9,441 research outputs found
Phase Diagram of the One Dimensional model with Ferromagnetic nearest-neighbor and Antiferromagnetic next-nearest neighbor interactions
We have studied the phase diagram of the one dimensional model
with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest neighbor
interactions. We have applied the quantum renormalization group (QRG) approach
to get the stable fixed points and the running of coupling constants. The
second order QRG has been implemented to get the self similar Hamiltonian. This
model shows a rich phase diagram which consists of different phases which
possess the quantum spin-fluid and dimer phases in addition to the classical
N\'{e}el and ferromagnetic ones. The border between different phases has been
shown as a projection onto two different planes in the phase space
The elementary excitations of the exactly solvable Russian doll BCS model of superconductivity
The recently proposed Russian doll BCS model provides a simple example of a
many body system whose renormalization group analysis reveals the existence of
limit cycles in the running coupling constants of the model. The model was
first studied using RG, mean field and numerical methods showing the Russian
doll scaling of the spectrum, E(n) ~ E0 exp(-l n}, where l is the RG period. In
this paper we use the recently discovered exact solution of this model to study
the low energy spectrum. We find that, in addition to the standard
quasiparticles, the electrons can bind into Cooper pairs that are different
from those forming the condensate and with higher energy. These excited Cooper
pairs can be described by a quantum number Q which appears in the Bethe ansatz
equation and has a RG interpretation.Comment: 36 pages, 12 figure
Leptogenesis in the presence of exact flavor symmetries
In models with flavor symmetries in the leptonic sector leptogenesis can take
place in a very different way compared to the standard leptogenesis scenario.
We study the generation of a asymmetry in these kind of models in the
flavor symmetric phase pointing out that successful leptogenesis requires (i)
the right-handed neutrinos to lie in different representations of the flavor
group; (ii) the flavons to be lighter at least that one of the right-handed
neutrino representations. When these conditions are satisfied leptogenesis
proceeds due to new contributions to the CP violating asymmetry and -depending
on the specific model- in several stages. We demonstrate the validity of these
arguments by studying in detail the generation of the asymmetry in a
scenario of a concrete flavor model realization.Comment: 25 pages, 7 figures; version 2: A few clarifications added. Version
matches publication in JHE
Exact renormalization in quantum spin chains
We introduce a real-space exact renormalization group method to find exactly
solvable quantum spin chains and their ground states. This method allows us to
provide a complete list for exact solutions within SU(2) symmetric quantum spin
chains with and nearest-neighbor interactions, as well as examples
with S=5. We obtain two classes of solutions: One of them converges to the
fixed points of renormalization group and the ground states are matrix product
states. Another one does not have renormalization fixed points and the ground
states are partially ferromagnetic states.Comment: 8 pages, 5 figures, references added, published versio
Rigid N=2 superconformal hypermultiplets
We discuss superconformally invariant systems of hypermultiplets coupled to
gauge fields associated with target-space isometries.Comment: Invited talk given at the International Seminar "Supersymmetries and
Quantum Symmetries", July 1997, Dubna. Latex, 9 p
Recurrent Variational Approach to the Two-Leg Hubbard Ladder
We applied the Recurrent Variational Approach to the two-leg Hubbard ladder.
At half-filling, our variational Ansatz was a generalization of the resonating
valence bond state. At finite doping, hole pairs were allowed to move in the
resonating valence bond background. The results obtained by the Recurrent
Variational Approach were compared with results from Density Matrix
Renormalization Group.Comment: 10 pages, 14 Postscript figure
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