2,171 research outputs found
Structural precursor to the metal-insulator transition in V_2O_3
The temperature dependence of the local structure of V_2O_3 in the vicinity
of the metal to insulator transition (MIT) has been investigated using hard
X-ray absorption spectroscopy. It is shown that the vanadium pair distance
along the hexagonal c-axis changes abruptly at the MIT as expected. However, a
continuous increase of the tilt of these pairs sets in already at higher
temperatures and reaches its maximum value at the onset of the electronic and
magnetic transition. These findings confirm recent theoretical results which
claim that electron-lattice coupling is important for the MIT in V_2O_3. Our
results suggest that interactions in the basal plane play a decisive role for
the MIT and orbital degrees of freedom drive the MIT via changes in
hybridization.Comment: 6 pages, 5 figures, 2 table
Mirror Symmetry, Mirror Map and Applications to Calabi-Yau Hypersurfaces
Mirror Symmetry, Picard-Fuchs equations and instanton corrected Yukawa
couplings are discussed within the framework of toric geometry. It allows to
establish mirror symmetry of Calabi-Yau spaces for which the mirror manifold
had been unavailable in previous constructions. Mirror maps and Yukawa
couplings are explicitly given for several examples with two and three moduli.Comment: 59 pages. Some changes in the references, a few minor points have
been clarifie
A Search for Non-Perturbative Dualities of Local Yang--Mills Theories from Calabi--Yau Threefolds
The generalisation of the rigid special geometry of the vector multiplet
quantum moduli space to the case of supergravity is discussed through the
notion of a dynamical Calabi--Yau threefold. Duality symmetries of this
manifold are connected with the analogous dualities associated with the
dynamical Riemann surface of the rigid theory. N=2 rigid gauge theories are
reviewed in a framework ready for comparison with the local case. As a
byproduct we give in general the full duality group (quantum monodromy) for an
arbitrary rigid gauge theory, extending previous explicit
constructions for the cases. In the coupling to gravity, R--symmetry
and monodromy groups of the dynamical Riemann surface, whose structure we
discuss in detail, are embedded into the symplectic duality group
associated with the moduli space of the dynamical Calabi--Yau threefold.Comment: Latex. Version of previous paper with enlarged and revised appendix
35 pages, plain LaTe
Clustering properties of a generalised critical Euclidean network
Many real-world networks exhibit scale-free feature, have a small diameter
and a high clustering tendency. We have studied the properties of a growing
network, which has all these features, in which an incoming node is connected
to its th predecessor of degree with a link of length using a
probability proportional to . For , the
network is scale free at with the degree distribution and as in the Barab\'asi-Albert model (). We find a phase boundary in the plane along which
the network is scale-free. Interestingly, we find scale-free behaviour even for
for where the existence of a new universality class
is indicated from the behaviour of the degree distribution and the clustering
coefficients. The network has a small diameter in the entire scale-free region.
The clustering coefficients emulate the behaviour of most real networks for
increasing negative values of on the phase boundary.Comment: 4 pages REVTEX, 4 figure
Time Correlation Functions of Three Classical Heisenberg Spins on an Isosceles Triangle and on a Chain: Strong Effects of Broken Symmetry
At arbitrary temperature , we solve for the dynamics of single molecule
magnets composed of three classical Heisenberg spins either on a chain with two
equal exchange constants , or on an isosceles triangle with a third,
different exchange constant . As T\rightrarrow\infty, the Fourier
transforms and long-time asymptotic behaviors of the two-spin time correlation
functions are evaluated exactly. The lack of translational symmetry on a chain
or an isosceles triangle yields time correlation functions that differ
strikingly from those on an equilateral trinagle with . At low ,
the Fourier transforms of the two autocorrelation functions with
show one and four modes, respectively. For a semi-infinite range, one
mode is a central peak. At the origin of this range, this mode has a novel
scaling form.Comment: 9 pages, 14 figures, accepted for publication in Phys. Rev.
