Seminar by Dr. Louis-Paul Henry (ENS Lyon): “Stability of the Coulomb phase in square ice"

Theory of Quantum Matter Unit would like to invite you to a seminar presented by Dr. Louis-Paul Henry, ENS Lyon :

• Date: Wed, Feb 5th
• Time: 10:30-11:30 am
• Location: C016, Lab 1

“Stability of the Coulomb phase in square ice : topological defects, loop and membrane dynamics”

Dr. Louis-Paul Henry, Laboratoire de Physique, ENS de Lyon

Frustration – namely the presence of competing interactions – gives rise to highly complex effects in physics. Ice (be it the well-known water ice or its magnetic equivalent, the so-called spin-ice) offers a remarkable example in this context. For short-range interactions and classical degrees of freedom, its ground state is infinitely degenerate, and exhibits long-range correlations induced by a local constraint, characterizing the so-called Coulomb phase.

We study the stability of this Coulomb phase in the two-dimensional ice – realized in the form of a proton ice in organic compounds as well as of spin ice in nanomagnetic systems.

In the classical case, dipolar interactions – present in the experimental systems – destabilize the Coulomb phase in the ground state. However, a slight deformation of the simple planar geometry allows to recover this phase in a regime where different ordered states compete with each other. In the quantum case, fluctuations due to a transverse magnetic field induce a symmetry breaking in the ground state, that melts at low temperature into a quantum Coulomb phase, with fractionalized excitations. In the classical Coulomb phase, low-temperature fluctuations are governed by the flip of closed loops of spins. In presence of quantum fluctuations, these loops expand in imaginary time and turn into “membranes”. These membranes allow to design a new Monte Carlo algorithm to investigate the quantum Coulomb phase at finite temperature.