55 research outputs found
The antiferromagnetic order in an F-AF random alternating quantum spin chain : (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3
A possibility of the uniform antiferromagnetic order is pointed out in an
S=1/2 ferromagnetic (F) - antiferromagnetic (AF) random alternating Heisenberg
quantum spin chain compound: (CH_3)_2 CHNH_3 Cu(Cl_x Br_{1-x})_3. The system
possesses the bond alternation of strong random bonds that take +/- 2J and weak
uniform AF bonds of -J. In the pure concentration limits, the model reduces to
the AF-AF alternation chain at x=0 and to the F-AF alternation chain at x=1.
The nonequilibrium relaxation of large-scale quantum Monte Carlo simulations
exhibits critical behaviors of the uniform AF order in the intermediate
concentration region, which explains the experimental observation of the
magnetic phase transition. The present results suggest that the uniform AF
order may survive even in the presence of the randomly located ferromagnetic
bonds.Comment: 4 pages, 3 figure
Localization length of a soliton from a non-magnetic impurity in a general double-spin-chain model
A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ( model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length is found to obey a power law of the
dimerization with an exponent -1/3; .
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.Comment: 4 pages, REVTeX, Figures are in eps-file
Order by disorder from non-magnetic impurities in a two-dimensional quantum spin liquid
We consider doping of non-magnetic impurities in the spin-1/2, 1/5-depleted
square lattice. This structure, whose undoped phase diagram offers both
magnetically ordered and spin-liquid ground states, is realized physically in
CaV_4O_9. Doping into the ordered phase results in a progressive loss of order,
which becomes complete at the percolation threshold. By contrast, non-magnetic
impurities introduced in the spin liquids create a phase of weak but
long-ranged antiferromagnetic order coexisting with the gapped state. The
latter may be viewed as a true order-by-disorder phenomenon. We study the phase
diagram of the doped system by computing the static susceptibility and
staggered magnetization using a stochastic series-expansion quantum Monte Carlo
technique.Comment: 4 pages, 5 figure
Local magnetic structure due to inhomogeneity of interaction in S=1/2 antiferromagnetic chain
We study the magnetic properties of antiferromagnetic Heisenberg
chains with inhomogeneity of interaction. Using a quantum Monte Carlo method
and an exact diagonalization method, we study bond-impurity effect in the
uniform chain and also in the bond-alternating chain. Here `bond
impurity' means a bond with strength different from those in the bulk or a
defect in the alternating order. Local magnetic structures induced by bond
impurities are investigated both in the ground state and at finite
temperatures, calculating the local magnetization, the local susceptibility and
the local field susceptibility. We also investigate the force acting between
bond impurities and find the force generally attractive.Comment: 15pages, 34figure
Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System
We investigate thermodynamic properties of a one-dimensional S=1/2
antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum
Monte Carlo method. In particular we study how spin and lattice dimerize as a
function of the temperature, which gives a fundamental process of the
spin-Peierls transition in higher dimensions. The degree of freedom of the
lattice is taken into account adiabatically and the thermal distribution of the
lattice distortion is obtained by the thermal bath algorithm. We find that the
dimerization develops as the temperature decreases and it converges to the
value of the dimerization of the ground state at T=0. Furthermore we find that
the coupling constants of spins fluctuate quite largly at high temperature and
there thermodynamic properties deviate from those of the uniform chain. Doping
of non-magnetic impurities causes cut of the chain into short chains with open
boundary. We investigate thermodynamic properties of open chains taking
relaxation of the lattice into consideration. We find that strong bonds locate
at the edges and a defect of the bond alternation appears in the chain with odd
number of sites, which causes enhancement of the staggered magnetic order. We
find a spreaded staggered structure which indicates that the defect moves
diffusively in the chain even at very low temperature.Comment: 7 pages, 17 figures; added comments on section 2 and 3, corrected
typo
Excitation Spectra and Thermodynamic Response of Segmented Heisenberg Spin Chains
The spectral and thermodynamic response of segmented quantum spin chains is
