  Isolated attosecond pulses (IAPs) produced through laser-driven high-harmonic
generation (HHG) hold promise for unprecedented insight into biological
processes via attosecond x-ray diffraction with tabletop sources. However,
efficient scaling of HHG towards x-ray energies has been hampered by
ionization-induced plasma generation impeding the coherent buildup of
high-harmonic radiation. Recently, it has been shown that these limitations can
be overcome in the so-called 'overdriven regime' where ionization loss and
plasma dispersion strongly modify the driving laser pulse over small distances,
albeit without demonstrating IAPs. Here, we report on experiments comparing the
generation of IAPs in argon and neon at 80 eV via attosecond streaking
measurements. Contrasting our experimental results with numerical simulations,
we conclude that IAPs in argon are generated through ionization-induced
transient phase-matching gating effective over distances on the order of 100
$\mu$m. We show that the decay of the intensity and blue-shift due to plasma
defocussing are crucial for allowing phase-matching close to the XUV cutoff at
high plasma densities. We perform simulations for different gases and
wavelengths and show that the mechanism is important for the phase-matching of
long-wavelength, tightly-focused laser beams in high-pressure gas targets,
which are currently being employed for scaling isolated attosecond pulse
generation to x-ray photon energies.
