Quantum Magnetism in the Shastry Sutherland Lattice Family R2Be2SiO7

Geometrically frustrated magnetism arises when competing exchange interactions are incompatible with the geometrical arrangement of magnetic ions in the lattice. This leads to a large ground state degeneracy and the system typically has difficulty establishing a unique ground state often resulting in unusual magnetic ground states characterized by quantum entanglement and complex field dependent phase diagrams. The rare earth compound family R2Be2SiO7 garnered our attention when we realized that the R3+ ions formed a structure equivalent to the geometrically frustrated Shastry-Sutherland lattice. The competing interactions and unique geometry of this lattice produces interesting magnetic phases both in zero field, in the form of unique ground states lacking long range order, and in field, where an out of plane field induces magnetization plateau in several members. An investigation into three members of the R2Be2SiO7 family, Er2Be2SiO7, Dy2Be2SiO7, and Yb2Be2SiO7 is presented in this thesis. Both Er2Be2SiO7 and Dy2Be2SiO7 compounds order into non-collinear antiferromagnetic states at 0.85 K and 1.1 K respectively. While Yb2Be2SiO7 shows no signs of long range order and instead appears to realize a unique entangled dimer ground state. For Er2Be2SiO7 field applied perpendicular to the plane produces two successive magnetic transitions but nor magnetization plateau. The behavior of Er2Be2SiO7 is broadly consistent with classical anisotropic moments. In Dy2Be2SiO7 a field applied perpendicular to the plane produces several magnetization plateau at: 1/7,4/9, and 2/3 fractions of

the saturation magnetization. Both Er2Be2SiO7 and Dy2Be2SiO7 have different zero- field spin structures with the same magnetic propagation vector of (0,0,1/2) and with moments primarily oriented in the ab-plane. The spin structure of the plateau phases in Dy2Be2SiO7 remains to be determined. For all three compounds inelastic neutron scattering experiments were done to gain insight into the CEF spectrum of the compound. The ground state doublet is found to be well isolated in Dy2Be2SiO7 and Yb2Be2SiO7 with the first excited level at 7.9 meV and 11 meV respectively. For Er2Be2SiO7 the first excited level is at only 1.7 meV indicating an Sef f = 1/2 model is not appropriate. In Yb2Be2SiO7 inelastic measurements with cold neutrons reveal several excitations related to gaps in the spin excitation spectrum at energy transfers of 0.11 meV and 0.19 meV. Subsequent analysis determines that these gaps are due to excitations from the novel entangled dimer unit ground state: (↑↑ − ↓↓)/√2.