How Large is the Intrinsic Flux Noise of a Magnetic Flux Quantum, of Half a Flux Quantum and of a Vortex-Free Superconductor?

This article addresses the question whether the magnetic flux of stationary vortices or of half flux quanta generated by frustrated superconducting rings is noisy. It is found that the flux noise generated intrinsically by a superconductor is, in good approximation, not enhanced by stationary vortices. Half flux quanta generated by $\pi$-rings are characterized by considerably larger noise.Comment: 11 pages, 3 figures. in: A. Bussmann-Holder, H. Keller (Eds.) High Tc Superconductors and Related Transition Metal Oxides, Springer, 237-242; also to be published in: Journal of Superconductivity (2007

Quasiparticle Interface States in Junctions Involving d-Wave Superconductors

Influence of surface pair breaking, barrier transmission and phase difference on quasiparticle bound states in junctions with d-wave superconductors is examined. Based on the quasiclassical theory of superconductivity, an approach is developed to handle interface bound states. It is shown in SIS' junctions that low energy bound states get their energies reduced by surface pair breaking, which can be taken into account by introducing an effective order parameter for each superconductor at the junction barrier. More interestingly, for the interface bound states near the continuous spectrum the effect of surface pair breaking may result in a splitting of the bound states. In the tunneling limit this can lead to a square root dependence of a nonequilibrium Josephson current on the barrier transmision, which means an enhancement as compared to the conventional critical current linear in the transmission. Reduced broadening of bound states in NIS junctions due to surface pair breaking is found.Comment: 27 pages, Latex fil

Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results

The dispersive interaction between nanotubes is investigated through ab initio theory calculations and in an analytical approximation. A van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to determine and compare the binding of a pair of nanotubes as well as in a nanotube crystal. To analyze the interaction and determine the importance of morphology, we furthermore compare results of our ab initio calculations with a simple analytical result that we obtain for a pair of well-separated nanotubes. In contrast to traditional density functional theory calculations, the vdW-DF study predicts an intertube vdW bonding with a strength that is consistent with recent observations for the interlayer binding in graphitics. It also produce a nanotube wall-to-wall separation which is in very good agreement with experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal binding energy can be approximated by a sum of nanotube-pair interactions when these are calculated in vdW-DF. This observation suggests a framework for an efficient implementation of quantum-physical modeling of the CNT bundling in more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure

Electronic structure of d-wave superconducting quantum wires

We present analytical and numerical results for the electronic spectra of wires of a d-wave superconductor on a square lattice. The spectra of Andreev and other quasiparticle states, as well as the spatial and particle-hole structures of their wave functions, depend on interference effects caused by the presence of the surfaces and are qualitatively different for half-filled wires with even or odd number of chains. For half-filled wires with an odd number of chains N at (110) orientation, spectra consist of N doubly degenerate branches. By contrast, for even N wires, these levels are split, and all quasiparticle states, even the ones lying above the maximal gap, have the characteristic properties of Andreev bound states. These Andreev states above the gap can be interpreted as a consequence of an infinite sequence of Andreev reflections experienced by quasiparticles along their trajectories bounded by the surfaces of the wire. Our microscopic results for the local density of states display atomic-scale Friedel oscillations due to the presence of the surfaces, which should be observable by scanning tunneling microscopy. For narrow wires the self-consistent treatment of the order parameter is found to play a crucial role. In particular, we find that for small wire widths the finite geometry may drive strong fluctuations or even stablilize exotic quasi-1D pair states with spin triplet character.Comment: 21 pages, 20 figures. Slightly modified version as published in PR

Casimir torque

We develop a formalism for the calculation of the flow of angular momentum carried by the fluctuating electromagnetic field within a cavity bounded by two flat anisotropic materials. By generalizing a procedure employed recently for the calculation of the Casimir force between arbitrary materials, we obtain an expression for the torque between anisotropic plates in terms of their reflection amplitude matrices. We evaluate the torque in 1D for ideal and realistic model materials.Comment: 8 pages, 4 figs, Submitted to Proc. of QFEXT'05, to appear in J. Phys.

Dispersion interactions in stratified anisotropic and optically active media at all separations

We propose a method to calculate dispersion interactions in a system composed of a one dimensional layering of finite thickness anisotropic and optically active slabs. The result is expressed within the algebra of 4x4 matrices and is demonstrated to be equivalent to the known limits of isotropic, nonretarded and uniaxial dispersion interactions. The method is also capable of handling dielectric media with smoothly varying anisotropy axes.Comment: 10 pages, 3 figures, revte

Dispersion Interactions between Optically Anisotropic Cylinders at all Separations: Retardation Effects for Insulating and Semiconducting Single Wall Carbon Nanotubes

We derive the complete form of the van der Waals dispersion interaction between two infinitely long anisotropic semiconducting/insulating thin cylinders at all separations. The derivation is based on the general theory of dispersion interactions between anisotropic media as formulated in [J. N. Munday, D. Iannuzzi, Yu. S. Barash and F. Capasso, {\sl Phys. Rev. A} {\bf 71}, 042102 (2005)]. This formulation is then used to calculate the dispersion interactions between a pair of single walled carbon nanotubes at all separations and all angles. Non-retarded and retarded forms of the interactions are developed separately. The possibility of repulsive dispersion interactions and non-monotonic dispersion interactions is discussed within the framework of the new formulation

Zero-energy Andreev surface bound states in the lattice model

The conditions for zero-energy Andreev surface bound states to exist are found for the lattice model of d-wave superconductor with arbitrary surface orientation. Both nearest neighbors and next nearest neighbors models are considered. It is shown that the results are very sensitive to the surface orientation. In particular, for half-filled $(hl0)$-surface zero-energy Andreev surface states only appear under the condition that $h$ and $l$ are odd simultaneouslyComment: 9 pages, 1 figur

Self-consistent interface properties of d and s-wave superconductors

We develop a method to solve the Bogoliubov de Gennes equation for superconductors self-consistently, using the recursion method. The method allows the pairing interaction to be either local or non-local corresponding to s and d-wave superconductivity, respectively. Using this method we examine the properties of various S-N and S-S interfaces. In particular we calculate the spatially varying density of states and order parameter for the following geometries (i) s-wave superconductor to normal metal, (ii) d-wave superconductor to normal metal, (iii) d-wave superconductor to s-wave superconductor. We show that the density of states at the interface has a complex structure including the effects of normal surface Friedel oscillations, the spatially varying gap and Andeev states within the gap, and the subtle effects associated with the interplay of the gap and the normal van Hove peaks in the density of states. In the case of bulk d-wave superconductors the surface leads to mixing of different order parameter symmetries near the interface and substantial local filling in of the gap.Comment: 20 pages, Latex and 8 figure

Josephson effect in quasi one-dimensional unconventional superconductors

Josephson effect in junctions of quasi one-dimensional triangular lattice superconductors is discussed, where the theoretical model corresponds to organic superconductors (TMTSF)_2PF_6. We assume the quarter-filling electron band and p, d and f wave like pairing symmetries in organic superconductors. To realize the electronic structures in organic superconductors, we introduce the asymmetric hopping integral, (t') among second nearest lattice sites. At t'=0, the Josephson current in the d wave symmetry saturates in low temperatures, whereas those in the p and the f wave symmetries show the low-temperature anomaly due to the zero-energy state at the junction interfaces. The low-temperature anomaly appears even in the d wave symmetry in the presence of t', whereas the anomaly is suppressed in the f wave symmetry. The shape of the Fermi surface is an important factor for the formation of the ZES in the quarter-filling electron systems.Comment: 10 page