Search

Vortex motion in ferromagnetic-superconducting bilayers constituted by thin YBa$_2$Cu$_3$O$_{7-\delta}$ film deposited on the top of thin manganite La$_{0.67}$Sr$_{0.33}$MnO$_3$ film has been investigated. Both films were deposited on uniformly twinned single-crystalline substrates and patterned into strips directed at different angles to the twin lines pattern. These conditions lead to the guided vortex motion at different angles to the periodic pinning potential. It has been established that critical current and flux flow resistance in such an anisotropic pinning landscape-scale follow the elliptic dependence on the angle of the direction of the strip. For vortex flow at the directions close to the perpendicular to magnetic domain walls, signatures of the coherence in the vortex motion have been observed., Comment: 10 pages, 6 figures

We provide a systematic study of the position-dependent correlation function in weak lensing convergence maps and its relation to the squeezed limit of the three-point correlation function (3PCF) using state-of-the-art numerical simulations. We relate the position-dependent correlation function to its harmonic counterpart, i.e., the position-dependent power spectrum or equivalently the integrated bispectrum. We use a recently proposed improved fitting function, BiHalofit, for the bispectrum to compute the theoretical predictions as a function of source redshifts. In addition to low redshift results ($z_s=1.0-2.0$), we also provide results for maps inferred from lensing of the cosmic microwave background, i.e., $z_s=1100$. We include a {\em Euclid}-type realistic survey mask and noise. In agreement with the recent studies on the position-dependent power spectrum, we find that the results from simulations are consistent with the theoretical expectations when appropriate corrections are included. Performing a rough estimate, we find that the (S/N) for the detection of the position-dependent correlation function from {\em Euclid}-type mask with $f_{sky}=0.35$, can range between $6-12$ depending on the value of the intrinsic ellipticity distribution parameter $\sigma_{\epsilon} = 0.3-1.0$. For reconstructed $\kappa$ maps using an ideal CMB survey the (S/N) $\approx 1.8$. We also found that a $10\%$ deviation in $\sigma_8$ can be detected using IB for the optimistic case of $\sigma_\epsilon=0.3$ with a (S/N) $\approx 5$. The (S/N) for such detection in case of $\Omega_M$ is lower., Comment: 7 pages, 7 figures (PRD in press)

We analyse the Planck full-mission cosmic microwave background (CMB) temperature and E-mode polarization maps to obtain constraints on primordial non-Gaussianity (NG). We compare estimates obtained from separable template-fitting, binned, and modal bispectrum estimators, finding consistent values for the local, equilateral, and orthogonal bispectrum amplitudes. Our combined temperature and polarization analysis produces the following results: f_NL^local = -0.9 +\- 5.1; f_NL^equil = -26 +\- 47; and f_NL^ortho = - 38 +\- 24 (68%CL, statistical). These results include the low-multipole (4 <= l < 40) polarization data, not included in our previous analysis, pass an extensive battery of tests, and are stable with respect to our 2015 measurements. Polarization bispectra display a significant improvement in robustness; they can now be used independently to set NG constraints. We consider a large number of additional cases, e.g. scale-dependent feature and resonance bispectra, isocurvature primordial NG, and parity-breaking models, where we also place tight constraints but do not detect any signal. The non-primordial lensing bispectrum is detected with an improved significance compared to 2015, excluding the null hypothesis at 3.5 sigma. We present model-independent reconstructions and analyses of the CMB bispectrum. Our final constraint on the local trispectrum shape is g_NLl^local = (-5.8 +\-6.5) x 10^4 (68%CL, statistical), while constraints for other trispectra are also determined. We constrain the parameter space of different early-Universe scenarios, including general single-field models of inflation, multi-field and axion field parity-breaking models. Our results provide a high-precision test for structure-formation scenarios, in complete agreement with the basic picture of the LambdaCDM cosmology regarding the statistics of the initial conditions (abridged)., Comment: 50 pages, 20 figures

