734 research outputs found
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 -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 -surface zero-energy Andreev
surface states only appear under the condition that and 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
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