Gruppo di Fisica Statistica

The low-energy excitation spectrum of an Ising quantum chain in a transverse field which is critical (h=J=1) for 1 less or equal n and n less or equal L/2 and off-critical (h not=J) for L/2 (n less or equal L) is calculated up to O(L-1). Interface critical exponents are determined numerically in the odd sector and exactly in the even sector using finite-size scaling. One gets an ordinary surface transition when h)J and an extraordinary surface transition when h(J. The gap-exponent relation is verified, the mass gaps and the level degeneracy are in agreement with conformal invariance.