12,665 research outputs found
The dimer-RVB State of the Four-Leg Heisenberg Ladder: Interference among Resonances
We study the ground state of the 4-leg spin ladder using a dimer-RVB ansatz
and the Lanczos method. Besides the well known resonance mechanism between
valence bond configurations we find novel interference effects among nearby
resonances.Comment: 4 pages, RevTex, 7 eps fig
The Matrix Product Approach to Quantum Spin Ladders
We present a manifestly rotational invariant formulation of the matrix
product method valid for spin chains and ladders. We apply it to 2 legged spin
ladders with spins 1/2, 1 and 3/2 and different magnetic structures labelled by
the exchange coupling constants, which can be ferromagnetic or
antiferromagnetic along the legs and the rungs of the ladder We compute ground
state energy densities, correlation lengths and string order parameters. We
present numerical evidence of the duality properties of the 3 different non
ferromagnetic spin 1/2 ladders. We show that the long range topological order
characteristic of isolated spin 1 chains is broken by the interchain coupling.
The string order correlation function decays exponentially with a finite
correlation length that we compute. A physical picture of the spin 1 ladder is
given in terms of a collection of resonating spin 1 chains. Finally for ladders
with spin equal or greater than 3/2 we define a class of AKLT states whose
matrix product coefficients are given by 9-j symbols.Comment: REVTEX file, 16 pages, 12 figures, 6 Table
Universality Classes of Diagonal Quantum Spin Ladders
We find the classification of diagonal spin ladders depending on a
characteristic integer in terms of ferrimagnetic, gapped and critical
phases. We use the finite algorithm DMRG, non-linear sigma model and
bosonization techniques to prove our results. We find stoichiometric contents
in cuprate planes that allow for the existence of weakly interacting
diagonal ladders.Comment: REVTEX4 file, 3 color figures, 1 tabl
Interactions and thermoelectric effects in a parallel-coupled double quantum dot
We investigate the nonequilibrium transport properties of a double quantum
dot system connected in parallel to two leads, including intradot
electron-electron interaction. In the absence of interactions the system
supports a bound state in the continuum. This state is revealed as a Fano
antiresonance in the transmission when the energy levels of the dots are
detuned. Using the Keldysh nonequilibrium Green's function formalism, we find
that the occurrence of the Fano antiresonance survives in the presence of
Coulomb repulsion. We give precise predictions for the experimental detection
of bound states in the continuum. First, we calculate the differential
conductance as a function of the applied voltage and the dot level detuning and
find that crossing points in the diamond structure are revealed as minima due
to the transmission antiresonances. Second, we determine the thermoelectric
current in response to an applied temperature bias. In the linear regime,
quantum interference gives rise to sharp peaks in the thermoelectric
conductance. Remarkably, we find interaction induced strong current
nonlinearities for large thermal gradients that may lead to several nontrivial
zeros in the thermocurrent. The latter property is especially attractive for
thermoelectric applications.Comment: 9 pages, 8 figure
Dualities in Spin Ladders
We introduce a set of discrete modular transformations and
in order to study the relationships between the different phases of
the Heisenberg ladders obtained with all possible exchange coupling constants.
For the 2 legged ladder we show that the phase is invariant under the
transformation, while the Haldane phase is invariant under .
These two phases are related by . Moreover there is a "mixed" phase,
that is invariant under , and which under becomes the RVB
phase, while under becomes the Haldane phase. For odd ladders there
exists only the transformation which, for strong coupling, maps the
effective antiferromagnetic spin 1/2 chain into the spin 3/2 chain.Comment: REVTEX file, 5 pages, 2 EPS figure
Critical Lines and Massive Phases in Quantum Spin Ladders with Dimerization
We determine the existence of critical lines in dimerized quantum spin
ladders in their phase diagram of coupling constants using the finite-size DMRG
algorithm. We consider both staggered and columnar dimerization patterns, and
antiferromagnetic and ferromagnetic inter-leg couplings. The existence of
critical phases depends on the precise combination of these patterns. The
nature of the massive phases separating the critical lines are characterized
with generalized string order parameters that determine their valence bond
solid (VBS) content.Comment: 9 pages 10 figure
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