  In X-ray binaries, compact jets are known to commonly radiate at radio to
infrared frequencies, whereas at optical to gamma-ray energies, the
contribution of the jet is debated. The total luminosity, and hence power of
the jet is critically dependent on the position of the break in its spectrum,
between optically thick (self-absorbed) and optically thin synchrotron
emission. This break, or turnover, has been reported in just one black hole
X-ray binary (BHXB) thus far, GX 339-4, and inferred via spectral fitting in
two others, A0620-00 and Cyg X-1. Here, we collect a wealth of multiwavelength
data from the outbursts of BHXBs during hard X-ray states, in order to search
for jet breaks as yet unidentified in their spectral energy distributions. In
particular, we report the direct detection of the jet break in the spectrum of
V404 Cyg during its 1989 outburst, at nu_b = (1.8 +- 0.3) x 10^14 Hz (1.7 +-
0.2 microns). We increase the number of BHXBs with measured jet breaks from
three to eight. Jet breaks are found at frequencies spanning more than two
orders of magnitude, from nu_b = (4.5 +- 0.8) x 10^12 Hz for XTE J1118+480
during its 2005 outburst, to nu_b > 4.7 x 10^14 Hz for V4641 Sgr in outburst. A
positive correlation between jet break frequency and luminosity is expected
theoretically; nu_b \propto L_nu,jet^~0.5 if other parameters are constant.
With constraints on the jet break in a total of 12 BHXBs including two
quiescent systems, we find a large range of jet break frequencies at similar
luminosities and no obvious global relation (but such a relation cannot be
ruled out for individual sources). We speculate that different magnetic field
strengths and/or different radii of the acceleration zone in the inner regions
of the jet are likely to be responsible for the observed scatter between
sources. (abridged)
