Monte Carlo Study of the S=1/2 and S=1 Heisenberg Antiferromagnet on a Spatially Anisotropic Square Lattice

We present a quantum Monte Carlo study of a Heisenberg antiferromagnet on a spatially anisotropic square lattice, where the coupling strength in the x-direction ($J_x$) is different from that in the y-direction ($J_y$). By varying the anisotropy $\alpha$ from 0 to 1, we interpolate between the one-dimensional chain and the two-dimensional isotropic square lattice. Both $S=1/2$ and S=1 systems are considered separately in order to facilitate comparison. The temperature dependence of the uniform susceptibility and the spin-spin correlation length are computed down to very low temperatures for various values of $\alpha$. For S=1, the existence of a quantum critical point at $\alpha^{S=1}_c=0.040(5)$ as well as the scaling of the spin gap is confirmed. Universal quantities predicted from the ${\cal O}(3)$ nonlinear $\sigma$ model agree with our results at $\alpha=0.04$ without any adjustable parameters. On the other hand, the $S=1/2$ results are consistent with $\alpha^{S=1/2}_c=0$, as discussed by a number of previous theoretical studies. Experimental implications for $S=1/2$ compounds such as Sr$_2$CuO$_3$ are also discussed.Comment: 8 pages, 7 figures, to be published in Phys. Rev.

X-Ray Diffuse Scattering

Contains research summary and reports on six research projects.Joint Services Electronics Program (Contract DAAG29-78-C-0020

X-Ray Diffuse Scattering

Contains research summary and reports on three research projects.Joint Services Electronics Program (Contract DAAG29-83-K-0003

X-Ray Diffuse Scattering

Contains summary of resarch and reports on three research projects.Joint Services Electronics Program (Contract DAALO03-86-K-0002

X-Ray Diffuse Scattering

Contains summary of research and reports on three research projects.Joint Services Electronics Program (Contract DAAG29-83-K-0003

Monte Carlo Study of Correlations in Quantum Spin Chains at Non-Zero Temperature

Antiferromagnetic Heisenberg spin chains with various spin values ($S=1/2,1,3/2,2,5/2$) are studied numerically with the quantum Monte Carlo method. Effective spin $S$ chains are realized by ferromagnetically coupling $n=2S$ antiferromagnetic spin chains with $S=1/2$. The temperature dependence of the uniform susceptibility, the staggered susceptibility, and the static structure factor peak intensity are computed down to very low temperatures, $T/J \approx 0.01$. 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 $S$. For $S$=2 chains which have a gap $\Delta$, the correlation length and the uniform susceptibility in the temperature range $\Delta < T < J$ are well predicted by a semi-classical theory due to Damle and Sachdev.Comment: LaTeX EPJ macr

X-Ray Diffuse Scattering

Contains summary of resarch and reports on three research projects.Joint Services Electronics Program (Contract DAALO03-86-K-0002

X-Ray Scattering Spectroscopy

Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAABO7-76-C-1400

First and second order magnetic and structural transitions in BaFe2(1−x)_{2(1-x)}Co2x_{2x}As2_{2}

We present here high resolution magnetization measurements on high-quality BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$, 0$\leq$x$\leq$0.046 as-grown single crystals. The results confirm the existence of a magnetic tricritical point in the ($x$,$T$) plane at x$^{m}_{tr}$$\approx$0.022 and reveal the emergence of the heat capacity anomaly associated with the onset of the structural transition at x$^{s}$$\approx$0.0064. We show that the samples with doping near x$^{m}_{tr}$ do not show superconductivity, but rather superconductivity emerges at a slightly higher cobalt doping, x$\approx$0.0315Comment: 4 pages, 5 figure