Spin dynamics in a hole-doped S=1/2 Heisenberg antiferromagnet with a disordered ground state

Only 3% hole doping by Li is sufficient to suppress the long-range antiferromagnetic order in La2CuO4. Spin dynamics in such a disordered state was investigated with measurements of the dynamic magnetic structure factor S(omega,q), using cold neutron spectroscopy, for La2(Cu0.94Li0.06)O4. The S(omega,q) is found to sharply peak at (pi,pi), and its dynamics to be relaxational. Confirming theoretical expectation for the quantum disordered 2D S=1/2 Heisenberg antiferromagnet, the energy scale saturates at a finite value at low temperatures. Possible connection to the ``pseudo spin gap'' phenomenon observed in the NMR/NQR studies on underdoped cuprates is discussed.