GKZ-Generalized Hypergeometric Systems in Mirror Symmetry of Calabi-Yau Hypersurfaces
We present a detailed study of the generalized hypergeometric system
introduced by Gel'fand, Kapranov and Zelevinski (GKZ-hypergeometric system) in
the context of toric geometry. GKZ systems arise naturally in the moduli theory
of Calabi-Yau toric varieties, and play an important role in applications of
the mirror symmetry. We find that the Gr\"obner basis for the so-called toric
ideal determines a finite set of differential operators for the local solutions
of the GKZ system. At the special point called the large radius limit, we find
a close relationship between the principal parts of the operators in the GKZ
system and the intersection ring of a toric variety. As applications, we
analyze general three dimensional hypersurfaces of Fermat and non-Fermat types
with Hodge numbers up to . We also find and analyze several non
Landau-Ginzburg models which are related to singular models.Comment: 55 pages, 3 Postscript figures, harvma
-cofiniteness of 2-cyclic permutation orbifold models
In this article, we consider permutation orbifold models of -cofinite
vertex operator algebras of CFT type. We show the -cofiniteness of the
2-cyclic permutation orbifold model for an arbitrary
-cofinite simple vertex operator algebra of CFT type. We also give a
proof of the -cofiniteness of a -orbifold model of the
lattice vertex operator algebra associated with a rank one positive
definite even lattice by using our result and the -cofiniteness of
.Comment: 25 pages, no figure, some typo are correcte
The rigid limit in Special Kahler geometry; From K3-fibrations to Special Riemann surfaces: a detailed case study
The limiting procedure of special Kahler manifolds to their rigid limit is
studied for moduli spaces of Calabi-Yau manifolds in the neighbourhood of
certain singularities. In two examples we consider all the periods in and
around the rigid limit, identifying the nontrivial ones in the limit as periods
of a meromorphic form on the relevant Riemann surfaces. We show how the Kahler
potential of the special Kahler manifold reduces to that of a rigid special
Kahler manifold. We extensively make use of the structure of these Calabi-Yau
manifolds as K3 fibrations, which is useful to obtain the periods even before
the K3 degenerates to an ALE manifold in the limit. We study various methods to
calculate the periods and their properties. The development of these methods is
an important step to obtain exact results from supergravity on Calabi-Yau
manifolds.Comment: 79 pages, 8 figures. LaTeX; typos corrected, version to appear in
Classical and Quantum Gravit
Gaussian Tunneling Model of c-Axis Twist Josephson Junctions
We calculate the critical current density for c-axis Josephson
tunneling between identical high temperature superconductors twisted an angle
about the c-axis. We model the tunneling matrix element squared as a
Gaussian in the change of wavevector q parallel to the junction, . The
obtained for the s- and extended-s-wave order parameters (OP's) are consistent
with the BiSrCaCuO data of Li {\it et al.}, but only
for strongly incoherent tunneling, . A -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 -wave.Comment: 6 pages, 6 figure
Muon-spin relaxation measurements on the dimerized spin-1/2 chains NaTiSi2O6 and TiOCl
We report muon spin relaxation (muSR) and magnetic susceptibility
investigations of two Ti3+ chain compounds which each exhibit a spin gap at low
temperature, NaTiSi2O6 and TiOCl. From these we conclude that the spin gap in
NaTiSi2O6 is temperature independent, with a value of 2*Delta=660(50)K, arising
from orbital ordering at Too = 210K; the associated structural fluctuations
activate the muon spin relaxation rate up to temperatures above 270K. In TiOCl
we find thermally activated spin fluctuations corresponding to a spin gap
2*Delta=420(40)K below Tc1=67K. We also compare the methods used to extract the
spin gap and the concentration of free spins within the samples from muSR and
magnetic susceptibility data.Comment: 4 pages, 3 figure
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