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Scanning tunneling spectroscopy studies of high-temperature cuprate superconductors

Citation

Chen, Ching-Tzu (2006) Scanning tunneling spectroscopy studies of high-temperature cuprate superconductors. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05222006-124257

Abstract

This thesis presents the scanning tunneling spectroscopic studies of the non-universal electronic properties among electron- and hole-doped cuprates. Tunneling spectra of the electron-doped Sr0.9La0.1CuO2 and the hole-doped YBa2Cu3O6+delta reveal distinctly different behavior in the pairing symmetries, pseudogap phenomena, satellite features, and low-energy excitations. While underdoped and optimally doped YBa2Cu3O6+delta exhibits d-wave and overdoped Ca-doped YBa2Cu3O6+delta (d+s)-wave pairing symmetry, the electron-doped Sr0.9La0.1CuO2 shows fully gapped s-wave pairing symmetry. The absence of the satellite features and pseudogap in tunneling spectra of electron-doped cuprates sharply contrasts with their general presence in hole-doped cuprates. Furthermore, the subgap low-energy spectral characteristics of Sr0.9La0.1CuO2 deviate substantially from the mean-field Bardeen-Cooper-Schrieffer theory, while those of YBa2Cu3O6+delta can be fully accounted for by the mean-field generalized Blonder-Tinkham-Klapwijk formalism. Despite the aforementioned disparities, several experimental results reveal important connections between the two types of cuprates. For instance, the coexistence of the pseudogap and superconducting spectra in hole-doped cuprates and the observations of the current- and field-induced pseudogap in electron-doped cuprates suggest that competing orders, manifested as the pseudogap, coexist with superconductivity in both types of cuprates. In addition, by comparing the tunneling spectra with the high-field vortex dynamics measurements, we find that the quasiparticle spectral characteristics of Sr0.9La0.1CuO2 and YBa2Cu3O6+delta correlate with the degree of field-induced quantum phase fluctuations of the two compounds. Based on these findings, we propose a simple model of coexisting density waves with superconductivity to unify the apparent non-universal phenomena among cuprate superconductors. By incorporating quantum phase fluctuations and adopting realistic band structures, numerical simulations of the quasiparticle tunneling spectra demonstrate excess subgap low-energy excitations, which is consistent with the empirical observations in Sr0.9La0.1CuO2. Furthermore, by tuning the ratio of the density waves to superconductivity, the theoretical calculations reproduce the absence of pseudogap phenomena in electron-doped cuprates and the general presence of he pseudogap in hole-doped cuprates. Thereby, we conclude that the competing orders that coexist with superconductivity in cuprate superconductors contribute to the rich cuprate phenomenology.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:competing orders; high-temperature superconductivity; scanning tunneling spectroscopy; Sr_{0.9}La_{0.1}CuO_2; YBa_2Cu_3O_{6+delta}
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
Thesis Committee:
  • Yeh, Nai-Chang (chair)
  • Refael, Gil
  • Eisenstein, James P.
  • Zmuidzinas, Jonas
Defense Date:19 May 2006
Record Number:CaltechETD:etd-05222006-124257
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05222006-124257
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1943
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:31 May 2006
Last Modified:02 Dec 2014 23:22

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