  Antiferromagnetically coupled S=1/2 spins on an isotropic triangular lattice
is the paradigm of frustrated quantum magnetism, but structurally ideal
realizations are rare. Here we investigate NaYbO$_2$, which hosts an ideal
triangular lattice of $J_{eff}=1/2$ moments with no inherent site disorder. No
signatures of conventional magnetic order appear down to 50 mK, strongly
suggesting a quantum spin liquid ground state. We observe a two-peak specific
heat and a nearly quadratic temperature dependence in accord with expectations
for a two-dimensional Dirac spin liquid. Application of a magnetic field
strongly perturbs the quantum disordered ground state and induces a clear
transition into a collinear ordered state consistent with a long-predicted
up-up-down structure for a triangular lattice XXZ Hamiltonian driven by quantum
fluctuations. The observation of spin liquid signatures in zero field and
quantum-induced ordering in intermediate fields in the same compound
demonstrate an intrinsically quantum disordered ground state. We conclude that
NaYbO$_2$ is a model, versatile platform for exploring spin liquid physics with
full tunability of field and temperature.
