Algebraic correlations and dispersive excitations in the pyrochlore magnet Tb2Ti2O7

Orateur : Sylvain PETIT (LLB, CEA Saclay)

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. In the latter case, an e-ective ferromagnetic interaction leads to a degenerate ground state where two spins point in and two spins point out each tetrahedron. No long range order is observed but the spins experience algebraic correlations. The ground state is then a "spin-ice", as for instance in Ho2Ti2O7 and Dy2Ti2O7 and exhibits fractionalized excitations (the celebrated magnetic monopoles). In contrast, Tb2Ti2O7 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]. To explain this spectrum, we recently proposed that a breaking of the trigonal symmetry of the Tb crystal -eld develops at very low temperature [5, 6]. This scenario is still debated, but it might be the consequence of a cooperative tetragonal distortion, thought to be precursor to a JahnTeller transition. These considerations will be developed in this presentation and discussed in the context of recent theoretical studies. References [1] J Gardner, M. Gingras and J. Greedan, Reviews of modern Physics, Vol 82 (2010) [2] J. Gardner et al PRL 82, 1012, (1999) [3] T. Fennell et al, PRL 109, 017201 (2012) [4] S. Guitteny et al., in preparation. [5] P. Bonville et al. PRB 84, 184409 (2011) [6] S. Petit et al., PRB 85, 054428 (2012)

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