M. Shepard

Thermodynamic properties of perovskite ARuO3 (A=Ca, Sr, and Ba) single crystals

Magnetic susceptibility, transport properties (including magnetoresistivity and the Hall effect), and specific heat of perovskite ARuO3 (A=Ca, Sr, and Ba) single crystals were measured. These compounds, particularly CaRuO3 (paramagnetic conductor) and SrRuO3 (ferromagnetic conductor) are believed to have a narrow π* band and their thermodynamic properties depend in an extremely sensitive way on the degree of the band filling and band width. This study reveals that the π* bandwidth of these compounds is largely determined by the ionic radii of the alkaline-earth A cations, i.e., the interaction between A s and O 2p orbitals and the electron correlation becomes progressively stronger in the series BaRuO3, SrRuO3, and CaRuO3.

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.

Thermal transport in Sr1−xCaxRuO3

We have studied the thermal conductivity K(T,B) (6<T<250 K, 0<B<15 T) of single crystals of SrRuO3, a ferromagnet with Tc=160 K, and isostructural but paramagnetic CaRuO3, as well as the intermediate crystals (Sr, Ca)RuO3 for the purpose of probing charge carrier (Ke), phonon (Kp), and magnon (Km) excitations in these systems. The contributions of electron and phonon scattering processes to the thermal conductivities have been estimated using a Wiedemann–Franz relationship to estimate Ke and thus obtain Kp at higher temperatures. Roughly 50% of the thermal conductivity is estimated to be phononic near room temperature. Amorphous behavior is observed in the doped samples, and no break in the thermal conductivity is seen at the ferromagnetic onset. A comparison of magnetothermal conductivity and magnetoresistivity measurements at lower temperatures in SrRuO3 and CaRuO3 reveals that additional scattering mechanisms have become important. These investigations argue for the importance of phonon and magnon cont...

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.

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).