Surface reactions with oxygen are a fundamental cause of the degradation of phosphorene. Using first-principles calculations, we show that for each oxygen atom adsorbed onto phosphorene there is an energy release of about 2 eV. Although the most stable oxygen adsorbed forms are electrically inactive and lead only to minor distortions of the lattice, there are low energy metastable forms which introduce deep donor and/or acceptor levels in the gap. We also propose a mechanism for phosphorene oxidation involving reactive dangling oxygen atoms and we suggest that dangling oxygen atoms increase the hydrophilicity of phosphorene.

We report a combination of Fe $K\beta$ x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in ${\mathrm{CaFe}}_2{({\mathrm{As}}_{1-x}{\mathrm{P}}_x)}_2$. The puzzling temperature behavior of the local moment found in rare earth-doped ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in ${\mathrm{CaFe}}_2{({\mathrm{As}}_{1-x}{\mathrm{P}}_x)}_2$. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both ${\mathrm{CaFe}}_2{({\mathrm{As}}_{1-x}{\mathrm{P}}_x)}_2$ ($x=0.055$) and ${\mathrm{Ca}}_{0.78}{\mathrm{La}}_{0.22}{\mathrm{Fe}}_2{\mathrm{As}}_2$ at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the $c$-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

The interfacial Dzyaloshinskii-Moriya interaction in an in-plane anisotropic $\mathrm{Pt}(4\mathrm{nm})/\mathrm{Co}(1.6\mathrm{nm})/\mathrm{Ni}(1.6\mathrm{nm})$ film has been directly observed by Brillouin spectroscopy. It is manifested as the asymmetry of the measured magnon dispersion relation, from which the Dzyaloshinskii-Moriya interaction constant has been evaluated. Linewidth measurements reveal that the lifetime of the magnons is asymmetric with respect to their counter-propagating directions. The lifetime asymmetry is dependent on the magnon frequency, being more pronounced, the higher the frequency. Analytical calculations of the magnon dispersion relation and linewidth agree well with experiments.

We study the linear and nonlinear acoustic response of ${\mathrm{SrTiO}}_3$ across its ferroelastic transition at $T_a=105\mathrm{K}$ by time domain Brillouin scattering. Above $T_a$ we observe that for a strain amplitude of $\sim 0.18\%$ the sound velocity for compressive strain exceeds the tensile strain velocity by 3%. Below $T_a$ we find a giant slowing down of the sound velocity by 12% and attribute this to the coupling of GHz phonons to ferroelastic twin domain walls. We propose a new mechanism for this coupling on the ultrafast time scale, providing an important new test ground for theories used to simulate atomic motion in domain forming crystals.

We study a specific type of lifetime broadening resulting in the well-known exponential “Urbach tail” density of states within the energy gap of an insulator. After establishing the frequency and temperature dependence of the Urbach edge in GaAs quantum wells, we show that the broadening due to the zero-point optical phonons is the fundamental limit to the Urbach slope in high-quality samples. In rough analogy with Welton’s heuristic interpretation of the Lamb shift, the zero-temperature contribution to the Urbach slope can be thought of as arising from the electric field of the zero-point longitudinal-optical phonons. The value of this electric field is experimentally measured to be $3\mathrm{kV}{\mathrm{cm}}^{-1}$, in excellent agreement with the theoretical estimate.

In this Letter, we study the electronic structures and optical properties of partially and fully fluorinated graphene by a combination of ab initio $G_0{W}_0$ calculations and large-scale multiorbital tight-binding simulations. We find that, for partially fluorinated graphene, the appearance of paired fluorine atoms is more favorable than unpaired atoms. We also show that different types of structural disorder, such as carbon vacancies, fluorine vacancies, fluorine vacancy clusters and fluorine armchair and zigzag clusters, will introduce different types of midgap states and extra excitations within the optical gap. Furthermore, we argue that the local formation of $s{p}^3$ bonds upon fluorination can be distinguished from other disorder inducing mechanisms which do not destroy the $s{p}^2$ hybrid orbitals by measuring the polarization rotation of passing polarized light.

