Observation of electronic and atomic shell effects in gold nanowires

The formation of gold nanowires in vacuum at room temperature reveals a periodic spectrum of exceptionally stable diameters. This is identified as shell structure similar to that which was recently discovered for alkali metals at low temperatures. The gold nanowires present two competing `magic' series of stable diameters, one governed by electronic structure and the other by the atomic packing.Comment: 4 pages, 4 figure

Theoretical study of the (3x2) reconstruction of beta-SiC(001)

By means of ab initio molecular dynamics and band structure calculations, as well as using calculated STM images, we have singled out one structural model for the (3x2) reconstruction of the Si-terminated (001) surface of cubic SiC, amongst several proposed in the literature. This is an alternate dimer-row model, with an excess Si coverage of 1/3, yielding STM images in good accord with recent measurements [F.Semond et al. Phys. Rev. Lett. 77, 2013 (1996)].Comment: To be published in PRB Rapid. Com

Hole Doping Dependence of the Coherence Length in La2−xSrxCuO4La_{2-x}Sr_xCuO_4 Thin Films

By measuring the field and temperature dependence of magnetization on systematically doped $La_{2-x}Sr_xCuO_4$ thin films, the critical current density $j_c(0)$ and the collective pinning energy $U_p(0)$ are determined in single vortex creep regime. Together with the published data of superfluid density, condensation energy and anisotropy, for the first time we derive the doping dependence of the coherence length or vortex core size in wide doping regime directly from the low temperature data. It is found that the coherence length drops in the underdoped region and increases in the overdoped side with the increase of hole concentration. The result in underdoped region clearly deviates from what expected by the pre-formed pairing model if one simply associates the pseudogap with the upper-critical field.Comment: 4 pages, 4 figure

Coupling between Light and Terahertz-Frequency Acoustic Phonons in Ferroelectric BaTiO3 /SrTiO3 Superlattices

The acoustic phonons in epitaxial ferroelectric (BaTiO3 )n /(SrTiO3 )m superlattices (SLs) are investigated by high-resolution ultraviolet Raman scattering. The temperature dependence of the folded acoustic (FA) phonon Raman intensity through the ferroelectric transition is addressed. A comparison of this behavior between SLs with different number of ferroelectric BaTiO3 unit cells n and spacer SrTiO3 unit cells m is presented. A mechanism involving the strain modulation of the spatially varying ferroelectric polarization is introduced to explain the temperature dependence of the FA phonon scattering. The temperature dependence of the polarization can be derived from an analysis of the first-order optical phononspectra. Using this information, the observed temperature dependence of the whole set of SLs with different n can be consistently accounted for with the presented model. Atomistic shell-model simulations of the spatial pattern of the SL polarization are presented to explain the variation of the FA-spectral intensity for SLs with different m and the experimental fact that no high-order FA-replicas are observed. These results demonstrate the strong coupling between THz hypersound, charge, and light in these multifunctional nanoscale ferroelectrics.Fil: Bruchhausen, Axel Emerico. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. University of Konstanz; AlemaniaFil: Fainstein, Alejandro. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química (i); ArgentinaFil: Soukiassian, A.. University Park; Estados UnidosFil: Schlom, D. G.. Cornell University; Estados UnidosFil: Xi, X. X.. Cornell University; Estados Unidos. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentin

First direct observation of a nearly ideal graphene band structure

Angle-resolved photoemission and X-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(000-1) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films cause adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds to an individual macro-scale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.Comment: 5 pages, 4 figure

Ferroelectricity in Ultrathin Strained BaTiO\u3csub\u3e3\u3c/sub\u3e Films: Probing the Size Effect by Ultraviolet Raman Spectroscopy

We demonstrate a dramatic effect of film thickness on the ferroelectric phase transition temperature, Tc, in strained BaTiO3 films grown on SrTiO3 substrates. Using variable temperature ultraviolet Raman spectroscopy enables measuring Tc in films as thin as 1.6 nm, and film thickness variation from 1.6 to 10 nm leads to Tc tuning from 70 to about 925K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of of 180◦ domains below Tc, and thermodynamic phase-field model calculations of Tc as a function of thickness

Modeling the series of (n x 2) Si-rich reconstructions of beta-SiC(001): a prospective atomic wire?

We perform ab initio plane wave supercell density functional calculations on three candidate models of the (3 x 2) reconstruction of the beta-SiC(001) surface. We find that the two-adlayer asymmetric-dimer model (TAADM) is unambiguously favored for all reasonable values of Si chemical potential. We then use structures derived from the TAADM parent to model the silicon lines that are observed when the (3 x 2) reconstruction is annealed (the (n x 2) series of reconstructions), using a tight-binding method. We find that as we increase n, and so separate the lines, a structural transition occurs in which the top addimer of the line flattens. We also find that associated with the separation of the lines is a large decrease in the HOMO-LUMO gap, and that the HOMO state becomes quasi-one-dimensional. These properties are qualititatively and quantitatively different from the electronic properties of the original (3 x 2) reconstruction.Comment: 22 pages, including 6 EPS figure