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Scanning Tunneling Spectroscopic Studies on High-Temperature Superconductors and Dirac Materials


Teague, Marcus Lawrence (2013) Scanning Tunneling Spectroscopic Studies on High-Temperature Superconductors and Dirac Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/M8FW-S641.


This thesis details the investigations of the unconventional low-energy quasiparticle excitations in electron-type cuprate superconductors and electron-type ferrous superconductors as well as the electronic properties of Dirac fermions in graphene and three-dimensional strong topological insulators through experimental studies using spatially resolved scanning tunneling spectroscopy (STS) experiments.

Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type cuprate La0.1Sr0.9CuO2 (La-112) TC = 43 K, are investigated experimentally. For temperature (T) less than the superconducting transition temperature (TC), and in zero field, the quasiparticle spectra of La-112 exhibits gapped behavior with two coherence peaks and no satellite features. For magnetic field measurements at T < TC, first ever observation of vortices in La-112 are reported. Moreover, pseudogap-like spectra are revealed inside the core of vortices, where superconductivity is suppressed. The intra-vortex pseudogap-like spectra are characterized by an energy gap of VPG = 8.5 ± 0.6 meV, while the inter-vortex quasiparticle spectra shows larger peak-to-peak gap values characterized by Δpk-pk(H) >VPG, and Δpk-pk (0)=12.2 ± 0.8 meV > Δpk-pk (H > 0). The quasiparticle spectra are found to be gapped at all locations up to the highest magnetic field examined (H = 6T) and reveal an apparent low-energy cutoff at the VPG energy scale.

Magnetic-field- and temperature-dependent evolution of the spatially resolved quasiparticle spectra in the electron-type "122" iron-based Ba(Fe1-xCox)2As2 are investigated for multiple doping levels (x = 0.06, 0.08, 0.12 with TC= 14 K, 24 K, and 20 K). For all doping levels and the T < TC, two-gap superconductivity is observed. Both superconducting gaps decrease monotonically in size with increasing temperature and disappear for temperatures above the superconducting transition temperature, TC. Magnetic resonant modes that follow the temperature dependence of the superconducting gaps have been identified in the tunneling quasiparticle spectra. Together with quasiparticle interference (QPI) analysis and magnetic field studies, this provides strong evidence for two-gap sign-changing s-wave superconductivity.

Additionally spatial scanning tunneling spectroscopic studies are performed on mechanically exfoliated graphene and chemical vapor deposition grown graphene. In all cases lattice strain exerts a strong influence on the electronic properties of the sample. In particular topological defects give rise to pseudomagnetic fields (B ~ 50 Tesla) and charging effects resulting in quantized conductance peaks associated with the integer and fractional Quantum Hall States.

Finally, spectroscopic studies on the 3D-STI, Bi2Se3 found evidence of impurity resonance in the surface state. The impurities are in the unitary limit and the spectral resonances are localized spatially to within ~ 0.2 nm of the impurity. The spectral weight of the impurity resonance diverges as the Fermi energy approaches the Dirac point and the rapid recovery of the surface state suggests robust topological protection against perturbations that preserve time reversal symmetry.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Graphene, Topological Insulators, Iron Pnictide, High Temperature Superconductivity, Cuprate Superconductivity, Scanning Tunneling Microscopy, STM
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Yeh, Nai-Chang
Group:Institute for Quantum Information and Matter
Thesis Committee:
  • Yeh, Nai-Chang (chair)
  • Eisenstein, James P.
  • Libbrecht, Kenneth George
  • Motrunich, Olexei I.
Defense Date:8 June 2012
Non-Caltech Author Email:mindtex (AT)
Record Number:CaltechTHESIS:05142013-151159910
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7709
Deposited By: Marcus Teague
Deposited On:17 May 2013 22:57
Last Modified:02 Jun 2020 21:55

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