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 $\rho_{zz}$ in quasi two-dimensional metals for a magnetic field perpendicular to the layers. Giant Shubnikov-de Haas oscillations are found when the cyclotron energy $\hbar \omega_{c}$ is much larger than the interlayer transfer integral $t$ (the two-dimensional limit). In large magnetic fields and at low temperatures, the minima of the magnetoconductivity $\sigma_{zz}=\rho_{zz}^{-1}$ 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 $d=4$, 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 $G_{2,2}/SU(2)\times SU(2)$ 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 $(A,B_\star,\phi)$, where $A=\{A^s|s\in S\}$ are families of commutative associative Frobenius algebras, $B_\star = \bigoplus_{\sigma\in\Sigma} B_\sigma$ is an graduated by graphes, associative algebras of Frobenius type and $\phi=\{\phi_\sigma^s: A^s\to (B_\sigma)|s\in S,\sigma\in \Sigma\}$ 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 $Bi_2Sr_2CaCu_2O_y$ magneto-thermopower (TEP) $\Delta S(T,H)$ by asymmetric linear triangle of the form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T_c-T)$ with positive $B ^{-}(H)$ and $B ^{+}(H)$ defined below and above $T_c$, we observe that $B ^{+}(H)\simeq 2B ^{-}(H)$. In order to account for this asymmetry, we explicitly introduce the field-dependent chemical potential of holes $\mu (H)$ into the Ginzburg-Landau theory and calculate both an average $\Delta S_{av}(T,H)$ and fluctuation $\Delta S_{fl}(T,H)$ contributions to the total magneto-TEP $\Delta S(T,H)$. 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 $T_c$, viz. $\Delta \mu (H)\propto S_p(H)$.Comment: REVTEX (epsf), 4 pages, 2 PS figures; to be published in JET