Charge density wave physics in shape memory alloys
Gabi Schierning, University of Duisburg-Essen, Germany
Very different materials exhibit displacive phase transitions, for example the famous shape memory alloy NiTi. But how does this phase transition work in detail? For the prototype system NiTi, there are many experimental as well as theoretical indications that point to an electronic mechanism of this displacive phase transition: Transport anomalies, nesting of the Fermi surface, phonon softening and also (giant) Kohn anomalies place this phase transition close to metal-insulator transitions of the charge density wave type. In particular, the giant Kohn anomalies, which have been known in NiTi for decades, are often interpreted as experimental evidence for charge density wave physics. In this talk, using transport data from a series of NiTi samples (with varying nickel content) and Cu-substituted samples, I will show that the martensitic phase transition in NiTi is accompanied by a reduction in charge carrier density at the phase transition, as is typical for metal-insulator transitions. Values for the electronic entropy (and enthalpy) at the phase transition can be estimated, which shows the relevance of the electronic driving forces. However – and this is where the discussion can only begin – it remains completely unclear from these macroscopically averaged transport data how microstructural aspects and electronic aspects of this phase transition interact.