  CuAl2O4 is a normal spinel oxide having quantum spin, S=1/2 for Cu2+. It is a
rather unique feature that the Cu2+ ions of CuAl2O4 sit 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 CuAl2O4 shows 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 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 = Theta_CW/Tm ~67, one of the largest values
reported for spinel oxides. Our observations suggest that CuAl2O4 should be a
rare example of a frustrated quantum spin glass with a good candidate for an
orbital liquid state.
