C. C. Fu

Effects of lattice distortion and Jahn-Teller coupling on the magnetoresistance of La0.7Ca0.3MnO3 and La0.5Ca0.5CoO3 epitaxial films

Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T → TC, TC being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T ≪ TC/, the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn–Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn–Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn–Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites.

Effects of lattice distortion on the physical properties and surface morphology of magnetoresistive perovskite epitaxial films

The effects of lattice distortion on the physical properties of La0.7Ca0.3MnO3 epitaxial films are investigated. Our results suggest that larger substrate-induced lattice distortion gives rise to larger zero-field resistivity and larger negative magnetoresistance. Similar effects are also observed in samples of different thicknesses and on the same substrate material, with larger resistivity and magnetoresistance associated with thinner samples. In addition to x-ray diffraction spectroscopy, the degrees of lattice distortion in different samples are further verified by the surface topography taken with a low-temperature scanning tunneling microscope. Quantitative analyses of the transport properties suggest that the high-temperature (T→TC) colossal magnetoresistance (CMR) in the manganites is consistent with the conduction of lattice polarons induced by the Jahn–Teller coupling, and that the low-temperature (T≪TC) magnetoresistance may be attributed to the magnetic domain wall scattering. In contrast, the...

Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting YBa 2 Cu 3 O 7 - Φ (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-l-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density (J c ) as a function of the injection current density (J i n j ), temperature (T), and the thickness (d) of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures. with strong suppression of J c and a rapidly increasing characteristic transport length near the superconducting transition temperature T c only in the F-I-S samples The temperature dependence of the efficiency (η≡ΔJ c /J i n j ; ΔJ c : the suppression of critical current due to finite J i n j ) in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift μ* in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO 2 planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity.

Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-pola...

Investigating the pairing state of cuprate superconductors via quasiparticle tunneling and spin injection

Abstract Scanning tunneling spectroscopic studies of cuprate superconductors revealed that the most eminent phenomena of hole-doped (p-type) cuprates, including the predominant d x 2 − y 2 pairing symmetry, spin fluctuations and pseudogap, are absent in the infinite-layer electron-doped (n-type) cuprates Sr 1− x La x CuO 2 . The optimally doped Sr 0.9 La 0.1 CuO 2 exhibits s-wave pairing symmetry, with a superconducting transition temperature T c =43.0 K, a large energy gap Δ =13.0±1.0 meV, and no pseudogap in the normal state. The response of Sr 0.9 La 0.1 CuO 2 to quantum impurities is also conventional, being less sensitive to non-magnetic impurities than to magnetic impurities, in sharp contrast to the strong effects induced by static non-magnetic impurities in the p-type cuprates. The dynamic injection of spin-polarized quasiparticles into the p-type cuprates is found to incur significant suppression of superconductivity, probably due to the strong influence on the spin fluctuations.

Giant spontaneous Hall effect and magnetoresistance in La1−xCaxCoO3 (0.1⩽x⩽0.5)

Results of resistivity and Hall effect measurements in La1−xCaxCoO3 (0.1⩽x⩽0.5) epitaxial films and ceramics are presented. The spontaneous Hall effect in La1−xCaxCoO3 (LCCO) is observed for ferromagnetic samples with x⩾0.2. The Hall effect is largest near the magnetic percolation threshold x=0.2. For x=0.2, the low-field slope of the Hall resistivity, ρxy/(μ0H), attains a large value of 2×10−6 m3/C below the Curie temperature Tc, which may be applied to sensitive low-field detection. Except near the magnetic percolation threshold, the longitudinal resistivity of LCCO decreases with increasing field at all temperatures. Anomalous temperature-dependent magnetoresistance occurs in the sample with x=0.2, which may be associated with the spin-state transition in LCCO.

Dynamic Pair Breaking in Cuprate Superconductors Via Injection of Spin-Polarized Quasiparticles in Perovskite F-I-S Heterostructures

Abstract We report experimental evidence of dynamic Cooper pair breaking induced by spin-polarized quasiparticles in cuprate superconductors by studying the critical current density and quasiparticle density of states of ferromagnet–insulator–superconductor (F–I–S) heterostructures. The spin diffusion length and relaxation time are also estimated.

Magnetic Percolation Effect on the Spontaneous Hall Resistivity and Magnetoresistance of La(1-x)A(x)CoO(3) (A = Ca, Sr; 0.1 <= x <= 0.5)

Abstract The Hall resistivity and magnetoresistance of La 1−x A x CoO 3 ( A = Ca , Sr ) are investigated. The spontaneous Hall coefficient reaches maximum near the Curie temperature for each doping level, and achieves the largest magnitude near the magnetic percolation threshold. The physical significance of these results are discussed.

Investigation of the current-induced nonlinear vortex dynamics in YBa2Cu3O7 single crystals using transport and transmittivity measurements

Abstract The current-induced nonlinear vortex dynamics of high-temperature superconductors are investigated in two YBa 2 Cu 3 O 7 single crystals: one untwinned and the other with canted columnar defects. Both the electric transport properties and harmonic transmittivities demonstrate unique current-driven effects associated with the intrinsic pinning forH¯⊥ĉin layered superconductors, in contrast to the defect-dependent current-driven effects forH¯|ĉ.

Observation of defect-independent ab-plane vortex dynamics in YBa2Cu3O7 single crystals with the third harmonic transmittivity measurements

AbstractAnisotropic vortex dynamics of YBa2Cu3O7 single crystals with heavy-ion induced parallel and canted columnar defects are investigated and compared with that of the as-grown single crystals. Novel vortex dynamics are observed when magnetic field is applied close to the ab-plane