109 research outputs found
Exact ground states of spin-2 chains
We use the matrix product approach to construct all optimum ground states of
general anisotropic spin-2 chains with nearest neighbour interactions and
common symmetries. These states are exact ground states of the model and their
properties depend on up to three parameters. We find three different
antiferromagnetic Haldane phases, one weak antiferromagnetic and one weak
ferromagnetic phase. The antiferromagnetic phases can be described as spin
liquids with exponentially decaying correlation functions. The variety of
phases found with the matrix product ansatz also gives insight into the
behaviour of spin chains with arbitrary higher spins.Comment: 7 pages, 2 figures, to be published in europhysics letters, uses
epl.cl
Andreev Bound States in High Temperature Superconductors
Andreev bound states (ABS) at the surface of superconductors are expected for
any pair potential showing a sign change in different k-directions with their
spectral weight depending on the relative orientation of the surface and the
pair potential. We report on the observation of ABS in HTS employing tunneling
spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The
tunneling spectra were studied as a function of temperature and applied
magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO
show a pronounced zero bias conductance peak that can be interpreted in terms
of Andreev bound states at zero energy that are expected at the surface of HTS
having a d-wave symmetry of the order parameter. In contrast, for the most
likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a
magnetic field results in a shift of spectral weight from zero to finite
energy. This shift is found to depend nonlinearly on the applied magnetic
field. Further consequences of the Andreev bound states are discussed and
experimental evidence for anomalous Meissner currents is presented.Comment: 17 pages, 10 figures, to appear in Eur. Phys. J.
Nucleation of superconducting pairing states at mesoscopic scales at zero temperature
We find the spin polarized disordered Fermi liquids are unstable to the
nucleation of superconducting pairing states at mesoscopic scales even when
magnetic fields which polarize the spins are substantially higher than the
critical one. We study the probability of finding superconducting pairing
states at mesoscopic scales in this limit. We find that the distribution
function depends only on the film conductance. The typical length scale at
which pairing takes place is universal, and decreases when the magnetic field
is increased. The number density of these states determines the strength of the
random exchange interactions between mesoscopic pairing states.Comment: 11 pages, no figure
Matrix-product-groundstates for one-dimensional spin-1 quantum antiferromagnets
We have found the exact groundstate for a large class of antiferromagnetic
spin-1 models with nearest-neighbour interactions on a linear chain. All
groundstate properties can be calculated. The groundstate is determined as a
matrix product of individual site states and has the properties of the Haldane
scenario.Comment: 8 pages (plain tex), preprint cologne-93-471
Spin-flip Effects in the Mesoscopic Spin-Interferometer
We investigate the properties of the electron spin-transmission through an
Aharonov-Bohm interferometer with an embedded multilevel quantum dot containing
magnetic impurities. A suitable formalism is developed. The amplitude and the
phase of the flip- and nonflip-transmittance are calculated numerically as
function of the magnetic field and the gate potential applied on the dot. The
effects induced by the exchange interaction to spin-dependent
magnetoconductance fluctuations and transmittance phase are shown.Comment: 10 pages, 9 figure
Quantum and Thermal Depinning of a String from a Linear Defect
The problem of a massive elastic string depinning from a linear defect under
the action of a small driving force is considered. To exponential accuracy the
decay rate is calculated with the help of the instanton method; then,
fluctuations of the quasiclassical solution are taken into account to determine
the preexponential factor. The decay rate exhibits a kind of first order
transition from quantum tunneling to thermal activation with vanishing
crossover region. The model may be applied to describe nucleation in
2-dimensional first order quantum phase transitions.Comment: Revtex. 11 pages + 4 PS figures. Accepted for publication in PR
Pair correlation functions in one-dimensional correlated-hopping models
We investigate ground-state properties of two correlated-hopping electron
models, the Hirsch and the Bariev model. Both models are of recent interest in
the context of hole superconductivity. Applying the Lanczos technique to small
clusters, we numerically determine the binding energy, the spin gaps,
correlation functions, and other properties for various values of the
bond-charge interaction parameter. Our results for small systems indicate that
pairing is favoured in a certain parameter range. However, in contrast to the
Bariev model, superconducting correlations are suppressed in the Hirsch model,
for a bond-charge repulsion larger than a critical value.Comment: 7 pages (LaTeX) + 6 postcript figures in a separate uuencoded fil
Gap States in Dilute Magnetic Alloy Superconductors
We study states in the superconducting gap induced by magnetic impurities
using self-consistent quantum Monte Carlo with maximum entropy and formally
exact analytic continuation methods. The magnetic impurity susceptibility has
different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0}
(: Kondo temperature, : superconducting transition temperature)
due to the crossover between a doublet and a singlet ground state. We
systematically study the location and the weight of the gap states and the gap
parameter as a function of and the concentration of the
impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure
Transfer-matrix DMRG for stochastic models: The Domany-Kinzel cellular automaton
We apply the transfer-matrix DMRG (TMRG) to a stochastic model, the
Domany-Kinzel cellular automaton, which exhibits a non-equilibrium phase
transition in the directed percolation universality class. Estimates for the
stochastic time evolution, phase boundaries and critical exponents can be
obtained with high precision. This is possible using only modest numerical
effort since the thermodynamic limit can be taken analytically in our approach.
We also point out further advantages of the TMRG over other numerical
approaches, such as classical DMRG or Monte-Carlo simulations.Comment: 9 pages, 9 figures, uses IOP styl
Loss of Pi-Junction Behaviour in an Interacting Impurity Josephson Junction
Using a generalization of the non-crossing approximation which incorporates
Andreev reflection, we study the properties of an infinite-U Anderson impurity
coupled to two superconducting leads. In the regime where and
are comparable, we find that the position of the sub-gap resonance in the
impurity spectral function develops a strong anomalous phase dependence-- its
energy is a minimum when the phase difference between the superconductors is
equal to . Calculating the Josephson current through the impurity, we find
that -junction behaviour is lost as the position of the bound-state moves
above the Fermi energy.Comment: 4 pages, 4 figures; labelling of Fig. 3 corrected; final published
form, only trivial change
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