Recent progress in the physics of highly frustrated pyrochlore magnets

Orateur : Sylvain PETIT (Laboratoire Léon Brillouin - CEA Saclay - Gif sur Yvette - France)

Geometrical magnetic frustration is a central concept in condensed matter physics, at the heart of a number of issues. In this -eld, rare earth pyrochlore magnets R2Ti2O7 (R is a rare earth) play a prominent role, as they form model systems showing a rich variety of ground states, depending on the balance between dipolar, exchange interactions and crystal -eld [1]. Actually, the CEF scheme drives di-erent kinds of anisotropies, for instance XY or Ising-like, with magnetic moments constrained along the local <111> axes. These considerations will be reviewed in this presentation and discussed in the context of recent theoretical studies emphasizing the existence of an emergent Gauge structure. We will especially discuss the case of Tb2Ti2O7, which remains a cooperative paramagnet (a spin liquid), with strongly correlated moments still uctuating at 50 mK [2]. The reason for this behavior remains so far a theoretical puzzle and an experimental challenge [1]. According to our neutron experiments, the magnetic correlations in Tb2Ti2O7 are characterized by pinch points, especially visible around Q=(111) positions. These features might be the signature of algebraic correlations, reminiscent of a local constraint but likely di-erent from those observed in spin ices [3]. Neutron data also reveal that Tb2Ti2O7 supports strong low energy uctuations, below the -rst crystal -eld transition located at about 1.5 meV. Polarized neutron experiments show that these uctuations correspond to well-de-ned and dispersive collective excitations [4]. We also report anomalies of the phonon modes as well as of the -rst CEF level, unveiling a strong dynamical coupling to the lattice likely driven by quadrupolar e-ects.

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