  Quantum spin liquid (QSL) is a novel state of matter which refuses the
conventional spin freezing even at 0 K. Experimentally searching for the
structurally perfect candidates is a big challenge in condensed matter physics.
Here we report the successful synthesis of a new spin-1/2 triangular
antiferromagnet YbMgGaO$_4$ with R$\bar{3}$m symmetry. The compound with an
ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no
site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM)
interactions. No spin freezing down to 60 mK (despite $\Theta$$_w$ $\sim$ -4
K), the low-T power-law temperature dependence of heat capacity and nonzero
susceptibility suggest that YbMgGaO$_4$ is a promising gapless ($\leq$
$|$$\Theta$$_w$$|$/100) QSL candidate. The residual spin entropy, which is
accurately determined with a non-magnetic reference LuMgGaO$_4$, approaches
zero ($<$ 0.6 \%). This indicates that the possible QSL ground state (GS) of
the frustrated spin system has been experimentally achieved at the lowest
measurement temperatures.
