  Heat transport mediated by Majorana edge modes in a magnetic insulator leads
to a half-integer thermal quantum Hall conductance, which has recently been
reported for the two-dimensional honeycomb material $\alpha$-RuCl$_3$. While
the conventional electronic Hall effect requires a perpendicular magnetic
field, we find that this is not the case in $\alpha$-RuCl$_3$. Strikingly, the
thermal Hall plateau appears even for a magnetic field with no out-of-plane
components. The field-angular variation of the quantized thermal Hall
conductance has the same sign structure of the topological Chern number, which
is either $\pm$1, as the Majorana band structure of the pure Kitaev spin
liquid. This observation of a half-integer anomalous thermal Hall effect firmly
establishes that the Kitaev interaction is primarily responsible and that the
non-Abelian topological order associated with fractionalization of the local
magnetic moments persists even in the presence of non-Kitaev interactions in
$\alpha$-RuCl$_3$.
