Dear Tony, The model used in A.7 is the model of a nanowire with spin-orbit interaction subject to external Zeeman field and coupled to an s-wave superconductor. It was introduced in these two papers: R. Lutchyn; J. Sau; S. Das Sarma (2010)."Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures". Physical Review Letters 105(7): 077001. arXiv:1002.4033. doi:10.1103/PhysRevLett.105.077001. Y. Oreg; G. Refael; F. von Oppen (2010). "Helical Liquids and Majorana Bound States in Quantum Wires". Physical Review Letters 105 (17): 177002. arXiv:1003.1145. doi:10.1103/PhysRevLett.105.177002. The 4 degrees of freedom per site correspond accordingly to spin x electron-hole. For more information about Majorana fermions take a look at e.g. one of these reviews: C. W. J. Beenakker. Search for Majorana fermions in superconductors. arXiv:1112.1950. J. Alicea. New directions in the pursuit of Majorana fermions in solid state systems. arXiv:1202.1293, doi:10.1088/0034-4885/75/7/076501. Best regards, Anton Akhmerov On Mon, Jan 27, 2014 at 10:19 AM, Tony <liuchx1993@163.com> wrote:

Dear all,

There is some code dealing with Majorana fermions: "A.7 Majorana Fermion" on the paper "Kwant: a software package for quantum transport", by Christoph et al. I am still a novice and I'm not familiar with the physical system the code is written for. Neither do I understand why the onsite and hopping matrices are both 4*4 instead of 2*2 or something. May I know if there is some paper describing the model for this code, and why the onsite and hopping matrices are both 4*4?

Thanks and regards,

Tony