645 research outputs found
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
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
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
Quantum vs. Geometric Disorder in a Two-Dimensional Heisenberg Antiferromagnet
We present a numerical study of the spin-1/2 bilayer Heisenberg
antiferromagnet with random interlayer dimer dilution. From the temperature
dependence of the uniform susceptibility and a scaling analysis of the spin
correlation length we deduce the ground state phase diagram as a function of
nonmagnetic impurity concentration p and bilayer coupling g. At the site
percolation threshold, there exists a multicritical point at small but nonzero
bilayer coupling g_m = 0.15(3). The magnetic properties of the single-layer
material La_2Cu_{1-p}(Zn,Mg)_pO_4 near the percolation threshold appear to be
controlled by the proximity to this new quantum critical point.Comment: minor changes, updated figure
Optical determination of the relation between the electron-boson coupling function and the critical temperature in high T cuprates
We take advantage of the connection between the free carrier optical
conductivity and the glue function in the normal state, to reconstruct from the
infrared optical conductivity the glue-spectrum of ten different high-Tc
cuprates revealing a robust peak in the 50-60 meV range and a broad con- tinuum
at higher energies for all measured charge carrier concentrations and
temperatures up to 290 K. We observe that the strong coupling formalism
accounts fully for the known strong temperature dependence of the optical
spectra of the high Tc cuprates, except for strongly underdoped samples. We
observe a correlation between the doping trend of the experimental glue spectra
and the critical temperature. The data obtained on the overdoped side of the
phase diagram conclusively excludes the electron-phonon coupling as the main
source of superconducting pairing.Comment: Accepted for publication in Phys. Rev.
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
Dynamical Spin Response Functions for Heisenberg Ladders
We present the results of a numerical study of the 2 by L spin 1/2 Heisenberg
ladder. Ground state energies and the singlet-triplet energy gaps for L =
(4-14) and equal rung and leg interaction strengths were obtained in a Lanczos
calculation and checked against earlier calculations by Barnes et al. (even L
up to 12). A related moments technique is then employed to evaluate the
dynamical spin response for L=12 and a range of rung to leg interaction
strength ratios (0 - 5). We comment on two issues, the need for
reorthogonalization and the rate of convergence, that affect the numerical
utility of the moments treatment of response functions.Comment: Revtex, 3 figure
- …