Order Parameter fluctuations dominating the Charge Density Wave transition in Quasi-1D Systems: Emergence of Critical Singularities

Abstract: 

When a system is in a critical state, its order parameter tends to experience large fluctuations, which can significantly impact its thermodynamic properties. However, we still know very little about these fluctuations and how they behave.

In this talk, I’ll share our recent findings on the critical fluctuations of the charge-density wave (CDW) order parameter in the quasi-one-dimensional compound (TaSe4)2I. Instead of focusing on average properties like resistivity, magnetization, or specific heat, we directly measured these fluctuations. This approach has uncovered a wealth of complex and fascinating behaviours as the system nears its critical point. What’s particularly interesting is that these critical fluctuations are slow enough to persist even in the thermodynamic limit. We observed a dramatic increase in noise variance and relaxation time, which indicates critical opalescence and slowing down over a wide temperature range around the transition. As the temperature gets very close to the critical point, we see a shift from the expected mean-field behaviour to a regime dominated by fluctuations, with some unusual critical exponents. Additionally, the distribution of these fluctuations is skewed and non-Gaussian, suggesting that the central limit theorem breaks down as the coherence volume expands to match the sample size.

These observations not only enhance our understanding of CDW systems, may also provide valuable insights into other second-order phase transitions, such as those seen in Bose-Einstein (BE) and Bardeen-Cooper- Schrieffer (BCS) condensations