Since its prediction 15 years ago, hydrodynamic instability in concentration polarization at a charge-selective interface has been attributed to nonequilibrium electro-osmosis related to the extended space charge which develops at the limiting current. This attribution had a double basis. On the one hand, it has been recognized that neither equilibrium electro-osmosis nor bulk electroconvection can yield instability for a perfectly charge-selective solid. On the other hand, it has been shown that nonequilibrium electro-osmosis can. The first theoretical studies in which electro-osmotic instability was predicted and analyzed employed the assumption of perfect charge selectivity for the sake of simplicity and so did the subsequent studies of various time-dependent and nonlinear features of electro-osmotic instability. In this Letter, we show that relaxing the assumption of perfect charge selectivity (tantamount to fixing the electrochemical potential of counterions in the solid) allows for the equilibrium electroconvective instability. In addition, we suggest a simple experimental test for determining the true, either equilibrium or nonequilibrium, origin of instability in concentration polarization.

Recent advances in fabrication techniques to create mesoscopic 3D structures have led to significant developments in a variety of fields including biology, photonics, and magnetism. Further progress in these areas benefits from their full quantitative and structural characterization. We present resonant ptychographic tomography, combining quantitative hard x-ray phase imaging and resonant elastic scattering to achieve ab initio element-specific 3D characterization of a cobalt-coated artificial buckyball polymer scaffold at the nanoscale. By performing ptychographic x-ray tomography at and far from the Co $K$ edge, we are able to locate and quantify the Co layer in our sample to a 3D spatial resolution of 25 nm. With a quantitative determination of the electron density we can determine that the Co layer is oxidized, which is confirmed with microfluorescence experiments.

In graphene growth, island symmetry can become lower than the intrinsic symmetries of both graphene and the substrate. First-principles calculations and Monte Carlo modeling explain the shapes observed in our experiments and earlier studies for various metal surface symmetries. For equilibrium shape, edge energy variations $\delta E$ manifest in distorted hexagons with different ground-state edge structures. In growth or nucleation, energy variation enters exponentially as $\sim e^{\delta E/k_{B}T}$, strongly amplifying the symmetry breaking, up to completely changing the shapes to triangular, ribbonlike, or rhombic.

We demonstrate that honeycomb arrays of microcavity pillars behave as an optical-frequency two-dimensional photonic topological insulator. We show that the interplay between the photonic spin-orbit coupling natively present in this system and the Zeeman splitting of exciton polaritons in external magnetic fields leads to the opening of a nontrivial gap characterized by a $C=\pm 2$ set of band Chern numbers and to the formation of topologically protected one-way edge states.

We investigate a system of multiple Majorana states at the end of a topological superconducting wire coupled to a normal lead. For a minimum of three Majorana fermions at the interface, we find nontrivial renormalization physics. Interface tunneling processes can be classified in terms of spin-$1/2$ and spin-$3/2$ irreducible representations of the SU(2) group. We show that the renormalization of the tunneling amplitudes belonging to different representations is completely different in that one type is suppressed, whereas the other is enhanced, depending on the sign of the Kondo-type interaction coupling. This results in distinct temperature dependencies of the tunneling current through the interface and different spin polarizations of this current.

Coherent-control protocols are introduced to selectively transport electrons, excitons, or pure two-particle correlations through semiconductor interfaces. The scheme is tested in a double-quantum-well structure where a sequence of terahertz pulses is applied to induce the vertical excitation transfer between the wells. Using a microscopic theory, it is shown that efficient and highly selective transfer can be realized even in the presence of the unavoidable scattering and dephasing processes.

