465 research outputs found
Coupled Heisenberg antiferromagnetic chains in an effective staggered field
We present a systematic study of coupled Heisenberg antiferromagnetic
chains in an effective staggered field. We investigate several effects of the
staggered field in the {\em higher} ({\em two or three}) {\em dimensional} spin
system analytically. In particular, in the case where the staggered field and
the inter-chain interaction compete with each other, we predict, using
mean-field theory, a characteristic phase transition. The spin-wave theory
predicts that the behavior of the gaps induced by the staggered field is
different between the competitive case and the non-competitive case. When the
inter-chain interactions are sufficiently weak, we can improve the mean-field
phase diagram by using chain mean-field theory and the analytical results of
field theories. The ordered phase region predicted by the chain mean-field
theory is substantially smaller than that by the mean-field theory.Comment: 13pages, 12figures, to be published in PR
Frustration of decoherence in -shaped superconducting Josephson networks
We examine the possibility that pertinent impurities in a condensed matter
system may help in designing quantum devices with enhanced coherent behaviors.
For this purpose, we analyze a field theory model describing Y- shaped
superconducting Josephson networks. We show that a new finite coupling stable
infrared fixed point emerges in its phase diagram; we then explicitly evidence
that, when engineered to operate near by this new fixed point, Y-shaped
networks support two-level quantum systems, for which the entanglement with the
environment is frustrated. We briefly address the potential relevance of this
result for engineering finite-size superconducting devices with enhanced
quantum coherence. Our approach uses boundary conformal field theory since it
naturally allows for a field-theoretical treatment of the phase slips
(instantons), describing the quantum tunneling between degenerate levels.Comment: 11 pages, 5 .eps figures; several changes in the presentation and in
the figures, upgraded reference
Theory of Low Temperature Electron Spin Resonance in Half-integer Spin Antiferromagnetic Chains
A theory of low temperature (T) electron spin resonance (ESR) in half-integer
spin antiferromagnetic chains is developed using field theory methods and
avoiding previous approximations. It is compared to experiments on Cu benzoate.
Power laws are predicted for the line-width broadening due to various types of
anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The
second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate
Neel order as previously claimed, but to correspond to a sine-Gordon "breather"
excitation.Comment: 4 pages, REVTEX, 3 PostScript figures embedded in tex
String Junctions and Holographic Interfaces
In this paper we study half-BPS type IIB supergravity solutions with multiple
asymptotic regions, where is either or
. These solutions were first constructed in [1] and have geometries given
by the warped product of over , where
is a Riemann surface. We show that the holographic boundary has the
structure of a star graph, i.e. half-lines joined at a point. The attractor
mechanism and the relation of the solutions to junctions of self-dual strings
in six-dimensional supergravity are discussed. The solutions of [1] are
constructed introducing two meromorphic and two harmonic functions defined on
. We focus our analysis on solutions corresponding to junctions of
three different conformal field theories and show that the conditions for
having a solution charged only under Ramond-Ramond three-form fields reduce to
relations involving the positions of the poles and the residues of the relevant
harmonic and meromorphic functions. The degeneration limit in which some of the
poles collide is analyzed in detail. Finally, we calculate the holographic
boundary entropy for a junction of three CFTs and obtain a simple expression in
terms of poles and residues.Comment: 54 pages, 6 figures, pdf-LaTeX, v2: minor change
Direct perturbation theory on the shift of Electron Spin Resonance
We formulate a direct and systematic perturbation theory on the shift of the
main paramagnetic peak in Electron Spin Resonance, and derive a general
expression up to second order. It is applied to one-dimensional XXZ and
transverse Ising models in the high field limit, to obtain explicit results
including the polarization dependence for arbitrary temperature.Comment: 5 pages (no figures) in REVTE
The Origin of Magnetic Interactions in Ca3Co2O6
We investigate the microscopic origin of the ferromagnetic and
antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt
compound Ca3Co2O6. In particular, we establish a local model which stabilizes a
ferromagnetic alignment of the S=2 spins on the cobalt sites with trigonal
prismatic symmetry, for a sufficiently strong Hund's rule coupling on the
cobalt ions. The exchange is mediated through a S=0 cobalt ion at the
octahedral sites of the chain structure. We present a strong coupling
evaluation of the Heisenberg coupling between the S=2 Co spins on a separate
chain. The chains are coupled antiferromagnetically through super-superexchange
via short O-O bonds.Comment: 5 Pages, 3 Figures; added anisotropy term in eq. 9; extended
discussion of phase transitio
Metallic Ferromagnetism in the Kondo Lattice
Metallic magnetism is both ancient and modern, occurring in such familiar
settings as the lodestone in compass needles and the hard drive in computers.
