  We unveil a previously overlooked wave propagation regime in magnetized
plasmonic (gyrotropic) materials with comparable plasma and cyclotron
frequencies, which enables a giant and broadband (nondispersive) nonreciprocal
response. We show that this effect is due to a natural form of dispersion
compensation that ultimately originates from the subtle implications of the
principle of causality for gyrotropic plasmonic media, which allows the
existence of a low-loss frequency window with anomalous nonmonotonic dispersion
for the extraordinary mode. This is in stark contrast with conventional
nongyrotropic passive materials, for which the frequency derivative of the
permittivity dispersion function is always positive in low-loss regions. These
findings pave the way for superior nonreciprocal components in terms of
bandwidth of operation and compactness, with orders-of-magnitude reductions in
size. As a relevant example, we consider indium antimonide (InSb) to
theoretically demonstrate a deeply subwavelength, broadband, THz isolator
operating under moderate magnetic bias and at room temperature.
