I. Kurosawa

Size dependence and mesoscopic effect of YBa2Cu3O7−δ/La0.67Sr0.33MnO3 junctions

Abstract The size dependence of transport properties is studied in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 (YBCO/LSMO) junctions. When the junction size is 40×30 μm 2 , a large zero-bias conductance peak originated from d-wave pairing symmetry has clearly been observed. However, as the junction size becomes smaller (15×15 μm 2 ) , the conductance spectra are drastically changed. The large peak becomes broader, and a small zero-bias peak with a pair of dips whose energy levels are symmetric with respect to zero-bias appears only below 6 K. The features peculiar to the small junction reflect the presence of the phase coherent transport due to the mesoscopic size of effective junction area.

Composition and phase variations in Y–Ba–Cu–O thin films grown by off-axis sputtering as a function of deposition parameters

Abstract The deposition condition of superconducting YBa 2 Cu 3 O y (YBCO) thin films grown by 90° off-axis dc sputtering was investigated to obtain large and smooth YBCO thin films. The composition of the deposited Y–Ba–Cu–O thin films strongly depended on the distance from the target and sputtering pressure. The composition and phase variations in Y–Ba–Cu–O thin films was consistently described in the simple model. Under the optimized deposition condition, single-phase YBCO thin films with stoichiometric composition could be obtained. YBCO thin films with critical temperatures above 85 K and surface roughnesses of 5 nm were successfully obtained in an area approximately 2-in. in diameter by rotating the substrate.

Mesoscopic effect observed in strong correlated electron materials

Abstract The conductance spectra in YBa 2 Cu 3 O 7− δ /La 0.67 Sr 0.33 MnO 3 (YBCO/LSMO) tunnel junctions are studied in ultra-low-temperature region in order to clarify the influences of strong correlation on transport. The conductance spectra vary from usual Blonder–Tinkham–Klapwijk type to phase coherent transport as the junction size becomes smaller. Conductance spectra obtained on small junctions (less than 15×15 μm 2 ) show a periodic response to the applied magnetic field, which suggests the presence of the Aharanov–Bohm effect in the LSMO electrode. From these features, we conclude that the mesoscopic effect due to phase coherency of conducting electrons survives even in the strongly correlated electron system. Estimated phase coherence length l φ of LSMO is between 0.19 and 9 μm at 150 mK.

Magnetic-field dependence of tunneling conductance spectra of La0.67Sr0.33MnO3/YBa2Cu3O7−δ junctions in ultra-low temperatures

We precisely measured conductance spectra near the zero-bias conductance peak (ZBCP) in La 0.67 Sr 0.33 MnO 3 / YBa 2 Cu 3 O 7-δ junctions, which reflects the formation of the surface bound states in the d-wave superconductors. When the conductance spectra of the junctions were measured at ultra-low temperatures (<150 mK), very sharp ZBCPs were observed. We found the notable features of the magnetic field response of the ZBCP, i.e., the coexistence of spin-dependent and spin-independent components in the ZBCP and the apparent absence of the Zeeman splitting.

Mesoscopic effect observed in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 tunnel junctions

Abstract Transport properties of YBa2Cu3O7−δ/La0.67Sr0.33MnO3 tunnel junction are studied at low temperature. The conductance spectra showed pronounced size dependence, i.e., the sharp zero-bias peak transited to a peak-and-dip structure as the junction size becomes smaller (∼10 μm ) . The peak-and-dip structure is sensitive to applied magnetic field, and the dip position moves as the magnetic field increases. These results suggest that mesoscopic effects such as the phase coherent effect and Aharanov–Bohm effect coexist in the present junction, even though YBCO and LSMO have a strongly correlated electron system.

Spin‐polarized Tunneling Spectroscopy of YBCO/LSMO Tunnel Junction

The Zeeman magnetic field responses of eptaxially grown YBa2Cu3O7−δ/La0.67Sr0.33MnO3 (YBCO/LSMO) tunnel junction are studied for various doping rates of YBCO. We have fabricated underdoped, optimally doped, and overdoped edge junctions, and the magnetic field is applied along the junction interface (perpendicular to the c‐axis). The measured conductance spectra at below 1 K showed neither splitting nor broadening up to 14 T with μV resolution for all doping rates. Also the magnetic field response at higher temperature showed no Zeeman splitting. This result apparently contradicts the model based on the weak coupling theory for d‐wave, and implies the presence of anomalous quasiparticle states in YBCO.