SPECTRAL AND MAGNETIC TRANSITIONS IN THE QUANTUM ISING-MODEL

The quantum Ising model in a transverse field is studied where the exchange interaction is a modulating function of sites with many harmonics. The magnetically disordered phase is found to exhibit a mixed spectrum containing both critical and localized states, while the magnetically ordered phase has a pure spectrum with all states localized. Therefore, the spectral transition, although broadened, occurs simulataneously with the magnetic transition, and the magnetic disorder is accompanied by spectral disorder. © 1990 The American Physical Society.