We investigate the low-energy electron dynamics in two-dimensional ultrathin film of magnetically doped topological insulators in the context of gate-tuned coherent spin manipulation. Our first-principles calculations for such film unambiguously identify its spin-resolved topological band structure arising from spin-orbit coupling and time-reversal symmetry breaking. Exploiting this characteristic, we predict a negative birefraction for chiral electron tunneling through a gate-controlled $p\mathrm{\text{−}}n$ interface in the film, analogous to optical birefringence. By fine-tuning the gate voltage, a series of unusual phenomena, including electron double focusing, spatial modulation of spin polarizations, and quantum-interference-induced beating patterns, could be efficiently implemented, offering a powerful platform to establish spin-resolved electron optics by all-electrical means.

Raman spectroscopy has been used in chemistry and physics to investigate the fundamental process involving light and phonons. The carbon nanohorn introduces a new subject to Raman spectroscopy, namely topology. We show theoretically that a photoexcited carrier with a nonzero winding number activates a topological $D$ Raman band through the Aharonov-Bohm effect. The topology-induced $D$ Raman band can be distinguished from the ordinary $D$ Raman band for a graphene edge by its peak position.

The optical properties of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates ($1/\tau$), well describe the real part of the optical conductivity ${\sigma }_1(\omega )$. A “broad” Drude component results in an incoherent background with a $T$-independent $1/{\tau }_b$, while a “narrow” Drude component reveals a $T$-linear $1/{\tau }_n$ resulting in a resistivity ${\rho }_n\equiv 1/{\sigma }_{1n}(\omega \to 0)$ also linear in temperature. An $\arctan (T)$ low-frequency spectral weight is also strong evidence for a $T$-linear $1/\tau$. A comparison to other materials with similar behavior suggests that the $T$-linear $1/{\tau }_n$ and ${\rho }_n$ in ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.

We present a new tool for imaging spin properties. We show that a spatially averaged spin signal, measured as a function of a scanned magnetic probe’s position, contains information about the local spin properties. In this first demonstration we map the injected spin density in GaAs by measuring spin photoluminescence with a resolution of $1.2\mu \mathrm{m}$. The ultimate limit of the technique is set by the gradient of the probe’s field, allowing for a resolution beyond the optical diffraction limit. Such probes can also be integrated with other detection methods. This generality allows the technique to be extended to buried interfaces and optically inactive materials.

A thermodynamically consistent gradient dynamics model for the evolution of thin layers of liquid mixtures, solutions, and suspensions on solid substrates is presented which is based on a film-height- and mean-concentration-dependent free energy functional. It is able to describe a large variety of structuring processes, including coupled dewetting and decomposition processes. As an example, the model is employed to investigate the dewetting of thin films of liquid mixtures and suspensions under the influence of effective long-range van der Waals forces that depend on solute concentration. The occurring fluxes are discussed, and it is shown that spinodal dewetting may be triggered through the coupling of film height and concentration fluctuations. Fully nonlinear calculations provide the time evolution and resulting steady film height and concentration profiles.

The flow properties of a continuum model for an active nematic are studied and compared with recent experiments on suspensions of microtubule bundles and molecular motors. The velocity correlation length is found to be independent of the strength of the activity while the characteristic velocity scale increases monotonically as the activity is increased, both in agreement with the experimental observations. We interpret our results in terms of the creation and annihilation dynamics of a gas of topological defects.

We use graph theory to analyze chromatin interaction (Hi-C) data in the human genome. We show that a key functional feature of the genome—“master” replication origins—corresponds to DNA loci of maximal network centrality. These loci form a set of interconnected hubs both within chromosomes and between different chromosomes. Our results open the way to a fruitful use of graph theory concepts to decipher DNA structural organization in relation to genome functions such as replication and transcription. This quantitative information should prove useful to discriminate between possible polymer models of nuclear organization.

We address the collective dynamics of non-Brownian particles cruising in a confined microfluidic geometry and provide a comprehensive characterization of their spatiotemporal density fluctuations. We show that density excitations freely propagate at all scales, and in all directions even though the particles are neither affected by potential forces nor by inertia. We introduce a kinetic theory which quantitatively accounts for our experimental findings, demonstrating that the fluctuation spectrum of this nonequilibrium system is shaped by the combination of truly long-range hydrodynamic interactions and local collisions. We also demonstrate that the free propagation of density waves is a generic phenomenon which should be observed in a much broader range of hydrodynamic systems.

