2 research outputs found
Ab initio determination of exchange integrals and Neel temperature in the chain cuprates
We report ab initio quantum chemical cluster calculations of the chain (J_a)
and the largest interchain (J_b) Heisenberg exchange of the chain cuprates
Ca_2CuO_3 and Sr_2CuO_3. We find that J_a is comparable to the in-plane J in
layered cuprates and J_a/J_b ~250-400. Using recent theory we obtain close
agreement with experiment for the staggered moments and the critical
temperatures. This implies that T_N does not depend on the third parameter J_c
<< J_b, and cannot be calculated using spin-wave theory. We propose an
explanation for this interms of a 1D->2D cross-over.Comment: ps, 19 pages. To appear in Chemical Physics Letter
Optical Absorption of CuO antiferromagnetic chains at finite temperatures
We use a high-statistic quantum Monte Carlo and Maximum Entropy
regularization method to compute the dynamical energy correlation function
(DECF) of the one-dimensional (1D) antiferromagnetic Heisenberg model
at finite temperatures. We also present a finite temperature analytical ansatz
for the DECF which is in very good agreement with the numerical data in all the
considered temperature range. From these results, and from a finite temperature
generalisation of the mechanism proposed by Lorenzana and Sawatsky [Phys. Rev.
Lett. {\bf 74}, 1867 (1995)], we compute the line shape for the optical
absorption spectra of multimagnon excitations assisted by phonons for quasi 1D
compounds. The line shape has two contributions analogous to the Stokes and
anti-Stokes process of Raman scattering. Our low temperature data is in good
agreement with optical absorption experiments of CuO chains in
SrCuO. Our finite temperature results provide a non trivial prediction
on the dynamics of the Heisenberg model at finite temperatures that is easy to
verify experimentally.Comment: 7 pages, 5 figure