Observing the emergence of phase biaxiality in a polar smectic A system via polarised Raman spectroscopy

We report polarised Raman spectroscopy, optical and dielectric properties of an asymmetric bent-core compound derived from 3-hydroxybenzoic acid with a long terminal chain at one end and a nitro group at the other. Earlier X-ray scattering experiments on the compound suggested a partial bilayer smectic A phase (SmA_d) and a partial bilayer biaxial antiferroelectric smectic A phase (SmA_d P_A) in the material. The dielectric behaviour, the microscopic textures and conoscopy experiments all explicitly show that the compound exhibits two different phases, with the lower temperature phase biaxial in nature. Raman spectroscopy was used to determine the temperature evolution of the uniaxial order parameters 〈P_2 〉 and 〈P_4 〉, deduced from analysis of the depolarisation ratio, informed by modelling the bent-core structure. Anomalously low values were measured (less than 0.5 and 0.15 respectively) which could suggest that the smectic A phase may be de Vries like in nature, rather than a partial bilayer structure. Raman spectroscopy was also used to investigate the biaxial nature of the SmA_d P_A phase. The effect that the biaxial order parameters 〈P_220 〉,〈P_420 〉 and 〈P_440 〉 has have on the depolarisation ratio is calculated. By making the assumption of an approximately continuous increase in the 〈P_2 〉 and 〈P_4 〉 order parameters, it was possible to deduce the behaviour of the biaxial order parameters in the biaxial SmA_d P_A phase; the emergence of biaxial order in the system is clearly demonstrated as all of the biaxial order parameters increase in magnitude as the temperature decreases in the (SmA_d P_A) phase. The dielectric studies show that the perpendicular component of the dielectric permittivity increases from 10 to 70 in the SmA_d phase and decreases from 70 to 45 in the SmA_d P_A phase. A strongly temperature dependent relaxation frequency with a large value ~400 kHz is observed in the SmA_d phase. On the other, the SmA_d P_A phase exhibits a weakly temperature dependent relaxation frequency at ~100 kH

Harmonic emission from cluster nanoplasmas subject to intense short laser pulses

Harmonic emission from cluster nanoplasmas subject to short intense infrared laser pulses is studied. In a previous publication [M. Kundu et al., Phys. Rev. A 76, 033201 (2007)] we reported particle-in-cell simulation results showing resonant enhancements of low-order harmonics when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. Simultaneously we found that high-order harmonics were barely present in the spectrum, even at high intensities. The current paper is focused on the analytical modeling of the process. We show that dynamical stochasticity owing to nonlinear resonance inhibits the emission of high order harmonics.Comment: 12 pages, 7 figures, RevTe

Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton

We have identified four new black hole candidates in M31 globular clusters using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts from the neutron star transients Aql X-1 or 4U 1608-452; the outburst apparently started ~45 days earlier than the observed peak, hence the luminosity could have been considerably higher. We identified XB082, XB153 and XB185 as BHCs by observing "low state" emission spectra at luminosities that exceed the threshold for neutron star binaries. The probability that these are neutron star systems with anisotropic emission beamed toward us is <4E-4, and their variability suggests emission from a single source. We therefore conclude that these systems likely contain black holes rather than neutron stars. We have now identified 4 persistently bright BHCs in the region; the probability that these are all background AGN is <1E-20. According to theory, the donors could be tidally captured main sequence stars, or white dwarves in ultra-compact binaries. We find that GCs that are particularly massive (XB082) or metal rich (XB144) can host bright X-ray sources in addition to those that are both (XB163). Our method may reveal BHCs in other bright X-ray sources.Comment: Accepted for publication in ApJ. 17 pages, 5 figure

Static solitons with non-zero Hopf number

We investigate a generalized non-linear O(3) $\sigma$-model in three space dimensions where the fields are maps $S^3 \mapsto S^2$. Such maps are classified by a homotopy invariant called the Hopf number which takes integer values. The model exhibits soliton solutions of closed vortex type which have a lower topological bound on their energies. We explicitly compute the fields for topological charge 1 and 2 and discuss their shapes and binding energies. The effect of an additional potential term is considered and an approximation is given for the spectrum of slowly rotating solitons.Comment: 13 pages, RevTeX, 7 Postscript figures, minor changes have been made, a reference has been corrected and a figure replace