analyzed using a combination of numerical techniques and finite-size scaling
arguments. Various distributions of segment lengths are considered, including
the two extreme cases of quenched and annealed averages. As the impurity
concentration is increased, it is found that (i) the integrated spectral weight
is rapidly reduced, (ii) a pseudo-gap feature opens up at small frequencies,
and (iii) at larger frequencies a discrete peak structure emerges, dominated by
the contributions of the smallest cluster segments. The corresponding
low-temperature thermodynamic response has a divergent contribution due to the
odd-site clusters and a sub-dominant exponentially activated component due to
the even-site segments whose finite-size gap is responsible for the spectral
weight suppression at small frequencies. Based on simple scaling arguments,
approximate low-temperature expressions are derived for the uniform
susceptibility and the heat capacity. These are shown to be in good agreement
with numerical solutions of the Bethe ansatz equations for ensembles of
open-end chains.Comment: RevTex, 9 pages with 6 figure
Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(CuMg)Cl
Neutron elastic and inelastic scattering measurements have been performed in
order to investigate the spin structure and the magnetic excitations in the
impurity-induced antiferromagnetic ordered phase of the doped spin gap system
Tl(CuMg)Cl with . The magnetic Bragg reflections
indicative of the ordering were observed at with integer
and odd below K. It was found that the spin structure
of the impurity-induced antiferromagnetic ordered phase on average in
Tl(CuMg)Cl with is the same as that of the
field-induced magnetic ordered phase for in the parent
compound TlCuCl. The triplet magnetic excitation was clearly observed in
the - plane and the dispersion relations of the triplet excitation
were determined along four different directions. The lowest triplet excitation
corresponding to the spin gap was observed at with integer
and odd , as observed in TlCuCl. It was also found that the spin gap
increases steeply below upon decreasing temperature. This strongly
indicates that the impurity-induced antiferromagnetic ordering coexists with
the spin gap state in Tl(CuMg)Cl with .Comment: 24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys.
Rev.
Magnetic polarons in weakly doped high-Tc superconductors
We consider a spin Hamiltonian describing - exchange interactions
between localized spins of a finite antiferromagnet as well as -
interactions between a conducting hole () and localized spins. The spin
Hamiltonian is solved numerically with use of Lanczos method of
diagonalization. We conclude that - exchange interaction leads to
localization of magnetic polarons. Quantum fluctuations of the antiferromagnet
strengthen this effect and make the formation of polarons localized in one site
possible even for weak - coupling. Total energy calculations, including
the kinetic energy, do not change essentially the phase diagram of magnetic
polarons formation. For parameters reasonable for high- superconductors
either a polaron localized on one lattice cell or a small ferron can form. For
reasonable values of the dielectric function and - coupling, the
contributions of magnetic and phonon terms in the formation of a polaron in
weakly doped high- materials are comparable.Comment: revised, revtex-4, 12 pages 8 eps figure
Study of impurities in spin-Peierls systems including lattice relaxation
The effects of magnetic and non-magnetic impurities in spin-Peierls systems
are investigated allowing for lattice relaxation and quantum fluctuations. We
show that, in isolated chains, strong bonds form next to impurities, leading to
the appearance of magneto-elastic solitons. Generically, these solitonic
excitations do not bind to impurities. However, interchain elastic coupling
produces an attractive potential at the impurity site which can lead to the
formation of bound states. In addition, we predict that small enough chain
segments do not carry magnetic moments at the ends
Thermal conductivity and specific heat of the linear chain cuprate SrCuO: Evidence for thermal transport via spinons
We report measurements of the specific heat and the thermal conductivity of
the model Heisenberg spin-1/2 chain cuprate SrCuO at low
temperatures. In addition to a nearly isotropic phonon heat transport, we find
a quasi one-dimensional excess thermal conductivity along the chain direction,
most likely associated with spin excitations (spinons). The spinon energy
current is limited mainly by scattering on defects and phonons. Analyzing the
specific heat data, the intrachain magnetic exchange is estimated to
be 2650 K.Comment: 4 RevTeX pages, 3 figures, to appear in Phys. Rev.
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