Anomalous relaxation kinetics and charge density wave correlations in underdoped BaPb1-xBixO3

TL;DR

This study uses ultrafast terahertz spectroscopy to reveal short-range charge density wave correlations and their dynamics in underdoped BaPb1-xBixO3, elucidating the interplay between CDW order and superconductivity.

Contribution

It provides new insights into the transient charge dynamics and short-range CDW correlations above Tc in BaPb1-xBixO3, a material near a competing CDW phase.

Findings

Presence of a robust gap up to 40 K linked to short-range CDW correlations

Different relaxation behaviors below and above 40 K indicating phase crossover

Evidence of melting of CDW correlations at higher temperatures

Abstract

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.