第6回　物理工学科教室談話会（講師：Dr. Ludovic Jaubert）

---

講演要旨：
Frustrated magnets are a class of materials that have kept exhibiting intriguing new phases of matter for the past 30 years !
While magnetic ordering is usually expected at a temperature of the order of the coupling “J” between spins, geometric frustration can prevent such phase transition down to much lower temperature. Even if this has been widely used as a test of frustration in susceptibility experiments, its general behaviour still remains relatively unclear. Comparing analytical theory and Monte Carlo simulations to neutron scattering and bulk susceptibility measurements on the Ho2Ti2O7 spin ice compound, we have recently shown how “pushing down” the transition is accompanied by a crossover between the standard high temperature Curie law and a low temperature “Spin Liquid Curie law” with a different pre-factor, hence using local and bulk measurements as a signature of a topological phase, and shining light on the experimental difficulty to measure a precise Curie-Weiss temperature in frustrated materials.
The second aspect of my talk will focus on what is happening once we have reached this classical Spin Liquid phase at low temperature, characterised by strong fluctuations and magnetic correlations at the same time; the emergent gauge field Coulomb phase. Here we shall make a brief review of this Coulomb phase physics, with phase transitions beyond the Ginzburg-Landau paradigm, comparing our theory with in-field and under-pressure experiments on spin ice, making contact with 2D-artificial spin ice, Heisenberg antiferromagnets, percolation physics [2] and the influence of itinerant electrons on the pyrochlore lattice [3].
[1] Jaubert, Harris, Fennel, Melko, Bramwell & Holdsworth, Phys. Rev. X (2013)
[2] Jaubert, Haque, Moessner, Phys. Rev. Lett. (2011)
[3] Jaubert, Pitaecki, Haque & Moessner, Phys. Rev. B (2012)
紹介教員：今田教授、求准教授