  The Kitaev model is a rare example of an analytically solvable and physically
instantiable Hamiltonian yielding a topological quantum spin liquid ground
state. Here we report signatures of Kitaev spin liquid physics in the honeycomb
magnet $Li_3Co_2SbO_6$, built of high-spin $\it{d^7}$ ($Co^{2+}$) ions, in
contrast to the more typical low-spin $\it{d^5}$ electron configurations in the
presence of large spin-orbit coupling. Neutron powder diffraction measurements,
heat capacity, and magnetization studies support the development of a
long-range antiferromagnetic order space group of $\it{C_C}2/\it{m}$, below
$\it{T_N}$ = 11 K at $\it{\mu_0H}$ = 0 T. The magnetic entropy recovered
between $\it{T}$ = 2 K and 50 K is estimated to be 0.6Rln2, in good agreement
with the value expected for systems close to a Kitaev quantum spin liquid
state. The temperature-dependent magnetic order parameter demonstrates a
$\beta$ value of 0.19(3), consistent with XY anisotropy and in-plane ordering,
with Ising-like interactions between layers. Further, we observe a spin-flop
driven crossover to ferromagnetic order with space group of $\it{C}2/\it{m}$
under an applied magnetic field of $\it{\mu_0H}$ $\approx$ 0.7 T at $\it{T}$ =
2 K. Magnetic structure analysis demonstrates these magnetic states are
competing at finite applied magnetic fields even below the spin-flop
transition. Both the $\it{d^7}$ compass model, a quantitative comparison of the
specific heat of $Li_3Co_2SbO_6$, and related honeycomb cobaltates to the
anisotropic Kitaev model further support proximity to a Kitaev spin liquid
state. This material demonstrates the rich playground of high-spin $\it{d^7}$
systems for spin liquid candidates, and complements known $\it{d^5}$ Ir- and
Ru-based materials.

Strongly Correlated Electrons