Quantum XX chain with interface
  The quantum XX chain - or rather ring - is studied as a toy model of an interface. Two transverse field patterns are used to define the interface, on the one hand a staggered field, on the other hand a step-like configuration, from -h to +h. The interface leads to Friedel oscillations and proximity effects, in particular close to the quantum phase transition of the bulk, which is a metal-insulator transition in the fermionic language. The most prominent interface effects appear for odd-numbered rings, for which - in contrast to even chains - the ground state is doubly degenerate. In the regime where the bulk energy spectrum is gapped a level appears close to midgap, with a wave function localized in the region of the interface. The two members of the ground state doublet have two different particle number parities and spin components S_z=+-1/2. They also have different energy levels and thus the degeneracy does not originate from different occupancies of a rigid band structure, but rather from a global symmetry. The unitary transformation linking the two degenerate ground states resembles a Majorana operator. Coherent superpositions of the two states may be suitable candidates for well protected qubits. 
D. Baeriswyl and G. Ferraz (University of Fribourg, Switzerland, and   International Institute of Physics, Natal, Brazil)
cond-mat.mes-hall cond-mat.str-el