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The B1g Raman spectrum of the two-dimensional S=1/2 Heisenberg model isdiscussed within Loudon-Fleury theory at both zero and finite temperature. Theexact T=0 spectrum for lattices with up to 6*6 sites is computed using Lanczosexact diagonalization. A quantum Monte Carlo (QMC) method is used to calculatethe corresponding imaginary-time correlation function and its first twoderivatives for lattices with up to 16*16 spins. The imaginary-time data iscontinued to real frequency using the maximum-entropy method, as well as a fitbased on spinwave theory. The numerical results are compared with spinwavecalculations for finite lattices. There is a surprisingly large change in theexact spectrum going from 4*4 to 6*6 sites. In the former case there is asingle dominant two-magnon peak at frequency w/J appr. 3.0, whereas in thelatter case there are two approximately equal-sized peaks at w/J appr. 2.7 and3.9. This is in good qualitative agreement with the spinwave calculationsincluding two-magnon processes on the same lattices. Both the Lanczos and theQMC results indicate that the actual infinite-size two-magnon profile isbroader than the narrow peak obtained in spinwave theory, but the positions ofthe maxima agree to within a few percent. The higher-order contributionspresent in the numerical results are merged with the two-magnon profile andextend up to frequencies w/J appr. 7. The first three frequency cumulants ofthe spectrum are in excellent agreement with results previously obtained from aseries expansion around the Ising limit. Typical experimental B1g$ spectra forLa2CuO4 are only slightly broader than what we obtain here. The exchangeconstant extracted from the peak position is J appr. 1400K, in good agreementwith values obtained from neutron scattering and NMR experiments.Comment: 15 pages, Revtex, 13 PostScript figure

physical review. b, condensed matter

Numerical calculations of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>B</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>g</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>Raman spectrum of the two-dimensional Heisenberg model

Numerical calculations of theB1gRaman spectrum of the two-dimensional Heisenberg model