3,055 research outputs found
Chain breaks and the susceptibility of Sr_2Cu_{1-x}Pd_xO_{3+\delta} and other doped quasi one-dimensional antiferromagnets
We study the magnetic susceptibility of one-dimensional S=1/2
antiferromagnets containing non-magnetic impurities which cut the chain into
finite segments. For the susceptibility of long anisotropic Heisenberg
chain-segments with open boundaries we derive a parameter-free result at low
temperatures using field theory methods and the Bethe Ansatz. The analytical
result is verified by comparing with Quantum-Monte-Carlo calculations. We then
show that the partitioning of the chain into finite segments can explain the
Curie-like contribution observed in recent experiments on
Sr_2Cu_{1-x}Pd_xO_{3+\delta}. Possible additional paramagnetic impurities seem
to play only a minor role.Comment: 4 pages, 3 figures, final versio
Spin- and charge-density oscillations in spin chains and quantum wires
We analyze the spin- and charge-density oscillations near impurities in spin
chains and quantum wires. These so-called Friedel oscillations give detailed
information about the impurity and also about the interactions in the system.
The temperature dependence of these oscillations explicitly shows the
renormalization of backscattering and conductivity, which we analyze for a
number of different impurity models. We are also able to analyze screening
effects in one dimension. The relation to the Kondo effect and experimental
consequences are discussed.Comment: Final published version. 15 pages in revtex format including 22
epsf-embedded figures. The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/density-osc.pd
Universal cross-over behavior of a magnetic impurity and consequences for doping in spin-1/2 chains
We consider a magnetic impurity in the antiferromagnetic spin-1/2 chain which
is equivalent to the two-channel Kondo problem in terms of the field
theoretical description. Using a modification of the transfer-matrix density
matrix renormalization group (DMRG) we are able to determine local and global
properties in the thermodynamic limit. The cross-over function for the impurity
susceptibility is calculated over a large temperature range, which exhibits
universal data-collapse. We are also able to determine the local
susceptibilities near the impurity, which show an interesting competition of
boundary effects. This results in quantitative predictions for experiments on
doped spin-1/2 chains, which could observe two-channel Kondo physics directly.Comment: 5 pages in revtex format including 3 embedded figures (using epsf).
The latest version in PDF format is available from
http://fy.chalmers.se/~eggert/papers/crossover.pdf . Accepted by PR
Transport and scattering in inhomogeneous quantum wires
We consider scattering and transport in interacting quantum wires that are
connected to leads. Such a setup can be represented by a minimal model of
interacting fermions with inhomogeneities in the form of sudden changes in
interaction strength and/or velocity. The inhomogeneities generally cause
relevant backscattering, so it is a priori unclear if a perfectly ballistic
quantum wire can exist in the low temperature limit. We are able to identify
such a perfectly conducting fixed point even for large abrupt changes, which in
the considered model corresponds to a velocity matching condition. The general
position dependent Green's function is calculated in the presence of a sudden
change, which is confirmed numerically with high accuracy. The exact form of
the interference pattern in the form of density oscillations around
inhomogeneities can be used to estimate the effective strength of local
backscattering sources.Comment: 5 pages, 3 figures. Published version. For more information and the
latest version see http://www.physik.uni-kl.de/eggert/papers/index.htm
Edge Logarithmic Corrections probed by Impurity NMR
Semi-infinite quantum spin chains display spin autocorrelations near the
boundary with power-law exponents that are given by boundary conformal field
theories. We show that NMR measurements on spinless impurities that break a
quantum spin chain lead to a spin-lattice relaxation rate 1/T_1^edge that has a
temperature dependence which is a direct probe of the anomalous boundary
exponents. For the antiferromagnetic S=1/2 spin chain, we show that 1/T_1^edge
behaves as T (log T)^2 instead of (log T)^1/2 for a bulk measurement. We show
that, in the case of a one-dimensional conductor described by a Luttinger
liquid, a similar measurement leads to a relaxation rate 1/T_1^{edge} behaving
as T, independent of the anomalous exponent K_rho.Comment: 4 pages, 1 encapsulated figure, corrected typo
Universal alternating order around impurities in antiferromagnets
The study of impurities in antiferromagnets is of considerable interest in
condensed matter physics. In this paper we address the elementary question of
the effect of vacancies on the orientation of the surrounding magnetic moments
in an antiferromagnet. In the presence of a magnetic field, alternating
magnetic moments are induced, which can be described by a universal expression
that is valid in any ordered antiferromagnet and turns out to be independent of
temperature over a large range. The universality is not destroyed by quantum
fluctuation, which is demonstrated by quantum Monte Carlo simulations in the
two-dimensional Heisenberg antiferromagnet. Physical predictions for finite
doping are made, which are relevant for experiments probing Knight shifts and
the order parameter.Comment: 5 pages, 2 figures. The most recent version in PDF format can be
found at http://www.physik.uni-kl.de/eggert/papers
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