4,043 research outputs found
Andreev Tunneling in Strongly Interacting Quantum Dots
We review recent work on resonant Andreev tunneling through a strongly
interacting quantum dot connected to a normal and to a superconducting lead. We
derive a general expression for the current flowing in the structure and
discuss the linear and non-linear transport in the nonperturbative regime. New
effects associated to the Kondo resonance combined with the two-particle
tunneling arise. The Kondo anomaly in the characteristics depends on the
relative size of the gap energy and the Kondo temperature.Comment: 8 pages, 4 figures; submitted to Superlattices and Microstructure
Non-magnetic impurities in two- and three- dimensional Heisenberg antiferromagnets
In this paper we study in a large-S expansion effects of substituting spins
by non-magnetic impurities in two- and three- dimensional Heisenberg
antiferromagnets in a weak magnetic field. In particular, we demonstrate a
novel mechanism where magnetic moments are induced around non-magnetic
impurities when magnetic field is present. As a result, Curie-type behaviour in
magnetic susceptibility can be observed well below the Neel temperature, in
agreement with what is being observed in and
compounds.Comment: Latex fil
Topological spin excitations of Heisenberg antiferromagnets in two dimensions
In this paper we discuss the construction and the dynamics of vortex-like
topological spin excitations in the Schwinger-boson description of Heisenberg
antiferromagnets in two dimensions. The topological spin excitations are Dirac
fermions (with gap) when spin value is a half-integer. Experimental and
theoretical implications of these excitations are being investigated.Comment: Latex file, no figur
Topological term in the non-linear model of the SO(5) spin chains
We show that there is a topological (Berry phase) term in the non-linear
model description of the SO(5) spin chain. It distinguishes the linear
and projective representations of the SO(5) symmetry group, in exact analogy to
the well-known -term of the SO(3) spin chain. The presence of the
topological term is due to the fact that . We discuss the implication of our results on the spectra
of the SO(5) spin chain, and connect it with a recent solvable SO(5) spin model
which exhibits valence bond solid ground state and edge degeneracy.Comment: 12 pages, 1 figure; the publication versio
Andreev tunnelling in quantum dots: A slave-boson approach
We study a strongly interacting quantum dot connected to a normal and to a
superconducting lead. By means of the slave-boson technique we investigate the
low temperature regime and discuss electrical transport through the dot. We
find that the zero bias anomaly in the current-voltage characteristics which is
associated to the occurance of the Kondo resonance in the quantum dot, is
enhanced in the presence of superconductivity, due to resonant Andreev
scattering.Comment: 4 pages, 1 figur
Tristability in a non-equilibrium double-quantum-dot in Kondo regime
Electron tunneling through a non-equilibrium double quantum dot in the Kondo
regime is studied. In the region of negative differential resistance, it is
shown that this system possesses a complex response to the applied potential
characterized by a tristable solution for the current. Increasing the applied
potential or reducing the inter-dot coupling, the system goes through a
transition from a coherent inter-dot regime to an incoherent one. The different
nature of the solutions are characterized and it is shown that the effects of
the asymmetry in the dot-lead coupling can be used to control the region of
multistability. The mean-field slave-boson formalism is used to obtain the
solution of the problem.Comment: 4 pages, 4 figures. To appear in Sol. State. Com
Edge states and conformal boundary conditions in super spin chains and super sigma models
The sigma models on projective superspaces CP^{N+M-1|N} with topological
angle theta=pi mod 2pi flow to non-unitary, logarithmic conformal field
theories in the low-energy limit. In this paper, we determine the exact
spectrum of these theories for all open boundary conditions preserving the full
global symmetry of the model, generalizing recent work on the particular case
M=0 [C. Candu et al, JHEP02(2010)015]. In the sigma model setting, these
boundary conditions are associated with complex line bundles, and are labelled
by an integer, related with the exact value of theta. Our approach relies on a
spin chain regularization, where the boundary conditions now correspond to the
introduction of additional edge states. The exact values of the exponents then
follow from a lengthy algebraic analysis, a reformulation of the spin chain in
terms of crossing and non-crossing loops (represented as a certain subalgebra
of the Brauer algebra), and earlier results on the so-called one- and
two-boundary Temperley Lieb algebras (also known as blob algebras). A
remarkable result is that the exponents, in general, turn out to be irrational.
The case M=1 has direct applications to the spin quantum Hall effect, which
will be discussed in a sequel.Comment: 50 pages, 18 figure
Chiral Correction to the Spin Fluctuation Feedback in two-dimensional p-wave Superconductors
We consider the stability of the superconducting phase for spin-triplet
p-wave pairing in a quasi-two-dimensional system. We show that in the absence
of spin-orbit coupling there is a chiral contribution to spin fluctuation
feedback which is related to spin quantum Hall effect in a chiral
superconducting phase. We show that this mechanism supports the stability of a
chiral p-wave state.Comment: 8 pages. The final version is accepted for publication in Europhys
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Supersymmetry in models with strong on-site Coulomb repulsion - application to t-J model
A supersymmetric way of imposing the constraint of no double occupancy in
models with strong on-site Coulomb repulsion is presented in this paper. In
this formulation the physical operators in the constrainted Hilbert space are
invariant under local unitary transformations mixing boson and fermion
representations. As an illustration the formulation is applied to the
model. The model is studied in the mean-field level in the J=0 limit where we
show how both the slave-boson and slave-fermion formulations are included
naturally in the present approach and how further results beyond both
approaches are obtained.Comment: 12 pages, Latex file, 1 figur
Vortices in Schwinger-Boson Mean-Field Theory of Two-Dimensional Quantum Antiferromagnets
In this paper we study the properties of vortices in two dimensional quantum
antiferromagnets with spin magnitude S on a square lattice within the framework
of Schwinger-boson mean field theory. Based on a continuum description, we show
that vortices are stable topological excitations in the disordered state of
quantum antiferromagnets. Furthermore, we argue that vortices can be divided
into two kinds: the first kind always carries zero angular momentum and are
bosons, whereas the second kind carries angular momentum S under favourable
conditions and are fermions if S is half-integer. A plausible consequence of
our results relating to RVB theories of High-Tc superconductors is pointed out.Comment: 25 page
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