We demonstrate nanoscale x-ray holographic imaging using optimized illumination wave fronts emitted by x-ray waveguide channels. Mode filtering minimizes wave-front distortions and artifacts encountered in most hard x-ray focusing schemes, enabling quantitative reconstruction of the projected density, as evidenced by a test pattern imaged with a field of view of about $20\times 40\mu \mathrm{m}$ and at 22 nm resolution. The dose efficiency and contrast sensitivity make the optical scheme compatible with samples of intrinsically low contrast, typical for hydrated soft matter. This is demonstrated by imaging bacteria in the hydrated and living state, with quantitative phase contrast revealing dense structures of the bacterial nucleoids associated with compactified DNA. In response to continued irradiation, characteristic changes in these dense structures are observed.

Phenomenologically important quantum dissipative processes include blackbody friction (an atom absorbs counterpropagating blueshifted photons and spontaneously emits them in all directions, losing kinetic energy) and noncontact van der Waals friction (in the vicinity of a dielectric surface, the mirror charges of the constituent particles inside the surface experience drag, slowing the atom). The theoretical predictions for these processes are modified upon a rigorous quantum electrodynamic treatment, which shows that the one-loop “correction” yields the dominant contribution to the off-resonant, gauge-invariant, imaginary part of the atom’s polarizability at room temperature, for typical atom-surface interactions. The tree-level contribution to the polarizability dominates at high temperature.

We show that the timed Dicke states of a collection of three-level atoms can form a tight-binding lattice in momentum space. This lattice, coined the superradiance lattice (SL), can be constructed based on electromagnetically induced transparency (EIT). For a one-dimensional SL, we need the coupling field of the EIT system to be a standing wave. The detuning between the two components of the standing wave introduces an effective uniform force in momentum space. The quantum lattice dynamics, such as Bloch oscillations, Wannier-Stark ladders, Bloch band collapsing, and dynamic localization can be observed in the SL. The two-dimensional SL provides a flexible platform for Dirac physics in graphene. The SL can be extended to three and higher dimensions where no analogous real space lattices exist with new physics waiting to be explored.

Microtubules and motor proteins are building blocks of self-organized subcellular biological structures such as the mitotic spindle and the centrosomal microtubule array. These same ingredients can form new “bioactive” liquid-crystalline fluids that are intrinsically out of equilibrium and which display complex flows and defect dynamics. It is not yet well understood how microscopic activity, which involves polarity-dependent interactions between motor proteins and microtubules, yields such larger-scale dynamical structures. In our multiscale theory, Brownian dynamics simulations of polar microtubule ensembles driven by cross-linking motors allow us to study microscopic organization and stresses. Polarity sorting and cross-link relaxation emerge as two polar-specific sources of active destabilizing stress. On larger length scales, our continuum Doi-Onsager theory captures the hydrodynamic flows generated by polarity-dependent active stresses. The results connect local polar structure to flow structures and defect dynamics.

During the division of animal cells, an actomyosin ring is formed in the cell cortex. The contraction of this ring induces shape changes of the cell and the formation of a cytokinesis furrow. In many cases, a cell-cell interface forms that separates the two new cells. Here we present a simple physical description of the cell shape changes and the dynamics of the interface closure, based on force balances involving active stresses and viscous friction. We discuss conditions in which the interface closure is either axially symmetric or asymmetric. We show that our model can account for the observed dynamics of ring contraction and interface closure in the C. elegans embryo.

A novel scheme is proposed to generate an isolated monocycle x-ray pulse in free-electron lasers, which is based on coherent emission from a chirped microbunch passing through a strongly tapered undulator. In this scheme, the pulse lengthening by optical slippage, being intrinsic to the lasing process of free-electron lasers, can be effectively suppressed through destructive interference of electromagnetic waves emitted at individual undulator periods. Calculations show that an isolated monocycle x-ray pulse with a wavelength of 8.6 nm and a peak power of 1.2 GW can be generated if this scheme is applied to a 2-GeV and 2-kA electron beam.

