Field-dependent dynamics of the Anderson impurity model

Single-particle dynamics of the Anderson impurity model in the presence of a magnetic field H are considered, using a recently developed local moment approach that encompasses all energy scales, field and interaction strength s. For strong coupling in particular, the Kondo scaling regime is recovered . Here the frequency (omega)/omega (K)) and field (H/omega (K)) dependence of the resultant universal scaling spectrum is obtained in large part analy tically, and the field-induced destruction of the Kondo resonance investiga ted. The scaling spectrum is found to exhibit the slow logarithmic tails re cently shown to dominate the zero-field scaling spectrum. At the opposite e xtreme of the Fermi level, it gives asymptotically exact agreement with res ults for statics known from the Bethe ansatz. Good agreement is also found with the frequency and field-dependence of recent numerical renormalization group calculations. Differential conductance experiments on quantum dots i n the presence of a magnetic field are likewise considered, and appear to b e well accounted for by the theory. Some new exact results for the problem are also established.