Direct experimental evidence of half-metallic density of states (DOS) is observed by scanning tunneling spectroscopy on ferromagnetic La 0.7 Ca 0.3 MnO 3 , which exhibits colossal magnetoresistance (CMR). Tunneling conductance data taken at 77 K, well below the Curie temperature T C ഠ 260 K, show close resemblance to the spin-split DOS spectrum calculated for the itinerant bands in the ferromagnetic state. The half-metallic spectral characteristics are absent in the paramagnetic state at room temperature, as well as in the undoped antiferromagnetic compound LaMnO 3 , which shows no CMR. These results directly implicate the half-metallic ferromagnetism in the phenomenon of CMR.

Epitaxial diodes of a half-metallic ferromagnet on an oxide semiconductor J. Appl. Phys. 95, 7324 (2004); 10.1063/1.1669255 Enhancement of room temperature magnetoresistance in double perovskite ferrimagnets

By spatially mapping the Doppler effect of an in-plane magnetic field on the quasiparticle tunneling spectrum, we have laterally imaged the vortex lattice in superconducting 2H-NbSe 2 . Cryomagnetic scanning tunneling spectroscopy was performed at 300 mK on the ab-surface oriented parallel to the field H. Conductance images at zero bias show stripe patterns running along H, with the stripe separation varying as H −0.5 . Regions of higher zero-bias conductance show lower gap-edge conductance, consistent with spectral redistribution by spatially-modulated superfluid momentum. Our results are interpreted in terms of the interaction between vortical and screening currents, and demonstrate a general method for probing subsurface vortices.PACS numbers: 74.55.+v, 74.25.Uv, 74.70.Ad In response to an applied magnetic field, type-II superconductors experience a diamagnetic current that circulates along the sample edge. Above the lower critical field, field can penetrate into the superconductor via a lattice of vortices, each consisting of a paramagnetic current loop enclosing a flux quantum.1 The vortex lattice can be imaged using techniques sensitive to variations in the local magnetic field such as Bitter decoration 2 and Lorentz microscopy, 3 or with scanning tunneling microscopy (STM) which probes the local quasiparticle density of states (DOS). STM imaging of the vortices is possible by virtue of bound states and suppressed superconducting gap in the vortex core.4,5 Because of this reliance on vortex-core states, STM imaging has been largely limited to the cross-sectional geometry, i.e. with the vortices piercing the sample surface. An earlier STM study versus field direction has shown the density of vortices to decrease as the field is tilted away from the surface normal.6,7 For fields parallel to the surface, the vortex cores become buried in the bulk, making them difficult to probe directly. In this lateral field geometry, vortex lattices have been imaged by Lorentz microscopy, but only in highly 2D superconductors where pancake vortices decorate in-plane flux lines. 8,9In this letter, we report on lateral imaging of the superconducting vortex lattice using cryomagnetic scanning tunneling spectroscopy. By mapping the zero-bias tunneling conductance over the ab-surface of superconducting 2H-NbSe 2 in an in-plane magnetic field and at 300 mK, we have observed distinct stripe patterns whose orientation and spacing versus the field can be directly attributed to the in-plane flux lattice. Our observations are interpreted in terms of the interaction between the diamagnetic screening current and the paramagnetic vortical currents, which results in a spatially-modulated Doppler effect on the quasiparticle DOS spectrum.The STM used in our experiment was specially designed for the magnetic field to be applied parallel to the sample surface, as shown in Fig. 1(e). The STM is mounted inside a 3 He cryostat which is inserted into a superconducting solenoid. The Pt-Ir tips used were fieldemitted in situ to ensure stable vac...

Microstructures and resistivity of cuprate/manganite bilayer deposited on SrTiO 3 substrate Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa 2 Cu 3 O 7Ϫ␦ and La 0.7 Ca 0.3 MnO 3 , to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high T c cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum ⌬ 0 Ϸ22 meV, up to at least 35 mA (7ϫ10 3 A/cm 2 ) 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-polarized quasiparticles.

The effects of lattice distortion on the physical properties of La 0.7 Ca 0.3 MnO 3 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→T C) colossal magnetoresistance ͑CMR͒ in the manganites is consistent with the conduction of lattice polarons induced by the Jahn-Teller coupling, and that the low-temperature (TӶT C) magnetoresistance may be attributed to the magnetic domain wall scattering. In contrast, the absence of the Jahn-Teller coupling and the large conductivity in La 0.5 Ca 0.5 CoO 3 epitaxial films yield much smaller negative magnetoresistance, which may be attributed to disorder-spin scattering.