The effect of competing orders and quantum criticality on the macroscopic vortex dynamics and microscopic low-energy excitations of cuprate superconductors is investigated using high-field magnetic measurements and low-temperature scanning tunneling spectroscopy. Our experimental results suggest that significant field-induced quantum fluctuations at low temperatures are present in all cuprates investigated, suggesting that cuprate superconductors are in close proximity to a quantum critical point (QCP) that separates a pure superconducting phase (SC) from a phase consisting of coexisting SC and a competing order. The proximity of a cuprate to the QCP can be determined from a normalized characteristic field in the zero-temperature limit, and the characteristic field correlates well with the quasiparticle tunneling spectra, showing increasing spectral deviation from the mean-field behavior for samples of closer proximity to the QCP. Macroscopically, the presence of competing order can induce strong fluctuation effects in the cuprate superconductors, which is consistent with the extreme type-II nature of the cuprates. The relevant competing orders in different cuprates are examined by comparing theory with experimental data, and the physics implications of these studies are discussed.
We report scanning tunneling spectroscopic studies of the effects of quantum impurities on cuprate superconductors. The samples include p-type YBa2Cu3O7-δ single crystals with spinless impurities of Zn2+ and Mg2+ ((Zn,Mg)-YBCO) and n-type infinite-layer system Sr0.9La0.1CuO2 with 1% magnetic Ni2+- or 1% non-magnetic Zn2+-impurities that substitute the Cu2+ in the CuO2 plane. The local effects of spinless impurities on the quasiparticles spectra of (Zn,Mg)-YBCO are analogous to those of Zn-substituted Bi2Sr2CaCu2O8+x, and the global effect is manifested by the suppression of the pairing potential Δd and of the spin excitation energy. In contrast, spectroscopic studies of Sr0.9La0.1CuO2 reveal momentum-independent spectra and superconducting gap Δ , with (2Δ/kBTc) ~ 7 for Tc = 43 K and no pseudogap above Tc. The global response of Sr0.9La0.1CuO2 to quantum impurities is similar to that of s-wave superconductors, being insensitive to small concentrations of spinless impurities (Zn) while showing rapid degradation in Tc with increasing magnetic impurities (Ni). Moreover, the spectra of the Ni-substituted Sr0.9La0.1CuO2 reveal strong electron-hole asymmetry and long-range impurity effects, in contrast to the localized impurity effects in the p-type cuprates, and the introduction of Zn yield no reduction in either Δ or Tc. The physical implications of these findings are discussed.
The pairing symmetry and the superconducting gap in high- temperature superconducting cuprates are investigated as a function of the hole doping level (x) and temperature (T), using directional scanning tunneling spectroscopy (STS). It is found that the predominant pairing symmetry is (Formula available in paper), which is insensitive to the variations in T and x. In contrast, the maximum superconducting gap ((Delta) d) in YBa2Cu3O7-(delta ) and La2-xSrxCuO4-(delta ) scales with the superconducting transition temperature (Tc), and the ratio of (2 (Delta) d/kBTc) increases with decreasing doping level. The dominance of dx2-y2 pairing is consistent with strong spatial variations in the local quasiparticle spectra near non-magnetic impurities such as Zn and Mg in a (Zn,Mg)-doped YBa2Cu3O7-(delta ) single crystal. To further elucidate the nature of the pairing state, the c-axis spin- polarized quasiparticle transport in the superconducting state of YBa2Cu3O7-(delta ) is investigated by studying the critical currents and STS under the injection of electrical currents from the underlying ferromagnetic La0.7Sr0.3MnO3 layer in various ferromagnet-insulator-superconductor (F-I-S) heterostructures. The temperature dependent spin diffusion length ((delta) s) and signatures of nonequilibrium quasiparticle distribution under spin injection in d-wave superconductors are determined for the first time.
Vortex critical dynamics of superconducting amorphous Mo3Si films are investigated at microwave frequencies from 8 to 19 GHz by using a high-Q sapphire ring resonator at the whispering gallery (WG) modes. It is found that the microwave surface resistance R3 in the vortex-state of a-Mo3Si follows the scaling relation R3 e- 611(2-2).R „(I), where 6 E 11 (TITm)I, TM (H) is the vortex-glass melting temperature in a constant magnetic field H, R3 is a scaling function, f _---F-_- fb-" , and v P.,- 2/3, z :::.: 3.0 are the static and dynamics exponents associated with the vortex- glass transition1'2. In contrast, the vortex dynamics at microwave frequencies for high-temperature superconductors (HTS) such as Nd1.85Ce0.15Cu0.4 and YBa2Cu307 is associated with vortex depinning and diffusion1'2'3 rather than the critical dynamics of the vortex-glass transition1'2. Physical quantities associated with the viscous motion of vortex-liquid in high-temperature superconductors are derived from the microwave measurements.
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