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LiFeAs is one of the new class of iron superconductors with a bulk onset Tc in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared from self-flux growth techniques with significantly improved properties, including a much decreased residual gamma (proportional to C/T as T->0) in the superconducting state. Thus, in contrast to previous explanations of a finite residual gamma in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual gamma in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe0.98Cu0.02As, which has the reduced Tc of 9 K and Hc2 of 15 T. Interestingly, although presumably both LiFeAs and LiFe0.98Cu0.02As are nodeless, they clearly show a non-linear dependence of the electronic density of states (proportional to the specific heat gamma) at the Fermi energy in the mixed state with applied field similar to the Volovik effect for nodal superconductors. However, rather than nodal behavior, the satisfactory comparison with a recent theory for gamma(H) for a two isotropic gap superconductor in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor., Comment: Accepted for publication in Journal of Physics: Condensed Matter

We use transport, inelastic neutron scattering, and angle resolved photoemission experiments to demonstrate that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor similar to those of the electron-overdoped NaFe1-xTxAs and BaFe2-xTxAs2 (T = Co,Ni). Furthermore, we show that although transport properties of the stoichiometric superconducting LiFeAs and Li-deficient nonsuperconducting Li1-xFeAs are different, their electronic and magnetic properties are rather similar. Therefore, the nonsuperconducting Li1-xFeAs is also in the electron overdoped regime, where small Li deficiencies near the FeAs octahedra can dramatically suppress superconductivity through the impurity scattering effect., Comment: 5 figures,5 pages

We demonstrate the remote all-optical control of the spontaneous emission (SE) of quantum dots using coupled photonic crystal cavities. By spectrally tuning a Fabry-Perot cavity in resonance with a target cavity, the quality factor and the local density of states experienced by emitters in the target cavity are modified, leading to a change in the SE rate. From the theoretical analysis of the coupled-cavity quantum electrodynamics system, the SE rate change can be higher than the quality factor change due to a reduction of the vacuum field at the emitter's position when the two cavities are brought in resonance. Both the weak and strong coupling regimes of two cavities have been observed experimentally and the SE decay rate has been modified by more than a factor of three with remote optical control., Comment: 5 pages, 4 figures

We aim to characterize the nature of galaxies whose optical emission line diagnostics are consistent with star formation, but whose X-ray properties strongly point towards the presence of an AGN. Understanding these sources is of particular importance in assessing the completeness of AGN samples derived from large galaxy surveys, selected solely on the basis of their optical spectral properties.We construct a large sample of 211 NELGs, which have FWHMs Hb emission line <1200 km/s from the SDSS-DR7 galaxy spectroscopic catalogue, for which we are able to construct a classical diagnostic diagram, [OIII]/Hb versus [NII]/Ha (hence z<0.4), and that are also detected in the hard energy band and present in the 2XMM catalogue. This sample offers a large database by which to investigate potential mismatches between optical diagnostics and X-ray emission. Among these 211 objects, which based on our selection criteria are all at z<0.4, we find that 145 galaxies are diagnosed as AGNs, having 2-10 keV X-ray luminosities that span a wide range, from 10^40 erg/s to above 10^44 erg/s. Out of the remaining 66 galaxies, which are instead diagnosed as SF, we find a bimodal distribution in which 28 have X-ray luminosities in excess of 10^42 erg/s, large T (>1), and large X/O ratio (>0.1), while the rest are consistent with being simply SF galaxies. Those 28 galaxies exhibit the broadest Hb line FWHMs, from ~300 to 1200 km/s, and their X-ray spectrum is steeper than average and often displays a soft excess. We therefore conclude that the population of X-ray luminous NELGs with optical lines consistent with those of a starforming galaxy (which represent 19% of our whole sample) is largely dominated by NLS1s. The occurrence of such sources in the overall optically selected sample is small (<2%), hence the contamination of optically selected galaxies by NLS1s is very small., Comment: 16 pages, 8 figures, 4 tables, accepted for publication in A&A

