arXiv subjects

R. J. Birgeneau

Publications and source records attributed to R. J. Birgeneau.

131 records · Page 8Linked to original sources

arXiv1997

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 $ξ\approx 350,000$ 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.

cond-mat.str-el↗

arXiv1996

Sr impurity effects on the magnetic correlations of LaSrCuO

We examine the low-temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low-temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. This behaviour is explained in terms of our theoretical work that finds small clusters of AFM correlated regions separated by disordered domain walls. The domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as $1/(2 ξ(x,T=0)^2)$, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states in the SG phase are commensurate, and that this behaviour should persist at higher temperatures where the holes should move along the domain walls. However, our results show that incommensurate correlations develop continuously around 5 % doping, consistent with recent measurements by Yamada.

cond-mat.str-el↗

arXiv1996

A Monte Carlo Study of Correlations in Quantum Spin Ladders

We study antiferromagnetic spin--1/2 Heisenberg ladders, comprised of $n_c$ chains ($2 \leq n_c \leq 6$) with ratio $J_{\bot}/J_{\|}$ of inter-- to intra--chain couplings. From measurements of the correlation function we deduce the correlation length $ξ(T)$. For even $n_c$, the static structure factor exhibits a peak at a temperature below the corresponding spin gap. Results for isotropically coupled ladders ($J_{\bot}/J_{\|} = 1$) are compared to those for the single chain and the square lattice. For $J_{\bot}/J_{\|} \leq 0.5$, the correlation function of the two--chain ladder is in excellent agreement with analytic results from conformal field theory, and $ξ(T)$ exhibits simple scaling behavior.

cond-mat↗

arXiv1995

Intercalation and Staging Behavior in Super-Oxygenated $La_2CuO_{4 + δ}$

A high temperature electrochemical oxidation process has been used to produce large single crystals of $La_2CuO_{4 + δ}$ suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for $La_2NiO_{4 + δ}$ can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO$_6$ octahedra. Within this model, the changes in CuO$_6$ tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes $La_2CuO_{4 + δ}$ as a unique lamellar superconducting system with annealed disorder.

cond-mat↗

arXiv1995

Spin Dependence of Correlations in Two-Dimensional Quantum Heisenberg Antiferromagnets

We present a series expansion study of spin-S square-lattice Heisenberg antiferromagnets. The numerical data are in excellent agreement with recent neutron scattering measurements. Our key result is that the correlation length for S>1/2 strongly deviates from the exact T->0 (renormalized classical, or RC) scaling prediction for all experimentally and numerically accessible temperatures. We note basic trends with S of the experimental and series expansion correlation length data and propose a scaling crossover scenario to explain them.

cond-mat↗