Colloque IQ : Michel Gingras

Michel Gingras

Université de Waterloo

Invité de André-Marie Tremblay 

Title:     Frustrated Quantum Rare-Earth Pyrochlores

Abstract:

The search for magnetic systems that remain disordered down to absolute zero temperature —so-called quantum spin liquids — is a forefront research activity in contemporary condensed matter physics.  Systems of antiferromagnetically interacting magnetic moments (spins) that reside on the sites of lattice architectures made of corner-shared triangles or tetrahedra in two and three-dimensions have been on centre stage of this activity for the past thirty-five years. In this context, materials possessing spin S=1/2 degrees of freedom, expected to have the largest level of quantum fluctuations for having the smallest S value, have attracted foremost interest.  From a materials perspective, the pyrochlore oxides of chemical formula R2M2O7 (where R is trivalent R^{3+} rare-earth ion and M is a tetravalent M^{4+} transition metal ion) would seem particularly attractive  when the interactions are overall antiferromagnetic since the two sets of ions reside on two interpenetrating three-dimensional lattices of corner-shared tetrahedra. Most often, the R^{3+}ions are magnetic — and many theorists and experimentalists have argued that those systems should not host any interesting quantum effects since rare earth ions often possess total angular momenta J=L+S much larger than 1, and should therefore behave essentially  classically. In this talk, I will argue that this perspective is incorrect and discuss few examples of R2M2O7 rare-earth pyrochlore compounds that display interesting quantum effects (e.g. quantum spin ice/spin liquid), even when the system ultimately orders at non-zero temperature (e.g. quantum order-by-disorder).