Finite-temperature and dynamical properties of the random transverse-field Ising spin chain

We study numerically the paramagnetic phase of the spin-1/2 random transverse-field Ising chain, using a mapping to noninteracting fermions. We extend our earlier work, Phys. Rev. B 53, 8486 (1996), to finite temperatures and to dynamical properties. Our results are consistent with the idea that there are Griffiths-McCoy singularities in the paramagnetic phase described by a continuously varying exponent z(delta), where delta measures the deviation from criticality. There are some discrepancies between the values of z(delta) obtained from different quantities, but this may be due to corrections to scaling. The average on-site time dependent correlation function decays with a power law in the paramagnetic phase, namely, tau(-1/z(delta)), where ris imaginary time. However, the typical value decays with a stretched exponential behavior, exp(-c tau(1/mu)), where mu may be related to z(delta). We also obtain results for the full probability distribution of time dependent correlation functions at different points in the paramagnetic phase. [S0163-1829(97)07641-8].