Generic 2D Hamiltonian systems possess partial barriers in their chaotic phase space that restrict classical transport. Quantum mechanically, the transport is suppressed if Planck’s constant $h$ is large compared to the classical flux, $h\gg \Phi$, such that wave packets and states are localized. In contrast, classical transport is mimicked for $h\ll \Phi$. Designing a quantum map with an isolated partial barrier of controllable flux $\Phi$ is the key to investigating the transition from this form of quantum localization to mimicking classical transport. It is observed that quantum transport follows a universal transition curve as a function of the expected scaling parameter $\Phi /h$. We find this curve to be symmetric to $\Phi /h=1$, having a width of 2 orders of magnitude in $\Phi /h$, and exhibiting no quantized steps. We establish the relevance of local coupling, improving on previous random matrix models relying on global coupling. It turns out that a phenomenological $2\times 2$ model gives an accurate analytical description of the transition curve.

Nanostructured thin plastic foils have been used to enhance the mechanism of laser-driven proton beam acceleration. In particular, the presence of a monolayer of polystyrene nanospheres on the target front side has drastically enhanced the absorption of the incident 100 TW laser beam, leading to a consequent increase in the maximum proton energy and beam charge. The cutoff energy increased by about 60% for the optimal spheres’ diameter of 535 nm in comparison to the planar foil. The total number of protons with energies higher than 1 MeV was increased approximately 5 times. To our knowledge this is the first experimental demonstration of such advanced target geometry. Experimental results are interpreted and discussed by means of $2\frac{1}{2}$-dimensional particle-in-cell simulations.

In the process of dissipative relaxation, there is strong astrophysical and laboratory evidence that plasmas tend to evolve towards the well-known Woltjer-Taylor state, specified by $\nabla \times \mathit{B}=\alpha \mathit{B}$ for constant $\alpha$. To explain how such a state is reached, Taylor developed his famous theory based on the conjecture that relaxation is dominated by short wavelength fluctuations. However, there is no conclusive experimental or numerical evidence in support of Taylor’s conjecture. A new theory is developed, which predicts that the system will evolve towards the Woltjer-Taylor state for an arbitrary fluctuation spectrum.

We report clear experimental signatures of the theoretically unexpected gas-liquid transition in the first three monolayer systems of ${}^3\mathrm{He}$ adsorbed on graphite. The transition is inferred from the linear density dependence of the gamma coefficient of the heat capacity measured in the degenerate region ($2\le T\le 80\mathrm{mK}$) below a critical liquid density (${\rho }_{c0}$). Surprisingly, the measured ${\rho }_{c0}$ values ($0.6–0.9{\mathrm{nm}}^{-2}$) are nearly the same for all these layers in spite of their quite different environments. We conclude that the ground state of ${}^3\mathrm{He}$ in strictly two dimensions is not a dilute quantum gas but a self-bound quantum liquid with the lowest density ever found.

We present low field magnetization and susceptibility measurements made on a single crystal of the ferromagnetic superconductor UCoGe. The interplay between ferromagnetism and superconductivity comes into view in the study of hysteresis along the $c$ axis (easy magnetization axis). The Meissner state (perfect diamagnetism) could not be observed in very low magnetic fields for all three crystallographic directions, implying that the sample is always in the mixed state. Notwithstanding, the Meissner-Ochsenfeld effect (reversible flux expulsion) occurs and is found to be anisotropic. For the $c$ axis in low fields, it is proportional to the bulk magnetization $M$ (and thus to the population of domains) and not to the applied magnetic field $H$. On a microscopic level, our interpretation of these results implies that flux is expelled independently from each domain proportional to its volume.

Quantum phase transitions in the two-dimensional Kugel-Khomskii model on a square lattice are studied using the plaquette mean field theory and the entanglement renormalization Ansatz. When $3z^2\mathrm{\text{−}}{r}^2$ orbitals are favored by the crystal field and Hund’s exchange is finite, both methods give a noncollinear exotic magnetic order that consists of four sublattices with mutually orthogonal nearest-neighbor and antiferromagnetic second-neighbor spins. We derive an effective frustrated spin model with second- and third-neighbor spin interactions which stabilize this phase and follow from spin-orbital quantum fluctuations involving spin singlets entangled with orbital excitations.