In this paper we study the properties of the optical spectra of Type 1 active galactic nuclei (AGNs) by using the unobscured hard X-ray emission as a diagnostic. We develop the `Correlation Spectrum Technique' (CST) and use this to show the strength of correlation between the hard X-ray luminosity and each wavelength of the optical spectrum. This shows that for Broad Line Seyfert 1s all the strong emission lines (broad component of H\alpha and H\beta, [NeIII] \lambda\lambda 3869/3967, [OI] \lambda\lambda 6300/6364, [OII] \lambda\lambda 3726/3729, [OIII] \lambda\lambda 4959/5007) and the optical underlying continuum all strongly correlate with the hard X-ray emission. But the NLS1s appear to be somewhat different. Among the various Balmer line components and the broadband SED components, the best correlation exists between the hard X-ray component and broad component (BC) of the Balmer lines, which supports the view that broad line region (BLR) has the closest link with the AGN's compact X-ray emission. The equivalent widths of Balmer line IC and BC are found to correlate with L$_{2-10keV}$, $\kappa_{2-10keV}^{-1} = L_{bol}/L_{2-10keV}$, Balmer line FWHM and black hole mass. There is a non-linear dependence of the Balmer line IC and BC luminosities with L$_{2-10keV}$ and L$_{5100}$, which suggests that a second-order factor such as the ILR and BLR covering factors affect the Balmer line component luminosities. The Balmer decrement is found to decrease from ~5 in the line core to ~2 in the extended wings, with mean decrements of 2.1 in BLR and 4.8 in ILR. This suggests different physical conditions in these regions. The [OIII] line is composed of a narrow core together with a blue-shifted component with average outflow velocity of $130^{+230}_{-80} km s^{-1}$. The total luminosity of [OIII] \lambda 5007 well correlates with the hard X-ray luminosity., Comment: 23 pages, 14 figures, accepted by MNRAS

Single crystalline CaFe2As2 and (Ca1-xNax)Fe2As2 polycrystals (0 < x < 0.66) are synthesized and characterized using structural, magnetic, electronic transport, and heat capacity measurements. These measurements show that the structural/magnetic phase transition in CaFe2As2 at 165 K is monotonically suppressed by the Na doping and that superconductivity can be realized over a wide doping region. For 0.3 < x < 0.36, the magnetic susceptibilities indicate the possible coexistence of the spin density wave (SDW) and superconductivity. Superconducting phases corresponding to the Na doping level in (Ca1-xNax)Fe2As2 for nominal x = 0.36, 0.4, 0.5, 0.6, and 0.66, with Tc = 17 K, 19 K, 22 K, 33 K, and 33 K, respectively, are identified. The effects of the magnetic field on the superconductivity transitions for x = 0.66 samples with high upper critical fields Hc2 \approx 103 T are studied, and a phase diagram of the SDW and superconductivity as a function of the doping level is thus presented.

We present experimental results showing the appearance of a near-continuum in the high-order harmonic generation (HHG) spectra of atomic and molecular species as the driving laser intensity of an infrared pulse increases. Detailed macroscopic simulations reveal that these near-continuum spectra are capable of producing IAPs in the far field if a proper spatial filter is applied. Further, our simulations show that the near-continuum spectra and the IAPs are a product of strong temporal and spatial reshaping (blue shift and defocusing) of the driving field. This offers a possibility of producing IAPs with a broad range of photon energy, including plateau harmonics, by mid-IR laser pulses even without carrier-envelope phase stabilization., Comment: 7 pages, 5 figures, submitted to J.Phys. B (Oct 2011)

