Colloquium

Are Continuous Structural Transitions Really Screened Ferroelectric Transitions?

Dr. Peter S. Riseborough
Physics Department
Temple University
Wednesday, September 30, 2009
Higgins 310, 4 pm

Structural transitions usually occur from a high temperature phase characterized by one symmetry group G1 to a low temperature structure whose symmetry group G2 is not a subgroup of G1. Such transitions usually occur discontinuously, and are only expected to occur continuously if the coefficients of the second-order and third-order terms in the Landau free energy function vanish simultaneously. This special case of continuous transitions is expected to occur with probability zero. Recently, a class of materials (containing V3Si, AuZn alloys and others to be discussed) has been discovered that usually exhibit first-order transitions but which can, to within experimental precision, become continuous. The structural transition temperatures are sensitive to stoichiometry and pressure and can be driven towards absolute zero, where the critical fluctuations are expected to cross over from classical to quantum character. Superconducting phases are found in the vicinity of the quantum critical point. 

Since the continuous structural transitions are associated with a loss of inversion symmetry, it is possible that the continuous transitions are examples of ferroelectric transitions in metals, in which the order parameter is hidden by the screening of conduction electrons.  In this talk we shall examine the effects of the dielectric response on the lattice dynamics and the structural transition, as ascertained through the application of high magnetic fields. We will also suggest a possible cause of the superconducting phases.