406 research outputs found
Giant Quantum Oscillations of the Longitudinal Magnetoresistance in Quasi two-dimensional Metals
We have investigated in frame of the quantum transport theory the magnetic
quantum oscillations of the longitudinal magnetoresistance in quasi
two-dimensional metals for a magnetic field perpendicular to the layers.
Giant Shubnikov-de Haas oscillations are found when the cyclotron energy
is much larger than the interlayer transfer integral
(the two-dimensional limit). In large magnetic fields and at low temperatures,
the minima of the magnetoconductivity exhibit a
thermally activated behavior in presence of negligibly small chemical potential
oscillations, as observed in the organic layered conductor
\beta''\mathrm{-(BEDT-TTF)}_{2}\mathrm{SF}_{5}\mathrm{CH}_{2}\mathrm{CF}_{2}\m
athrm{SO}_{3}.
The questions concerning the absence of strong chemical potential
oscillations in such compound and the impurity self-energy are discussed.Comment: 4 pages, intended for publication in special issue of Physica B for
RHMF 2003 Conference, Toulous
Instantons and Wormholes in N=2 supergravity
In this paper, we construct Euclidean instanton and wormhole solutions in
, N=2 supergravity theories with hypermultiplets. The analytic
continuation of the hypermultiplet action, involving pseudoscalar axions, is
discussed using the approach originally developed by Coleman which determines
the apparence of boundary terms. In particular, we investigate the conditions
obtained by requiring the action to be positive-definite once the boundary
terms are taken into account. The case of two hypermultiplets parameterizing
the coset is studied in detail. Orientifold
projections which reduce the supersymmetry to N=1 are also discussed.Comment: 57 pages, 4 figures, pdflatex, v2: references and some clarifications
added,v3: reference corrected and figures modifie
Supersymmetry and First Order Equations for Extremal States: Monopoles, Hyperinstantons, Black-Holes and p-Branes
In this lecture I review recent results on the first order equations
describing BPS extremal states, in particular N=2 extremal black-holes. The
role of special geometry is emphasized also in the rigid theory and a
comparison is drawn with the supersymmetric derivation of instantons and
hyperinstantons in topological field theories. Work in progress on the
application of solvable Lie algebras to the discussion of BPS states in
maximally extended supergravities is outlined.Comment: LaTeX, {article.sty, espcrc2.sty} 11 twocolumn pages. Invited Seminar
given at Santa Margerita Conference on Contrained Dynamics and Quantum
Gravity September 1995. In the replaced version misprints have been corrected
and some sentences have been modifie
Cyclic Foam Topological Field Theories
This paper proposes an axiomatic for Cyclic Foam Topological Field theories.
That is Topological Field theories, corresponding to String theories, where
particles are arbitrary graphs. World surfaces in this case are two-manifolds
with one-dimensional singularities. We proved that Cyclic Foam Topological
Field theories one-to-one correspond to graph-Cardy-Frobenius algebras, that
are families , where are families of
commutative associative Frobenius algebras, is an graduated by graphes, associative
algebras of Frobenius type and is a family of special representations. There are
constructed examples of Cyclic Foam Topological Field theories and its
graph-Cardy-Frobenius algebrasComment: 14 page
Finite-size and pressure effects in YBa_2Cu_4O_8 probed by magnetic field penetration depth measurements
We explore the combined pressure and finite-size effects on the in-plane
penetration depth \lambda_{ab} in YBa_2Cu_4O_8. Even though this cuprate is
stoichiometric the finite-size scaling analysis of \lambda_{ab}^{-2}(T)
uncovers the granular nature and reveals domains with nanoscale size L_{c}
along the c-axis. L_{c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures
from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence
of a phase coherent macroscopic superconducting state in cuprate
superconductors.Comment: 7 pages, 6 figure
Scherk-Schwarz Reduction of D=5 Special and Quaternionic Geometry
We give the N=2 gauged supergravity interpretation of a generic D=4, N=2
theory as it comes from generalized Scherk-Schwarz reduction of D=5, N=2
(ungauged) supergravity. We focus on the geometric aspects of the D=4 data such
as the general form of the scalar potential and masses in terms of the gauging
of a ``flat group''. Higgs and super-Higgs mechanism are discussed in some
detail.Comment: final version to be published on Class.Quant.Gra
M-theory on `toric' G_2 cones and its type II reduction
We analyze a class of conical G_2 metrics admitting two commuting isometries,
together with a certain one-parameter family of G_2 deformations which
preserves these symmetries. Upon using recent results of Calderbank and
Pedersen, we write down the explicit G_2 metric for the most general member of
this family and extract the IIA reduction of M-theory on such backgrounds, as
well as its type IIB dual. By studying the asymptotics of type II fields around
the relevant loci, we confirm the interpretation of such backgrounds in terms
of localized IIA 6-branes and delocalized IIB 5-branes. In particular, we find
explicit, general expressions for the string coupling and R-R/NS-NS forms in
the vicinity of these objects. Our solutions contain and generalize the field
configurations relevant for certain models considered in recent work of Acharya
and Witten.Comment: 45 pages, references adde
c--Map,very Special Quaternionic Geometry and Dual Ka\"hler Spaces
We show that for all very special quaternionic manifolds a different N=1
reduction exists, defining a Kaehler Geometry which is ``dual'' to the original
very special Kaehler geometry with metric G_{a\bar{b}}= - \partial_a \partial_b
\ln V (V={1/6}d_{abc}\lambda^a \lambda^b \lambda^c). The dual metric
g^{ab}=V^{-2} (G^{-1})^{ab} is Kaehler and it also defines a flat potential as
the original metric. Such geometries and some of their extensions find
applications in Type IIB compactifications on Calabi--Yau orientifolds.Comment: 9 pages, LaTeX, typos corrected, comments adde
Probing the field-induced variation of the chemical potential in Bi(2)Sr(2)CaCu(2)O(y) via the magneto-thermopower measurements
Approximating the shape of the measured in
magneto-thermopower (TEP) by asymmetric linear triangle of the
form with positive and defined below and above , we observe that . In order to account for this asymmetry, we
explicitly introduce the field-dependent chemical potential of holes
into the Ginzburg-Landau theory and calculate both an average and fluctuation contributions to the total
magneto-TEP . As a result, we find a rather simple relationship
between the field-induced variation of the chemical potential in this material
and the above-mentioned magneto-TEP data around , viz. .Comment: REVTEX (epsf), 4 pages, 2 PS figures; to be published in JET
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