Gapless Spin-Fluid Ground State in a Random Quantum Heisenberg Magnet
Abstract
We examine the spin-S quantum Heisenberg magnet with Gaussian-random, infinite-range exchange interactions. The quantum-disordered phase is accessed by generalizing to SU(M) symmetry and studying the large M limit. For large S the ground state is a spin-glass, while quantum fluctuations produce a spin-fluid state for small S. The spin-fluid phase is found to be generically gapless - the average, zero temperature, local dynamic spin-susceptibility obeys (ω ) (1/|ω|) + i (π/2) sgn (ω) at low frequencies. This form is identical to the phenomenological `marginal' spectrum proposed by Varma et. al.\/ for the doped cuprates.
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