The electronic and transport properties of a molecular junction studied by an integrated piecewise thermal equilibrium approach

An integrated piecewise thermal equilibrium approach based on the first-principles calculation method has been developed to calculate bias dependent electronic structures and current- and differential conductance-voltage characteristics of the gold-benzene-1,4-dithiol-gold molecular junction. The calculated currents and differential conductance have the same order of magnitude as experimental ones. An electron transfer was found between the two electrodes when a bias is applied, which renders the two electrodes to have different local electronic structures. It was also found that when Au 5d electrons were treated as core electrons the calculated currents were overestimated, which can be understood as an underestimate of the Au-S covalent bonding and consequently the contact potential barrier and the replacement of delocalized Au 5d carriers by more itinerant delocalized Au 6sp carriers in the electrodes

Extended Dynamical Mean Field Theory Study of the Periodic Anderson Model

We investigate the competition of the Kondo and the RKKY interactions in heavy fermion systems. We solve a periodic Anderson model using Extended Dynamical Mean Field Theory (EDMFT) with QMC. We monitor simultaneously the evolution of the electronic and magnetic properties. As the RKKY coupling increases the heavy fermion quasiparticle unbinds and a local moment forms. At a critical RKKY coupling there is an onset of magnetic order. Within EDMFT the two transitions occur at different points and the disapparence of the magnetism is not described by a local quantum critical point.Comment: 4 pages, 4 figure

Vortex State of Tl2_2Ba2_2CuO6+δ_{6+\delta} via 205^{205}Tl NMR at 2 Tesla

We report a $^{205}$Tl NMR study of vortex state for an aligned polycrystalline sample of an overdoped high-$T_c$ superconductor Tl$_2$Ba$_2$CuO$_{6+\delta}$ ($T_{c}\sim$85 K) with magnetic field 2 T along the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at $T$=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and vortex melting between 65 and 85 K. No evidence for local antiferromagnetic ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure

Antiferromagnetic order driven by the molecular orbital order of C60_{60} in α′\alpha'--tetra--kiskis--(dimethylamino)--ethylene--C60_{60}

We have studied the ground state of a fullerene--based magnet, the $\alpha'$--phase tetra--$kis$--(dimethylamino)--ethylene--C$_{60}$ ($\alpha'$--TDAE--C$_{60}$), by electron spin resonance (ESR) and magnetic torque measurements. Below T$_N=7$ K, non--paramagnetic field dependent resonances with a finite excitation gap (1.7 GHz) are observed along the $a$--axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the $\alpha'$--phase ground state. Below T$_N$, non--quadratic field dependence of magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle--dependent torque signals below T$_N$ indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferromagnetic ground state driven by the cooperative orientational ordering of C$_{60}$ in the $\alpha'$--TDAE--C$_{60}$.Comment: 4 pages, 3 figure

Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis

Because of our relatively low spectral resolution, we compare our observations with Gray's line bisector data by fitting observed line profiles to an expansion in terms of orthogonal (Hermite) functions. To obtain an accurate comparison, we model the emergent line profiles from rotating and pulsating stars, taking the instrumental point spread function into account. We describe this modeling process in detail. We find no evidence for line profile or strength variations at the radial velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for line shape variations with 4.23-day periodicity is small enough to exclude with 10 sigma confidence the bisector curvature signal reported by Gray & Hatzes; the bisector span and relative line depth signals reported by Gray (1997) are also not seen, but in this case with marginal (2 sigma) confidence. We cannot, however, exclude pulsations as the source of 51 Peg's radial velocity variation, because our models imply that line shape variations associated with pulsations should be much smaller than those computed by Gray & Hatzes; these smaller signals are below the detection limits both for Gray & Hatzes' data and for our own. Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test for pulsations in this star. Again we find no evidence for periodic line-shape changes, at a level that rules out pulsations as the source of the radial velocity variability. We conclude that the planet hypothesis remains the most likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to letter astro-ph/9712279