A Study Of A New Class Of Discrete Nonlinear Schroedinger Equations

A new class of 1D discrete nonlinear Schr${\ddot{\rm{o}}}$dinger Hamiltonians with tunable nonlinerities is introduced, which includes the integrable Ablowitz-Ladik system as a limit. A new subset of equations, which are derived from these Hamiltonians using a generalized definition of Poisson brackets, and collectively refered to as the N-AL equation, is studied. The symmetry properties of the equation are discussed. These equations are shown to possess propagating localized solutions, having the continuous translational symmetry of the one-soliton solution of the Ablowitz-Ladik nonlinear Schr${\ddot{\rm{o}}}$dinger equation. The N-AL systems are shown to be suitable to study the combined effect of the dynamical imbalance of nonlinearity and dispersion and the Peierls-Nabarro potential, arising from the lattice discreteness, on the propagating solitary wave like profiles. A perturbative analysis shows that the N-AL systems can have discrete breather solutions, due to the presence of saddle center bifurcations in phase portraits. The unstaggered localized states are shown to have positive effective mass. On the other hand, large width but small amplitude staggered localized states have negative effective mass. The collison dynamics of two colliding solitary wave profiles are studied numerically. Notwithstanding colliding solitary wave profiles are seen to exhibit nontrivial nonsolitonic interactions, certain universal features are observed in the collison dynamics. Future scopes of this work and possible applications of the N-AL systems are discussed.Comment: 17 pages, 15 figures, revtex4, xmgr, gn

Soft X-ray analysis of a loop flare on the Sun

We present the results of an analysis of soft X-ray images for a solar flare which occurred on 1992 July 11. This flare, as seen in Yohkoh Soft X-ray Telescope (SXT) images was of comparatively simple geometry, consisting of two bright footpoints early in the flare with a bright loop seen later in the flare. We examine how closely this flare compares with the supposed paradigm of a confined simple-loop flare. Closer examination of the SXT images reveals that the flare structure consisted of at least two adjacent loops, one much fainter than the other. We examine the brighter of the two soft X-ray loops. The SXT images reveal an apparent slow, northward motion of this loop (roughly transverse to its major axis). Examination of derived emission measure and temperature images also indicate an apparent northward motion. In addition, we find an increase in the cross-sectional width at the top of the loop with time. Emission measure maps derived from the SXT images also indicates an apparent broadening of the loop-top region. We infer that the apparent northward motion and the apparent broadening of the soft X-ray emission can be explained in a reconnection scenario where successive magnetic field structures do not lie in a plane but are tilted to the south of the line of sight but with successively brightening loops oriented at less tilted angles. Halpha images for this flare reveal an evolution from a few brilliant points to a short two- ribbon-like appearance. Comparison of the SXT images with the Halpha images shows that the Halpha patches are aligned with the footpoints of the soft X-ray loops, suggesting the presence of a small arcade structure. There is no clear evidence for an eruptive signature in our observations nor in reports from other observations. The lack of an eruptive signature could suggest that the flare may have been a confined simple-loop flare, but this is not compelling due to a gap in the coronal observations prior to and early in the event. Analysis of our observations indicate that the flare exhibited characteristics suggesting that it may be better understood as a mini-arcade flare. These results casts doubt on the validity of the supposed paradigm of a confined simple-loop flare, at least for this flare. They indicate that even an apparently simple-loop flare may be considered to be a variety of arcade flare. We also find an effect which, to our knowledge, has not been reported before: the hot flaring regions later become cooler than the surrounding quiescent corona. That is, the flare loops do not evolve into bright active region loops, but into cooler loops. This may indicate an increase in the efficiency of the cooling mechanism or a transformed equilibrium state within the flaring loops