We have measured the spin-wave spectrum of the half-doped bilayer manganite $\mathrm{Pr}(\mathrm{Ca},\mathrm{Sr}{)}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ in its spin, charge, and orbital ordered phase. The measurements, which extend throughout the Brillouin zone and cover the entire one-magnon spectrum, are compared critically with spin-wave calculations for different models of the electronic ground state. The data are described very well by the Goodenough model, which has weakly interacting ferromagnetic zig-zag chains in the CE-type arrangement. A model that allows ferromagnetic dimers to form within the zigzags is inconsistent with the data. The analysis conclusively rules out the strongly bound dimer (Zener polaron) model.

We study the effects of motor-generated stresses in disordered three-dimensional fiber networks using a combination of a mean-field theory, scaling analysis, and a computational model. We find that motor activity controls the elasticity in an anomalous fashion close to the point of marginal stability by coupling to critical network fluctuations. We also show that motor stresses can stabilize initially floppy networks, extending the range of critical behavior to a broad regime of network connectivities below the marginal point. Away from this regime, or at high stress, motors give rise to a linear increase in stiffness with stress. Finally, we demonstrate that our results are captured by a simple, constitutive scaling relation highlighting the important role of nonaffine strain fluctuations as a susceptibility to motor stress.

A Zeeman-insensitive optical clock atomic transition is engineered when nuclear spins are dressed by a nonresonant radio-frequency field. For fermionic species as ${}^{87}\mathrm{Sr}$, ${}^{171}\mathrm{Yb}$, and ${}^{199}\mathrm{Hg}$, particular ratios between the radio-frequency driving amplitude and frequency lead to “magic” magnetic values where a net cancelation of the Zeeman clock shift and a complete reduction of first-order magnetic variations are produced within a relative uncertainty below the ${10}^{-18}$ level. An Autler-Townes continued fraction describing a semiclassical radio-frequency dressed spin is numerically computed and compared to an analytical quantum description including higher-order magnetic field corrections to the dressed energies.

In the last 15 years hundreds of papers have been devoted to the study of positron-atom or positron-molecule interaction. A large body of evidence has accumulated showing that many atoms in their ground state can bind a positron forming an electronically stable system. Studies on the possibility that a positron binds to an atomic excited state, however, are scarce. The first atom that was proved able to bind a positron in its ground state is lithium. Surprisingly, nothing is known on the possibility that a positron could bind to one of its excited states. In this Letter we study the positron attachment to the $1s^{2}2p$ ${}^{2}P^o$, $1s2s2p$ ${}^{2}P^o$ and $2p^3$ ${}^{4}S^o$ excited states of the lithium atom. While the ${}^{2}P^o$ state cannot bind a positron, and the ${}^{4}S^o$ could at most form a metastable state, a positron can attach to the ${}^{4}P^o$ state of lithium forming a bound state with a binding energy of about 0.003 hartree. This state can alternatively be considered an excited state of the system $e^{+}\mathrm{Li}$ and it could be, in principle, exploited in an experiment to detect $e^{+}\mathrm{Li}$, whose existence has been predicted theoretically but has not yet been observed experimentally.

We have experimentally observed optical multistability (OM) in an optical ring cavity containing three-level $\Lambda$-type Doppler-broadened rubidium atoms. The shape of the OM curve can be significantly modified by changing the power of the control laser field. An all-optical multistate switching or coding element is realized and flexibly controlled by adding a pulse sequence to the input (probe) intensity.

We show that a system of hard disks confined to a narrow channel exhibits a fragile-strong fluid crossover located at the maximum of the isobaric heat capacity and that the relaxation times for different channel widths fall onto a single master curve when rescaled by the relaxation times and temperatures of the crossover. Calculations of the configurational entropy and the inherent structure equation of state find that the crossover is related to properties of the jamming landscape for the model but that the Adam-Gibbs relation does not predict the relaxation behavior. We also show that a facilitated dynamics description of the system, where kinetically excited regions are identified with local packing arrangements of the disks, successfully describes the fragile-strong crossover.

In this Letter, we develop a continuum theory for the Cassie-Baxter–Wenzel (CB–W) transition. The proposed model accounts for the metastabilities in the wetting of rough hydrophobic surfaces, allows us to reconstruct the transition mechanism, and identifies the free energy barriers separating the CB and W states as a function of the liquid pressure. This information is crucial in the context of superhydrophobic surfaces, where there is interest in extending the duration of the metastable superhydrophobic CB state. The model is validated against free energy atomistic simulations.

Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. In ${\mathrm{Cu}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$, the prime candidate of topological superconductors, quantum oscillations are still not observed in magnetotransport measurement. However, using torque magnetometry, quantum oscillations (the de Haas–van Alphen effect) were observed in ${\mathrm{Cu}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$. The doping of Cu in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ increases the carrier density and the effective mass without increasing the scattering rate or decreasing the mean free path. In addition, the Fermi velocity remains the same in ${\mathrm{Cu}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ as that in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Our results imply that the insertion of Cu does not change the band structure and that conduction electrons in Cu doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ sit in the linear Dirac-like band.

A method is derived to solve the massless Dirac-Weyl equation describing electron transport in a monolayer of graphene with a scalar potential barrier $U(x,t)$, homogeneous in the $y$ direction, of arbitrary space and time dependence. Resonant enhancement of both electron backscattering and currents, across and along the barrier, is predicted when the modulation frequencies satisfy certain resonance conditions. These conditions resemble those for Shapiro steps of driven Josephson junctions. Surprisingly, we find a nonzero $y$ component of the current for carriers of zero momentum along the $y$-axis.

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating ${\mathrm{BaTiO}}_3$ barrier to the electrons injected from the ${\mathrm{SrRuO}}_3$ electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here ${\mathrm{SrTiO}}_3$, at one of the ${\mathrm{SrRuO}}_3/{\mathrm{BaTiO}}_3$ interfaces. As the complex band structure of ${\mathrm{SrTiO}}_3$ has the same symmetry as that of ${\mathrm{BaTiO}}_3$, the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the ${\mathrm{SrTiO}}_3$ layer. The ${\mathrm{SrTiO}}_3$ thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

Recent experiments in semiconductor nanowires with a spin-orbit coupling and proximity-induced superconductivity exhibit signatures of Majorana bound states predicted to exist in the topological phase. In this Letter we predict that these nanowire systems exhibit unconventional magnetoelectric effects showing a sharp crossover behavior at the topological phase transition. We find that magnetic fields with a component parallel to the spin-orbit field can give rise to currents in equilibrium. Surprisingly, also fields perpendicular to the spin-orbit field may induce currents and can be employed in adiabatic charge pumping. The perpendicular field magnetoelectric effect may be regarded as a manifestation of the anomalous Hall effect in one dimension. We discuss how the predicted phenomena could be observed in experiments and employed in probing the topological phase transition.

We studied the atomic and electronic structures of ultrathin Bi(111) films grown on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by means of angle-resolved photoemission, first-principles calculations, and low-energy electron diffraction. These Bi films were found to be strained due to the influence of the substrate. Accordingly, the band structure is affected and Bi undergoes a topological phase transition; it is shown that the $Z_2$ topological invariant in three dimensions switches from $+1$ (trivial) to $-1$ (nontrivial or topological). This was clearly confirmed from the change in the surface-state dispersion near the Fermi level. Our discovery offers a method to produce novel topological systems from simple materials.

We describe experiments and simulations carried out to investigate spinodal decomposition in a vibrated, dry granular system. The dynamics is found to be similar to that of systems evolving under curvature-driven diffusion, which suggests the presence of an effective surface tension. By studying quasi-2D droplets in the steady state, we find behavior consistent with Laplace’s equation, demonstrating the existence of an actual surface tension. Detailed measurements of the pressure tensor in the interfacial region show that the surface tension results predominantly from an anisotropy in the kinetic energy part of the pressure tensor, in contrast to thermodynamic systems where it arises from either the attractive interaction between particles or entropic considerations.

Although mechanical properties of DNA are well characterized at the kilobase-pair range, a number of recent experiments have suggested that DNA is more flexible at shorter length scales, which correspond to the regime that is crucial for cellular processes such as DNA packaging and gene regulation. Here, we perform a systematic study of the effective elastic properties of DNA at different length scales by probing the conformation and fluctuations of DNA from the single base-pair level up to four helical turns, using trajectories from atomistic simulation. We find evidence that supports cooperative softening of the stretch modulus and identify the essential modes that give rise to this effect. The bend correlation exhibits modulations that reflect the helical periodicity, while it yields a reasonable value for the effective persistence length, and the twist modulus undergoes a smooth crossover—from a relatively smaller value at the single base-pair level to the bulk value—over half a DNA turn.