2,168 research outputs found
The Spin-SAF transition in NaV2O5 induced by spin-pseudospin coupling
We present microscopic estimates for the spin-spin and spin-speudospin
interactions of the quarter-filled ladder compound NaV2O5, obtained by exactly
diagonalizing appropriate clusters of the underlying generalized Hubbard
Hamiltonian. We present evidence for a substantial interladder spin-pseudospin
interaction term which would allow simultaneously for the
superantiferroelectric (SAF) charge (pseudospin) ordering and spin
dimerization. We discuss the values of the coupling constants appropriate for
NaV2O5 and deduce the absence of a soft antiferroelectric mode
Semiclassical Particle Spectrum of Double Sine-Gordon Model
We present new theoretical results on the spectrum of the quantum field
theory of the Double Sine Gordon model. This non-integrable model displays
different varieties of kink excitations and bound states thereof. Their mass
can be obtained by using a semiclassical expression of the matrix elements of
the local fields. In certain regions of the coupling-constants space the
semiclassical method provides a picture which is complementary to the one of
the Form Factor Perturbation Theory, since the two techniques give information
about the mass of different types of excitations. In other regions the two
methods are comparable, since they describe the same kind of particles.
Furthermore, the semiclassical picture is particularly suited to describe the
phenomenon of false vacuum decay, and it also accounts in a natural way the
presence of resonance states and the occurrence of a phase transition.Comment: 32 pages, latex, 8 figure
Unity of Supersymmetry Breaking Models
We examine the models with gauge group U(1)^{k-1}\times\prod_{i=1}^k SU(n_i),
which are obtained from decomposing the supersymmetry breaking model of
Affleck, Dine and Seiberg containing an antisymmetric tensor field. We note
that all of these models are distinct vacua of a single SU(N) gauge theory with
an adjoint superfield. The dynamics of this model may be analyzed using the
duality of Kutasov and Schwimmer and the deconfinement trick of Berkooz. This
analysis leads to a simple picture for supersymmetry breaking for k=2,
complementing that of previous work. We examine the flat directions of these
models, and give straightforward criteria for lifting them, explaining the
requisite peculiar form of the superpotential. For all cases with k>2, the
duality argument fails to give supersymmetry breaking dynamics, and we identify
a class of problematic flat directions, which we term 2m-baryons. We study in
some detail the requirements for lifting these directions, and uncover some
surprising facts regarding the relationship between R-symmetry and
supersymmetry breaking in models with several gauge groups.Comment: harvmac, 40 page
Non-Fermi liquid behavior in Kondo models
Despite the fact that the low energy behavior of the basic Kondo model cannot
be studied perturbatively it was eventually shown by Wilson, Anderson, Nozieres
and others to have a simple "local Fermi liquid theory" description. That is,
electronic degrees of freedom become effectively non-interacting in the zero
energy limit. However, generalized versions of the Kondo model involving more
than one channel or impurity may exhibit low energy behavior of a less trivial
sort which can, nonetheless, be solved exactly using either Bethe ansatz or
conformal field theory and bosonization techniques. Now the low energy limit
exhibits interacting many body behavior. For example, processes in which a
single electron scatters off the impurity into a multi electron-hole state have
a non-vanishing (and sometimes large) amplitude at zero energy. This
corresponds to a rare solveable example of non-Fermi liquid behavior. Essential
features of these phenomena are reviewed.Comment: A brief review submitted to the special issue of J. Phys. Soc. of
Japan, "Kondo effect -- 40 years after the discovery
Junctions of three quantum wires and the dissipative Hofstadter model
We study a junction of three quantum wires enclosing a magnetic flux. This is
the simplest problem of a quantum junction between Tomonaga-Luttinger liquids
in which Fermi statistics enter in a non-trivial way. We present a direct
connection between this problem and the dissipative Hofstadter problem, or
quantum Brownian motion in two dimensions in a periodic potential and an
external magnetic field, which in turn is connected to open string theory in a
background electromagnetic field. We find non-trivial fixed points
corresponding to a chiral conductance tensor leading to an asymmetric flow of
the current.Comment: 4 pages, 1 figur
Vortex Penetration into a Type II Superconductor due to a Mesoscopic External Current
Applying the London theory we study curved vortices produced by an external
current near and parallel to the surface of a type II superconductor. By
minimizing the energy functional we find the contour describing the hard core
of the flux line, and predict the threshold current for entrance of the first
vortex. We assume that the vortex entrance is allowed due to surface defects,
despite the Bean-Livingston barrier. Compared to the usual situation with a
homogeneous magnetic field, the main effect of the present geometry is that
larger magnetic fields can be applied locally before vortices enter the
superconducting sample. It is argued that this effect can be further enhanced
in anisotropic superconductors.Comment: 9 pages, 14 figure
Phase diagram of a 1 dimensional spin-orbital model
We study a 1 dimensional spin-orbital model using both analytical and
numerical methods. Renormalization group calculations are performed in the
vicinity of a special integrable point in the phase diagram with SU(4)
symmetry. These indicate the existence of a gapless phase in an extended region
of the phase diagram, missed in previous studies. This phase is SU(4) invariant
at low energies apart from the presence of different velocities for spin and
orbital degrees of freedom. The phase transition into a gapped dimerized phase
is in a generalized Kosterlitz-Thouless universality class. The phase diagram
of this model is sketched using the density matrix renormalization group
technique.Comment: 11 pages, 5 figures, new references adde
Solitonic excitations in the Haldane phase of a S=1 chain
We study low-lying excitations in the 1D antiferromagnetic
valence-bond-solid (VBS) model. In a numerical calculation on finite systems
the lowest excitations are found to form a discrete triplet branch, separated
from the higher-lying continuum. The dispersion of these triplet excitations
can be satisfactorily reproduced by assuming approximate wave functions. These
wave functions are shown to correspond to moving hidden domain walls, i.e. to
one-soliton excitations.Comment: RevTex 3.0, 24 pages, 2 figures on request by fax or mai
Bosonization in the two-channel Kondo model
The bosonization of the anisotropic two-channel Kondo model is shown
to yield two equivalent representations of the original problem. In a straight
forward extension of the Emery-Kivelson approach, the interacting resonant
level model previously derived by the Anderson-Yuval technique is obtained. In
addition, however, a ``(,)'' description is also found. The
strong coupling fixed point of the (,) model was originally
postulated to be related to the intermediate coupling fixed point of the
two-channel Kondo model. The equivalence of the , model to the
two-channel Kondo model is formally established. A summary of what one may
learn from a simple study of these different representations is also given.Comment: 5 pages, latex (uses revtex and epsf macros) with 1 postscript figur
N\'eel and disordered phases of coupled Heisenberg chains with to S=4
We use the two-step density-matrix renormalization group method to study the
effects of frustration in Heisenberg models for to S=4 in a
two-dimensional anisotropic lattice. We find that as in studied
previously, the system is made of nearly disconnected chains at the maximally
frustrated point, , i.e., the transverse spin-spin
correlations decay exponentially. This leads to the following consequences: (i)
all half-integer spins systems are gapless, behaving like a sliding Luttinger
liquid as in ; (ii) for integer spins, there is an intermediate
disordered phase with a spin gap, with the width of the disordered state is
roughly proportional to the 1D Haldane gap.Comment: 13 pages, 22 figure
- …