University of Toronto
Physics Department
Quantum Optics and Condensed Matter
SPECIAL MONDAY SEMINAR

In  phase transitions in solids it is often the case that domain growth is frozen at some stage. Many authors have ascribed its origin to some quenched disorder.  However, domain pinning can occur intrinsically due to cooperative anharmonic elasticity.

I will give two examples investigated by our group.

(1) In elastically inhomogeneous alloys [1], in which the shear modulus depends on the composition, coarsening in phase separation is pinned even without quennched disorder. Here networks in the soft phase enclose the domains in the hard phase, where highly asymmetric strains in the two regions create large free energy barriers that prevent further coarsening. Thus the phase transition occurs between one-phase and glassy two-phase states. We numerically calculate the phase diagram and show that this transition is discontinuous at any composition. Therefore there is no critical point.

(2) Heterogeneous coexistence of high- and low-temperature phases (premartensitic effect) has been observed around structural phase transitions as stable or metastable states [2]. Such intermediate states can be produced by third-order elastic anharmonicity in the Landau expansion of the free energy density. For example, in the presence of a third-order coupling term of the form e₁e₂², the dilation strain e₁ can modify the effective temperature for the order parameter (tetragonal strain) e₂.  We show that e₁ is asymmetrically induced in the high- and low-temperature phases, leading to elastic stabilization of tetragonally strained domains  coherently embedded in the cubic matrix.

1)  Phys.Rev. Lett. 86 (2001) 452.
2)  J. Phys. Soc.Jpn. 68 (1999) 5.

Local Host: Professor Rashmi Desai   (416-978-5191)
See   http://www.physics.utoronto.ca/~qocmp