Events

Next Quantum Material Seminar:

Thursday, November 24th 2022 at 2pm

Geoffroy Haeseler  (Laboratoire de physique, ENS Lyon)

“Phase transitions in topologically constraints kagome and pyrochlore lattices”

To access this seminar:

Institut Néel CNRS, Building E, 3rd floor, Room Louis Weil (E424)

Abstract: Spin ice is a frustrated ferromagnet on a pyrochlore lattice whose lowest energy states have two spins pointing in and two out of each tetrahedron. This “ice rule” constraint can be written as an emergent field whose net flux is conserved through each tetrahedron. Magnetisation is then divergence free corresponding to an emergent field satisfying the Coulomb gauge – the Coulomb phase. Excitations are spin flips that locally break this ice rule creating quasi–particles that behave like magnetic monopoles.

When a strong field is applied along one of the preferred spin orientations (a [111] axis), one finds an ordered ground state respecting a 3–in–1–out rule in which magnetisation is saturated. If this constraint is enforced, disordered states are obtain through loops excitations spanning through the boundaries. In this case, the system presents a Kasteleyn phase transition between the fully ordered phase and a high temperature Coulomb phase. This transition will be extensively discuss during this seminar.

A closely related system is kagome ice, constrained to a phase with positive charges on an up triangle and negative charges on down triangles. If internal energy is added in the form of a “plaquette” term of strength V, favouring states with short spin loops, ordering is via a Kosterlitz–Thouless Berezinski phase transition. Under a magnetic field, with V=0, the system also exhibits a Kasteleyn transition. We’ll discuss the competition between these two topological transitions when both magnetic field and plaquette term are added

The organizers:

Elsa Lhotel (elsa.lhotel@neel.cnrs.fr)
Florence Levy-Bertrand (florence.levy-bertrand@neel.cnrs.fr)