The plume from the 2011 eruption of Grímsvötn was highly electrically charged, as shown by the considerable lightning activity measured by the United Kingdom Met Office’s low-frequency lightning detection network. Previous measurements of volcanic plumes have shown that ash particles are electrically charged up to hundreds of kilometers away from the vent, which indicates that the ash continues to charge in the plume [R. G. Harrison, K. A. Nicoll, Z. Ulanowski, and T. A. Mather, Environ. Res. Lett. 5, 024004 (2010); H. Hatakeyama J. Meteorol. Soc. Jpn. 27, 372 (1949)]. In this Letter, we study triboelectric charging of different size fractions of a sample of volcanic ash experimentally. Consistently with previous work, we find that the particle size distribution is a determining factor in the charging. Specifically, our laboratory experiments demonstrate that the normalized span of the particle size distribution plays an important role in the magnitude of charging generated. The influence of the normalized span on plume charging suggests that all ash plumes are likely to be charged, with implications for remote sensing and plume lifetime through scavenging effects.

We present a model for cosmological inflation based on a no-scale supergravity sector with an $\mathrm{SU}(2,1)/\mathrm{SU}(2)\times \mathrm{U}(1)$ Kähler potential, a single modulus $T$, and an inflaton superfield $\Phi$ described by a Wess-Zumino model with superpotential parameters ($\mu$, $\lambda$). When $T$ is fixed, this model yields a scalar spectral index $n_s$ and a tensor-to-scalar ratio $r$ that are compatible with the Planck measurements for values of $\lambda \simeq \mu /3M_P$. For the specific choice $\lambda =\mu /3M_P$, the model is a no-scale supergravity realization of the $R+R^2$ Starobinsky model.

We argue that the observed large-scale cosmic microwave anomalies, discovered by WMAP and confirmed by the Planck satellite, are most naturally explained in the context of a marginally open universe. Particular focus is placed on the dipole power asymmetry, via an open universe implementation of the large-scale gradient mechanism of Erickcek et al. Open inflation models, which are motivated by the string landscape and which can excite “supercurvature” perturbation modes, can explain the presence of a very-large-scale perturbation that leads to a dipole modulation of the power spectrum measured by a typical observer. We provide a specific implementation of the scenario which appears compatible with all existing constraints.

We measure the mass difference $\Delta m_0$ between the $D^{*}(2010{)}^{+}$ and the $D^0$ and the natural linewidth $\Gamma$ of the transition $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$. The data were recorded with the BABAR detector at center-of-mass energies at and near the ${\rm Y}(4S)$ resonance, and correspond to an integrated luminosity of approximately $477{\mathrm{fb}}^{-1}$. The $D^0$ is reconstructed in the decay modes $D^0\to K^{-}{\pi }^{+}$ and $D^0\to K^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$. For the decay mode $D^0\to K^{-}{\pi }^{+}$ we obtain $\Gamma =(83.4\pm 1.7\pm 1.5)\mathrm{keV}$ and $\Delta m_0=(145425.6\pm 0.6\pm 1.8)\mathrm{keV}$, where the quoted errors are statistical and systematic, respectively. For the $D^0\to K^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$ mode we obtain $\Gamma =(83.2\pm 1.5\pm 2.6)\mathrm{keV}$ and $\Delta m_0=(145426.6\pm 0.5\pm 2.0)\mathrm{keV}$. The combined measurements yield $\Gamma =(83.3\pm 1.2\pm 1.4)\mathrm{keV}$ and $\Delta m_0=(145425.9\pm 0.4\pm 1.7)\mathrm{keV}$; the width is a factor of approximately 12 times more precise than the previous value, while the mass difference is a factor of approximately 6 times more precise.

We search for bottomonium states in ${\rm Y}(2S)\to (b\overline{b})\gamma$ decays with an integrated luminosity of $24.7{\mathrm{fb}}^{-1}$ recorded at the ${\rm Y}(2S)$ resonance with the Belle detector at KEK, containing $(157.8\pm 3.6)\times {10}^6$ ${\rm Y}(2S)$ events. The $(b\overline{b})$ system is reconstructed in 26 exclusive hadronic final states composed of charged pions, kaons, protons, and $K_S^0$ mesons. We find no evidence for the state recently observed around 9975 MeV ($X_{b\overline{b}}$) in an analysis based on a data sample of $9.3\times {10}^6$ ${\rm Y}(2S)$ events collected with the CLEO III detector. We set a 90% confidence level upper limit on the branching fraction $\mathcal{B}[{\rm Y}(2S)\to X_{b\overline{b}}\gamma ]\times \sum _i\mathcal{B}[X_{b\overline{b}}\to h_i]<4.9\times {10}^{-6}$, summed over the exclusive hadronic final states employed in our analysis. This result is an order of magnitude smaller than the measurement reported with CLEO data. We also set an upper limit for the ${\eta }_b(1S)$ state of $\mathcal{B}[{\rm Y}(2S)\to {\eta }_b(1S)\gamma ]\times \sum _i\mathcal{B}[{\eta }_b(1S)\to h_i]<3.7\times {10}^{-6}$.

