Frustrated Bonds and Long Range Order in Quasi-2D Magnets

I. Ya. Korenblit

doi:10.1103/PhysRevB.51.12551

paper

Frustrated Bonds and Long Range Order in Quasi-2D Magnets

arXiv:cond-mat/9407111 · doi:10.1103/PhysRevB.51.12551

Abstract

We employ the Schwinger boson mean-field approach to study the effects of arbitrary frustrated bonds and plaquettes (formed from four frustrated bonds) in two-dimensional ferro- and antiferromagnets on the spin-wave spectrum and the correlation length at finite temperatures. We distinguish between strongly frustrated bonds (plaquettes), when the frustrated coupling $J^{'}$ exceeds the spin canting threshold $J_{c}$ , and weakly frustrated bonds (plaquettes), with $J^{'} < J_{c}, (J_{c} - J^{'}) / J_{c} \sim 1$ . It is shown that in antiferromagnets the amplitude of spin-wave scattering on strongly frustrated bonds or plaquettes grows with the decrease of the temperature. A small amount of such defects reduces significantly the spin-wave stiffness and the correlation length at low temperatures. As a result, the quasi-2D Néel temperature is sharply suppressed. Quantum fluctuations are also considered and their effect on the spin-wave spectrum is shown to be of the order of $(2 S)^{- 2} ln^{- 1} 2 S$ in the large spin limit. For weakly frustrated (nonfrustrated) defect bonds (plaquettes) the spin-wave stiffness renormalization is of the order of the dopant concentration and does not depend on the temperature. The results account for the observed properties of doped quasi-2D $L a_{2} C u O_{4 + x}$ .

REVTEX, 50 pages and one Postscript file of a figure appended

Frustrated Bonds and Long Range Order in Quasi-2D Magnets

Abstract

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