The electric noise can be an important limitation for applications of conducting elements of size in the nanometer range. The intrinsic electrical noise of prospective materials for opto-spintronics applications like ZnO has not been characterized yet. In this study we have investigated the conductivity fluctuations in 10~nm thick current paths produced by proton implantation of ZnO microwires at room temperature. The voltage noise under a constant dc current bias in undoped as well as in Li-doped microwires is characterized by $1/f^a$ power spectra with $a \sim 1$. The noise intensity scales with the square of the bias current pointing out to bias-independent resistivity fluctuations as a source of the observed noise. The normalized power spectral density appears inversely proportional to the number of carriers in the probed sample volume, in agreement with the phenomenological Hooge law. For the proton-implanted ZnO microwire and at 1~Hz we obtain a normalized power spectral density as low as $\sim 10^{-11}~$Hz$^{-1}$., Comment: 10 pages,3 figures to be published in Nanotechnology (2016)

Conductivity noise in dc current biased La_{0.82}Ca_{0.18}MnO_{3} single crystals has been investigated in different metastable resistivity states enforced by applying voltage pulses to the sample at low temperatures. Noise measured in all investigated resistivity states is of 1/f-type and its intensity at high temperatures and low dc bias scales as a square of the bias. At liquid nitrogen temperatures for under bias exceeding a threshold value, the behavior of the noise deviates from above quasi- equilibrium modulation noise and depends in a non monotonic way on applied bias. The bias range of nonequilibrium 1/f noise coincides with the range at which the conductance increases linearly with bias voltage. This feature is attributed to a broad continuity of states enabling indirect inelastic tunneling across intrinsic tunnel junctions. The nonequilibrium noise has been ascribed to indirect intrinsic tunneling mechanism while resistivity changes in metastable states to variations in the energy landscape for charge carriers introduced by microcracks created by the pulse procedures employed, Comment: 10 pages, 5 figures

Low frequency noise in current biased La$_{0.82}$Ca$_{0.18}$MnO$_{3}$ single crystals has been investigated in a wide temperature range from 79 K to 290 K. Despite pronounced changes in magnetic properties and dissipation mechanisms of the sample with changing temperature, the noise spectra were found to be always of the 1/f type and their intensity (except the lowest temperature studied) scaled as a square of the bias. At liquid nitrogen temperatures and under bias exceeding some threshold value, the behavior of the noise deviates from the quasi-equilibrium modulation noise and starts to depend in a non monotonic way on bias. It has been verified that the observed noise obeys Dutta and Horn model of 1/f noise in solids. The appearance of nonequilibrium 1/f noise and its dependence on bias have been associated with changes in the distribution of activation energies in the underlying energy landscape. These changes have been correlated with bias induced changes in the intrinsic tunneling mechanism dominating dissipation in La$_{0.82}$Ca$_{0.18}$MnO$_{3}$ at low temperatures., Comment: Accepted for publication in the Journal of Applied Physics

A collective electron transfer (ET) process was discovered by studying the current noise in a field effect transistor with light-sensitive gate formed by nanocrystals linked by organic molecules to its surface. Fluctuations in the ET through the organic linker are reflected in the fluctuations of the transistor conductivity. The current noise has an avalanche character. Critical exponents obtained from the noise power spectra, avalanche distributions, and the dependence of the average avalanche size on avalanche duration are consistent with each other. A plausible model is proposed for this phenomenon, Comment: 15 pages 4 figures. Accepted for publication in Physical Review Letters

1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77 K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multi-step indirect tunneling., Comment: 4pages, 3figures,APL accepted

The shape of the V-I characteristics leading to a peak in the differential resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r_d is large when measured by dc current. However, for a small Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r_d are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning., Comment: 12 pages 4 figures. Accepted for publication in PRB rapid

The large increase in voltage noise, commonly observed in the vicinity of the peak-effect in superconductors, is ascribed to a novel noise mechanism. A strongly pinned metastable disordered vortex phase, which is randomly generated at the edges and annealed into ordered phase in the bulk, causes large fluctuations in the integrated critical current of the sample. The excess noise due to this dynamic admixture of two distinct phases is found to display pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears, Comment: 5 pages 3 figures. Accepted for publication in Europhysics letters

Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La$_{0.82}$Ca$_{0.18}$MnO$_3$ single crystals. At low temperatures, below the Curie point, application of specific bias procedures switches the crystal into metastable resistivity state characterized by appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantum well within the double tunnel barrier of intrinsic weak-links associated with twin-like defect boundaries., Comment: 4 pages, 4 figures, to appear in Phys. Rev. B