We present modeling and interpretation of the continuum and emission lines for a sample of 51 unobscured Type 1 active galactic nuclei (AGN). All of these AGNs have high quality spectra from both XMM-Newton and Sloan Digital Sky Survey (SDSS). We extend the wavelength coverage where possible by adding simultaneous UV data from the OM onboard XMM-Newton. Our sample is selected based on low reddening in the optical and low gas columns implied by their X-ray spectra. They also lack clear signatures for the presence of a warm absorber. Therefore the observed characteristics of this sample are likely to be directly related to the intrinsic properties of the central engine. We perform multi-component spectral fitting for strong optical emission lines and the whole optical spectra. We fit the combined optical, UV and X-ray data by applying a new broadband SED model which comprises the accretion disc emission, low temperature optically thick Comptonisation and a hard X-ray tail by introducing the a corona radius (Done et al. 2011). We find that in order to fit the data, the model often requires an additional long wavelength optical continuum component, whose origin is discussed in this paper. We also find that the Photo-recombination edge of Balmer continuum shifts and broadens beyond the standard limit of 3646{\AA}, implying an electron number density which is far higher than that in the broad line region clouds. Our results indicate that the Narrow Line Seyfert 1s in this sample tend to have lower black hole masses, higher Eddington ratios, softer 2-10 keV band spectra, lower 2-10 keV luminosities and higher \alpha_{ox}, compared with typical broad line Seyfert 1s (BLS1), although their bolometric luminosities are similar. We illustrate these differences in properties by forming an average SED for three subsamples, based on the FWHM velocity width of the H{\beta} emission line., Comment: 47 pages, 13 figures, accepted by MNRAS

By directly measuring electrical hysteresis loops using the Positive-Up Negative-Down (PUND) method, we accurately determined the remanent ferroelectric polarization Pr of orthorhombic RMnO3 (R = Ho, Tm, Yb, and Lu) compounds below their E-type spin ordering temperatures. We found that LuMnO3 has the largest Pr of 0.17 uC/cm^2 at 6 K in the series, indicating that its single-crystal form can produce a Pr of at least 0.6 \muuC/cm^2 at 0 K. Furthermore, at a fixed temperature, Pr decreases systematically with increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong correlation with the variations in the in-plane Mn-O-Mn bond angle and Mn-O distances. Our experimental results suggest that the contribution of the Mn t2g orbitals dominates the ferroelectric polarization., Comment: 16 pages, 4 figures

We use neutron scattering to determine spin excitations in single crystals of nonsuperconducting Li1-xFeAs throughout the Brillouin zone. Although angle resolved photoemission experiments and local density approximation calculations suggest poor Fermi surface nesting conditions for antiferromagnetic(AF) order, spin excitations in Li1-xFeAs occur at the AF wave vectors Q = (1, 0) at low energies, but move to wave vectors Q = (\pm 0.5, \pm0.5) near the zone boundary with a total magnetic bandwidth comparable to that of BaFe2As2. These results reveal that AF spin excitations still dominate the low-energy physics of these materials and suggest both itinerancy and strong electron-electron correlations are essential to understand the measured magnetic excitations., Comment: 8 pages, 8 figures

With the unique database from Michelson Doppler Imager aboard the Solar and Heliospheric Observatory in an interval embodying solar cycle 23, the cyclic behavior of solar small-scale magnetic elements is studied. More than 13 million small-scale magnetic elements are selected, and the following results are unclosed. (1) The quiet regions dominated the Sun's magnetic flux for about 8 years in the 12.25 year duration of Cycle 23. They contributed (0.94 - 1.44) $\times 10^{23}$ Mx flux to the Sun from the solar minimum to maximum. The monthly average magnetic flux of the quiet regions is 1.12 times that of active regions in the cycle. (2) The ratio of quiet region flux to that of the total Sun equally characterizes the course of a solar cycle. The 6-month running-average flux ratio of quiet region had been larger than 90.0% for 28 continuous months from July 2007 to October 2009, which characterizes very well the grand solar minima of Cycles 23-24. (3) From the small to large end of the flux spectrum, the variations of numbers and total flux of the network elements show no-correlation, anti-correlation, and correlation with sunspots, respectively. The anti-correlated elements, covering the flux of (2.9 - 32.0)$\times 10^{18}$ Mx, occupies 77.2% of total element number and 37.4% of quiet Sun flux. These results provide insight into reason for anti-correlated variations of small-scale magnetic activity during the solar cycle., Comment: 21 pages, 6 figures, Accepted by ApJ

