Surface Phases of Complex Oxides: Ca2-xSrxRuO4

E. Ward Plummer, University of Tennessee, DMR 0451163 (funded jointly by NSF and DOE/DMS&E )

The exotic properties displayed by transition-metal oxides (TMO) are intimately related to the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom—charge, lattice, orbital, and spin states. Creating a surface by cleaving a layered TMO is a controlled way to explore the manifestation of broken symmetry on the functionality of these materials. In a detailed study of Ca2‑xSrxRuO4, we find that the broken symmetry at the surface enhances the structural instability against the RuO6 rotation, killing superconductivity for x = 2. These RuO6 octrahedra are the building blocks for this structure. In contrast, the surface diminishes the instability against the RuO6 tilt, leading to some interesting and unique surfaces properties, such as an inherent Mott transition for x = 0.1 and the likely displacement or even destruction of the quantum critical point at xc = 0.5.

Science 318, 615 (2007); Phys. Rev. Lett. 100, 066102 (2008)

Surface phase diagram for Ca2‑xSrxRuO4. The red arrows indicate the different concentrations investigated.

Surface Phases of Complex Oxides: Ca2-xSrxRuO4

E. Ward Plummer, University of Tennessee, DMR 0451163(funded jointly by NSF and DOE/DMS&E)

Science 318, 615 (2007); Phys. Rev. Lett. 100, 066102 (2008)

Signatures of the metal-to-insulator transition in the bulk (Tc = 154 K) and at the [001] surface (Tc,s = 130 K) of same single crystal. T dependence of the bulk resistivity (o) measured using a physicall property measurement system and surface energy gap () measured by scanning tunneling spectroscopy on cooling.

(A) LEED pattern from Ca1.9Sr0.1RuO4 surface.(B) Increased buckling of top-most Ca/Sr-O

plane by inward motion of Ca/Sr ions.

Through the Collegiate Research Scholars Program (launched Summer of 2007), top scientists from UTK are refining the talents of exceptional students from two Knox County High Schools, Farragut and Fulton. In 2007–2008, there were three Farragut students working in Plummer’s laboratory (Emma Stockdale, Michael McCormick, and Kyle Peterson) (top picture), and three new students will start this fall. All three students stated that working in the laboratory increased their interest in science careers. Emma is enrolled at Rice, Kyle at Vanderbilt, and Michael at Duke this fall.

The “Distinguished Faculty Lecture Series” was also initiated with presentations about the global energy and environmental crisis and the need for discovery of new materials in the 21st century given at the two high schools. This theme was also a showcase lecture preceding a UTK football game. The bottom picture shows Plummer giving the inaugural lecture at Farragut on November 1, 2007, to an audience of ~150 students, teachers, and parents.

Cultivating the Next Generation of Scientists

E. Ward Plummer,University of Tennessee, DMR 0451163 (funded jointly by NSF and DOE/DMS&E )