Quantum phase transition between one-channel and two-channel Kondo polarons

For a mobile spin-1/2 impurity, coupled antiferromagnetically to a one-dimensional gas of fermions, perturbative ideas have been used to argue in favor of two-channel Kondo behavior of the impurity spin. Here we combine general considerations and extensive numerical simulations to show that the problem displays a novel quantum phase transition between two-channel and one-channel Kondo screening upon increasing the Kondo coupling. We construct a ground-state phase diagram and discuss the various non-trivial crossovers as well as possible experimental realizations.Comment: 5+4 pages, 5+3 fig

Superconductivity with s and p-symmetries in an extended Hubbard model with correlated hopping

We consider a generalized Hubbard model with on-site and nearest-neighbour repulsions U and V respectively, and nearest-neighbour hopping for spin up (down) which depends on the total occupation n_b of spin down (up) electrons on both sites involved. The hopping parameters are t_{AA}, t_{AB} and t_{BB} for n_b=0,1,2 respectively. We briefly summarize results which support that the model exhibits s-wave superconductivity for certain parameters and extend them by studying the Berry phases. Using a generalized Hartree-Fock(HF) BCS decoupling of the two and three-body terms, we obtain that at half filling, for t_{AB}<t_{AA}=t_{BB} and sufficiently small U and V the model leads to triplet p-wave superconductivity for a simple cubic lattice in any dimension. In one dimension, the resulting phase diagram is compared with that obtained numerically using two quantized Berry phases (topological numbers) as order parameters. While this novel method supports the previous results, there are quantitative differences.Comment: Latex file, 14 pages, 2 postscript figure

Critical behavior of the S=3/2 antiferromagnetic Heisenberg chain

Using the density-matrix renormalization-group technique we study the long-wavelength properties of the spin S=3/2 nearest-neighbor Heisenberg chain. We obtain an accurate value for the spin velocity v=3.8+- 0.02, in agreement with experiment. Our results show conclusively that the model belongs to the same universality class as the S=1/2 Heisenberg chain, with a conformal central charge c=1 and critical exponent eta=1Comment: RevTeX (version 3.0), 4 twocolumn pages with 4 embedded figure