SF-Net: Single-Frame Supervision for Temporal Action Localization

In this paper, we study an intermediate form of supervision, i.e., single-frame supervision, for temporal action localization (TAL). To obtain the single-frame supervision, the annotators are asked to identify only a single frame within the temporal window of an action. This can significantly reduce the labor cost of obtaining full supervision which requires annotating the action boundary. Compared to the weak supervision that only annotates the video-level label, the single-frame supervision introduces extra temporal action signals while maintaining low annotation overhead. To make full use of such single-frame supervision, we propose a unified system called SF-Net. First, we propose to predict an actionness score for each video frame. Along with a typical category score, the actionness score can provide comprehensive information about the occurrence of a potential action and aid the temporal boundary refinement during inference. Second, we mine pseudo action and background frames based on the single-frame annotations. We identify pseudo action frames by adaptively expanding each annotated single frame to its nearby, contextual frames and we mine pseudo background frames from all the unannotated frames across multiple videos. Together with the ground-truth labeled frames, these pseudo-labeled frames are further used for training the classifier. In extensive experiments on THUMOS14, GTEA, and BEOID, SF-Net significantly improves upon state-of-the-art weakly-supervised methods in terms of both segment localization and single-frame localization. Notably, SF-Net achieves comparable results to its fully-supervised counterpart which requires much more resource intensive annotations. The code is available at https://github.com/Flowerfan/SF-Net

Electronic band structure of three-dimensional topological insulators with different stoichiometry composition

We report on a comparative theoretical and experimental investigation of the electronic band structure of a family of three-dimensional topological insulators, AIVBi4Te7−xSex (AIV= Sn, Pb;x = 0, 1). We prove by means of density functional theory calculations and angle-resolved photoemission spectroscopy measurements that partial or total substitution of heavy atoms by lighter isoelectronic ones affects the electronic properties of topological insulators. In particular, we show that the modification of the Dirac cone position relative to the Fermi level and the bulk band gap size can be controlled by varying the stoichiometry of the compound. We also demonstrate that the investigated systems are inert to oxygen exposure.The authors acknowledge financial support from the Saint Petersburg State University (Grant No. 40990069), the Tomsk State University competitiveness improvement program (Grant No. 8.1.01.2018), the Fundamental Research Program of the State Academies of Sciences (line of research III.23.2.9), and the project EUROFEL-ROADMAP ESFRI. This work was also partly supported by the Italian Ministry of Education, Universities and Research (MIUR) through project PON03PE_00092_1 (EOMAT) and by the Science Development Foundation under the President of the Republic of Azerbaijan (Grant No. EIF/MQM/Elm-Tehsil-1-2016- 1(26)-71/01/4-M-33). S.V.E. acknowledges support from the Russian Science Foundation (Grant No. 18-12-00169) for part of the electronic band structure calculations.Peer reviewe

Topological insulator quantum dot with tunable barriers

Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to realize semiconducting barriers which may be tuned from Ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV, with additional features implying excited states

Upregulated sirtuin 1 by miRNA-34a is required for smooth muscle cell differentiation from pluripotent stem cells

© 2015 Macmillan Publishers Limited. All rights reserved. microRNA-34a (miR-34a) and sirtuin 1 (SirT1) have been extensively studied in tumour biology and longevityaging, but little is known about their functional roles in smooth muscle cell (SMC) differentiation from pluripotent stem cells. Using well-established SMC differentiation models, we have demonstrated that miR-34a has an important role in SMC differentiation from murine and human embryonic stem cells. Surprisingly, deacetylase sirtuin 1 (SirT1), one of the top predicted targets, was positively regulated by miR-34a during SMC differentiation. Mechanistically, we demonstrated that miR-34a promoted differentiating stem cells' arrest at G0G1 phase and observed a significantly decreased incorporation of miR-34a and SirT1 RNA into Ago2-RISC complex upon SMC differentiation. Importantly, we have identified SirT1 as a transcriptional activator in the regulation of SMC gene programme. Finally, our data showed that SirT1 modulated the enrichment of H3K9 tri-methylation around the SMC gene-promoter regions. Taken together, our data reveal a specific regulatory pathway that miR-34a positively regulates its target gene SirT1 in a cellular context-dependent and sequence-specific manner and suggest a functional role for this pathway in SMC differentiation from stem cells in vitro and in vivo