R. L. Glenister | Searcharxiv

arXiv1994

Scaling Regimes, Crossovers, and Lattice Corrections in 2D Heisenberg Antiferromagnets

We study scaling behavior in 2D, S=1/2 and S=1 Heisenberg antiferromagnets using the data on full q-dependences of the equal time structure factor and the static susceptibility, calculated through high temperature expansions. We also carry out comparisons with a model of two coupled S=1/2 planes with the interlayer coupling tuned to the T=0 critical point. We separately determine the spin-wave velocity c and mass $m=c/ξ$, in addition to the correlation length, $ξ$, and find that c is temperature dependent; only for $T\alt JS$, it approaches its known T=0 value $c_0$. Despite this temperature dependent spin-wave velocity, full q- and $ω$-dependences of the dynamical susceptibility $χ(\bf q,ω)$ agree with the universal scaling functions computable for the $σ$-model, for temperatures upto $T_0 \sim 0.6c_0/a$. Detailed comparisons show that below $T_0$ the S=1 model is in the renormalized classical (RC) regime, the two plane model is in the quantum critical (QC) regime, and the S=1/2 model exhibits a RC-QC crossover, centered at T=0.55J. In particular, for the S=1/2 model above this crossover and for the two-plane model at all T, the spin-wave mass is in excellent agreement with the universal QC prediction, $m\simeq 1.04\,T$. In contrast, for the S=1/2 model below the RC-QC crossover, and for the S=1 model at all T, the behavior agrees with the known RC expression. For all models nonuniversal behavior occurs above $T\sim 0.6c_0/a$. Our results strongly support the conjecture of Chubukov and Sachdev that the S=1/2 model is close to the T=0 critical point to exhibit QC behavior.

cond-mat↗

arXiv1993

Evidence for Spin-Charge Separation in the Two-Dimensional T-J Model

We have calculated high temperature expansions for the momentum distribution function $n_{\scriptscriptstyle\bf k}$ and the equal time spin and density correlation functions $S({\bf q})$ and $N({\bf q})$ of the two-dimensional $t$-$J$ model. On extrapolation to low temperatures we find that $n_{\scriptscriptstyle\bf k}$ has a step-like feature at ${\bf k}_{\scriptscriptstyle F}$ and $S({\bf q})$ has $2{\bf k}_{\scriptscriptstyle F}$ as a characteristic wavevector, whereas $N({\bf q})$ has $2{\bf k}_{\scriptscriptstyle F}^{\scriptscriptstyle SF}$ as a characteristic wavevector. Here ${\bf k}_{\scriptscriptstyle F}$ and ${\bf k}_{\scriptscriptstyle F}^{\scriptscriptstyle SF}$ are the Fermi wavevectors of the nearest-neighbor square lattice tight-binding and spinless fermion models, respectively. By comparison to the known results for one dimension this suggests spin-charge separation in the two-dimensional $t$-$J$ model.

cond-mat↗