  Superconductivity often emerges in proximity of other symmetry-breaking
ground states, such as antiferromagnetism or charge-density-wave (CDW) order.
However, the subtle inter-relation of these phases remains poorly understood,
and in some cases even the existence of short-range correlations for
superconducting compositions is uncertain. In such circumstances, ultrafast
experiments can provide new insights, by tracking the relaxation kinetics
following excitation at frequencies related to the broken symmetry state. Here,
we investigate the transient terahertz conductivity of BaPb1-xBixO3 - a
material for which superconductivity is adjacent to a competing CDW phase -
after optical excitation tuned to the CDW absorption band. In insulating BaBiO3
we observed an increase in conductivity and a subsequent relaxation, which are
consistent with quasiparticles injection across a rigid semiconducting gap. In
the doped compound BaPb0.72Bi0.28O3 (superconducting below Tc=7K), a similar
response was also found immediately above Tc. This observation evidences the
presence of a robust gap up to T=40 K, which is presumably associated with
short-range CDW correlations. A qualitatively different behaviour was observed
in the same material fo T>40 K. Here, the photo-conductivity was dominated by
an enhancement in carrier mobility at constant density, suggestive of melting
of the CDW correlations rather than excitation across an optical gap. The
relaxation displayed a temperature dependent, Arrhenius-like kinetics,
suggestive of the crossing of a free-energy barrier between two phases. These
results support the existence of short-range CDW correlations above Tc in
underdoped BaPb1-xBixO3, and provide new information on the dynamical interplay
between superconductivity and charge order.