The coupling of weak light fields to Rydberg states of atoms under conditions of electromagnetically induced transparency leads to the formation of Rydberg polaritons which are quasiparticles with tunable effective mass and nonlocal interactions. Confined to one spatial dimension their low energy physics is that of a moving-frame Luttinger liquid which, due to the nonlocal character of the repulsive interaction, can form a Wigner crystal of individual photons. We calculate the Luttinger $K$ parameter using density-matrix renormalization group simulations and find that under typical slow-light conditions kinetic energy contributions are too strong for crystal formation. However, adiabatically increasing the polariton mass by turning a light pulse into stationary spin excitations allows us to generate true crystalline order over a finite length. The dynamics of this process and asymptotic correlations are analyzed in terms of a time-dependent Luttinger theory.

Initiating the gain process in a free-electron laser (FEL) from an external highly coherent source of radiation is a promising way to improve the pulse properties such as temporal coherence and synchronization performance in time-resolved pump-probe experiments at FEL facilities, but this so-called “seeding” suffers from the lack of adequate sources at short wavelengths. We report on the first successful seeding at a wavelength as short as 38.2 nm, resulting in GW-level, coherent FEL radiation pulses at this wavelength as well as significant second harmonic emission at 19.1 nm. The external seed pulses are about 1 order of magnitude shorter compared to previous experiments allowing an ultimate time resolution for the investigation of dynamic processes enabling breakthroughs in ultrafast science with FELs. The seeding pulse is the 21st harmonic of an 800-nm, 15-fs (rms) laser pulse generated in an argon medium. Methods for finding the overlap of seed pulses with electron bunches in spatial, longitudinal, and spectral dimensions are discussed and results are presented. The experiment was conducted at FLASH, the FEL user facility at DESY in Hamburg, Germany.

We investigate the momentum-dependent transport of 1D quasicondensates in quasiperiodic optical lattices. We observe a sharp crossover from a weakly dissipative regime to a strongly unstable one at a disorder-dependent critical momentum. In the limit of nondisordered lattices the observations suggest a contribution of quantum phase slips to the dissipation. We identify a set of critical disorder and interaction strengths for which such critical momentum vanishes, separating a fluid regime from an insulating one. We relate our observation to the predicted zero-temperature superfluid-Bose glass transition.

We study various realizations of collective coordinates, e.g., the position of a particle, the charge of a Coulomb box, or the phase of a Bose or a superconducting condensate, coupled to Luttinger liquids with $N$ flavors. We find that for a Luttinger parameter $(1/2)<K<1$ there is a phase transition from a delocalized phase into a phase with a periodic potential at strong coupling. In the delocalized phase the dynamics is dominated by an effective mass, i.e., diffusive in imaginary time, while on the transition line it becomes dissipative. At $K=(1/2)$ there is an additional transition into a localized phase with no diffusion at zero temperature.

Recently, the possibility of quantum simulation of dynamical gauge fields was pointed out by using a system of cold atoms trapped on each link in an optical lattice. However, to implement exact local gauge invariance, fine-tuning the interaction parameters among atoms is necessary. In the present Letter, we study the effect of violation of the U(1) local gauge invariance by relaxing the fine-tuning of the parameters and showing that a wide variety of cold atoms is still a faithful quantum simulator for a U(1) gauge-Higgs model containing a Higgs field sitting on sites. The clarification of the dynamics of this gauge-Higgs model sheds some light upon various unsolved problems, including the inflation process of the early Universe. We study the phase structure of this model by Monte Carlo simulation and also discuss the atomic characteristics of the Higgs phase in each simulator.

Nanoclusters with extra stability at certain cluster sizes are known as magic clusters with exotic properties. The classic Wulff construction principle, which stipulates that the preferred structure of a cluster should minimize its total surface energy, is often invoked in determining the cluster magicity, resulting in close-shelled Mackay icosahedronal clusters with odd-numbered magic sizes of 13, 55, 147, etc. Here we use transition metal clusters around size 55 as prototypical examples to demonstrate that, in the nanometer regime, the classic Wulff construction principle needs to be generalized to primarily emphasize the edge atom effect instead of the surface energy. Specifically, our detailed calculations show that nanoclusters with much shorter total edge lengths but substantially enlarged total surface areas are energetically much more stable. As a consequence, a large majority of the nanoclusters within the $3d$-, $4d$-, and $5d$-transition metal series are found to be fcc or hcp crystal fragments with much lower edge energies, and the widely perceived magic size of 55 is shifted to its nearby even numbers.

It has been highly debated whether the thermal conductivity $\kappa$ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to $\kappa$ values not only lower than the alloy’s, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that $\kappa$ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower $\kappa$ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of $\sim 3\mathrm{THz}$. Our calculated $\kappa$’s quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.