On the occurrence of Berezinskii-Kosterlitz-Thouless behavior in highly anisotropic cuprate superconductors

The conflicting observations in the highly anisotropic Bi2Sr2CaCu2O8+x, vidence for BKT behavior emerging from magnetization data and smeared 3D-xy behavior, stemming form the temperature dependence of the magnetic in-plane penetration depth are traced back to the rather small ratio, gsic+/gsic-=0.45, between the c-axis correlation length probed above (+) and below (-) Tc, and the comparatively large anisotropy. The latter leads to critical amplitudes gsic0+,-which are substantially smaller than the distance between two CuO2 double layers. In combination with gsic+/gsic-=0.45 and in contrast to the situation below Tc the c-axis correlation length gsic exceeds the distance between two CuO2 double layers very close to Tc only. Below this narrow temperature regime where 3D-xy fluctuations dominate, there is then an extended temperature regime where the units with two CuO2 double layers are nearly uncoupled so that 2D thermal fluctuations dominate and BKT features are observable.Comment: 4 pages, 4 figure

Kondo effect of Co adatoms on Ag monolayers on noble metal surfaces

The Kondo temperature $T_K$ of single Co adatoms on monolayers of Ag on Cu and Au(111) is determined using Scanning Tunneling Spectroscopy. $T_K$ of Co on a single monolayer of Ag on either substrate is essentially the same as that of Co on a homogenous Ag(111) crystal. This gives strong evidence that the interaction of surface Kondo impurities with the substrate is very local in nature. By comparing $T_K$ found for Co on Cu, Ag, and Au (111)-surfaces we show that the energy scale of the many-electron Kondo state is insensitive to the properties of surface states and to the energetic position of the projected bulk band edges.Comment: 4 pages, 3 figure

A general interpolation scheme for thermal fluctuations in superconductors

We present a general interpolation theory for the phenomenological effects of thermal fluctuations in superconductors. Fluctuations are described by a simple gauge invariant extension of the gaussian effective potential for the Ginzburg-Landau static model. The approach is shown to be a genuine variational method, and to be stationary for infinitesimal gauge variations around the Landau gauge. Correlation and penetration lengths are shown to depart from the mean field behaviour in a more or less wide range of temperature below the critical regime, depending on the class of material considered. The method is quite general and yields a very good interpolation of the experimental data for very different materials.Comment: some misprints have been corrected in Eq.(15),(19); more references and comments have been adde

Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits

This work is concerned with Al/Al-oxide(AlO$_{x}$)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlO$_{x}$ tunnel barrier. The study is focused on the correlation of the fabrication conditions, nanostructural and nanochemical properties and the occurrence of two-level tunneling systems with particular emphasis on the AlO$_{x}$-layer. Electron-beam evaporation with two different processes and sputter deposition were used for structure fabrication, and the effect of illumination by ultraviolet light during Al-oxide formation is elucidated. Characterization was performed by analytical transmission electron microscopy and low-temperature dielectric measurements. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the layer systems and the properties of two-level tunneling systems. Based on the understanding of the observed structural characteristics, routes are derived towards the fabrication of Al/AlO$_{x}$/Al-layers systems with improved properties.Comment: 28 pages, 4 figure

Jahn-Teller Distortion and Ferromagnetism in the Dilute Magnetic Semiconductors GaN:Mn

Using first-principles total-energy methods, we investigate Jahn-Teller distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the cubic zinc blende structure. The results for an isolated Mn impurity on a Ga site show that there is no appreciable effect in GaAs, whereas, in GaN there is a Jahn-Teller effect in which the symmetry around the impurity changes from T$_{d}$ to D$_{2d}$ or to C$_{2v}$. The large effect in GaN occurs because of the localized d$^4$ character, which is further enhanced by the distortion. The lower symmetry should be detectable experimentally in cubic GaN with low Mn concentration, and should be affected by charge compensation (reductions of holes and conversion of Mn ions to d$^5$ with no Jahn-Teller effect). Jahn-Teller effect is greatly reduced because the symmetry at each Mn site is lowered due to the Mn-Mn interaction. The tendency toward ferromagnetism is found to be stronger in GaN:Mn than in GaAs:Mn and to be only slightly reduced by charge compensation.Comment: 6 pages, 3 figure

LSD: Lyman-break galaxies Stellar populations and Dynamics. I: Mass, metallicity and gas at z~3.1

We present the first results of a project, LSD, aimed at obtaining spatially-resolved, near-infrared spectroscopy of a complete sample of Lyman-Break Galaxies at z~3. Deep observations with adaptive optics resulted in the detection of the main optical lines, such as [OII], Hbeta and [OIII], which are used to study sizes, SFRs, morphologies, gas-phase metallicities, gas fractions and effective yields. Optical, near-IR and Spitzer/IRAC photometry is used to measure stellar mass. We obtain that morphologies are usually complex, with the presence of several peaks of emissions and companions that are not detected in broad-band images. Typical metallicities are 10-50% solar, with a strong evolution of the mass-metallicity relation from lower redshifts. Stellar masses, gas fraction, and evolutionary stages vary significantly among the galaxies, with less massive galaxies showing larger fractions of gas. In contrast with observations in the local universe, effective yields decrease with stellar mass and reach solar values at the low-mass end of the sample. This effect can be reproduced by gas infall with rates of the order of the SFRs. Outflows are present but are not needed to explain the mass-metallicity relation. We conclude that a large fraction of these galaxies are actively creating stars after major episodes of gas infall or merging.Comment: MNRAS, in pres

Robust surface electronic properties of topological insulators: Bi2Te3 films grown by molecular beam epitaxy

The surface electronic properties of the important topological insulator Bi2Te3 are shown to be robust under an extended surface preparation procedure which includes exposure to atmosphere and subsequent cleaning and recrystallization by an optimized in-situ sputter-anneal procedure under ultra high vacuum conditions. Clear Dirac-cone features are displayed in high-resolution angle-resolved photoemission spectra from the resulting samples, indicating remarkable insensitivity of the topological surface state to cleaning-induced surface roughness.Comment: 3 pages, 3 figure