  Radio frequency tunable oscillators are vital electronic components for
signal generation, characterization, and processing. They are often constructed
with a resonant circuit and a 'negative' resistor, such as a Gunn-diode,
involving complex structure and large footprints. Here we report that a piece
of SmB6, 100 micron in size, works as a current-controlled oscillator in the 30
MHz frequency range. SmB6 is a strongly correlated Kondo insulator that was
recently found to have a robust surface state likely to be protected by the
topology of its electronics structure. We exploit its non-linear dynamics, and
demonstrate large AC voltage outputs with frequencies from 20 Hz to 30 MHz by
adjusting a small DC bias current. The behaviors of these oscillators agree
well with a theoretical model describing the thermal and electronic dynamics of
coupled surface and bulk states. With reduced crystal size we anticipate the
device to work at higher frequencies, even in the THz regime. This type of
oscillator might be realized in other materials with a metallic surface and a
semiconducting bulk.