With the unique database from Michelson Doppler Imager aboard the Solar and Heliospheric Observatory in an interval embodying solar cycle 23, the cyclic behavior of solar small-scale magnetic elements is studied. More than 13 million small-scale magnetic elements are selected, and the following results are unclosed. (1) The quiet regions dominated the Sun\textsf{'}s magnetic flux for about 8 years in the 12.25 year duration of Cycle 23. They contributed (0.94 -- 1.44) $\times 10^{23}$ Mx flux to the Sun from the solar minimum to maximum. The monthly average magnetic flux of the quiet regions is 1.12 times that of active regions in the cycle. (2) The ratio of quiet region flux to that of the total Sun equally characterizes the course of a solar cycle. The 6-month running-average flux ratio of quiet region had been larger than 90.0% for 28 continuous months from July 2007 to October 2009, which characterizes very well the grand solar minima of Cycles 23-24. (3) From the small to large end of the flux spectrum, the variations of numbers and total flux of the network elements show no-correlation, anti-correlation, and correlation with sunspots, respectively. The anti-correlated elements, covering the flux of (2.9 - 32.0)$\times 10^{18}$ Mx, occupies 77.2% of total element number and 37.4% of quiet Sun flux. These results provide insight into reason for anti-correlated variations of small-scale magnetic activity during the solar cycle., Comment: 21 pages, 6 figures accepted by ApJ

We present extensive 75As NMR and NQR data on the superconducting arsenides PrFeAs0.89F0.11 (Tc=45 K), LaFeAsO0.92F0.08 (Tc=27 K), LiFeAs (Tc = 17 K) and Ba0.72K0.28Fe2As2 (Tc = 31.5 K) single crystal, and compare with the nickel analog LaNiAsO0.9F0.1 (Tc=4.0 K) . In contrast to LaNiAsO0.9F0.1 where the superconducting gap is shown to be isotropic, the spin lattice relaxation rate 1/T1 in the Fe-arsenides decreases below Tc with no coherence peak and shows a step-wise variation at low temperatures. The Knight shift decreases below Tc and shows a step-wise T variation as well. These results indicate spinsinglet superconductivity with multiple gaps in the Fe-arsenides. The Fe antiferromagnetic spin fluctuations are anisotropic and weaker compared to underdoped copper-oxides or cobalt-oxide superconductors, while there is no significant electron correlations in LaNiAsO0.9F0.1. We will discuss the implications of these results and highlight the importance of the Fermi surface topology., Comment: 6 pages, 11 figures

The upper critical field $\mu_0H_{c2}(T_c)$ of LiFeAs single crystals has been determined by measuring the electrical resistivity using the facilities of pulsed magnetic field at Los Alamos. We found that $\mu_0H_{c2}(T_c)$ of LiFeAs shows a moderate anisotropy among the layered iron-based superconductors; its anisotropic parameter $\gamma$ monotonically decreases with decreasing temperature and approaches $\gamma\simeq 1.5$ as $T\rightarrow 0$. The upper critical field reaches 15T ($H\parallel c$) and 24.2T ($H\parallel ab$) at $T=$1.4K, which value is much smaller than other iron-based high $T_c$ superconductors. The temperature dependence of $\mu_0H_{c2}(T_c)$ can be described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally and (likely) spin-paramagnetically limited upper critical field for $H\parallel c$ and $H\parallel ab$, respectively., Comment: 5 pages,5 figures

