"Tuning Frustration in the Hunt for Orbital and Spin Liquids "

S. L. Cooper

Dept. of Physics, U. of Illinois

The development at low temperatures of some form of long-range ordersuch as magnetism, orbital-order, charge-order, or superconductivityis ubiquitous in materials, and reflects the tendency of a material to lower its ground state degeneracy and minimize its entropy near T=0K. There has been substantial interest in materials in which atomic geometry, competing interactions, and applied fields conspire to frustrate the onset of long range magnetic and/or orbital order, even down to T=0. Frustrated systems are of interest because of the novel low temperature phase behavior they have been proposed to exhibitincluding orbital- and spin-liquids and -glasses. In this talk, I'll describe our efforts to use field- and pressure-dependent optical spectroscopy to create and study orbital- and spin-liquid phases in two materials, the orbital-ordering material KCuF3 and the ferrimagnetic spinel Mn3O4. Our results reveal some surprising routes by which the frustration of magnetic and orbital order are achieved in materials. 1I. Bozovic et al., Phys. Rev. Lett. 89, 107001 (2002); P. Abbamonte et al., Science 297, 581 (2002). 2I. Bozovic et al., Nature 422, 873 (2003); Phys. Rev. Lett. 93, 157002 (2004). 3N. Gedik et al., Science 316, 425 (2007); Z. Radovic et al., Phys. Rev. B 77, 092508 (2008); H. Shim et al., Phys. Rev. Lett. 101, 247004 (2008). 4A. Gozar et al., Nature 455, 782 (2008); S. Smadici et al., Phys. Rev. Lett. (2009) in press. Our results elucidate further the collective nature of electrons in 1D.