Proposal for a [111] Magnetization Plateau in the Spin Liquid State of Tb2Ti2O7

Despite a Curie-Weiss temperature $\theta_{\rm CW} \sim -14$ K, the Tb2Ti2O7 pyrochlore magnetic material lacks long range magnetic order down to at least $T^*\approx 50$ mK. It has recently been proposed that the low temperature collective paramagnetic or spin liquid regime of this material may be akin to a spin ice state subject to both thermal and quantum fluctuations $-$ a {\it quantum spin ice} (QSI) of sorts. Here we explore the effect of a magnetic field ${\bm B}$ along the $[111]$ direction on the QSI state. To do so, we investigate the magnetic properties of a microscopic model of Tb2Ti2O7 in an independent tetrahedron approximation in a finite ${\bm B}$ along $[111]$. Such a model describes semi-quantitatively the collective paramagnetic regime where nontrivial spin correlations start to develop at the shortest lengthscale, that is over a single tetrahedron, but where no long range order is yet present. Our results show that a magnetization plateau develops at low temperatures as the system develops ${\bm B}=0$ ferromagnetic spin-ice-like "two-in/two-out" correlations at the shortest lengthscale. From these results, we are led to propose that the observation of such a [111] magnetization plateau in Tb2Ti2O7 would provide compelling evidence for a QSI at ${\bm B}=0$ in this material and help guide the development of a theory for the origin of its spin liquid state.Comment: 6 pages, 3 figure

Absence of anomalous negative lattice-expansion for polycrystalline sample of Tb2Ti2O7

High resolution X-ray powder-diffraction experiments on a well-characterized polycrystalline sample of the spin liquid Tb2Ti2O7 reveal that it shows normal positive thermal-expansion above 4 K, which does not agree with the intriguing anomalous negative thermal-expansion due to a magneto-elastic coupling reported for a single crystal sample below 20 K. We also performed a Rietveld profile refinement of a powder-diffraction pattern taken at a room temperature, and confirmed that it is consistent with the fully ordered cubic pyrochlore structure.Comment: 2 pages, 3 figure

The Spin Liquid State of the Tb2Ti2O7 Pyrochlore Antiferromagnet: A Puzzling State of Affairs

The pyrochlore antiferromagnet Tb2Ti2O7 has proven to be an enigma to experimentalists and theorists working on frustrated magnetic systems. The experimentally determined energy level structure suggests a local Ising antiferromagnet at low temperatures, T < 10 K. An appropriate model then predicts a long-range ordered Q = 0 state below approximately 2 K. However, muon spin resonance experiments reveal a paramagnetic structure down to tens of milli-Kelvin. The importance of fluctuations out of the ground state effective Ising doublet has been recently understood, for the measured paramagnetic correlations can not be described without including the higher crystal field states. However, these fluctuations treated within the random phase approximation (RPA) fail to account for the lack of ordering in this system below 2 K. In this work, we briefly review the experimental evidence for the collective paramagnetic state of Tb2Ti2O7. The basic theoretical picture for this system is discussed, where results from classical spin models are used to motivate the investigation of quantum effects to lowest order via the RPA. Avenues for future experimental and theoretical work on Tb2Ti2O7 are presented.Comment: Latex2e,6 pages, IOP format, introduction shortened and other minor corrections, replaced with published version in the Proceedings of the Highly Frustrated Magnetism 2003 Conference, Grenobl