Giant enhancement of the nonlinear response of a single crystal of Nb placed in a sweeping magnetic field has been experimentally observed. The rectified signal from Nb ($Tc=9.15$ K) has been measured by means of an inductive method as a function of temperature, dc field, dc field sweep rate, and the amplitude of ac field. The Nb sample was excited by an amplitude modulated ac field. Under a stationary regime, the rectified signal appears only for magnetic fields ($H_0$) in the range $H_{c2}H_{c1}$. This experiment shows that the amplitude of the rectified signal is two orders of magnitude larger than the amplitude of the signal seen under stationary field conditions. Moreover, the amplitude of the rectified signal is a power function of the sweep rate, with the power exponent close to 1., Comment: 3 pages, 3 figures, presented to EUCAS 2001

Transport studies in a Corbino disk geometry suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows a sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe2 crystals. These features are found to be absent in the Corbino geometry, in which the circulating vortices do not cross the sample edges., Comment: 12 pages 3 figures. Accepted for publication in Physical Review Letters

The interaction between the telegraph noise and background voltage fluctuations in the current induced dissipative state of high-Tc BiSrCaCuO thin films has been investigated. Experimental time records of the voltage drop across current biased thin film strips show markedly different background noise traces in the up and down telegraph states. Detailed analysis demonstrates that fluctuations around the telegraph voltage levels are due to a unique background noise process. The apparent quiet and noisy voltage states are due only to differences in the effective frequency bandwidth at which background noise is seen at distinct telegraph levels. Changes of the background noise variance ratio with changing bias current follow changes of the statistical average lifetimes of the random telegraph process., Comment: submitted to Phys. Rev. B

The spectrum of the diffuse isotropic component of cosmic X-rays over the 13-180 keV range was determined by the UCSD/MIT Hard X-Ray and Gamma-Ray instrument (HEAO A4) on the High Energy Astronomical Observatory-1 (HEAO-1). The instrument consists of a complex of actively shielded and collimated scintillation counters, including the Low Energy Detector set from which the data reported here were obtained. These data join smoothly with the spectrum at lower energies reported by the GSFC HEAO A2 instrument and with that measured to 400 keV by the HEAO A4 Medium Energy Detectors. The HEAO data set also joins the recent results from COMPTEL on the Compton Gamma-Ray Observatory in the 1-10 MeV range, which failed to confirm the existence of an "MeV bump" in this range. Although the spectrum over the entire range 3 keV < E < 100 GeV can be fit by a simple empirical analytic expression, the origin is likely due to a number of distinct source components. The prevailing idea for the origin is that the hard X-ray spectrum is due to X-rays from various AGN components, particularly Seyfert galaxies extending to cosmological distances, and that the low energy gamma-rays may be due to emission from type 1a Supernovae, also integrated to cosmological distances. The higher energy gamma-ray spectrum defined by EGRET, also on the CGRO, may be due to unresolved gamma-ray emitting blazars. Models of production by these source components, extrapolated to the present epoch, must reproduce the observationally derived spectrum., Comment: LaTeX, 16pp, 3figs

We present the results of OSSE observations of the soft gamma ray continuum emission from the Galactic plane at longitude 95 degrees. Emission is detected between 50 and 600 keV where the spectrum is fit well by a power law with photon index -2.6+-0.3 and flux (4.0+-0.5) 10^{-2} photons/s/cm^2/rad/MeV at 100 keV. This spectral shape in this range is similar to that found for the continuum emission from the inner Galaxy but the amplitude is lower by a factor of four. This emission is either due to unresolved and previously unknown point sources or it is of diffuse origin, or a combination of the two. Simultaneous observations with OSSE and smaller field of view instruments operating in the soft gamma ray energy band, such as XTE or SAX, would help resolve this issue. If it is primarily diffuse emission due to nonthermal electron bremsstrahlung, as is the >1 MeV Galactic ridge continuum, then the power in low energy cosmic ray electrons exceeds that of the nuclear component of the cosmic rays by an order of magnitude. This would have profound implications for the origin of cosmic rays and the energetics of the interstellar medium. Alternatively, if the emission is diffuse and thermal, then there must be a component of the interstellar medium at temperatures near 10^9 K., Comment: 11 pages, Latex, requires AASTEX macros and psfig.tex, 2 postscript figures, Accepted for publication in the Astrophysical Journal Letters