  Graphene, a single atomic layer of covalently bonded carbon atoms, has been
investigated intensively for optoelectronics and represents a promising
candidate for high-speed electronics. Here we present a microwave mixer
constructed as an asymmetrically-contacted two-terminal graphene device based
on the thermoelectric effect. We report a 50 GHz (minimum) mixer bandwidth as
well as 130 V/W (163 mA/W) extrinsic direct-detection responsivity. Anomalous
second-harmonic generation due to self-mixing in our graphene detector is also
observed. Careful investigation of the responsivity from four different
approaches gives consistent results, confirming the exceptional performance of
our zero-bias device operating at room temperature. The 50 GHz bandwidth
indicates an extremely fast response time and our experimental results
represent an encouraging advance towards practical graphene microwave devices,
with anticipated future applications extended through millimeter wave and
terahertz frequencies.
