Scanning Tunneling Spectroscopic Studies of the Low-Energy Quasiparticle Excitations in Cuprate Superconductors

• Published in: Journal of superconductivity and novel magnetism 2013, Vol.26 (1), p.65-70
• Main Authors: Yeh, N.-C., Teague, M. L., Wu, R. T.-P., Feng, Z. J., Chu, H., Moehle, A. M.
• Format: Article
• Language: English
• Subjects: Carbon monoxide; Cations; Characterization and Evaluation of Materials; Condensed Matter Physics; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Cuprates; Cuprates superconductors (high tc and insulating parent compounds); Exact sciences and technology; Fluid flow; Homogeneity; Magnetic Materials; Magnetism; Materials science; Original Paper; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Physics and Astronomy; Properties of type I and type II superconductors; Strongly Correlated Systems; Superconductivity; Superconductors; Vortex lattices ,flux pinning, flux creep; Vortices; YBCO superconductors
• ISSN: 1557-1939; 1557-1947
• DOI: 10.1007/s10948-012-1706-y

We report scanning tunneling spectroscopic (STS) studies of the low-energy quasiparticle excitations of cuprate superconductors as a function of magnetic field and doping level. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition T c is associated with a CO energy Δ CO larger (smaller) than the SC gap Δ SC . Moreover, the spatial homogeneity of Δ SC and Δ CO depends on the type of disorder in different cuprates: For optimally and under-doped YBa 2 Cu 3 O 7− δ (Y-123), we find that Δ SC < Δ CO and that both Δ SC and Δ CO exhibit long-range spatial homogeneity, in contrast to the highly inhomogeneous STS in Bi 2 Sr 2 CaCu 2 O 8+ x (Bi-2212). We attribute this contrast to the stoichiometric cations and ordered apical oxygen in Y-123, which differs from the non-stoichiometric Bi-to-Sr ratio in Bi-2212 with disordered Sr and apical oxygen in the SrO planes. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder in the CuO 2 planes, giving rise to increasing inhomogeneity, decreasing Δ SC and Δ CO , and a suppressed vortex-solid phase. For electron-type cuprate Sr 0.9 La 0.1 CuO 2 (La-112), the homogeneous Δ SC and Δ CO distributions may be attributed to stoichiometric cations and the absence of apical oxygen, with Δ CO < Δ SC revealed only inside the vortex cores. Finally, the vortex-core radius ( ξ halo ) in electron-type cuprates is comparable to the SC coherence length ξ SC , whereas ξ halo ∼10 ξ SC in hole-type cuprates, suggesting that ξ halo may be correlated with the CO strength. The vortex-state irreversibility line in the magnetic field versus temperature phase diagram also reveals doping dependence, indicating the relevance of competing orders to vortex pinning.