377 research outputs found
Supersymmetric AdS_3 solutions of type IIB supergravity
For every positively curved Kahler-Einstein manifold in four dimensions we
construct an infinite family of supersymmetric solutions of type IIB
supergravity. The solutions are warped products of AdS_3 with a compact
seven-dimensional manifold and have non-vanishing five-form flux. Via the
AdS/CFT correspondence, the solutions are dual to two-dimensional conformal
field theories with (2,0) supersymmetry. The corresponding central charges are
rational numbers.Comment: Dedicated to Rafael Sorkin in celebration of his 60th birthday; 5
pages, latex. v2, typos corrected, to appear in PR
Bernoulli potential in type-I and weak type-II superconductors: III. Electrostatic potential above the vortex lattice
The electrostatic potential above the Abrikosov vortex lattice, discussed
earlier by Blatter {\em et al.} {[}PRL {\bf 77}, 566 (1996){]}, is evaluated
within the Ginzburg-Landau theory. Unlike previous studies we include the
surface dipole. Close to the critical temperature, the surface dipole reduces
the electrostatic potential to values below a sensitivity of recent sensors. At
low temperatures the surface dipole is less effective and the electrostatic
potential remains observable as predicted earlier.Comment: 8 pages 5 figure
Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions
We study the transport properties of a graphene ferromagnet-insulator
superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by
solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the
retro and specular Andreev reflections in the graphene FIS junction are
drastically modified in the presence of exchange interaction and that the
spin-polarization () of tunneling current can be tuned from the positive
to negative value by bias voltage (). In the thin-barrier limit, the
conductance of a graphene FIS junction oscillates as a function of barrier
strength . Both the amplitude and phase of the conductance oscillation
varies with the exchange energy . For (Fermi energy), the
amplitude of oscillation decreases with . For ,
the amplitude of oscillation increases with , where
( is the applied electrostatic potential on
the superconducting segment of the junction). For , the
amplitude of oscillation decreases with again. Interestingly, a
universal phase difference of in exists between the
curves for and . Finally, we find that the transitions
between retro and specular Andreev reflections occur at and
, and hence the singular behavior of the conductance near
these bias voltages results from the difference in transport properties between
specular and retro Andreev reflections.Comment: Accepted for publication in Physical Review
A Note on Einstein Sasaki Metrics in D \ge 7
In this paper, we obtain new non-singular Einstein-Sasaki spaces in
dimensions D\ge 7. The local construction involves taking a circle bundle over
a (D-1)-dimensional Einstein-Kahler metric that is itself constructed as a
complex line bundle over a product of Einstein-Kahler spaces. In general the
resulting Einstein-Sasaki spaces are singular, but if parameters in the local
solutions satisfy appropriate rationality conditions, the metrics extend
smoothly onto complete and non-singular compact manifolds.Comment: Latex, 13 page
The central charge of supersymmetric AdS_5 solutions of type IIB supergravity
We show that generic supersymmetric AdS_5 solutions of type IIB supergravity
admit a canonical contact structure. This structure determines the central
charge of the dual field theory and the conformal dimension of operators dual
to supersymmetric wrapped D3-branes. Hence both quantities can be calculated
using incomplete information about the solutions, allowing us to prove that
they are rational numbers for solutions with a U(1) R-symmetry, in agreement
with field theory expectations. We also discuss related Duistermaat-Heckman
integrals and localization formulae.Comment: 4 page
On the Ricci tensor in type II B string theory
Let be a metric connection with totally skew-symmetric torsion \T
on a Riemannian manifold. Given a spinor field and a dilaton function
, the basic equations in type II B string theory are \bdm \nabla \Psi =
0, \quad \delta(\T) = a \cdot \big(d \Phi \haken \T \big), \quad \T \cdot \Psi
= b \cdot d \Phi \cdot \Psi + \mu \cdot \Psi . \edm We derive some relations
between the length ||\T||^2 of the torsion form, the scalar curvature of
, the dilaton function and the parameters . The main
results deal with the divergence of the Ricci tensor \Ric^{\nabla} of the
connection. In particular, if the supersymmetry is non-trivial and if
the conditions \bdm (d \Phi \haken \T) \haken \T = 0, \quad \delta^{\nabla}(d
\T) \cdot \Psi = 0 \edm hold, then the energy-momentum tensor is
divergence-free. We show that the latter condition is satisfied in many
examples constructed out of special geometries. A special case is . Then
the divergence of the energy-momentum tensor vanishes if and only if one
condition \delta^{\nabla}(d \T) \cdot \Psi = 0 holds. Strong models (d \T =
0) have this property, but there are examples with \delta^{\nabla}(d \T) \neq
0 and \delta^{\nabla}(d \T) \cdot \Psi = 0.Comment: 9 pages, Latex2
Supersymmetric AdS_5 solutions of M-theory
We analyse the most general supersymmetric solutions of D=11 supergravity
consisting of a warped product of five-dimensional anti-de-Sitter space with a
six-dimensional Riemannian space M_6, with four-form flux on M_6. We show that
M_6 is partly specified by a one-parameter family of four-dimensional Kahler
metrics. We find a large family of new explicit regular solutions where M_6 is
a compact, complex manifold which is topologically a two-sphere bundle over a
four-dimensional base, where the latter is either (i) Kahler-Einstein with
positive curvature, or (ii) a product of two constant-curvature Riemann
surfaces. After dimensional reduction and T-duality, some solutions in the
second class are related to a new family of Sasaki-Einstein spaces which
includes T^{1,1}/Z_2. Our general analysis also covers warped products of
five-dimensional Minkowski space with a six-dimensional Riemannian space.Comment: 40 pages. v2: minor changes, eqs. (2.22) and (D.12) correcte
M-Theory solutions with AdS factors
Solutions of D=7 maximal gauged supergravity are constructed with metrics
that are a product of a n-dimensional anti-de Sitter (AdS) space, with
n=2,3,4,5, and certain Einstein manifolds. The gauge fields have the same form
as in the recently constructed solutions describing the near-horizon limits of
M5-branes wrapping supersymmetric cycles. The new solutions do not preserve any
supersymmetry and can be uplifted to obtain new solutions of D=11 supergravity,
which are warped and twisted products of the D=7 metric with a squashed
four-sphere. Some aspects of the stability of the solutions are discussed.Comment: 30 pages. References adde
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