Abstract
A consistent Schwinger-boson mean field theory is developed for a spin-1/2 2D antiferromagnet. It predicts that there are two branches of the Schwinger-boson excitation spectrum: an acoustic branch, essentially the same as that predicted by Arovas and Auerbach theory, and a new optical branch. The present theory provides a natural explanation of the mystery of the Raman “two magon” scattering from La2CuO4.
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References
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In the mean field Hamiltonian (2), there are two other terms:\(--J\chi \sum\limits_{k\sigma } ' \gamma _k (b_{k\sigma }^{A\dag } b_{k\sigma }^{B\dag } + b_{k\sigma }^{B\dag } b_{k\sigma }^A ) and --iJ\tilde \chi \sum\limits_{k\sigma } ' \tilde \gamma _k \sigma (b_{k\sigma }^{A\dag } b_{k--\sigma }^B + b_{k--\sigma }^{B\dag } b_{k\sigma }^A )\) in which\(\chi = \sum\limits_\sigma {\left\langle {b_{i\sigma }^\dag b_{i + \hat \eta \sigma } } \right\rangle } , \tilde \chi = \sum\limits_\sigma {\sigma \left\langle {b_{i\sigma }^\dag b_{i + \hat \eta --\sigma } } \right\rangle } \) and\(\tilde \gamma _k = \frac{1}{2}\) (sink x a + sink y a). Our numerical computation shows that\(\tilde \chi \) is always zero, and χ can be either zero or nonzero. However, it is only when χ=0 that there are two pointsk=0 and π at which the bose condensation occurs. So for the theory of antiferromagnet, we choose the solution χ=0
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