Research associate Rami Dana, University of Maryland and the Laboratory for Physical Sciences (LPS)

LT/mK-STM study on Bi2Se3 and CuxBi2Se3 with W and Nb tips

The search for Majorana Fermions (MF) in condense matter physics was getting a lot of attention in the last ~15 years. During this time different theoretical proposals suggested many experimental setups to identify these states (for a review see [1, 2]). Some of the ideas pointed towards finding a zero-bias conductance peak (ZBCP) in topological insulator - superconductor heterostructures, with and without the application of a magnetic field. At LPS we were using a homemade 4K and mK-STM [3] to look at the topological insulator Bi₂Se₃ and the topological superconductor Cu_xBi₂Se₃ using both normal W and superconducting Nb tips [4]. Our results include data from all four possible junctions. In this talk I will be discussing; gap and sub-gap features arising when Nb islands are grown on Bi₂Se₃, proximity induced superconductivity on the surface of Bi₂Se₃ with Nb tip, varying superconducting gaps on Cu_0.2Bi₂Se₃ and sub-gap states/ZBCP on the surface of Cu_0.2Bi₂Se₃ with applied magnetic field. Our SIS junctions display super-current peaks that are not bound to zero bias. Instead, they decay away from zero with extra peaks at finite voltage. I will explain this behavior by Cooper-pair current through ultra-small Josephson junctions and loss of energy to modes of the environment, both predicted by the P(E) theory [5]. I will also show microwave response of super-current in Nb-Nb junction and suggest its possible application in my search for (MF).

References

1. C.W.J. Beenakker, Search for Majorans Fermions in Superconductors, Annu. Rev. Condens. Matter Phys. 4, 113 2013
2. S. R. Elliott and M. Franz, Colloquium: Majorana fermions in nuclear, particle, and solid-state physics, Rev. Mod. Phys., 87 (1), 137 2015
3. Anita Roychowdhury, M. A. Gubrud, R. Dana, J. R. Anderson, C. J. Lobb, F. C. Wellstood, and M. Dreyer, A 30 mK, 13.5 T scanning tunneling microscope with two independent tips, Rev. Sci. Inst. 85, 043706 2014
4. A. Roychowdhury, R. Dana, M. Dreyer, J. R. Anderson, C. J. Lobb and F. C. Wellstood, Plasma etching of superconducting Niobium tips for scanning tunneling microscopy, J. Appl. Phys. 116, 014308 2014
5. G.-L. Ingold, H. Grabert, U. Eberhardt, Cooper-pair current through ultra-small Josephson junctions, PRB 50(1), 395 1994