What is the nature of the ground state in the stoichiometric pyrochlore Yb2Ti2O7 ?"

Orateur : Edwin KERMARREC (LNCMI Grenoble)

Rare-earth pyrochlores magnets are known to be prime examples of the rich physics generated by magnetic frustration [1]. Among them, Yb2T i2O7 has attracted a lot of attention recently due to the exciting possibility that it could host a Quantum Spin Liquid (QSL) state below a first-order transition observed in specific heat at Tc = 265mK [2]. However, the nature of the ground-state in Yb2Ti2O7 is greatly debated, and several studies find some evidences for long range ferromagnetic order below Tc [3,4,5]. Moreover, these experimental observations seem to vary from sample to sample, perhaps due to a slightly off-stoichiometry affecting real materials. We will present low temperature neutron scattering measurements carried out on a well characterized polycrystalline sample of stoichiometric Yb2Ti2O7. Our measurements reveal that there is change of intensity at nuclear Bragg positions upon warming which does not occur sharply at Tc, and which is consistent with an ordered moment size of 1.1 muB (58% of the saturation moment). The inelastic excitations at low temperature suggest the presence of dispersive modes coexisting with incoherent low energy fluctuations. Our results suggest that the ground state in nominally pure Yb2Ti2O7 is not a conventional ordered ferromagnet, but instead involves only partial polarization of the magnetic moments coexisting with a disordered component, a situation reminiscent of the partially polarized QSL called the Coulomb Ferromagnetic phase.

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