Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba

We report the structural transformation of hexagonal Ba3YIr 2O9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at ∼4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form γT+βT3 in the temperature (T) range of 2-10 K with the Sommerfeld coefficient γ=10 mJ/mol-Ir K2. The 89Y NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba3YIr 2O9. In this picture, the γT term in the heat capacity and the linear variation of 89Y 1/T1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice. © 2013 American Physical Society.