  Kitaev quantum spin liquids host Majorana fermions via the fractionalization
of spins. In a magnetic field, the Majorana fermions were predicted to comprise
a topological state, which has attracted great attention by the discovery of
the half-quantized thermal Hall conductivity. Nevertheless, a reliable theory
remains elusive for the field effect, especially at finite temperature. Here we
present unbiased large-scale numerical results for the Kitaev model in a wide
range of magnetic field and temperature. We find that the unconventional
paramagnetic region showing fractional spin dynamics extends at finite
temperature, far beyond the field range where the topological state is expected
at zero temperature. Our results show the confinement-deconfinement behavior
between the fractional Majorana excitations and the conventional magnons.
