Neutron Scattering Studies of the Quantum Spin Ice Material Yb2Ti2O7

Abstract

<p>Yb2Ti2O7 is one member of a series of magnetic compounds with the pyrochlore lattice structure. For specific types of single-ion anisotropy and exchange interactions, the geometry of the pyrochlore lattice frustrates near-neighbor interactions and coaxes a wide variety of unusual magnetic ground states from such compounds. Yb2Ti2O7 is unique among these compounds in that the source of the frustration is not immediately obvious when one considers the combination of single-ion anisotropy (XY-like) and the nature of the exchange interactions (ferromagnetic) present therein. A conventional magnetic transition was indeed initially expected based on the observation of specific heat anomaly near 200mK. However, many studies produced no signs of long-range magnetic order below this temperature. Intriguingly, above the transition, evidence for unusual two-dimensional correlations came in the form of rods of magnetic diffuse neutron scattering. This thesis contains four articles that detail the results of several neutron scattering studies on Yb2Ti2O7. The goal of these studies was to determine the nature of the static and dynamic spin correlations throughout the magnetic field vs. temperature phase diagram of Yb2Ti2O7.</p> <p>We first performed a time-of-flight neutron scattering experiment on a single crystal of Yb2Ti2O7, which we prepared using the optical floating zone method. This initial study provided a comprehensive survey of the phase diagram, including the previously unexplored response to a magnetic field. We found that the rods of diffuse scattering change qualitatively upon cooling below the temperature of the reported specific heat anomaly, showing signs for the development of short-range three-dimensional correlations. Additionally, we discovered that a relatively small magnetic field applied along the [110] direction could remove the diffuse scattering entirely, and produce sharp spin wave excitations in the inelastic channel, indicating long range spin correlations.</p> <p>We further quantified the temperature dependence of the diffuse scattering in zero-field using a triple-axis neutron spectrometer. The crossover from two-dimensional correlations to short-range three-dimensional correlations was found to begin at 400mK and reach completion near the temperature of the specific heat anomaly, ∼200mK. Our measurements of the low temperature specific heat of several single crystal samples, as well as a powder sample, revealed that significant sample-dependence of the magnetic properties exists. The single crystal samples were shown to have broader features in the specific heat at relatively low temperatures compared to the powder samples, pointing to some amount of structural disorder in the single crystals.</p> <p>To understand the nature of the structural defects in the single crystals, we compared the structure of a crushed single crystal of Yb2Ti2O7 to that of a powder sample using neutron powder diffraction. The major conclusion of that work was that the single crystal is non-stoichiometric, containing 2.3% excess ytterbium on the (non-magnetic) titanium sublattice. The introduction of additional magnetic moments into the system is expected to be the cause of the sample-dependence of the specific heat anomaly.</p> <p>Finally, we fit the spin wave dispersions in the field-polarized state, as measured by time-of-flight inelastic neutron scattering, to an effective spin-1/2 anisotropic exchange Hamiltonian. The microscopic parameters extracted from these fits place Yb2Ti2O7 close to exotic Quantum Spin Liquid phases predicted for the anisotropic spin-1/2 pryochlore model. The exchange parameters also reveal that the source of the frustration in Yb2Ti2O7 comes from the “quantum spin ice” nature of its exchange interactions.</p>