  Quasiparticle interference in a d-wave superconductor with weak disorder
produces distinctive peaks in the Fourier-transformed local density of states
measured by scanning tunneling spectroscopy. We predict that amplitudes of
these peaks can be enhanced or suppressed by applied magnetic field according
to a very specific pattern governed by the symmetry of the superconducting
order parameter. This calculated pattern agrees with the recent experimental
measurement and suggests that the technique could be useful for probing the
underlying normal state at high fields.
