Isospin effect on nuclear stopping in intermediate energy Heavy Ion Collisions

By using the Isospin Dependent Quantum Molecular Dynamics Model (IQMD), we study the dependence of nuclear stopping Q_{ZZ}/A and R in intermediate energy heavy ion collisions on system size, initial N/Z, isospin symmetry potential and the medium correction of two-body cross sections. We find the effect of initial N/Z ratio, isospin symmetry potential on stopping is weak. The excitation function of Q_{ZZ}/A and R depends on the form of medium correction of two-body cross sections, the equation of state of nuclear matter (EOS). Our results show the behavior of the excitation function of Q_{ZZ}/A and R can provide clearer information of the isospin dependence of the medium correction of two-body cross sections.Comment: 3 pages including 4 figure

Universal behavior of giant electroresistance in epitaxial La0.67Ca0.33MnO3 thin films

We report a giant resistance drop induced by dc electrical currents in La0.67Ca0.33MnO3 epitaxial thin films. Resistance of the patterned thin films decreases exponentially with increasing current and a maximum drop shows at the temperature of resistance peak Tp. Variation of resistance with current densities can be scaled below and above Tp, respectively. This work can be useful for the future applications of electroresistance.Comment: 13 pages, 4 figure

INTEGRAL/SPI γ -ray line spectroscopy : Response and background characteristics

© 2018 ESO. Reproduced with permission from Astronomy & Astrophysics. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Context. The space based γ-ray observatory INTEGRAL of the European Space Agency (ESA) includes the spectrometer instrument "SPI". This is a coded mask telescope featuring a 19-element Germanium detector array for high-resolution γ-ray spectroscopy, encapsulated in a scintillation detector assembly that provides a veto for background from charged particles. In space, cosmic rays irradiate spacecraft and instruments, which, in spite of the vetoing detectors, results in a large instrumental background from activation of those materials, and leads to deterioration of the charge collection properties of the Ge detectors.Aim. We aim to determine the measurement characteristics of our detectors and their evolution with time, that is, their spectral response and instrumental background. These incur systematic variations in the SPI signal from celestial photons, hence their determination from a broad empirical database enables a reduction of underlying systematics in data analysis. For this, we explore compromises balancing temporal and spectral resolution within statistical limitations. Our goal is to enable modelling of background applicable to spectroscopic studies of the sky, accounting separately for changes of the spectral response and of instrumental background.Methods. We use 13.5 years of INTEGRAL/SPI data, which consist of spectra for each detector and for each pointing of the satellite. Spectral fits to each such spectrum, with independent but coherent treatment of continuum and line backgrounds, provides us with details about separated background components. From the strongest background lines, we first determine how the spectral response changes with time. Applying symmetry and long-term stability tests, we eliminate degeneracies and reduce statistical fluctuations of background parameters, with the aim of providing a self-consistent description of the spectral response for each individual detector. Accounting for this, we then determine how the instrumental background components change in intensities and other characteristics, most-importantly their relative distribution among detectors.Results. Spectral resolution of Ge detectors in space degrades with time, up to 15% within half a year, consistently for all detectors, and across the SPI energy range. Semi-annual annealing operations recover these losses, yet there is a small long-term degradation. The intensity of instrumental background varies anti-correlated to solar activity, in general. There are significant differences among different lines and with respect to continuum. Background lines are found to have a characteristic, well-defined and long-term consistent intensity ratio among detectors. We use this to categorise lines in groups of similar behaviour. The dataset of spectral-response and background parameters as fitted across the INTEGRAL mission allows studies of SPI spectral response and background behaviour in a broad perspective, and efficiently supports precision modelling of instrumental background.Peer reviewedFinal Published versio

Impact of spatial resolution on the modelling of the Greenland ice sheet surface mass balance between 1990–2010, using the regional climate model MAR

With the aim to force an ice dynamical model, the Greenland ice sheet (GrIS) surface mass balance (SMB) was modelled at different spatial resolutions (15–50 km) for the period 1990–2010, using the regional climate model MAR (Modèle Atmosphérique Régional) forced by the ERA-INTERIM reanalysis. This comparison revealed that (i) the inter-annual variability of the SMB components is consistent within the different spatial resolutions investigated, (ii) the MAR model simulates heavier precipitation on average over the GrIS with decreasing spatial resolution, and (iii) the SMB components (except precipitation) can be derived from a simulation at lower resolution with an “intelligent” interpolation. This interpolation can also be used to approximate the SMB components over another topography/ice sheet mask of the GrIS. These results are important for the forcing of an ice dynamical model needed to enable future projections of the GrIS contribution to sea level rise over the coming centuries

Bias-Dependent Generation and Quenching of Defects in Pentacene

We describe a defect in pentacene single crystals that is created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at Ev + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (108 s-1) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.Comment: 10 pages, 3 figure

Fermi-surface induced modulation in an optimally doped YBCO superconductor

We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO superconductor with a wavevector along CuO chains, {\it i.e.} ${\bf q}_1$=(0,$\delta$,0). The value of $\delta\sim0.21$ is twice the Fermi wavevector ($2{\bf k}_F$) along {\bf b*} connecting nearly nested FS `ridges'. The ${\bf q}_1$ modulation exists only within O-vacancy-ordered islands (characterized by ${\bf q}_0$=$(\frac14,0,0))$ and persists well above and below $T_c$. Our results are consistent with the presence of a FS-induced charge-density wave

Disorder Effect on the Vortex Pinning by the Cooling Process Control in the Organic Superconductor κ\kappa-(BEDT-TTF)2_2Cu[N(CN)2_2]Br

We investigate the influence of disorders in terminal ethylene groups of BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br. Magnetization measurements are performed under different cooling-processes. The second peak in the magnetization hysteresis curve is observed for all samples studied, and the hysteresis width of the magnetization becomes narrower by cooling faster. In contradiction to the simple pinning effect of disorder, this result shows the suppression of the vortex pinning force by introducing more ethylene-disorders. The ethylene-disorder domain model is proposed for explaining the observed result. In the case of the system containing a moderate number of the ethylene-disorders, the disordered molecules form a domain structure and it works as an effective pinning site. On the contrary, an excess number of the ethylene-disorders may weaken the effect of the domain structure, which results in the less effective pinning force on the vortices.Comment: 6 pages, 6 figure

Variational energies and the Fermi contact term for the low-lying states of lithium: Basis-set completeness

Nonrelativistic energies for the low-lying states of lithium are calculated using the variational method in Hylleraas coordinates. Variational eigenvalues for the infinite nuclear mass case with up to 34020 terms are -7.478060323910147(1) a.u. for 1s22s2S, -7.35409842144437(1) a.u. for 1s23s2S, -7.31853084599891(1) a.u. for 1s24s2S, -7.41015653265241(4) a.u. for 1s22p2P, and -7.335523543524688(3) a.u. for 1s23d2D. The selection of the minimum set of angular momentum configurations is discussed, with the 2P and 3D states as examples to demonstrate the impact of various configurations on the variational energies. It is shown by numerical example that the second spin function (i.e., coupled to form a triplet intermediate state) has no significant effect on either the variational energies or the spin-dependent Fermi contact term. Results of greatly improved accuracy for the Fermi contact term are presented for all the states considered. © 2012 American Physical Society