  We present microscopic magnetic properties of a two dimensional triangular
lattice Sc2Ga2CuO7, consisting of single and double triangular Cu planes. A
Curie-Weiss temperature theta_CW = --44 K and an antiferromagnetic (AFM)
exchange interaction J/KB = 35 K between Cu2+ (S = 1/2) spins in the triangular
bi-plane are obtained from the analysis of magnetic susceptibility data. The
intrinsic magnetic susceptibility extracted from 71Ga NMR shift data displays
the presence of AFM short range spin correlations and remains finite down to 50
mK suggesting an non-singlet ground state. The nuclear spin-lattice relaxation
rate (1/T1) reveals a slowing down of Cu2+ spin fluctuations with decreasing T
down to 100 mK. Magnetic specific heat (Cm) and 1/T1 exhibit a power law
behavior at low temperatures implying gapless nature of the spin excitation
spectrum. Absence of long range magnetic ordering down to ~ J/700, nonzero spin
susceptibility at low T, and power law behavior of Cm and 1/T1 suggest a
gapless quantum spin liquid (QSL) state. Our results demonstrate that
persistent spin dynamics induced by frustration maintain a quantum-disordered
state at T -> 0 in this triangular lattice antiferromagnet. This suggests that
the low energy modes are dominated by spinon excitations in the QSL state due
to randomness engendered by disorder and frustration.
