Quasi-particle excitation spectrum of an XY spin chain in a random transverse field

Abstract

The quasi-particle excitation spectrum of an XY spin chain in a random transverse field was calculated by an exact singular value decomposition. The spectrum was found to be robust at small disorder strengths and deep within the paramagnetic phase. For larger disorder strengths, and about the ferromagnetic region, the spectrum deviated further from the quasi-excitation spectrum of the ordered chain. These deviations lead to the disorder-averaged ground state energy E₀ to increase with disorder strength, even as the average field strength remains constant.