title: Spin glass behavior in frustrated quantum spin system CuAl2O4with a possible orbital liquid state.
abstract: CuAl2O4is a normal spinel oxide having quantum spin, S  =  1/2 for Cu2+. It is a rather unique feature that the Cu2+ions of CuAl2O4sit at a tetrahedral position, not like the usual octahedral position for many oxides. At low temperatures, it exhibits all the thermodynamic evidence of a quantum spin glass. For example, the polycrystalline CuAl2O4shows a cusp centered at ~2 K in the low-field dc magnetization data and a clear frequency dependence in the ac magnetic susceptibility while it displays logarithmic relaxation behavior in a time dependence of the magnetization. At the same time, there is a peak at ~2.3 K in the heat capacity, which shifts towards a higher temperature with magnetic fields. On the other hand, there is no evidence of new superlattice peaks in the high-resolution neutron powder diffraction data when cooled from 40 to 0.4 K. This implies that there is no long-ranged magnetic order down to 0.4 K, thus confirming a spin glass-like ground state for CuAl2O4. Interestingly, there is no sign of structural distortion either although Cu2+is a Jahn-Teller active ion. Thus, we claim that an orbital liquid state is the most likely ground state in CuAl2O4. Of further interest, it also exhibits a large frustration parameter, f  =  |θCW/Tm| ~ 67, one of the largest values reported for spinel oxides. Our observations suggest that CuAl2O4should be a rare example of a frustrated quantum spin glass with a good candidate for an orbital liquid state.
authors.affiliation: Center for Strongly Correlated Materials Research, Seoul National University, Seoul 08826, Korea. Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India. Department of Physics, Sungkyunkwan University, Suwon 16419, Korea.
authors.forename: R
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