constrain eight linear combinations of these coefficients with extensive Monte Carlo simulations. It constitutes the first detailed and systematic test of the pure-gravity sector of minimal standard model extension with the state-of-the-art pulsar observations. No deviation from GR was detected. The limits of LV coefficients are expressed in the canonical Sun-centered celestial-equatorial frame for the convenience of further studies. They are all improved by significant factors of tens to hundreds with existing ones. As a consequence, Einstein’s equivalence principle is verified substantially further by pulsar experiments in terms of local Lorentz invariance in gravity.