103 research outputs found

### Theory of Coherent $c$-Axis Josephson Tunneling between Layered Superconductors

We calculate exactly the Josephson current for $c$-axis coherent tunneling
between two layered superconductors, each with internal coherent tight-binding
intra- and interlayer quasiparticle dispersions. Our results also apply when
one or both of the superconductors is a bulk material, and include the usually
neglected effects of surface states. For weak tunneling, our results reduce to
our previous results derived using the tunneling Hamiltonian. Our results are
also correct for strong tunneling. However, the $c$-axis tunneling results of
Tanaka and Kashiwaya are shown to be incorrect in any limit. In addition, we
consider the $c$-axis coherent critical current between two identical layered
superconductors twisted an angle $\phi_0$ about the $c$-axis with respect to
each other. Regardless of the order parameter symmetry, our coherent tunneling
results using a tight-binding intralayer quasiparticle dispersion are
inconsistent with the recent $c$-axis twist bicrystal
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ twist junction experiments of Li {\it et
al.}Comment: 11 pages, 13 figures, submitted to Physical Review

### Comment on "c-axis Josephson tunneling in $D_{x^2-y^2}$-wave superconductors''

This comment points out that the recent paper by Maki and Haas [Phys. Rev. B
{\bf 67}, 020510 (2003)] is completely wrong.Comment: 1 page, submittted to Phys. Rev.

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### Fluctuation phenomena in layered superconductors

Gaussian fluctuations in layered superconductors have been the subject of study for many years. Although the FD was studied in detail long ago, the FC (fluctuation conductivity) was studied only recently, since the MT and DOS diagrams were previously neglected. Recent comparisons with experiment on YBCO have shown that the DOS diagrams are important and can lead to qualitatively different behaviors for the FC parallel and perpendicular to the layers. In both cases, Gaussian fluctuations fit the data above {Tc} very well, even for YBCO. To date, nearly all calculations of fluctuation quantities were for B{parallel}{cflx c}. Nevertheless, it should be possible to treat an arbitrary B, but the evaluation of the required matrix elements for the fluctuation quantities will be more complicated

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### Why we still don`t know the symmetry of the order parameter in high temperature superconductors

One of the most elusive topics in the study of high temperature superconductors (HTCS) is the orbital symmetry of the superconducting order parameter {Delta}. The low temperature T behavior observed in many experiments suggests that {Delta} might exhibit line nodes, as expected for d{sub x}{sub {minus}y}2 orbital symmetry. However, such behavior also arises from intrinsic proximity coupling and surface states, etc. The paramagnetic Meissner effect, once cited as indirect evidence for a d-wave {Delta}, was recently observed in Nb. Some phase-sensitive Josephson junction experiments gave strong evidence for a substantial isotropic, or s-wave, component to {Delta}, whereas others supported the above d-wave form. However, such experiments are intrinsically unreliable in experimental geometries with sample corners, or with meandering grain boundary junctions and stoichiometry inhomogeneities. Finally, the pseudogap observed recently in underdoped HTCS could arise from a charge-density wave

### Gaussian Tunneling Model of c-Axis Twist Josephson Junctions

We calculate the critical current density $J^J_c$ for c-axis Josephson
tunneling between identical high temperature superconductors twisted an angle
$\phi_0$ about the c-axis. We model the tunneling matrix element squared as a
Gaussian in the change of wavevector q parallel to the junction, $<|t({\bf
q})|^2>\propto\exp(-{\bf q}^2a^2/2\pi^2\sigma^2)$. The $J^J_c(\phi_0)/J^J_c(0)$
obtained for the s- and extended-s-wave order parameters (OP's) are consistent
with the Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ data of Li {\it et al.}, but only
for strongly incoherent tunneling, $\sigma^2\ge0.25$. A $d_{x^2-y^2}$-wave OP
is always inconsistent with the data. In addition, we show that the apparent
conventional sum rule violation observed by Basov et al. might be
understandable in terms of incoherent c-axis tunneling, provided that the OP is
not $d_{x^2-y^2}$-wave.Comment: 6 pages, 6 figure

### Heisenberg Dimer Single Molecule Magnets in a Strong Magnetic Field

We calculate the static and dynamic properties of single crystal, single
molecule magnets consisting of equal spin $S=1/2$ or 5/2 dimers. The spins in
each dimer interact with each other via the Heisenberg exchange interaction and
with the magnetic induction ${\bf B}$ via the Zeeman interaction, and
interdimer interactions are negligible. For antiferromagnetic couplings, the
static magnetization and specific heat exhibit interesting low temperature $T$
and strong ${\bf B}$ quantum effects. We calculate the frequency spectrum of
the Fourier transform of the real part of the time autocorrelation function
${\cal C}_{11}(t)$ for arbitrary $T, {\bf B}$, and compare our results with
those obtained for classical spins. We also calculate the inelastic neutron
magnetic dynamical structure factor $S({\bf q},\omega)$ at arbitrary $T, {\bf
B}$.Comment: 11 pages, 14 figures, submitted to Phys. Rev.

### Properties of the Ideal Ginzburg-Landau Vortex Lattice

The magnetization curves M(H) for ideal type-II superconductors and the
maximum, minimum, and saddle point magnetic fields of the vortex lattice are
calculated from Ginzburg-Landau theory for the entire ranges of applied
magnetic fields Hc1 <= H < Hc2 or inductions 0 <= B < Hc2 and Ginzburg-Landau
parameters sqrt(1/2) <= kappa <= 1000. Results for the triangular and square
flux-line lattices are compared with the results of the circular cell
approximation. The exact magnetic field B(x,y) and magnetization M(H, kappa)
are compared with often used approximate expressions, some of which deviate
considerably or have limited validity. Useful limiting expressions and
analytical interpolation formulas are presented.Comment: 11 pages, 8 figure

### Non-ohmic critical fluctuation conductivity of layered superconductors in magnetic field

Thermal fluctuation conductivity for a layered superconductor in
perpendicular magnetic field is treated in the frame of the self-consistent
Hartree approximation for an arbitrarily strong in-plane electric field. The
simultaneous application of the two fields results in a slightly stronger
suppression of the superconducting fluctuations, compared to the case when the
fields are applied individually.Comment: 4 pages, 1 figure, to be published in Phys. Rev.

### Exact Mesonic Vacua From Matrix Models

We investigate in detail the structure of mesonic vacua of N=1 U(Nc)
supersymmetric gauge theory with Nf flavors from the matrix model. We show that
the Witten index from the matrix model calculation agrees with a result from
field theoretical analysis. We also discuss the relationship between a
diagrammatic summation and direct matrix integration with insertion of a
variable changing delta function. Using this formalism, we obtain the quantum
moduli space and evidence of the Seiberg duality from the matrix models.Comment: 14 pages, 1 figure, typos corrected and note on the quamtum moduli
space adde

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