Title: High-temperature terahertz optical diode effect without magnetic order in polar FeZnMo$_3$O$_8$,
Abstract: We present a terahertz spectroscopic study of polar ferrimagnet
FeZnMo$_3$O$_8$. Our main finding is a giant high-temperature optical diode
effect, or nonreciprocal directional dichroism, where the transmitted light
intensity in one direction is over 100 times lower than intensity transmitted
in the opposite direction. The effect takes place in the paramagnetic phase
with no long-range magnetic order in the crystal, which contrasts sharply with
all existing reports of the terahertz optical diode effect in other
magnetoelectric materials, where the long-range magnetic ordering is a
necessary prerequisite. In \fzmo, the effect occurs resonantly with a strong
magnetic dipole active transition centered at 1.27 THz and assigned as electron
spin resonance between the eigenstates of the single-ion anisotropy
Hamiltonian. We propose that the optical diode effect in paramagnetic
FeZnMo$_3$O$_8$ is driven by signle-ion terms in magnetoelectric free energy.