Quantum dynamics of an Ising spin-chain in a random transverse field

We consider an Ising spin-chain in a random transverse magnetic field and compute the zero temperature wave vector and frequency dependent dynamic structure factor numerically by using Jordan-Wigner transformation. Two types of distributions of magnetic fields are introduced. For a rectangular distribution, a dispersing branch is observed, and disorder tends to broaden the dispersion peak and close the excitation gap. For a binary distribution, a non-dispersing branch at almost zero energy is recovered. We discuss the relationship of our work to the neutron scattering measurement in $\mathrm{LiHoF_4}$.Comment: 4 pages and 6 eps figures; minor clarifications were made; the text was shortened to add an additional figur