The migration of liquids in porous media, such as sand, has been commonly considered at high saturation levels with liquid pathways at pore dimensions. In this Letter, we reveal a low saturation regime observed in our experiments with droplets of extremely low volatility liquids deposited on sand. In this regime, the liquid is mostly found within the grain surface roughness and in the capillary bridges formed at the contacts between the grains. The bridges act as variable-volume reservoirs and the flow is driven by the capillary pressure arising at the wetting front according to the roughness length scales. We propose that this migration (spreading) is the result of interplay between the bridge volume adjustment to this pressure distribution and viscous losses of a creeping flow within the roughness. The net macroscopic result is a special case of nonlinear diffusion described by a superfast diffusion equation for saturation with distinctive mathematical character. We obtain solutions to a moving boundary problem defined by superfast diffusion equation that robustly convey a time power law of spreading as seen in our experiments.

We show that monoenergetic ion beams can be accelerated by moderate Mach number collisionless, electrostatic shocks propagating in a long scale-length exponentially decaying plasma profile. Strong plasma heating and density steepening produced by an intense laser pulse near the critical density can launch such shocks that propagate in the extended plasma at high velocities. The generation of a monoenergetic ion beam is possible due to the small and constant sheath electric field associated with the slowly decreasing density profile. The conditions for the acceleration of high-quality, energetic ion beams are identified through theory and multidimensional particle-in-cell simulations. The scaling of the ion energy with laser intensity shows that it is possible to generate $\sim 200\mathrm{MeV}$ proton beams with state-of-the-art 100 TW class laser systems.

Motivated by new capabilities to realize artificial gauge fields in ultracold atomic systems, and by their potential to access correlated topological phases in lattice systems, we present a new strategy for designing topologically nontrivial band structures. Our approach is simple and direct: it amounts to considering tight-binding models directly in reciprocal space. These models naturally cause atoms to experience highly uniform magnetic flux density and lead to topological bands with very narrow dispersion, without fine-tuning of parameters. Further, our construction immediately yields instances of optical Chern lattices, as well as band structures with Chern numbers of magnitude larger than one.

While the formal valence and charge state concepts have been tremendously important in materials physics and chemistry, their very loose connection to actual charge leads to uncertainties in modeling behavior and interpreting data. We point out, taking several transition metal oxides (${\mathrm{La}}_2{\mathrm{VCuO}}_6$, ${\mathrm{YNiO}}_3$, ${\mathrm{CaFeO}}_3$, ${\mathrm{AgNiO}}_2$, ${\mathrm{V}}_4{\mathrm{O}}_7$) as examples, that while dividing the crystal charge into atomic contributions is an ill-posed activity, the $3d$ occupation of a cation (and more particularly, differences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or disproportionation) systems, where different “charge states” of cations have identical $3d$ orbital occupation. Implications for theoretical modeling of such charge states and charge-ordering mechanisms are discussed.

Cerium $4f$ electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation ($\mu \mathrm{SR}$). Short-range static magnetism occurs below the freezing temperature $T_g\approx 0.7\mathrm{K}$, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the $\mu \mathrm{SR}$, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at $E_F$ indicating that it is close to the metal-insulator phase boundary.

It is demonstrated that the noninstantaneous response of the optically induced coherent polarization tremendously influences the real-space shift of electronic charges in semiconductors. The possibility of coherently controlling this real-space shift with temporally nonoverlapping excitation pulses allows for the observation of a new type of shift current, which only exists for certain polarization-shaped excitation pulses and vanishes in the continuous-wave limit. In contrast to previously studied shift currents, the new current requires a phase mismatch between two orthogonal transition dipole moments and leads, within a nonlinear second-order description, to a tensor which is antisymmetric with respect to the order of the two exciting electric field amplitudes. These observations, which can even be made at room temperature and are expected to occur in a variety of semiconductor crystal classes, contribute to a better understanding of light-matter interaction involving degenerate bands. Thus, they are expected to prove important for future studies of coherent and nonlinear optical effects in semiconductors.

The quantum spin Hall insulator is characterized by the presence of gapless helical edge states where the spin of the charge carriers is locked to their direction of motion. In order to probe the properties of the edge modes, we propose a design of a tunable quantum impurity realized by a local gate under an external magnetic field. Using the integrability of the impurity model, the conductance is computed for arbitrary interactions, temperatures and voltages, including the effect of Fermi liquid leads. The result can be used to infer the strength of interactions from transport experiments.

