  We report the discovery of fast, frequency-dependent intensity variations
from the scintillating intra-day variable quasar J1819+3845 at a wavelength of
21cm which resemble diffractive interstellar scintillations observed in
pulsars. The timescale (down to 20 min) and the bandwidth (frequency
decorrelation bandwidth of 160 MHz) of the observed variations jointly imply
that the component of the source exhibiting this scintillation must possess a
brightness temperature well in excess of the inverse Compton limit. A specific
model in which both the source and scintillation pattern are isotropic implies
a brightness temperature 0.5 x 10^13 z_{pc} K, where previous estimates place
the distance to the scattering medium in the range z_{pc}=4-12pc, yielding a
minimum brightness temperature >20 times the inverse Compton limit. An
independent estimate of the screen distance using the 21cm scintillation
properties alone indicates a minimum screen distance of z approx 40pc and a
brightness temperature above 2 x 10^14 K. There is no evidence for anisotropy
in the scattering medium or source from the scintillation characteristics, but
these estimates may be reduced by a factor comparable to the axial ratio if the
source is indeed elongated.