Surprisingly, a rigorous theoretical basis for metallic ferromagnetism is still
largely missing. The Stoner approach perturbatively treates Coulomb
interactions when the latter need to be large, while the Nagaoka approach
incorporates thermodynamically negligible electrons into a half-filled band.
Here, we show that the ferromagnetic order of the Kondo lattice is amenable to
an asymptotically exact analysis over a range of interaction parameters. In
this ferromagnetic phase, the conduction electrons and local moments are
strongly coupled but the Fermi surface does not enclose the latter (i.e. it is
small). Moreover, non-Fermi liquid behavior appears over a range of frequencies
and temperatures. Our results provide the basis to understand some
long-standing puzzles in the ferromagnetic heavy fermion metals, and raises the
prospect for a new class of ferromagnetic quantum phase transitions.Comment: 21 pages, 9 figures, including Supporting Informatio
Pairing of Cooper pairs in a Josephson junction network containing an impurity
We show how to induce pairing of Cooper pairs (and, thus,
superconductivity) as a result of local embedding of a quantum impurity in a
Josephson network fabricable with conventional junctions. We find that a
boundary double Sine-Gordon model provides an accurate description of the dc
Josephson current patterns, as well as of the stable phases accessible to the
network. We point out that tunneling of pairs of Cooper pairs is robust against
quantum fluctuations, as a consequence of the time reversal invariance, arising
when the central region of the network is pierced by a dimensionless magnetic
flux . We find that, for , a stable attractive finite
coupling fixed point emerges and point out its relevance for engineering a two
level quantum system with enhanced coherence.Comment: 5 Pages, 5 Figures. Small modifications, ref.[11] added. To appear in
EP
Power dissipation for systems with junctions of multiple quantum wires
We study power dissipation for systems of multiple quantum wires meeting at a
junction, in terms of a current splitting matrix (M) describing the junction.
We present a unified framework for studying dissipation for wires with either
interacting electrons (i.e., Tomonaga-Luttinger liquid wires with Fermi liquid
leads) or non-interacting electrons. We show that for a given matrix M, the
eigenvalues of M^T M characterize the dissipation, and the eigenvectors
identify the combinations of bias voltages which need to be applied to the
different wires in order to maximize the dissipation associated with the
junction. We use our analysis to propose and study some microscopic models of a
dissipative junction which employ the edge states of a quantum Hall liquid.
These models realize some specific forms of the M-matrix whose entries depends
on the tunneling amplitudes between the different edges.Comment: 9 pages, 4 figures; made several minor changes; this is the published
versio
Duality between normal and superconducting junctions of multiple quantum wires
We study junctions of single-channel spinless Luttinger liquids using
bosonisation. We generalize earlier studies by allowing the junction to be
superconducting and find new charge non-conserving low energy fixed points. We
establish the existence of duality (where is the
Luttinger Liquid parameter) between the charge conserving (normal) junction and
the charge non-conserving (superconducting) junction by evaluating and
comparing the scaling dimensions of various operators around the fixed points
in normal and superconducting sectors of the theory. For the most general
two-wire junction, we show that there are two conformally invariant
one-parameter families of fixed points which are also connected by a duality
transformation. We also show that the stable fixed point for the two-wire
superconducting junction corresponds to the situation where the crossed Andreev
reflection is perfect between the wires. For the three-wire junction, we study,
in particular, the superconducting analogs of the chiral, and the
disconnected fixed points obtained earlier in the literature in the context of
charge conserving three-wire junctions. We show that these fixed points can be
stabilized for (repulsive electrons) within the superconducting sector
of the theory which makes them experimentally relevant.Comment: Figures added. Final version to appear in Phys. Rev.
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