CPM Seminar

Hidden, Composite and Emergent Order in heavy fermion materials

Piers Coleman

Center for Materials Theory
Rutgers University

The low transition temperature and high tunability of heavy electron materials make them an ideal test bed for exploring new forms of order and superconductivity. I will discuss the challenges posed by various heavy electron materials, in their normal, quantum critical and superconducting configurations, and the questions they pose.

In particular, we'll examine the case of the 115 superconductors, with Tc's ranging from 0.2 to 20K, which in their most extreme form undergo a direct transition from Curie paramagnet to Superconductor, indicating that the local moments directly entangle with the condensate to form “composite pairs”. I'll present a simple mean-field theory of this kind of behavior [1].

Another challenge is posed by the heavy fermion superconductor URu₂Si₂, in which a mysterious “hidden order” phase transition preceeds superconductivity, involving an order parameter that has eluded identification for almost 30 years. Here the recent observation of perfectly Ising quasiparticles suggests a new class of spinorial order we call “hastatic order” [2].

Finally, if time permits, I'll talk about the “spin dilemma”, and our most recent efforts to understand the bosonic and fermionic manifestations of spin order in terms of supersymmetry [3].

[1] “Composite pairing in a mixed valent two channel Anderson model”, R. Flint, A. Nevidomskyy and P. Coleman, Phys. Rev. B. 84, 064514 (2011).
[2] “Hastatic order: a theory for the hidden order in URu₂Si₂”, Premi Chandra, Rebecca Flint, Piers Coleman, Nature, 493, 621-626 (2013).
[3] “Supersymmetric symplectic spin operators and heavy fermion systems” Aline Ramires and Piers Coleman, preprint (2015).