F. Freibert

MAGNETIC AND TRANSPORT PROPERTIES OF NA DOPED SRRUO3 AND CARUO3

The magnetic susceptibility χ(T) (2≤T≤400 K) and electrical resistivity ρ(T) (2≤T≤300 K) of undoped and Na doped SrRuO3 and CaRuO3 single crystals were measured. These two perovskites are orthorhombic with space group Pbnm. While both of the undoped systems exhibit metallic conductivity, SrRuO3 is ferromagnetic with Tc≤160 K, but CaRuO3 is paramagnetic. When a small concentration of Na (≤12%) replaces Sr in the structure, SrRuO3 becomes an insulator with a depressed ferromagnetic response. In contrast, Na doped CaRuO3 (≤5%) remains metallic with the appearance of antiferromagnetism below 70 K. The dramatic changes in magnetic and transport properties resulting from Na replacement of Sr and Ca reflects the very subtle differences driving the magnetism in SrRuO3 and CaRuO3.

Itinerant-to-localized electron transition in perovskite CaRu1−xRhxO3

Magnetic susceptibility χ(T) (2⩽T⩽400 K), electrical resistivity ρ(T) (2⩽T⩽300 K), and Hall effect of the distorted perovskite CaRu1−xRhxO3 with 0⩽x⩽0.35 were measured. This study indicates that the magnetic and transport properties of CaRuO3, a paramagnetic conductor with a narrow 4d-band, depend in an extremely sensitive way on the degree of band filling and bandwidth. Slight Rh doping, while retaining the crystals structure, effectively narrows the 4d-band, and enhances the spin correlation. The occurrence of the enhanced spin correlation is accompanied by an itinerant-to-localized electron transition and a change in crystallographic constants. The results presented together with others strongly suggests that CaRuO3, is on the verge of the metal-insulator transition and magnetic ordering.

Effects of Pr, Tb, and Zn doping into YBa2Cu3O7 on magnetoresistivity and magnetic phase boundaries

Transverse AB‐plane magnetoresistivity ρ AB (C axis ∥H) of (Y1−X Pr X )Ba2Cu3O7−δ and YBa2(Cu1−X Zn X )3O7−δthin films and (Y1−X Tb X )Ba2Cu3O7−δthin films and single crystals has been measured as a function of 0.0<X<X C , 2 K<T<300 K, and 0 T<H<20 T. Mean‐field upper critical field H C2 and irreversibility field H I phase boundaries have been determined and are discussed in terms of proposed theories. Tb doping of YBa2Cu3O7 reflects an enhanced pinning or ‘‘stiffening’’ of the vortex system without apparent alteration to T C or the slope of the mean‐field normal phase‐mixed phase boundary line, dH C2/dT‖ T=T c . However, Pr and Zndoping of YBa2Cu3O7 tends to depress both T C and dH C2/dT‖ T=T c and shows a ‘‘softening’’ of the vortex system. This behavior may be understood in terms of the magnitude of the superconducting pair coherence length, which is controlled by doping, and the divergence of the vortex‐glass correlation length as it relates to flux line coupling of vortices in the CuO2 planes. These systems are compared with other high T C superconducting systems which possess similar phase boundary properties.

Anomalous magnetic properties of Tb doped YBa2Cu3O7 single crystals

Abstract High quality single crystals of Y1−xTbxBa2Cu3O7 (0 ⩽ x ⩽ 0.4) have been grown and the magnetic susceptibility χ(T, H) has been measured. These results reveal a wide range of unusual properties including the large temperature independent contribution to χ(T, H) which scales with the Tb concentration and is similar to that reported for (Y, Pr)Ba2Cu3O7, an insensitivity of the superconducting transition temperature Tc to Tb doping and an anomalously large anisotropy in χ(T, H). These observations largely deviate from the well-known behavior of the other rare earth substitute YBa2Cu3O7. The unusual magnetic behavior of Y1−xTbxBa2Cu3O7 may be attributed to a unique 4f-electron hybridization that is not typical of that seen in other rare substituted YBa2Cu3O7 including the anomalous (Y, Pr)Ba2Cu3O7 system.

Transport and thermodynamic studies of Y1−xTbxBa2Cu3O7 single crystals

The electrical resistivity ρ(T,x) and magnetic susceptibility χ(T,H), and specific heat C(T,x) have been measured on single crystals of Y1−x TbxBa2Cu3O7. The results strongly suggest the appearance of strong 4f electron hybridization. It becomes apparent that the Tb doped system possesses properties that largely deviate from the well-known behavior of the other high Tc and doped high Tc systems.

Anomalous low dimensional system: Study of magnetism and electrical conductivity in Na2Ru4O9−δ

The magnetic susceptibility χ∥(H∥b), χ⊥(H⊥b) (2≤T≤700 K) and electrical resistivity ρ(T) (2≤T≤300 K) of single crystalNa2Ru4O9−δ (δ≥0) with various oxygen content were measured. The crystal structure of this compound is monoclinic with single, double, and triple chains along the b axis. The most striking feature of this compound is the drastically large anisotropy reflected in the resistivity which exhibits metallic behavior along chains and semiconducting behavior perpendicular to the chains. The resistivity ratio for these two directions (ρ⊥/ρ∥) is larger than 105. This ratio is exceptionally large, indicating that the anisotropy of the bandwidth is substantial. It is remarkable that, after reduction of oxygen content in the system, the magnetic susceptibility undergoes a drastic change, whereas the electrical resistivity along the conducting chains is altered only slightly. It is argued that the different sensitivity to oxygen content reflected in magnetic and transport properties may be attributed to the complexity of the crystal structure. The striking observations presented here may suggest a one‐dimensional system that possesses unique physical properties.

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y1−xPrx)Ba2Cu3O7−gd and YBa2(Cu1−xZnx)3O7−δ thin films and (Y1−x. Tbx)Ba2Cu3O7−δ thin films and single crystals has been measured as a function of 0≤x≤xcrit, 2K≤T≤300K and OTH≤20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field Hc2 and irreversibility field Hirr values have been determined from these measurements. Increased Tb doping appears to shift Hirr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterTc, Pr and Zn doping of this system tends to depress bothTc and the slope of the mean field normal phase-mixed phase boundary line (dHc2/dT).