Bi2Te3 compound has been theoretically predicted (1) to be a topological insulator, and its topologically non-trivial surface state with a single Dirac cone has been observed in photoemission experiments (2). Here we report that superconductivity (Tc^~3K) can be induced in Bi2Te3 as-grown single crystal (with hole-carriers) via pressure. The first-principles calculations show that the electronic structure under pressure remains to be topologically nontrivial, and the Dirac-type surface states can be well distinguished from bulk states at corresponding Fermi level. The proximity effect between superconducting bulk states and Dirac-type surface state could generate Majorana fermions on the surface. We also discuss the possibility that the bulk state could be a topological superconductor., Comment: Published ahead of print December 20, 2010, doi:10.1073/pnas.1014085108

The spectra of AGN from the ultraviolet to the near infrared, exhibit emission lines covering a wide range of ionisation states, from neutral species such as [O I] 6300A, up to [Fe XIV] 5303A. Here we report on some recent studies of the properties of highly ionised lines (HILs), plus two case studies of individual objects. Future IFU observations at high spatial and good spectral resolution, will probe the excitation and kinematics of the gas in the zone between the extended NLR and unresolved BLR. Multi-component SED fitting can be used to link the source of photoionisation with the strengths and ratios of the HILs., Comment: Proceedings of the IAU Symposium: Co-evolution of Central Black Holes and Galaxies

By systematic first-principles calculations, we study the electronic structure and magnetic property of SrCrO$_3$. Our results suggest that SrCrO$_3$ is a weakly correlated antiferromagnetic (AF) metal, a very rare situation in transition-metal oxides. Among various possible AF states, the C-type spin ordering with small amount of orbital polarization (dxy orbital is more occupied than the d_{yz/zx} orbital) is favored. The detailed mechanism to stabilize the C-type AF state is analyzed based on the competition between the itinerant Stoner instability and superexchange, and our results suggest that the magnetic instability rather than the orbital or lattice instabilities plays an important role in this system. The experimentally observed compressive tetragonal distortion can be naturally explained from the C-type AF state. By applying the LDA+$U$ method to study this system, we show that wrong ground state will be obtained if $U$ is large., Comment: 17 pages 15 figures

We introduce a simple model to study movie competition in the recommender systems. Movies of heterogeneous quality compete against each other through viewers' reviews and generate interesting dynamics of box-office. By assuming mean-field interactions between the competing movies, we show that run-away effect of popularity spreading is triggered by defeating the average review score, leading to hits in box-office. The average review score thus characterizes the critical movie quality necessary for transition from box-office bombs to blockbusters. The major factors affecting the critical review score are examined. By iterating the mean-field dynamical equations, we obtain qualitative agreements with simulations and real systems in the dynamical forms of box-office, revealing the significant role of competition in understanding box-office dynamics., Comment: 8 pages, 6 figures

Whether large ferroelectric polarization P exists in the orthorhombic HoMnO3 with the E-type antiferromagnetic spin ordering or not remains as one of unresolved, challenging issues in the physics of multiferroics. The issue is closely linked to an intriguing experimental difficulty for determining P of polycrystalline specimens that conventional pyroelectric current measurements performed after a poling procedure under high dc electric fields are subject to large errors due to the problems caused by leakage currents or space charges. To overcome the difficulty, we employed the PUND method, which uses successively the two positive and two negative electrical pulses, to directly measure electrical hysteresis loops in several polycrystalline HoMnO3 specimens below their N\'eel temperatures. We found that all the HoMnO3 samples had similar remnant polarization Pr values at each temperature, regardless of their variations in resistivity, dielectric constant, and pyroelectric current levels. Moreover, Pr of ~0.07 \mu\C/cm2 at 6 K is consistent with the P value obtained from the pyroelectric current measurement performed after a short pulse poling. Our findings suggest that intrinsic P of polycrystalline HoMnO3 can be determined through the PUND method and P at 0 K may reach ~0.24 \mu\C/cm2 in a single crystalline specimen., Comment: 21 pages, 6 figures, submitted to New Journal of Physics

