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Fractionalised excitations in spin ice materials

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Modern physics rests on two fundamental pillars: the Standard Model of elementary particles and their interactions; and Landau theory of phase transitions based on local order parameters and spontaneous symmetry breaking. However, recent years have witnessed the discovery of a new class of systems that turned our understanding on its head, upending both our concept of particles and of phases of matter in an surprising and exciting way. These new phases of matter are generally dubbed `topological’ and exhibit unprecedented properties, such as (quasi)particles that behave neither like fermions nor bosons, and symmetries that are not inherited from those of the elementary degrees of freedom but rather emerge from the low temperature correlations. Spin ice models and materials — such as Dy2Ti2O7 and Ho2Ti2O7 — are a case in point. They provide a rare instance of emergent gauge symmetry and fractionalisation in three dimensions. Their elementary excitations carry a fraction of the magnetic moment of the microscopic spin degrees of freedom and they can be thought of as magnetic monopoles. The highly degenerate low temperature phase of spin ice can thus be described as an effective vacuum hosting itinerant charges — a Coulomb liquid. This description proves paramount to understand the thermodynamic behaviour. Moreover, spin ice ground states are locally constrained and, in the absence of monopole excitations, effectively no dynamics is possible. At low temperatures, when the monopoles are sparse, dynamics becomes very slow and these systems offer a rich playground for out of equilibrium behaviour, in a novel setting which combines kinematic constraints, emergent topological defects, and magnetic long range Coulomb interactions. Spin ice has since become a point of reference for fractionalised topological spin liquid behaviour in three dimensions, and a laboratory of choice for the study of tuneable, slow dynamics.

This talk is part of the Cavendish Physical Society series.

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