Regensburg 2002 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 6: Supraleitung: Elektronenstruktur, Phononen, Tunneln, Ordnungsparameter I

TT 6.1: Vortrag

Montag, 11. März 2002, 16:00–16:15, H19

DEFECTS IN TRANSITION METAL OXIDES: FROM SHEAR PLANES TO HTS JUNCTIONS AS MOTT INSULATORS AND YBCO IEJ’S — •Juergen Halbritter — Forschungszentrum Karlsruhe, IMF I, Postfach 3640, D-76021 Karlsruhe

The basic building unit of transition metal oxides are O-octaedra being edge connected to blocks (Nb₂O_5−y, . . . ) or planes (STO, . . . HTS) being separated by crystallographic shear planes. Shear planes allow easy creation of oxygen vacancies which then permit fast electron or hole hopping along those planes. The connected O-octaedra blocks or planes are structurally and electronically rigid, yielding, e.g., overdoped Mott insulators in HTS. In high T_c superconductors CuO-plane transport dominate and CuO-in-plane weak links occur easily in preparation and growth, being the major obstacles for HTS transport currents. CuO-plane weak links are tunnel junctions with reduced critical Josephson currents j_cJ(A/cm²), enhanced normal R_bn(Ωcm²), and leakage resistances R_bl(T < T_c) ≥ R_bn, where the degradation of j_cJR_bn ≪ Δ/e with Δ the superconducting energy gap is specific to HTS. Those degradations are consequences of the space-wise transition to a Mott-insulator seam with n_L^∥ localized states by reduced wave function overlap by surfaces and by spatial or bonding disorder, with d^∥> 0.2 nm as tunnel barrier width of height φ ≃ 2eV housing n_L^∥≃ 10²¹/cm³ localized states. To avoid the intrinsic Mott insulating seam, crystalline (YBCO)* is used in interface engineered junction (IEJ), which is a dielectric with a low barrier height φ < 0.1 eV and n^L ≪ 10²¹/cm³, simulating Nb/Al/AlO_x(OH)_y− junctions showing ways out of this interface chemistry dead lock.