We use numerical exact diagonalization to analyze which aspects of the many-body localization phenomenon survive in an imperfectly isolated setting, when the system of interest is weakly coupled to a thermalizing environment. We show that widely used diagnostics (such as many-body level statistics and expectation values in exact eigenstates) cease to show signatures of many-body localization above a critical coupling that is exponentially small in the size of the environment. However, we also identify alternative diagnostics for many-body localization, in the spectral functions of local operators. Diagnostics include a discrete spectrum and a hierarchy of energy gaps, including a universal gap at zero frequency. These alternative diagnostics are shown to be robust, and continue to show signatures of many-body localization as long as the coupling to the bath is weaker than the characteristic energy scales in the system. We also examine how these signatures disappear when the coupling to the environment becomes larger than the characteristic energy scales of the system.

It is established that the multiferroics $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ crystallize in the centrosymmetric $Pbam$ space group and that the magnetically induced electric polarization appearing at low temperature is accompanied by a symmetry breaking. However, both our present x-ray study—performed on compounds with $R=\mathrm{Pr},\mathrm{Nd},\mathrm{Gd},\mathrm{Tb}$, and Dy—and first-principles calculations unambiguously rule out this picture. Based on structural refinements, geometry optimization, and physical arguments, we demonstrate in this Letter that the actual space group is likely to be $Pm$. This turns out to be of crucial importance for $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ multiferroics since $Pm$ is not centrosymmetric. Ferroelectricity is thus already present at room temperature, and its enhancement at low temperature is a spin-enhanced process. This result is also supported by direct observation of optical second harmonic generation. This fundamental result calls into question the actual theoretical approaches that describe the magnetoelectric coupling in this multiferroic family.

Mott insulators with both spin and orbital degeneracy are pertinent to a large number of transition metal oxides. The intertwined spin and orbital fluctuations can lead to rather exotic phases such as quantum spin-orbital liquids. Here, we consider two-component (spin $1/2$) fermionic atoms with strong repulsive interactions on the $p$ band of the optical square lattice. We derive the spin-orbital exchange for quarter filling of the $p$ band when the density fluctuations are suppressed, and show that it frustrates the development of long-range spin order. Exact diagonalization indicates a spin-disordered ground state with ferro-orbital order. The system dynamically decouples into individual Heisenberg spin chains, each realizing a Luttinger liquid accessible at higher temperatures compared to atoms confined to the $s$ band.

In this Letter, we describe a new method to use baryon acoustic oscillations (BAO) to derive a constraint on the possible variation of the speed of light. The method relies on the fact that there is a simple relation between the angular diameter distance ($D_A$) maximum and the Hubble function ($H$) evaluated at the same maximum-condition redshift, which includes speed of light $c$. We note the close analogy of the BAO probe with a laboratory experiment: here we have $D_A$ which plays the role of a standard (cosmological) ruler, and $H^{-1}$, with the dimension of time, as a (cosmological) clock. We evaluate if current or future missions such as Euclid can be sensitive enough to detect any variation of $c$.

Strong-field ionization of aligned diatomic and polyatomic molecules such as ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, ${\mathrm{C}}_2{\mathrm{H}}_4$, and others in circularly polarized laser fields is investigated theoretically. By calculating the emission-angle-resolved lateral width of the momentum distribution perpendicular to the polarization plane, we show that nodal planes in molecular orbitals are directly imprinted on the angular dependence of the width. We demonstrate that orbital symmetries can be distinguished with the information obtained by observing the lateral width in addition to the angular distributions.

Radiation pressure acceleration is a highly efficient mechanism of laser-driven ion acceleration, with the laser energy almost totally transferrable to the ions in the relativistic regime. There is a fundamental limit on the maximum attainable ion energy, which is determined by the group velocity of the laser. In the case of tightly focused laser pulses, which are utilized to get the highest intensity, another factor limiting the maximum ion energy comes into play, the transverse expansion of the target. Transverse expansion makes the target transparent for radiation, thus reducing the effectiveness of acceleration. Utilization of an external guiding structure for the accelerating laser pulse may provide a way of compensating for the group velocity and transverse expansion effects.

