Spectroscopic Evidence for Competing Order-Induced Pseudogap Phenomena and Unconventional Low-Energy Excitations in High-TC Cuprate Superconductors

The low-energy excitations of cuprate superconductors exhibit various characteristics that differ from those of simple Bogoliubov quasiparticles for pure d_{x^2-y^2}-wave superconductors. Here we report experimental studies of spatially resolved quasiparticle tunnelling spectra of hole- and electron-type cuprate superconductors that manifest direct evidences for the presence of competing orders (COs) in the cuprates. In contrast to conventional type-II superconductors that exhibit enhanced local density of states (LDOS) peaking at zero energy near the centre of field-induced vortices, the vortex-state LDOS of YBa_2Cu_3O_{7-\delta} (Y-123) and La_{0.1}Sr_{0.9}CuO_2 (La-112) remains suppressed inside the vortex core, with pseudogap (PG)-like features at an energy larger (smaller) than the superconducting (SC) gap \Delta_{SC} in Y-123 (La-112). Energy histograms of the SC and PG features reveal steady spectral shifts from SC to PG with increasing magnetic field H. These findings may be explained by coexisting COs and SC: For hole-type cuprates with PG above T_c, the primary CO gap (V_{CO}) is larger than \Delta_{SC} and the corresponding COs are charge/pair-density waves with wave-vectors parallel to (\pi,0)/(0,\pi). For electron-type cuprates without PG above T_c, V_{CO} is smaller than \Delta_{SC} and the CO wave-vector is along (\pi,\pi). This CO scenario may be extended to the ARPES data to consistently account for the presence (absence) of Fermi arcs in hole- (electron)-type cuprates. Fourier transformation of the vortex-state LDOS in Y-123 further reveals multiple sets of energy-independent wave-vectors due to field-enhanced pair- and spin-density waves. These results imply important interplay of SC with low-energy collective excitations.
Comment: 8 pages, 8 figures. Accepted for publication in Journal of Superconductivity and Magnetism. [Manuscript received the Scientific and Technological Accomplishment Papers Award at the New3-SC7 Conference in Beijing, China, May 13 -- 16 (2009)]. Corresponding author: Nai-Chang Yeh (e-mail: ncyeh@caltech.edu)