Magnetochromism in the Quasi-2D Heisenberg Antiferromagnet, K$_2$V$_3$O$_8$

We present the optical and magneto-optical properties (0 - 32 T) of the S = 1/2 quasi-two-dimensional Heisenberg antiferromagnet, K$_2$V$_3$O$_8$. A large magnetochromic effect, centered at $\sim$2.6 eV, is observed at 4.2 K, and it is attributed to a change in the V$^{4+}$ on-site excitation. An additional very sharp magneto-optical feature near $\sim$1.2 eV may be associated with a vibronic excitation. Fine structure allows us to identify the coupling phonon. These field-induced color changes point toward a 12 T transition involving local distortions. \newline \newline *Work at the University of Tennessee is supported by the Materials Science Division, Basic Energy Sciences, U.S. Department of Energy (Grant DE-FG0201ER45885). Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725.