1,145 research outputs found
Monte Carlo Study of Correlations in Quantum Spin Chains at Non-Zero Temperature
Antiferromagnetic Heisenberg spin chains with various spin values
() are studied numerically with the quantum Monte Carlo
method. Effective spin chains are realized by ferromagnetically coupling
antiferromagnetic spin chains with . The temperature dependence
of the uniform susceptibility, the staggered susceptibility, and the static
structure factor peak intensity are computed down to very low temperatures,
. The correlation length at each temperature is deduced from
numerical measurements of the instantaneous spin-spin correlation function. At
high temperatures, very good agreement with exact results for the classical
spin chain is obtained independent of the value of . For =2 chains which
have a gap , the correlation length and the uniform susceptibility in
the temperature range are well predicted by a semi-classical
theory due to Damle and Sachdev.Comment: LaTeX EPJ macr
Resistivity phase diagram of cuprates revisited
The phase diagram of the cuprate superconductors has posed a formidable
scientific challenge for more than three decades. This challenge is perhaps
best exemplified by the need to understand the normal-state charge transport as
the system evolves from Mott insulator to Fermi-liquid metal with doping. Here
we report a detailed analysis of the temperature (T) and doping (p) dependence
of the planar resistivity of simple-tetragonal HgBaCuO
(Hg1201), the single-CuO-layer cuprate with the highest optimal . The
data allow us to test a recently proposed phenomenological model for the
cuprate phase diagram that combines a universal transport scattering rate with
spatially inhomogeneous (de)localization of the Mott-localized hole. We find
that the model provides an excellent description of the data. We then extend
this analysis to prior transport results for several other cuprates, including
the Hall number in the overdoped part of the phase diagram, and find little
compound-to-compound variation in (de)localization gap scale. The results point
to a robust, universal structural origin of the inherent gap inhomogeneity that
is unrelated to doping-related disorder. They are inconsistent with the notion
that much of the phase diagram is controlled by a quantum critical point, and
instead indicate that the unusual electronic properties exhibited by the
cuprates are fundamentally related to strong nonlinearities associated with
subtle nanoscale inhomogeneity.Comment: 22 pages, 5 figure
The Square-Lattice Heisenberg Antiferromagnet at Very Large Correlation Lengths
The correlation length of the square-lattice spin-1/2 Heisenberg
antiferromagnet is studied in the low-temperature (asymptotic-scaling) regime.
Our novel approach combines a very efficient loop cluster algorithm --
operating directly in the Euclidean time continuum -- with finite-size scaling.
This enables us to probe correlation lengths up to
lattice spacings -- more than three orders of magnitude larger than any
previous study. We resolve a conundrum concerning the applicability of
asymptotic-scaling formulae to experimentally- and numerically-determined
correlation lengths, and arrive at a very precise determination of the
low-energy observables. Our results have direct implications for the
zero-temperature behavior of spin-1/2 ladders.Comment: 12 pages, RevTeX, plus two Postscript figures. Some minor
modifications for final submission to Physical Review Letters. (accepted by
PRL
Spin Correlations in the Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2MnF4
We report a neutron scattering study of the instantaneous spin correlations
in the two-dimensional spin S=5/2 square-lattice Heisenberg antiferromagnet
Rb_2MnF_4. The measured correlation lengths are quantitatively described, with
no adjustable parameters, by high-temperature series expansion results and by a
theory based on the quantum self-consistent harmonic approximation. Conversely,
we find that the data, which cover the range from about 1 to 50 lattice
constants, are outside of the regime corresponding to renormalized classical
behavior of the quantum non-linear sigma model. In addition, we observe a
crossover from Heisenberg to Ising critical behavior near the Neel temperature;
this crossover is well described by a mean-field model with no adjustable
parameters.Comment: 8 pages, LaTeX, with 6 included EPS figures, submitted to EPJ
Quantum Impurities in the Two-Dimensional Spin One-Half Heisenberg Antiferromagnet
The study of randomness in low-dimensional quantum antiferromagnets is at the
forefront of research in the field of strongly correlated electron systems, yet
there have been relatively few experimental model systems. Complementary
neutron scattering and numerical experiments demonstrate that the spin-diluted
Heisenberg antiferromagnet La2Cu(1-z)(Zn,Mg)zO4 is an excellent model material
for square-lattice site percolation in the extreme quantum limit of spin
one-half. Measurements of the ordered moment and spin correlations provide
important quantitative information for tests of theories for this complex
quantum-impurity problem.Comment: 11 pages, 3 figures. NOTE: possible errors in PDF version of Fig. 1.
View postscript version of figure if possibl
Correlation Lengths in Quantum Spin Ladders
Analytic expressions for the correlation length temperature dependences are
given for antiferromagnetic spin-1/2 Heisenberg ladders using a finite-size
non-linear sigma-model approach. These calculations rely on identifying three
successive crossover regimes as a function of temperature. In each of these
regimes, precise and controlled approximations are formulated. The analytical
results are found to be in excellent agreement with Monte Carlo simulations for
the Heisenberg Hamiltonian.Comment: 5 pages LaTeX using RevTeX, 3 encapsulated postscript figure
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