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

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

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

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

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

Nitridation of InP(1 0 0) surface studied by synchrotron radiation

The nitridation of InP(1 0 0) surfaces has been studied using synchrotron radiation photoemission. The samples were chemically cleaned and then ion bombarded, which cleaned the surface and also induced the formation of metallic indium droplets. The nitridation with a Glow Discharge Cell (GDS) produced indium nitride by reaction with these indium clusters. We used the In 4d and P 2p core levels to monitor the chemical state of the surface and the coverage of the species present. We observed the creation of In-N and P-N bonds while the In-In metallic bonds decrease which confirm the reaction between indium clusters and nitrogen species. A theoretical model based on stacked layers allows us to assert that almost two monolayers of indium nitride are produced. The effect of annealing on the nitridated layers at 450 $^\circ$C has also been analysed. It appears that this system is stable up to this temperature, well above the congruent evaporation temperature (370 $^\circ$C) of clean InP(1 0 0): no increase of metallic indium bonds due to decomposition of the substrate is detected as shown in previous works [L. Bideux, Y. Ould-Metidji, B. Gruzza, V. Matolin, Surf. Interface Anal. 34 (2002) 712] studying the InP(1 0 0) surfaces

Direct Determination of the Effect of Strain on Domain Morphology in Ferroelectric Superlattices with Scanning Probe Microscopy

A variant of piezo force microscopy was used to characterize the effect of strain on polarization in [(BaTiO3)n/(SrTiO3)m]p superlattices. The measurements were compared to theoretical predictions based on phase-field calculations. When polarization is constrained to be perpendicular to the substrate, the measured polarization and domain morphology agree quantitatively with the predictions. This case allows the presence of an internal electric field in the thin film to be identified. The measured trend in piezoelectric response with strain state was in qualitative agreement with predictions, and the differences were consistent with the presence of internal electrical fields. Clear differences in domain morphology with strain were observed; and in some cases, the lateral anisotropic strain appeared to influence the domain morphology. The differences in magnitude and morphology were attributed to the internal electric fields and anisotropic strains

Interfacial coherency and ferroelectricity of BaTiO(3)/SrTiO(3) superlattice films

We studied the phase transitions, domain morphologies, and polarizations in BaTiO(3)/SrTiO(3) superlattices grown on SrTiO(3) substrates. Using the phase field approach, we discovered the remarkable influence of film/substrate interfacial coherency on the ferroelectricity of the SrTiO(3) layers within a superlattice: it is an orthorhombic ferroelectric for an incoherent interface while it exhibits only induced polarization by the adjacent BaTiO(3) layers for a coherent interface. We presented the domain morphologies within individual BaTiO(3) and SrTiO(3) layers which have different ferroelectric symmetries. The results are compared to ultraviolet Raman spectroscopy and variable temperature x-ray diffraction measurements.open312

Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics

We review our recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Our building block consists of silicon dangling bond on a H-Si(001) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro-to-atom connections are discussed.Comment: 28 pages, 19 figure

Prediction of ferroelectricity in BaTiO3/SrTiO3 superlattices with domains

The phase transitions of superlattices into single- and multidomain states were studied using a mesoscale phase-field model incorporating structural inhomogeneity, micromechanics, and electrostatics. While the predictions of transition temperatures of BaTiO3/SrTiO3 superlattices into multidomains show remarkably good, quantitative agreement with ultraviolet Raman spectroscopic and variable-temperature x-ray diffraction measurements, the single-domain assumption breaks down for superlattices in which the nonferroelectric layer thickness exceeds the characteristic domain size in the ferroelectric layers.open463