The search for Majorana fermions is one of the most prominent fundamental research tasks in physics today. Majorana fermions are an elusive class of fermions that act as their own antiparticles. Although an extensive effort has been made worldwide in particle physics, Majorana fermions have so far not been convincingly discovered in free space. In recent years, numerous proposals for probing Majorana fermion states in solid state systems have been suggested, ranging from exploring ν= 5/2 fractional quantum Hall systems, to exploring chiral p-wave superconductors, and to exploring hybrid systems of a topological insulator or a strong spin-orbit coupled semiconductor thin film or nanowire in the proximity of an external s-wave superconductor. These proposals have stimulated a wave of searches for Majorana fermions in solid state systems.
In this talk, I will review recent experimental observations of the signatures of Majorana fermions in topological superconductor mamowires realized using s-wave superconductor-contacted semiconductor nanowires in the presence of strong spin-orbit interaction. In particular, I will report on our recent work on the search for Majorana fermions in superconductor-semiconductor quantum dot-superconductor hybrid devices. The devices are made from high crystal-quality InSb nanowires and superconductor Nb contacts. The InSb nanowires are known to have excellent physical properties [1-3] and have therefore been considered as one of the most promising material systems for realizing topological superconductor systems in which Majorana fermions can be created. Due to the proximity effect, the InSb nanowire segments covered by superconductor Nb contacts turn to superconductors with a superconducting energy gap Δ* [4]. Under an applied magnetic field larger than a critical value for which the Zeeman energy in the InSb nanowire is Ez ~ Δ*, the entire InSb nanowire is found to be in a nontrivial topological superconductor phase, supporting a pair of Majorana fermions, and Cooper pairs can transport between the superconductor Nb contacts via the Majorana fermion states. This transport process will be suppressed when the applied magnetic field becomes larger than a second critical value at which the transition to a topologically trivial superconductor phase occurs in the system [4]. We have also found that the measured zero-bias conductance for our hybrid devices shows a conductance peak in a range of applied magnetic fields [4] and is independent of the even-odd parity of the occupation number of quasi-particles in the quantum dot.
I acknowledge collaborations with Mingtang Deng, Chunlin Yu, Guangyao Huang, Marcus Larsson, Philippe Caroff, and Kimberly Dick Thelander for this work.
[1] H. A. Nilsson, P. Caroff, C. Thelander, M. Larsson, J. B. Wagner, L.-E. Wernersson, L. Samuelson, and H. Q. Xu. Nano Lett. 9, 3151–3156 (2009).
[2] H. A. Nilsson, O. Karlström, M. Larsson, P. Caroff, J. N. Pedersen, L. Samuelson, A. Wacker, L.-E. Wernersson, and H. Q. Xu. Phys. Rev. Lett. 104, 186804 (2010).
[3] H. A. Nilsson, P. Samuelsson, P. Caroff, and H. Q. Xu, Nano Lett.12, 228-233 (2012).
[4] M. T. Deng, C. L. Yu, G. Y. Huang, M. Larsson, P. Caroff, and H. Q. Xu, Nano Lett. 12, 6414−6419 (2012); arXiv:1204.4130, http://arxiv.org/abs/1204.4130.
