Anomalous relaxation kinetics and charge-density-wave correlations in underdoped BaPb_1-x Bi _x O₃

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle interrelation 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 BaPb_1-x Bi _x O₃--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 BaPb_0.72Bi_0.28O₃ (superconducting below T_C = 7 K), a similar response was also found immediately above T_C This observation evidences the presence of a robust gap up to T [Formula: see text] 40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behavior was observed in the same material for [Formula: see text] 40 K. Here, the photoconductivity 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 T_C in underdoped BaPb_1-x Bi _x O_3, and provide information on the dynamical interplay between superconductivity and charge order.