  Due to their small mass, subsolar mass black hole binaries would have to be
primordial in origin instead of the result of stellar evolution. Soon after
formation in the early universe, primordial black holes can form binaries after
decoupling from the cosmic expansion. Alternatively, primordial black holes as
dark matter could also form binaries in the late universe due to dynamical
encounters and gravitational-wave braking. A significant feature for this
channel is the possibility that some sources retain nonzero eccentricity in the
LIGO/Virgo band. Assuming all dark matter is primordial black holes with a
delta function mass distribution, $1M_\odot-1M_\odot$ binaries formed in this
late-universe channel can be detected by Advanced LIGO and Virgo with their
design sensitivities at a rate of $\mathcal{O}(1)$/year, where $12\%(3\%)$ of
events have eccentricity at a gravitational-wave frequency of 10 Hz,
$e^\mathrm{10Hz}\geq0.01(0.1)$, and nondetection can constrain the binary
formation rate within this model. Third generation detectors would be expected
to detect subsolar mass eccentric binaries as light as $0.01 M_\odot$ within
this channel, if they accounted for the majority of the dark matter.
Furthermore, we use simulated gravitational-wave data to study the ability to
search for eccentric gravitational-wave signals using a quasi-circular waveform
template bank with Advanced LIGO design sensitivity. For a match-filtering
targeted search, assuming binaries with a delta function mass of
$0.1(1)M_\odot$ and the eccentricity distribution derived from this
late-universe formation channel, $41\%(6\%)$ of the signals would be missed
compared to the ideal detection rate due to the mismatch in the
gravitational-wave signal from eccentricity.