The phase of a single quantum state is undefined unless the history of its creation provides a reference point. Thus, quantum interference may seem hardly relevant for the design of deterministic single-electron sources which strive to isolate individual charge carriers quickly and completely. We provide a counterexample by analyzing the nonadiabatic separation of a localized quantum state from a Fermi sea due to a closing tunnel barrier. We identify the relevant energy scales and suggest ways to separate the contributions of quantum nonadiabatic excitation and back tunneling to the rare noncapture events. In the optimal regime of balanced decay and nonadiabaticity, our simple electron trap turns into a single-lead Landau-Zener back tunneling interferometer, revealing the dynamical phase accumulated between the particle capture and leakage. The predicted “quantum beats in back tunneling” may turn the error of a single-electron source into a valuable signal revealing essentially nonadiabatic energy scales of a dynamic quantum dot.

We have performed precise dc magnetization measurements for a single crystal of ${\mathrm{UBe}}_{13}$ down to 0.14 K, up to 80 kOe. We observed a magnetic anomaly in the superconducting (SC) mixed state at a field, named $H_{\mathrm{Mag}}^{*}$ ($\sim 26\mathrm{kOe}$, at 0.14 K), implying that ${\mathrm{UBe}}_{13}$ has a magnetically unusual SC state. We studied the magnetization curves of ${\mathrm{UBe}}_{13}$, assuming that the $H_{\mathrm{Mag}}^{*}$ anomaly originates from (1) and unusual SC diamagnetic response, or (2) a peculiarity of the normal-state magnetization due to vortices in the SC mixed state. The origin of the $H_{\mathrm{Mag}}^{*}$ anomaly is discussed.

We report superconductivity in defective pyrite-type iridium chalcogenides ${\mathrm{Ir}}_x{\mathrm{Ch}}_2$ ($\mathrm{Ch}=\mathrm{Se}$ and Te). Maximum values of $T_c$ of 6.4 K for ${\mathrm{Ir}}_{0.91}{\mathrm{Se}}_2$ and 4.7 K for ${\mathrm{Ir}}_{0.93}{\mathrm{Te}}_2$ were observed. It was found that ${\mathrm{Ir}}_{0.75}{\mathrm{Ch}}_2$ (${\mathrm{Ir}}_3{\mathrm{Ch}}_8$) is close to the boundary between metallic and insulating states and ${\mathrm{Ir}}_x{\mathrm{Ch}}_2$ systems undergo nonmetal to metal transitions as $x$ increases. On the basis of density functional theory calculations and the observed large variation in the Ch-Ch distance with $x$, we suggest that ${\mathrm{Ir}}_{0.75}{\mathrm{Ch}}_2$ (${\mathrm{Ir}}_3{\mathrm{Ch}}_8$) is the parent compound for the present superconductors.

We investigate the anisotropy in the in-plane optical spectra of detwinned $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$. The optical conductivity spectrum of $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{As}}_2$ shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc ($\omega =0$) resistivity is nearly isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering due to the presence of doped Co atoms, and it is extrinsic in origin. The intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by the Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase.

The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a charged massless particle in a uniform magnetic field. Similar dynamics is expected for other magnetic textures with a nonzero Skyrmion number. However, recent numerical simulations reveal that Skyrmion magnetic bubbles show significant deviations from this model. We show that a Skyrmion bubble possesses inertia and derive its mass from the standard theory of a thin-film ferromagnet. In addition to center-of-mass motion, other low energy modes are waves on the edge of the bubble traveling with different speeds in opposite directions.

We demonstrate experimentally that the energy from a highly localized free-electron-beam excitation can be converted via a planar plasmonic metamaterial to a low-divergence free-space light beam. This emission, which emanates from a collectively oscillating coupled metamolecule nanoantenna ensemble much larger in size than the initial excitation, is distinctly different from cathodoluminescence and bears some similarity with laser light. It offers a novel, flexible paradigm for the development of scalable, threshold-free light sources.