We report the $^{75}$As-NQR and NMR studies on the iron arsenide superconductor Li$_{x}$FeAs with $T_{\rm c} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\rm c}$ without a coherence peak, and can be fitted by gaps with s$^{\pm}$-wave symmetry in the presence of impurity scattering. In the normal state, both $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50 K$. Estimate of the Korringa ratio shows that the spin correlations are weaker than that in other families of iron arsenides, which may account for the lower $T_{\rm c}$ in this material., Comment: 10 pages 6 figures. to appear in J. Phys. Soc. Jpn., Vol.79, No.8

Electrical-resistivity and magnetic-susceptibility measurements under hydrostatic pressure up to p = 2.75 GPa have been performed on superconducting LiFeP. A broad superconducting (SC) region exists in the temperature - pressure (T-p) phase diagram. No indications for a spin-density-wave transition have been found, but an enhanced resistivity coefficient at low pressures hints at the presence of magnetic fluctuations. Our results show that the superconducting state in LiFeP is more robust than in the isostructural and isoelectronic LiFeAs. We suggest that this finding is related to the nearly regular [FeP_4] tetrahedron in LiFeP., Comment: 4 pages, 4 figures

The effect of pressure on superconductivity of 111 type NaxFeAs is investigated through temperature dependent electrical resistance measurement in a diamond anvil cell. The superconducting transition temperature (Tc) increases from 26 K to a maximum 31 K as the pressure increases from ambient to 3 GPa. Further increasing pressure suppresses Tc drastically. The behavior of pressure tuned Tc in NaxFeAs is much different from that in LixFeAs, although they have the same Cu2Sb type structure

A series of new Sr2-xBaxCuO3+y (0 x 0.6) superconductors were prepared using high-pressure and high-temperature synthesis. A Rietveld refinement based on powder x-ray diffraction confirms that the superconductors crystallize in the K2NiF4-type structure of a space group I4/mmm similar to that of La2CuO4 but with partially occupied apical oxygen sites. It is found that the superconducting transition temperature Tc of this Ba substituted Sr2CuO3+y superconductor with constant carrier doping level, i.e., constant d, is controlled not only by order/disorder of apical-O atoms but also by Ba content. Tcmax =98 K is achieved in the material with x=0.6 that reaches the record value of Tc among the single-layer copper oxide superconductors, and is higher than Tc=95K of Sr2CuO3+y with optimally ordered apical-O atoms. There is Sr-site disorder in Sr2-xBaxCuO3+y which might lead to a reduction of Tc. The result indicates that another effect surpasses the disorder effect that is related either to the increased in-plane Cu-O bond length or to elongated apical-O distance due to Ba substitution with larger cation size. The present experiment demonstrates that the optimization of local geometry out of the Cu-O plane can dramatically enhance Tc in the cuprate superconductors., Comment: 23 Pages, 1 Table, 5 Figures

A new iron pnictide LiFeP superconductor was found. The compound crystallizes into a Cu2Sb structure containing an FeP layer showing superconductivity with maximum Tc of 6K. This is the first 111 type iron pnictide superconductor containing no arsenic. The new superconductor is featured with itinerant behavior at normal state that could helpful to understand the novel superconducting mechanism of iron pnictide compounds., Comment: 3 figures + 1 table

With a collecting area of 70 000 m^2, the Five hundred metre Aperture Spherical Telescope (FAST) will allow for great advances in pulsar astronomy. We have performed simulations to estimate the number of previously unknown pulsars FAST will find with its 19-beam or possibly 100-beam receivers for different survey strategies. With the 19-beam receiver, a total of 5200 previously unknown pulsars could be discovered in the Galactic plane, including about 460 millisecond pulsars (MSPs). Such a survey would take just over 200 days with eight hours survey time per day. We also estimate that, with about 80 six-hour days, a survey of M31 and M33 could yield 50--100 extra-Galactic pulsars. A 19-beam receiver would produce just under 500 MB of data per second and requires about 9 tera-ops to perform the major part of a real time analysis. We also simulate the logistics of high-precision timing of MSPs with FAST. Timing of the 50 brightest MSPs to a signal-to-noise of 500 would take about 24 hours per epoch., Comment: 9 pages, 10 figures; accepted for publication in A&A

