Electron Tunneling and Point Contact Andreev Reflection Studies of Superconductors

Open Access
Author:
Dai, Wenqing
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 26, 2014
Committee Members:
  • Qi Li, Dissertation Advisor
  • Qi Li, Committee Chair
  • Moses Hung Wai Chan, Committee Member
  • Renee Denise Diehl, Committee Member
  • Nitin Samarth, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • Superconductor
  • Tunneling
  • Point contact
  • Topological superconductor
  • multiband superconductor
  • proximity effect
Abstract:
The energy gap is the most fundamental property of a superconductor. Electron tunneling spectroscopy and point contact spectroscopy (PCS) are powerful techniques for studying the density of states and energy gap features of superconductors. Two different superconducting systems, multiband superconductor MgB2 and proximity induced topological superconductor NbSe2/Bi2Se3 heterostructures were studied using either quasiparticle tunneling in planar tunnel junctions or PCS in this work. MgB2 is a conventional s-wave superconductor originated from electron-phonon interaction with a transition temperature of 39 K, the highest among conventional superconductors. It is also the first well established two-band superconductor. The two different superconducting gaps are caused by the electron-phonon interactions in two weakly interacting bands. Previous research in the group has established a method to fabricate high quality MgB2/native oxide/superconductor planar tunnel junctions and revealed a distribution of gap values in MgB2 from the quasiparticle tunneling spectra. Those results confirmed the importance of the anisotropic electron-phonon interaction. In this thesis, the effect of electron scattering in the tunnel junctions, either from the MgB2 film or the junction interface or barrier, are studied. The scattering was shown to smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering. In addition, the native oxide barrier composition and properties were obtained from the tunneling spectra. Another novel superconducting system, topological superconductor NbSe2/Bi2Se3 heterostructures were studied using the point contact Andreev reflection (PCAR) spectroscopy method at low temperatures down to 40 mK. According to theories, when the 3D topological insulator Bi2Se3 surface state is in proximity with an s-wave superconductor NbSe2, the resulting superconducting surface state resembles the spinless topological superconductor, which is predicted to host Majorana fermions. Scanning Tunneling Spectroscopy (STS) measurements first demonstrated an induced superconducting energy gap in topological insulator Bi2Se3 and Bi2Te3 in the NbSe2/Bi2Se3 and NbSe2/Bi2Te3 heterostructures. In this thesis, PCAR spectra revealed an induced energy gap in the bulk of Bi2Se3 due to the superconducting proximity effect from NbSe2. The gap values were consistent with the bulk superconducting gap observed by the ARPES and STS measurements. The gap was ~0.16 meV at low temperature in a NbSe2/16QL Bi2Se3 heterostructure. In addition, below 0.45 K, a plausible second energy gap ~0.12 meV appeared in the NbSe2/16QL Bi2Se3 PCAR spectra which could indicate a superconducting gap in the topological surface state. Besides spectroscopic measurements on superconducting energy gap features, a study of high-field properties of carbon-doped MgB2 superconducting thin films prepared by hybrid physical-chemical vapor deposition (HPCVD) using the trimethylboron (TMB) precursor source is included in Chapter 5. MgB2 holds great potentials for high magnetic field applications due to its high Tc. Previous research in the group on carbon-doped MgB2 thin films by HPCVD using a metal-organic source (MeCp)2Mg showed significantly enhanced upper critical field Hc2 above 70 T. In this work, another precursor source TMB, was used as the carbon source. The heavily C alloyed films using TMB source show better high parallel Hc2 than the films using (MeCp)2Mg source ( slope as high as ~ 8.3 T/K near Tc). The behavior is found to depend on the unique microstructure of the films, which consists of few-nanometers thick MgB2 layers separated by non-superconducting MgB2C2 layers. It leads to an increase in the parallel Hc2 by the geometrical effect, which is in addition to the significant enhancement of Hc2 due to changes of the scattering rates within and between the two bands. For the applications of MgB2 in high magnetic fields, the high Hc2 values observed are very promising.