Novel Orbital and Conducting Phases of Matter Induced with a Magnetic Field

“Strongly correlated” materials, i.e., materials whose phases and properties are governed by the competition among several quantum mechanical interactions, are known to exhibit exotic temperature- and magnetic-field-dependent phases in which the electronic charges and/or atomic orbitals spontaneously organize into distinctive patterns, known as charge- and orbital-order. Charge- and orbital-ordered phases are scientifically interesting because they can be associated with a variety of exotic and important phenomena, such as unconventional superconductivity and “colossal magnetoresistance.” These exotic phases of matter also have potential technological importance, as they exhibit electrical and structural properties that can be dramatically altered through the manipulation of the atomic spins.

(left graph) Direct current c-axis conductivities associated with different orbital configurations of Ca₃Ru₂O₇, including (red region) the high conducting state associated with the orbital-degenerate configuration (top illustration), and (blue region) the low conducting state associated with the orbital-ordered configuration (bottom illustration). The illustration at right shows the Ca₃Ru₂O₇crystal structure.

One class of materials that exhibit orbital-ordering are the layered ruthenates (e.g., Ca₃Ru₂O₇; right-most illustration). These materials undergo phase transitions from a high-temperature “orbital-degenerate” (OD) phase, in which the minority spin d-electron on every Ru site resides in a quantum-mechanical superposition of d-orbital configurations (top illustration), to a low-temperature orbital-ordered (OO) phase, in which the minority spin d-electron on every Ru site occupies a distinct (d_xy) orbital state (lower illustration in figure above). Much greater orbital-overlap (hybridization) occurs between adjacent atomic sites in the high-temperature “orbital degenerate” state than in the low-temperature “orbital-ordered” state. As a result, the orbital-degenerate state is metallic (“high conducting”; red region) while the orbital-ordered state is insulating (“low conducting”; blue region).

As reported in a paper published in Physical Review Letters (Phys. Rev. Lett. 93, 167205 [2004]), graduate students John Karpus and Harini Barath, postdoctoral research associate Rajeev Gupta, and Professor Lance Cooper used an applied magnetic field to manipulate the orbital configuration, and therefore the conducting state, of the layered ruthenate material Ca₃Ru₂O₇. In effect, the researchers used the orientation of the Ru-moments as a highly sensitive “spin-valve” for controlling the dc conductivity: the high-conducting OD state (top illustration) was induced by orienting the Ru-moments along the b-axis direction (red regions in Figures 2 and 3), while the low-conducting, orbital-ordered state (bottom illustrations) was induced by orienting the Ru-moments along the a-axis direction (blue regions in Figures 2 and 3). This work exploited the strong spin-orbit coupling between the ruthenium (Ru) atomic moments and the d-orbital states, which allowed the researchers to control the orbital population on the Ru sites by orienting the atomic moments along different spatial directions using an applied magnetic field.

In the study, the particular orbital state of the material was monitored by using inelastic light scattering to study the field- and temperature-dependent frequency w₀ of a vibrational mode associated with the RuO octahedra, whose frequency provides an extremely sensitive probe of the orbital configuration. Additionally, the field- and temperature-dependent alignment of the Ru moments was monitored by measuring the spin-wave (magnon) excitations, which are associated with coherent oscillations of Ru spin orientation on the atomic lattice.

This work was done in collaboration with Professor Gang Cao of the Department of Physics at the University of Kentucky.

This work was supported by the Department of Energy through the Frederick Seitz Materials Research Lab, under DEFG02-91ER45439, by the National Science Foundation under DMR02-44502, and by the Sony Faculty Scholar Fund. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.