Methods. We study an equatorial CH observed simultaneously by HINODE and STEREO on July 27, 2007. The HINODE/SP maps are adopted to derive the physical parameters of the photosphere and to research the magnetic field evolution and distribution. The G band and Ca II H images with high tempo-spatial resolution from HINODE/BFI and the multi-wavelength data from STEREO/EUVI are utilized to study the corresponding atmospheric response of different overlying layers. Results. We explore an emerging dipole locating at the CH boundary. Mini-scale arch filaments (AFs) accompanying the emerging dipole were observed with the Ca II H line. During the separation of the dipolar footpoints, three AFs appeared and expanded in turn. The first AF divided into two segments in its late stage, while the second and third AFs erupted in their late stages. The lifetimes of these three AFs are 4, 6, 10 minutes, and the two intervals between the three divisions or eruptions are 18 and 12 minutes, respectively. We display an example of mixed-polarity flux emergence of IN fields within the CH and present the corresponding chromospheric response. With the increase of the integrated magnetic flux, the brightness of the Ca II H images exhibits an increasing trend. We also study magnetic flux cancellations of NT fields locating at the CH boundary and present the obvious chromospheric and coronal response. We notice that the brighter regions seen in the 171 A images are relevant to the interacting magnetic elements. By examining the magnetic NT and IN elements and the response of different atmospheric layers, we obtain good positive linear correlations between the NT magnetic flux densities and the brightness of both G band (correlation coefficient 0.85) and Ca II H (correlation coefficient 0.58)., Comment: 9 pages, 9 figures. A&A, in press

We have carried out a survey of long 50ks XMM-Newton observations of a sample of bright, variable AGN. We found a distinctive energy dependence of the variability in RXJ0136.9-3510 where the fractional variability increases from 0.3 to 2 keV, and then remains constant. This is in sharp contrast to other AGN where the X-ray variability is either flat or falling with energy, sometimes with a peak at $\sim$~2 keV superimposed on the overall trend. Intriguingly these unusual characteristics of the variability are shared by one other AGN, namely RE J1034+396, which is so far unique showing a significant X-ray QPO. In addition the broad band spectrum of RXJ0136.9-3510 is also remarkably similar to that of RE J1034+396, being dominated by a huge soft excess in the EUV-soft X-ray bandpass. The bolometric luminosity of RX J0136.9-3510 gives an Eddington ratio of about 2.7 for a black hole mass (from the H beta line width) of $7.9 \times 10^{7}M_{\sun}$. This mass is about a factor of 50 higher than that of RE J1034+396, making any QPO undetectable in this length of observation. Nonetheless, its X-ray spectral and variability similarities suggest that RE J1034+396 is simply the closest representative of a new class of AGN spectra, representing the most extreme mass accretion rates., Comment: 5 pages, 4 figures

We present a first-order non-homogeneous Markov model for the interspike-interval density of a continuously stimulated spiking neuron. The model allows the conditional interspike-interval density and the stationary interspike-interval density to be expressed as products of two separate functions, one of which describes only the neuron characteristics, and the other of which describes only the signal characteristics. This allows the use of this model to predict the response when the underlying neuron model is not known or well determined. The approximation shows particularly clearly that signal autocorrelations and cross-correlations arise as natural features of the interspike-interval density, and are particularly clear for small signals and moderate noise. We show that this model simplifies the design of spiking neuron cross-correlation systems, and describe a four-neuron mutual inhibition network that generates a cross-correlation output for two input signals., Comment: Accepted for publication in Neural Computation

A new iron arsenide superconducting system LiFeAs was found that crystallizes into a tetragonal structure with space group P4/nmm. The superconductivity with Tc up to 18 K was observed in the compounds. This simple 111 type layered iron arsenide superconductor can be viewed as an analogue of the infinite layer structure of copper oxides., Comment: 11 pages 3 Figures

