Skyrmion crystal from RKKY interaction mediated by 2D electron gas*

We consider a C$_6$ invariant lattice of magnetic moments coupled via a Kondo exchange $J$ with a 2D electron gas (2DEG). The effective Ruderman-Kittel-Kasuya-Yosida interaction between the moments stabilizes a magnetic skyrmion crystal in the presence of magnetic field and easy-axis anisotropy. An attractive aspect of this mechanism is that the magnitude of the magnetic ordering wave vectors, $\bm{Q}_{\nu}$ ($\nu=1,2,3$), is dictated by the Fermi wave number $k_F$: $|\bm{Q}_{\nu} |=2k_F$. Consequently, the topological contribution to the Hall conductivity of the 2DEG becomes of the order of the quantized value, $e^2/h$, when $J$ is comparable to the Fermi energy $\epsilon_F$.