1202.3121 (H. Krull et al.)

Enhanced Perturbative Continuous Unitary Transformations    [PDF]

H. Krull, N. A. Drescher, G. S. Uhrig

Unitary transformations are an essential tool for the theoretical
understanding of many systems by mapping them to simpler effective models. A
systematically controlled variant to perform such a mapping are perturbative
continuous unitary transformations (pCUT) among others. So far, this approach
required an equidistant unperturbed spectrum. Here we pursue two goals: First,
we extend its applicability to non-equidistant spectra with the particular
focus on an efficient derivation of the differential flow equations, which
define the enhanced perturbative continuous unitary transformation (epCUT).
Second, we show that the numerical integration of the flow equations yields a
robust scheme to extract data from the epCUT. The method is illustrated by the
perturbation of two-leg spin ladders around the strong-rung-coupling limit for
uniform and alternating rung couplings. The latter case provides an example of
perturbation around a non-equidistant spectrum.

View original: http://arxiv.org/abs/1202.3121