We illustrate an iterative method for retrieving the internuclear separations of N$_2$, O$_2$ and CO$_2$ molecules using the high-order harmonics generated from these molecules by intense infrared laser pulses. We show that accurate results can be retrieved with a small set of harmonics and with one or few alignment angles of the molecules. For linear molecules the internuclear separations can also be retrieved from harmonics generated using isotropically distributed molecules. By extracting the transition dipole moment from the high-order harmonic spectra, we further demonstrated that it is preferable to retrieve the interatomic separation iteratively by fitting the extracted dipole moment. Our results show that time-resolved chemical imaging of molecules using infrared laser pulses with femtosecond temporal resolutions is possible., Comment: 14 pages, 9 figures

We address the question of whether the superconducting transition temperature (Tc) of high-Tc cuprates is enhanced when randomly distributed dopant atoms form an ordered array in the charge reservoir layers. This study is possible for the Sr2CuO3+d superconductor with K2NiF4-type structure in which oxygen atoms only partially occupy the apical sites next to the CuO2 planes and act as hole-dopants. We show that remarkable Tc enhancement up to 95K in this mono CuO2 layered HTS is associated with the apical oxygen ordering, not to the hole concentration change. The result points a route toward further enhancement of Tc in cuprate superconductors., Comment: 18 pages, 5 Figures

First-principles density-functional method is used to study the recently high-pressure synthesized compound PtC. It is confirmed by our calculations that the platinum carbide has a zinc-blende ground-state phase at zero pressure and the rock-salt structure is a high-pressure phase. The theoretical transition pressure from zinc-blende to rock-salt is determined to be 52GPa. Furthermore, our calculation shows the possibility that the experimentally synthesized PtC by Ono et al. under high pressure condition might undergo a transition from rock-salt structure to zinc-blende after the pressure quench to ambient condition., Comment: A revised version

We present the optical reflectance and conductivity spectra for non-oxide antiperovskite superconductor $MgCNi_{3}$ at different temperatures. The reflectance drops gradually over a large energy scale up to 33,000 cm$^{-1}$, with the presence of several wiggles. The reflectance has slight temperature dependence at low frequency but becomes temperature independent at high frequency. The optical conductivity shows a Drude response at low frequencies and four broad absorption features in the frequency range from 600 $cm^{-1}$ to 33,000 $cm^{-1}$. We illustrate that those features can be well understood from the intra- and interband transitions between different components of Ni 3d bands which are hybridized with C 2p bands. There is a good agreement between our experimental data and the first-principle band structure calculations., Comment: 4 pages, to be published in Phys. Rev. B

Recent radio VLBI observations of the complex gravitationally lensed system PKS1830-211 have thrown up some questions with regard to the processes occuring at the heart of the blazar source at a redshift of 2.51, which is viewed almost straight down the jet axis. This work links, by a model of a helical jet tracked by ballistically ejected plasmons from a precessing nozzle, observations on the scale of tens of microarcseconds to those on the scale of milliarcseconds. An observed jet precession period of 1.08 years is inferred from the model, translating to an intrinsic period of 30.8 years for a source at redshift 2.51 and an assumed jet bulk velocity of 0.99 c. This fits well with the picture of an active galactic nucleus hosting a binary black hole system at its centre, with the jet emitted by one member of the system, and the precession as being due to its orbital motion around its companion., Comment: Accepted for publication in the MNRAS

Raman spectroscopic studies have been carried out on CaCrO4 under pressure up to 26GPa at ambient temperature. The Raman spectra showed CaCrO4 experienced a continuous structural phase transition started at near 6GPa, and finished at about 10GPa. It is found that the high-pressure phase could be quenched to ambient conditions. Pressure dependence of the Raman peaks suggested there existed four pressure regions related to different structural characters. We discussed these characters and inferred that the nonreversible structural transition in CaCrO4, most likely was from a zircon-type (I41/amd) ambient phase to a scheelite-type high pressure structure (I41/a)., Comment: submitted