4,980 research outputs found

### Bi_2 Sr_2 CaCu_2 O_{8+delta} Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)] prepared atomically
clean Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (BSCCO) Josephson junctions between
identical single crystal cleaves stacked and twisted an angle $\phi_0$ about
the c axis. For each bicrystal, the ratio $J_c^J/J_c^S$ of the c-axis twist
junction critical current density to that across either single crystal part is
unity, independent of $\phi_0$ and the ratio $A^J/A^S$ of junction areas. From
extensive theoretical studies involving a variety of tunneling and
superconducting order parameter (OP) forms, we conclude that the results
provide strong evidence for incoherent c-axis tunneling and that the dominant
OP is s-wave for $T\le T_c$. Recently, Takano {\it et al.} [Phys. Rev. B {\bf
65}, 140315(R) (2002)] obtained results from BSCCO whisker twist junctions
which also rule out a pure d-wave OP, but which are surprisingly suggestive of
coherent c-axis tunneling form small Fermi surface hot spots.Comment: 5 pages, 11 figures, invited talk at LT23, to be published in Physica

### On the Monodromies of N=2 Supersymmetric Yang-Mills Theory with Gauge Group SO(2n)

We present families of algebraic curves describing the moduli-space of
$N\!=\!2$ supersymmetric Yang-Mills theory with gauge group $SO(2n)$. We test
our curves by computing the weak coupling monodromies and the number of
$N\!=\!1$ vacua.Comment: 14 pages, 5 Postscript figures, LaTeX file, uses epsf.st

### What has been the tax competition experience of the past 20 years?

This paper describes tax reforms in OECD countries over the last 20 years and how they are related to tax competition. Both individual countries? reforms and multilateralinitiatives and developments are covered. This is followed by an overview of theempirical evidence on tax competition. Our conclusion is that the evidence for someinterdependence in tax setting behaviour is strong, although the exact process driving thisremains unclear. While the most basic tax competition models fail to explain thedevelopment in OECD countries, there is more than one possible explanation for thereforms undertaken if more advanced models are considered. The multilateral initiativesthat were implemented however do not seem to be related to resource-based taxcompetition, instead they are about taxing rights. This paper describes tax reforms in OECD countries over the last 20 years and how they are related to tax competition. Both individual countries? reforms and multilateralinitiatives and developments are covered. This is followed by an overview of theempirical evidence on tax competition. Our conclusion is that the evidence for someinterdependence in tax setting behaviour is strong, although the exact process driving thisremains unclear. While the most basic tax competition models fail to explain thedevelopment in OECD countries, there is more than one possible explanation for thereforms undertaken if more advanced models are considered. The multilateral initiativesthat were implemented however do not seem to be related to resource-based taxcompetition, instead they are about taxing rights

### Mirror Maps and Instanton Sums for Complete Intersections in Weighted Projective Space

We consider a class of Calabi-Yau compactifications which are constructed as
a complete intersection in weighted projective space. For manifolds with one
K\"ahler modulus we construct the mirror manifolds and calculate the instanton
sum.Comment: 10 page

### Enhanced Gauge Symmetry in Three-Moduli Models of Type-II String and Hypergeometric Series

The conifold singularities in the type-II string are considered as the points
of phase transition. In some cases, these singularities can be understood in
the framework of the conventional fields theores as the points of enhanced
gauge symmetry. We consider a class of three moduli Type-II strings. It is
shown the periods can be written in the form of hypergeometric series around
the singular points in these models. The leading expansion around the conifold
locus turns out to be described by Appell functions. In one singular point, we
observe the enhanced gauge symmetry of $SU(2)\times SU(2)$ independent of the
models. Around another conifold locus, however, the resulting expression of the
Appell functions depends on the models. We examine the result by considering a
relation between these Appell functions and underlying Riemann surfaces.Comment: 20 pages, Late

### General (Anti-)de Sitter Black Holes in Five Dimensions

We find a general class of rotating charged black hole solutions to N=2, D=5
gauged supergravity coupled to vector supermultiplets. The supersymmetry
properties of these solutions are studied, and their mass and angular momenta
are obtained. We also compute the stress tensor of the dual D=4, N=4 super
Yang-Mills theory in the limit of strong `t Hooft coupling. It is shown that
closed timelike curves occur outside the horizon, indicating loss of unitarity
in the dual CFT. For imaginary coupling constant of the gravitini to the gauge
fields, one can obtain multi-centered rotating charged de Sitter black holes.
Some physical properties of these solutions are also discussed.Comment: 22 pages, LaTeX, some explanations concerning the appearance of
closed timelike curves outside the horizon added in section 5.1, updated
references, published versio

### 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

### Radiation from a Josephson STAR-emitter

We calculate the angular dependence of the radiation-zone output power and
electric polarization of stimulated terahertz amplified radiation (STAR)
emitted from a $dc$ voltage applied across cylindrical and rectangular stacks
of intrinsic Josephson junctions. During coherent emission, a spatially uniform
$ac$ Josephson current density in the stack acts as a surface electric current
density antenna source, leading to an harmonic radiation frequency spectrum, as
in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial
fluctuations of the $ac$ Josephson current cause its fundamental mode to lock
onto the lowest finite energy cylindrical cavity mode, causing it to resonate,
leading to a non-uniform magnetic surface current density radiation source, and
a non-trivial combined fundamental frequency output power with linear
polarization We also present a model of the superconducting substrate, and
present results for rectangular mesas.Comment: 18 pages, 26 figures, submitted to PR

### On the Moduli Space of SU(3) Seiberg-Witten Theory with Matter

We present a qualitative model of the Coulomb branch of the moduli space of
low-energy effective N=2 SQCD with gauge group SU(3) and up to five flavours of
massive matter. Overall, away from double cores, we find a situation broadly
similar to the case with no matter, but with additional complexity due to the
proliferation of extra BPS states. We also include a revised version of the
pure SU(3) model which can accommodate just the orthodox weak coupling
spectrum.Comment: 32 pages, 25 figures, uses JHEP.cls, added references, deleted joke

### Zero-field Time Correlation Functions of Four Classical Heisenberg Spins on a Ring

A model relevant for the study of certain molecular magnets is the ring of
N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange
interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly
for the time evolution of each of the spins. Exact triple integral
representations are derived for the auto, near-neighbor, and
next-nearest-neighbor time correlation functions for any temperature. At
infinite temperature, the correlation functions are reduced to quadrature. We
then evaluate the Fourier transforms of these functions in closed form, which
are double integrals. At low temperatures, the Fourier transform functions
explicitly demonstrate the presence of magnons. Our exact results for the
infinite temperature correlation functions in the long-time asymptotic limit
differ qualitatively from those obtained assuming diffusive spin dynamics.
Whether such explicitly non-hydrodynamic behavior would be maintained for
large-N rings is discussed.Comment: 18 pages, 21 figure

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