The metallization of high-pressure hydrogen, together with the associated molecular to atomic transition, is one of the most important problems in the field of high-pressure physics. It is also currently a matter of intense debate due to the existence of conflicting experimental reports on the observation of metallic hydrogen on a diamond-anvil cell. Theoretical calculations have typically relied on a mean-field description of electronic correlation through density functional theory, a theory with well-known limitations in the description of metal-insulator transitions. In fact, the predictions of the pressure-driven dissociation of molecules in high-pressure hydrogen by density functional theory is strongly affected by the chosen exchange-correlation functional. In this Letter, we use quantum Monte Carlo calculations to study the molecular to atomic transition in hydrogen. We obtain a transition pressure of 447(3) GPa, in excellent agreement with the best experimental estimate of the transition 450 GPa based on an extrapolation to zero band gap from experimental measurements. Additionally, we find that $C2/c$ is stable almost up to the molecular to atomic transition, in contrast to previous density functional theory (DFT) and $\mathrm{DFT}+\mathrm{quantum}$ Monte Carlo studies which predict large stability regimes for intermediary molecular phases.

Intrinsic spin Hall conductivities are calculated for strong spin-orbit ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ semimetals, from the Kubo formula and using Berry curvatures evaluated throughout the Brillouin zone from a tight-binding Hamiltonian. Nearly crossing bands with strong spin-orbit interaction generate giant spin Hall conductivities in these materials, ranging from 474 $(\hslash /\mathrm{e})(\mathrm{\Omega }\mathrm{cm}{)}^{-1}$ for bismuth to 96 $(\hslash /\mathrm{e})(\mathrm{\Omega }\mathrm{cm}{)}^{-1}$ for antimony; the value for bismuth is more than twice that of platinum. The large spin Hall conductivities persist for alloy compositions corresponding to a three-dimensional topological insulator state, such as ${\mathrm{Bi}}_{0.83}{\mathrm{Sb}}_{0.17}$. The spin Hall conductivity could be changed by a factor of 5 for doped Bi, or for ${\mathrm{Bi}}_{0.83}{\mathrm{Sb}}_{0.17}$, by changing the chemical potential by 0.5 eV, suggesting the potential for doping or voltage tuned spin Hall current.

The electric-field-induced modification in the Curie temperature of prototypical transition-metal thin films with the perpendicular magnetic easy axis, a freestanding Fe(001) monolayer and a Co monolayer on Pt(111), is investigated by first-principles calculations of spin-spiral structures in an external electric field ($E$ field). An applied $E$ field is found to modify the magnon (spin-spiral formation) energy; the change arises from the $E$-field-induced screening charge density in the spin-spiral states due to $p$-$d$ hybridizations. The Heisenberg exchange parameters obtained from the magnon energy suggest an $E$-field-induced modification of the Curie temperature, which is demonstrated via Monte Carlo simulations that take the magnetocrystalline anisotropy into account.

Ultrafast terahertz spectroscopy accesses the dark excitonic ground state in resonantly excited (6,5) single-walled carbon nanotubes via internal, direct dipole-allowed transitions between the lowest-lying dark-bright pair state of $\sim 6\mathrm{meV}$. An analytical model reproduces the response that enables the quantitative analysis of transient densities of dark excitons and $e\text{−}h$ plasma, oscillator strength, transition energy renormalization, and dynamics. Nonequilibrium, yet stable, quasi-one-dimensional quantum states with dark excitonic correlations rapidly emerge even with increasing off-resonance photoexcitation and experience a unique crossover to complex phase-space filling of both dark and bright pair states, different from dense two- and three-dimensional excitons influenced by the thermalization, cooling, and ionization to free carriers.

Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between ${\mathrm{MoS}}_2$ and ${\mathrm{MoSe}}_2$ single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.

We demonstrate that upstream swimming of sperm emerges via an orientation disorder-order transition. The order parameter, the average orientation of the sperm head against the flow, follows a 0.5 power law with the deviation from the critical flow shear rate ($\gamma -{\gamma }_c$). This transition is successfully explained by a hydrodynamic bifurcation theory, which extends the sperm upstream swimming to a broad class of near surface microswimmers that possess front-back asymmetry and circular motion.