Studies of the Low-Energy Quasiparticle Excitations in High-Temperature Superconducting Cuprates with Scanning Tunneling Spectroscopy and Magnetization Measurements

Citation

Beyer, Andrew David (2009) Studies of the Low-Energy Quasiparticle Excitations in High-Temperature Superconducting Cuprates with Scanning Tunneling Spectroscopy and Magnetization Measurements. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/MM6C-AS16. https://resolver.caltech.edu/CaltechETD:etd-06082009-200539

Abstract

This thesis details the investigation of the unconventional low-energy quasiparticle excitations in both hole-and electron-type cuprate superconductors through experimental studies and theoretical modeling. The experimental studies include spatially resolved scanning tunneling spectroscopy (STS) experiments and bulk magnetization measurements, and the theoretical modeling involves developing a phenomenology that incorporates coexisting competing orders and superconductivity in the ground state of the cuprates.

Magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the electron-type cuprate La_0.1Sr_0.9CuO₂ (La-112), the simplest structured cuprate superconductor with T_C = 43 K, are investigated experimentally for the first time. For temperature (T) less than the superconducting transition temperature (T_C), 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 << T_C, vortices are observed in La-112, which is the first direct observation of vortices among electron-type cuprate superconductors. 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 V_PG=(8.5±0.6)meV, while the inter-vortex quasiparticle spectra show larger peak-to-peak gap values characterized by Δ_pk-pk(H) ≥ V_PG, and Δ_pk-pk(0)=(12.2±0.8)meV ≥Δ_pk-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 V_PG energy scale. This finding is in stark contrast to the vortex-state quasiparticle spectra in conventional superconductors, where the intra-vortex spectra near vortex cores exhibit a sharp zero-bias conductance peak due to the complete suppression of superconductivity and the presence of continuous bound quasiparticle states. The lack of a zero-bias peak and the observation of pseudogap-like spectra in the intra-vortex quasiparticle spectra of La-112 suggest that superconductivity alone cannot describe the STS results.

Similar studies of the magnetic field and temperature dependent evolution of the spatially resolved quasiparticle excitation spectra in the hole-type cuprate YBa₂Cu₃O_7-δ (Y-123) have also been carried out. The quasiparticle spectra for T << T_C(~93 K) show satellite features at an energy higher than the superconducting gap, and the superconducting gap is found to be associated with a set of coherence peaks for H = 0. The coherence peaks are homogeneous, with a energy gap given by Δ_SC=(20±1)meV, and may be attributed to superconductivity. The satellite features are less homogeneous, with a effective gap energy Δ_eff=(37.8±2.0)meV. The application of magnetic fields reveal vortices in Y-123, and the intra-vortex quasiparticle spectra show two energy gaps, with one gap at the pseudogap energy scale V_PG~32meV and the other gap at the subgap energy scale Δ' ~ 7-12meV < Δ_SC. In contrast, the inter-vortex quasiparticle spectra reveal only one energy gap at Δ_SC~20meV. A dramatic shift in the peak-to-peak gaps, Δ_pk-pk(H), from Δ_SC to both V_PG and Δ' with increasing magnetic field is observed. In addition, higher spatial resolution STS measurements were performed in Y-123 to investigate the spatial dependence of the quasiparticle spectra in more detail. The experimental resolution allowed Fourier-transformed local density of states analysis to be performed. Energy-dependent dispersive diffraction modes attributable to quasiparticle scattering interferences (QPI) were seen, as well as three energy-independent modes not due to QPI. The energy-independent modes corresponded to periodic real-space conductance modulations along the Cu-O bonding and the nodal directions attributable to a pair-density wave, a charge-density wave, and a spin-density wave. The totality of data in Y-123 suggests that the ground state of Y-123 contains competing orders coexisting with superconductivity and not superconductivity alone.

In addition to the STS experiments, the effects of unconventional quasiparticle excitations on macroscopic superconductivity and vortex phase diagrams are investigated from bulk magnetization measurements on several different families of superconducting cuprate samples. Evidence for strong field-induced quantum phase fluctuations and quantum criticality are observed in the vortex phase diagrams of all samples considered. The origin of the apparent quantum criticality and strong field-induced quantum phase fluctuations due to the nearby presence of competing orders is discussed.

Finally, a "two-gap" phenomenological model, describing the excitations from a ground state of coexisting superconductivity and a competing order, is used to quantitatively model the unconventional quasiparticle excitations observed in the measurements of the local tunneling density of states and the angle-resolved photoemission spectroscopy (ARPES) experiments. The phenomenological model is found to provide consistent accounts for the quasiparticle tunneling data from our measurements in La-112 and Y-123, as well as experimental data by others on different cuprates.

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