We show in microwave measurements and computer simulations that the contribution of each eigenchannel of the transmission matrix to the density of states (DOS) is the derivative with angular frequency of a composite phase shift. The accuracy of the measurement of the DOS determined from transmission eigenchannels is confirmed by the agreement with the DOS found from the decomposition of the field into modes. The distribution of the DOS, which underlies the Thouless number, is substantially broadened in the Anderson localization transition. We find a crossover from constant to exponential scaling of fluctuations of the DOS normalized by its average value. These results illuminate the relationships between scattering, stored energy, and dynamics in complex media.

An autonomous oscillator synchronizes to an external harmonic force only when the forcing frequency lies within a certain interval—known as the synchronization range—around the oscillator’s natural frequency. Under ordinary conditions, the width of the synchronization range decreases when the oscillation amplitude grows, which constrains synchronized motion of micro- and nanomechanical resonators to narrow frequency and amplitude bounds. Here, we show that nonlinearity in the oscillator can be exploited to manifest a regime where the synchronization range increases with increasing oscillation amplitude. Experimental data are provided for self-sustained micromechanical oscillators operating in this regime, and analytical results show that nonlinearities are the key determinants of this effect. Our results provide a new strategy to enhance the synchronization of micromechanical oscillators by capitalizing on their intrinsic nonlinear dynamics.

Turbulent motions in a fluid decay at a certain rate once stirring has stopped. The role of the most basic parameter in fluid mechanics, the Reynolds number, in setting the decay rate is not generally known. This Letter concerns the high-Reynolds-number limit of the process. In a classical grid-turbulence wind-tunnel experiment that both reaches higher Reynolds numbers than ever before and covers a wide range of them ($10^4<\text{Re}=UM/\nu <5\times 10^6$), we measure the decay rate with the unprecedented precision of about 2%. Here $U$ is the mean speed of the flow, $M$ is the forcing scale, and $\nu$ is the kinematic viscosity of the fluid. We observe that the decay rate is Reynolds-number independent, which contradicts some models and supports others.

We probe the Ioffe-Regel limits of glasses with repulsions near the zero-temperature jamming transition by calculating the dynamical structure factors. The Ioffe-Regel limit (frequency) is reached when the phonon wavelength is comparable to the mean free path, beyond which phonons are no longer well defined. At zero temperature, the transverse Ioffe-Regel frequency vanishes at the jamming transition with a diverging length, but the longitudinal one does not, which excludes the existence of a diverging length associated with the longitudinal excitations. At low temperatures, the transverse and longitudinal Ioffe-Regel frequencies approach zero at the jamminglike transition and glass transition, respectively. As a consequence, glasses between the glass transition and the jamminglike transition, which are hard-sphere glasses in the low temperature limit, can only carry well-defined longitudinal phonons and have an opposite pressure dependence of the ratio of the shear modulus to the bulk modulus from glasses beyond the jamminglike transition.

By using broadband dielectric spectroscopy in the radio frequency and microwave range, we studied the magnetoelectric dynamics in the multiferroic chiral antiferromagnet ${\mathrm{MnWO}}_4$. Above the multiferroic phase transition at $T_{N2}\approx 12.6\mathrm{K}$ we observe a critical slowing of the corresponding magnetoelectric fluctuations resembling the soft-mode behavior in canonical ferroelectrics. This electric-field-driven excitation carries much less spectral weight than ordinary phonon modes. Also, the critical slowing down of this mode scales with an exponent larger than 1, which is expected for magnetic second-order phase transition scenarios. Therefore, the investigated dynamics have to be interpreted as the softening of an electrically active magnetic excitation, an electromagnon.