The Stabilization of Monopole Matter in Ising Pyrochlores

SLOBINSKY, D.; BORZI, R. A.; GRIGERA, S. A.; PILI, L.; G. BAGLIETTO

Buenos Aires

Conferencia; StatPhys 27; 2019

The so called Spin Ices are prototype materials for geometrical frustration. They show spectacular agreement between theory and experiments and had served as a playground for exotic behaviour such as residual entropy, Coulomb phases with pinch points correlations, and effective magnetic monopoles, among others. The inexhaustible Ising pyrochlore systems, where spin ices live, are yet compatible with another instance of geometrical frustration: Monopole Matter. The tuning of the nearest neighbor exchange constant and field direction can be used to obtain the most trivial forms of it: monopole gases and ordered crystals of staggered magnetic charge. However, other forms of order and disorder (including perfectly paramagnetic monopole liquids, dense fluids which are liquid both in the magnetic and in the structural sense, crystals of monopoles with magnetic moment fragmentation, or double layered crystals) have been proposed and even measured in experiments. In spite of this, it is perhaps even more remarkable that we do not generally know how these phases are (or could be) stabilized in nature.Among other examples, understanding Monopole Matter promise to be an essential step to shed light into the physics of $Tb_2Ti_2O_7$. Almost 20 years of experiments agree on the facts that this material presents a very high magneto-elastic coupling, pinch-points that could be related to a Fragmented Monopole Crystal, and a double layer crystal of monopoles when it is subject to a moderate field in the $[110]$ crystallographic direction. In this presentation we will introduce, for the first time, a realistic classical Hamiltonian that is able to stabilize a Monopole Liquid phase (and concomitantly other phases of Monopole Matter in the presence of different external fields and interactions) at low temperatures. To achieve this, we have taken into account the modulation of the interaction between neighboring spins when the central oxygen of the tetrahedron is allowed to move. We use this results as a starting point to obtain a high level of low temperature spin dynamics and a double layer crystal of monopoles in the presence of an applied field, needed to explain $Tb_2Ti_2O_7$ peculiar behavior.