We have measured the longitudinal and transverse sound velocities of La2-xCaxCoO4 (0.4≤x≤0.7) propagating along , , and  from 200 K down to 3 K with a frequency of 10 MHz. It was found that c66 shows a remarkable softening below Co 2+ spin-ordering temperature TN in the whole doping range, while c11-c12/2 exhibits a clear hardening below Co 3+ spin-ordering temperature TN2 only when x=0.5. To explain the observed anomaly, the Landau free energy for the La 2-xCaxCoO4 system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter (Q1, Q2), having a spatial modulation along  or [1̄ 10]. We argued that this represents orbital order, and it is this order that leads to the softening of c66.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Oct 27 2008|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics