Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors

We compare different scenarios for the low temperature splitting of the zero-energy peak in the local density of states at (110) surfaces of d_{x^2-y^2}-wave superconductors, observed by Covington et al. (Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the Bogolyubov-de Gennes treatment we find a surface phase transition towards a time-reversal symmetry breaking surface state carrying spontaneous currents and an s+id-wave state. Alternatively, we show that electron correlation leads to a surface phase transition towards a magnetic state corresponding to a local spin density wave state.Comment: 4 pages, 5 figure

Measurement of the Tip-Induced Potential in Scanning Gate Experiments

We present a detailed experimental study on the electrostatic interaction between a quantum dot and the metallic tip of a scanning force microscope. Our method allowed us to quantitatively map the tip-induced potential and to determine the spatial dependence of the tip's lever arm with high resolution. We find that two parts of the tip-induced potential can be distinguished, one that depends on the voltage applied to the tip and one that is independent of this voltage. The first part is due to the metallic tip while we interpret the second part as the effect of a charged dielectric particle on the tip. In the measurements of the lever arm we find fine structure that depends on which quantum state we study. The results are discussed in view of scanning gate experiments where the tip is used as a movable gate to study nanostructures.Comment: 7 pages, 5 figures, minor changes to fit published versio

Near-infrared resonant photoacoustic gas measurement using simultaneous dual-frequency excitation

The simultaneous dual-frequency operation of a resonant photoacoustic gas sensor based on the differential mode excitation photoacoustic (DME-PA) technique is presented. The DME-PA method uses the excitation of two different modes in a resonant photoacoustic cell and the gas concentration is derived from the amplitude ratio of these acoustic modes. With the simultaneous dual-frequency excitation, the amplitude ratio needed by the DME-PA technique is obtained instantaneously, in contrast to the sequential modulation scheme where additional time delays are introduced by changing the modulation frequency. For a given excitation power reaching the photoacoustic cell, and a total acquisition time longer than 7s, the simultaneous modulation scheme provides an improved measurement uncertainty compared to the sequential scheme. The proposed sensor allows measuring water vapour with a ±150 ppmV uncertainty using current-modulated near-infrared LEDs and a 15s total acquisition tim

Vortex avalanches in the non-centrosymmetric superconductor Li2Pt3B

We investigated the vortex dynamics in the non-centrosymmetric superconductor Li_2Pt_3B in the temperature range 0.1 K - 2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of a new, very effective pinning mechanism. At a certain time a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices. A possible mechanism based on the barrier effect of twin boundaries is briefly discussed

Asymmetric magnetic interference patterns in 0-pi Josephson junctions

We examine the magnetic interference patterns of Josephson junctions with a region of 0- and of pi-phase shift. Such junctions have recently been realized as c-axis YBCO-Pb junctions with a single twin boundary in YBCO. We show that in general the junction generates self-fields which introduces an asymmetry in the critical current under reversal of the magnetic field. Numerical calculations of these asymmetries indicate they account well for the unexplained features observed in single twin boundary junctions.Comment: 4 pages, 3 figure

Spontaneous flux in a d-wave superconductor with time-reversal-symmetry-broken pairing state at {110} boundaries

The induction of an s-wave component in a d-wave superconductor is considered. Near the {110}-oriented edges of such a sample, the induced s-wave order parameter together with d-wave component forms a complex combination d+e^{i\phi} s, which breaks the time reversal symmetry (BTRS) of the pairing state. As a result, the spontaneous current is created. We numerically study the current distribution and the formation of the spontaneous flux induced by the current. We show that the spontaneous flux formed from a number of defect lines with {110} orientation has a measurable strength. This result may provide a unambiguous way to check the existence of BTRS pairing state at {110}-oriented boundaries.Comment: 4 pages, 2 ps-figures, content changed, references adde

Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures

The anisotropic degenerate two-orbital Hubbard model is studied within dynamical mean-field theory at low temperatures. High-precision calculations on the basis of a refined quantum Monte Carlo (QMC) method reveal that two distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2 even in the absence of spin-flip contributions to the Hund exchange. The second transition -- not seen in earlier studies using QMC, iterative perturbation theory, and exact diagonalization -- is clearly exposed in a low-frequency analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure

Josephson tunneling in high-TcT_c superconductors

This article describes the Josephson tunneling from time-reversal symmetry-breaking states and compares it with that from time-reversal invariant states for both twinned and untwinned crystals and for both $c$-axis and basal-plane currents, in a model for orthorhombic YBCO. A macroscopic invariance group describing the superconducting state of a twinned crystal is introduced and shown to provide a useful framework for the discussion of the results for twinned crystals. In addition, a ring geometry, which allows $s$-wave and $d_{x^2-y^2}$-wave superconductivity in a tetragonal superconductor to be distinguished on the basis of symmetry arguments only, is proposed and analyzed. Finally, an appendix gives details of the experimental Josephson tunneling evidence for a superconducting state of orthorhombic $ux^2+vy^2$ symmetry in YBCO.Comment: Latex File, 18 pages, 6 Postscript figures, submitted to Phys. Rev.

Nonvanishing Local Moment in Triplet Superconductors

The Kondo effect in a $p_x + {\rm i} p_y$-wave superconductor is studied by applying the Wilson's numerical renormalization group method. In this type of superconductor with a full energy gap like a s-wave one, the ground state is always a spin doublet, while a local spin is shrunk by the Kondo effect. The calculated magnetic susceptibility indicates that the spin of the ground state is generated by the orbital effect of the $p_x + {\rm i} p_y$-wave Cooper pairs. The effect of spin polarization of the triplet superconductor is also discussed.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp