Criticality and metastability around first-order hysteretic phase transitions

Kundu, Satyaki (2023) Criticality and metastability around first-order hysteretic phase transitions. PhD thesis, Indian Institute of Science Education and Research, Kolkata.

Text (PhD thesis of Satyaki Kundu (15IP015)) 15IP015.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

Thermal hysteresis is often observed in solid state phase-change materials undergoing a structural transition. Examples include transition metal oxides, spin-crossover materials, charge-density-wave compounds, and shape-memory alloys. In this thesis, we experimentally investigate criticality and aspects of metastability in some such hysteretic phase transitions. We have chosen V₂O₃, NdNiO₃, and MAPbI3 as model systems to investigate these phenomena. Each of these exhibits a thermally-induced structural phase transition accompanied by phase coexistence, slow relaxation, and rate-dependent effects. The main empirical finding of this thesis is that the two boundaries of the hysteretic phase end at singularities. These are shown to have features similar to those seen at the critical point. We have experimentally observed a rapid growth in the magnitude and the time scales of the order parameter fluctuations in V₂O₃, NdNiO₃. These have been interpreted as critical opalescence and critical slowing down respectively. The reason for the significant difference between the “first-order” phase transition in such solid-state systems and the conventional (e.g. liquid-gas) transitions (where no such singularity is observed) is the long-ranged strain fields controlling the structural transition. These long-range interactions strongly suppress fluctuations and make the physics mean-field-like. The singularities are thus naturally identified with the spinodal points. We first systematically demonstrate spinodal singularities in bulk polycrystalline V₂O₃ and a highquality epitaxial NdNiO₃ film through low frequency resistance noise and phase-ordering measurements. For the thin film sample, we also find a rugged landscape (indicated by a separation of time scales) at the phase transition region. We have also studied the nature of the resistance fluctuations in the “deep” metastable region epitaxial NdNiO₃, accessed via minor hysteresis loops in the resistance-temperature plane. We find that the phase evolution is essentially arrested within the deep metastable region (around the middle of the hysteresis loop) and the resistance noise here is essentially indistinguishable from that seen in the non-critical regions far away from the transition. We next study MAPbI₃, which is currently a very popular halide perovskite material. MAPbI₃ is highly luminescent and has a hysteretic phase transition around 150 K.We show that many of the characteristics of this material are similar to the other two systems discussed in the thesis (V₂O₃ and NdNiO₃), with the added feature the system can be optically probed by photoluminescence (PL). Following up on the results of the previous chapters, we establish that the metastable states in MAPbI₃ are athermal, viz., the material does not show any phase evolution as long as the temperature is kept constant. But even a small (∼1 K) disturbance in temperature is recorded as an irreversible phase evolution that can be inferred from the change in the PL spectrum. We propose that this property can be utilized for thermal breach memory applications, where packages of materials like vaccines have to be transported under strict thermal control. Finally, via Langevin dynamics simulations, we have examined the effect of noise on the powerlaw scaling of the dynamic hysteresis in systems undergoing hysteretic first-order transitions. This study is motivated by our experimental observation (and many other groups’) of anomalously low values of the dynamic hysteresis scaling exponents in NdNiO₃ films. We show that even a small amount of thermal noise can lower the dynamical exponent from mean-field value of 2/3 to as low as 0.2.

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