With theoretical prediction and experimental validation, we propose a novel approach to significantly enhance the conversion efficiency of thermoacoustic (TA) imaging by using an ultrashort microwave pulse. The implementation of the ultrashort microwave pulse leads to orders of magnitude enhancement in excitation efficiency and spatial resolution, compared to that from existing TA imaging techniques. This allows high-resolution ($\sim 100$ micron resolution) TA imaging to be acquired noninvasively. The present work represents a major breakthrough in the conversion efficiency of the TA effect and the resolution of TA imaging, which can potentially be used for clinical imaging.

DNA wrapping is an important mechanism for chromosomal DNA packaging in cells and viruses. Previous studies of DNA wrapping have been performed mostly on torsionally unconstrained DNA, while in vivo DNA is often under torsional constraint. In this study, we extend a previously proposed theoretical model for wrapping of torsionally unconstrained DNA to a new model including the contribution of DNA twist energy, which influences DNA wrapping drastically. In particular, due to accumulation of twist energy during DNA wrapping, it predicts a finite amount of DNA that can be wrapped on a helical spool. The predictions of the new model are tested by single-molecule study of DNA wrapping under torsional constraint using magnetic tweezers. The theoretical predictions and the experimental results are consistent with each other and their implications are discussed.

Living cells often need to extract information from biochemical signals that are noisy. We study how accurately cells can measure chemical concentrations with signaling networks that are linear. For stationary signals of long duration, they can reach, but not beat, the Berg-Purcell limit, which relies on uniformly averaging in time the fluctuations in the input signal. For short times or nonstationary signals, however, they can beat the Berg-Purcell limit, by nonuniformly time averaging the input. We derive the optimal weighting function for time averaging and use it to provide the fundamental limit of measuring chemical concentrations with linear signaling networks.

We show that in the continuum limit watersheds dividing drainage basins are Schramm-Loewner evolution (SLE) curves, being described by one single parameter $\kappa$. Several numerical evaluations are applied to ascertain this. All calculations are consistent with ${\mathrm{SLE}}_{\kappa }$, with $\kappa =1.734\pm 0.005$, being the only known physical example of an SLE with $\kappa <2$. This lies outside the well-known duality conjecture, bringing up new questions regarding the existence and reversibility of dual models. Furthermore, it constitutes a strong indication for conformal invariance in random landscapes and suggests that watersheds likely correspond to a logarithmic conformal field theory with a central charge $c\approx -7/2$.

Although $CP$ violation in the $B$ meson system has been well established by the $B$ factories, there has been no direct observation of time-reversal violation. The decays of entangled neutral $B$ mesons into definite flavor states ($B^0$ or ${\overline{B}}^0$), and $J/\psi K_L^0$ or $c\overline{c}{K}_S^0$ final states (referred to as $B_{+}$ or $B_{-}$), allow comparisons between the probabilities of four pairs of $T$-conjugated transitions, for example, ${\overline{B}}^0\to B_{-}$ and $B_{-}\to {\overline{B}}^0$, as a function of the time difference between the two $B$ decays. Using $468\times {10}^6$ $B\overline{B}$ pairs produced in ${\rm Y}(4S)$ decays collected by the BABAR detector at SLAC, we measure $T$-violating parameters in the time evolution of neutral $B$ mesons, yielding $\Delta S_T^{+}=-1.37\pm 0.14(\mathrm{stat})\pm 0.06(\mathrm{syst})$ and $\Delta S_T^{-}=1.17\pm 0.18(\mathrm{stat})\pm 0.11(\mathrm{syst})$. These nonzero results represent the first direct observation of $T$ violation through the exchange of initial and final states in transitions that can only be connected by a $T$-symmetry transformation.

We have used torsional oscillators, containing disk-shaped slabs of superfluid ${}^3\mathrm{He}$-$A$, to probe the chiral orbital textures created by cooling into the superfluid state while continuously rotating. Comparing the observed flow-driven textural transitions with numerical simulations of possible textures shows that an oriented monodomain texture with $\widehat{\mathit{l}}$ antiparallel to the angular velocity ${\mathbf{\Omega }}_0$ is left behind after stopping rotation. The bias toward a particular chirality, while in the vortex state, is due to the inequivalence of energies of vortices of opposite circulation. When spun-up from rest, the critical velocity for vortex nucleation depends on the sense of rotation relative to that of $\widehat{\mathit{l}}$. A different type of vorticity, apparently linked to the slab’s rim by a domain wall, appears when the angular velocity, $\mathbf{\Omega }$, is parallel to $\widehat{\mathit{l}}$.