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

摘要

The low-energy excitations of cuprate superconductors exhibit variouscharacteristics that differ from those of simple Bogoliubov quasiparticles forpure d_{x^2-y^2}-wave superconductors. Here we report experimental studies ofspatially resolved quasiparticle tunnelling spectra of hole- and electron-typecuprate superconductors that manifest direct evidences for the presence ofcompeting orders (COs) in the cuprates. In contrast to conventional type-IIsuperconductors that exhibit enhanced local density of states (LDOS) peaking atzero energy near the centre of field-induced vortices, the vortex-state LDOS ofYBa_2Cu_3O_{7-delta} (Y-123) and La_{0.1}Sr_{0.9}CuO_2 (La-112) remainssuppressed inside the vortex core, with pseudogap (PG)-like features at anenergy larger (smaller) than the superconducting (SC) gap Delta_{SC} in Y-123(La-112). Energy histograms of the SC and PG features reveal steady spectralshifts from SC to PG with increasing magnetic field H. These findings may beexplained 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 correspondingCOs 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} issmaller than Delta_{SC} and the CO wave-vector is along (pi,pi). This COscenario may be extended to the ARPES data to consistently account for thepresence (absence) of Fermi arcs in hole- (electron)-type cuprates. Fouriertransformation of the vortex-state LDOS in Y-123 further reveals multiple setsof energy-independent wave-vectors due to field-enhanced pair- and spin-densitywaves. These results imply important interplay of SC with low-energy collectiveexcitations.