J. E. Crow

DESTRUCTION OF THE MOTT INSULATING GROUND STATE OF CA2RUO4 BY A STRUCTURALTRANSITION

We report a first-order phase transition at T_M=357 K in single crystal Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous decrease in electrical resistivity signals the near destruction of the Mott insulating phase and is triggered by a structural transition from the low temperature orthorhombic to a high temperature tetragonal phase. The magnetic susceptibility, which is temperature dependent but not Curie-like decreases abruptly at TM and becomes less temperature dependent. Unlike most insulator to metal transitions, the system is not magnetically ordered in either phase, though the Mott insulator phase is antiferromagnetic below T_N=110 K.

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.

63Cu NMR and hole depletion in the normal state of yttrium-rich Y1-xPrxBa2Cu3O7

The {sup 63}Cu Knight shift {ital K} and spin-lattice relaxation rate 1/{ital T}{sub 1} have been measured in the superconducting cuprate system Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7}, 0.05{le}{ital x}{le}0.20. With Pr doping {ital K} decreases and develops a temperature dependence at both plane and chain sites. This resembles the behavior of the Cu and Y Knight shifts as well as the bulk susceptibility in oxygen-deficient YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}. The orbital contribution to {ital K} and the anisotropy of the Cu hyperfine coupling remain essentially unchanged over the entire Pr concentration range. No appreciable direct effect of Pr magnetism on the conduction-band susceptibility was found. Instead, analysis of the bulk susceptibility and NMR data indicate that pair breaking and hole depletion both take part in the suppression of the superconducting transition temperature {ital T}{sub {ital c}}. The temperature dependence of 1/{ital T}{sub 1} for magnetic field parallel to the {ital c} axis is also similar to that for the oxygen-deficient compound. This agreement leads to a consistent picture of the role of antiferromagnetic fluctuations in these materials. An analysis of the data in the framework of the phenomenological theory of Millis, Monien, and Pines is given.

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.

Pressure-tuned collapse of the Mott-like state in Can+1RunO3n+1 (n = 1,2): Raman spectroscopic studies

We report a Raman scattering study of the pressure-induced collapse of the Mott-like phases of Ca3Ru2O7 (T(N)=56 K) and Ca2RuO4 (T(N)=110 K). The pressure dependence of the phonon and two-magnon excitations in these materials indicate (i) a T approximately 0 pressure-induced collapse of the antiferromagnetic (AF) insulating phase above P(*) approximately 55 kbar in Ca3Ru2O7 and P(*) approximately 5-10 kbar in Ca2RuO4, (ii) a remarkable insensitivity of the exchange interaction to pressure in both systems, and (iii) evidence for persistent AF correlations above the critical pressure of Ca2RuO4, suggestive of phase separation involving AF insulator and ferromagnetic metal phases.

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.

Magnetization and magnetotransport of LBaCo2O5.5(L=Gd, Eu) single crystals

The magnetization, resistivity, and magnetoresistance (MR) of single crystals of

Quantum oscillations, colossal magnetoresistance, and the magnetoelastic interaction in bilayeredCa3Ru2O7

\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}

Thermal transport in Sr1−xCaxRuO3

and

The United States National High Magnetic Field Laboratory: Facilities, science and technology

\mathrm{Eu}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}