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 χ¯(ω)∼ln(1/‖ω‖)+i(π/2)sgn(ω) at low frequencies.