Recent work has highlighted remarkable effects of classical thermal fluctuations in the dipolar spin ice compounds, such as ''artificial magnetostatics,'' manifesting as Coulombic power-law spin correlations and particles behaving as diffusive ''magnetic monopoles.'' In this paper, we address quantum spin ice, giving a unifying framework for the study of magnetism of a large class of magnetic compounds with the pyrochlore structure, and, in particular, discuss Rare-earth pyrochlores display a diverse set of fascinating physical phenomena [1]. One of the most interesting aspects of these materials from the point of view of fundamental physics is the strong frustration experienced by coupled magnetic moments on this lattice. The best explored materials exhibiting this frustration are the ''spin ice'' compounds, Ho 2 Ti 2 O 7 , Dy 2 Ti 2 O 7 , in which the moments can be regarded as classical spins with a strong easyaxis (Ising) anisotropy [2,3]. The frustration of these moments results in a remarkable classical spin liquid regime displaying Coulombic correlations and emergent ''magnetic monopole'' excitations that have now been studied extensively in theory and experiment [4][5][6].Strong quantum effects are absent in the spin ice compounds, but can be significant in other rare-earth pyrochlores. In particular, in many materials the low-energy spin dynamics may be reduced to that of an effective spin S ¼ 1=2 moment, with the strongest possible quantum effects expected. In this case symmetry considerations reduce the exchange constant phase space of the nearestneighbor exchange Hamiltonian to a maximum of three dimensionless parameters [7] [9,10]. This makes these materials beautiful examples of highly frustrated and strongly quantum magnets on the pyrochlore lattice. They are also nearly ideal subjects for detailed experimental investigation, existing as they do in large high-purity single crystals, and with large magnetic moments amenable to neutron scattering studies. Yb 2 Ti 2 O 7 is particularly appealing because its lowest Kramers doublet is extremely well separated from the first excited one [11], and a very large single-crystal neutron scattering data set is available, allowing us to determine the full Hamiltonian quantitatively, as we will show. Although we specialize to Yb 2 Ti 2 O 7 in the present article, the theoretical considerations and parameter determination method described here will very generally apply to all pyrochlore materials where exchange interactions dominate, and whose dynamics can be described by that of a single doublet.Theoretical studies have pointed to the likelihood of unusual ground states of quantum antiferromagnets on the pyrochlore lattice [12,13]. Most exciting is the possibility of a quantum spin liquid (QSL) state, which avoids magnetic ordering and freezing even at absolute zero temperature, and whose elementary excitations carry fractional quantum numbers and are decidedly different from spin waves [14]. Although one neutron study [15] supported ferromagnetic order in Yb 2...

Here we establish the systematic existence of a U(1) degeneracy of all symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice, at the mean-field level. By extracting the Hamiltonian of Er(2)Ti(2)O(7) from inelastic neutron scattering measurements, we then show that the U(1)-degenerate states of Er(2)Ti(2)O(7) are its classical ground states, and unambiguously show that quantum fluctuations break the degeneracy in a way which is confirmed by experiment. The degree of symmetry protection of the classical U(1) degeneracy in Er(2)Ti(2)O(7) is unprecedented in other materials. As a consequence, our observation of order by disorder is unusually definitive. We provide further verifiable consequences of this phenomenon, and several additional comparisons between theory and experiment.

Recent neutron scattering and specific heat studies on the pyrochlore Yb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In the best samples, a sharp anomaly in the specific heat is observed at T=265mK. Other samples, especially single crystals, have broad features in the specific heat which vary in sharpness and temperature depending on the sample, indicating that the magnetic ground state may be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverised single crystal of Yb2Ti2O7, grown by the floating zone technique, to a sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a "stuffed" pyrochlore, Yb2(Ti2−xYbx)O 7−x/2 with x = 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb 3+ on the non-magnetic Ti 4+ sublattice would introduce random exchange bonds and local lattice deformations. These are expected to be the mechanism leading to of the variation of the delicate magnetic ground state of Yb2Ti2O7. Determination of the cubic cell length, a, could be useful as a method for characterizing the stoichiometry of nonpulverised single crystals at room temperature.PACS numbers:

Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two-dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along 111 directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of approximately 0.5 T applied along the [110] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.

The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T=265mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short range three dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasi-elastic (QE) scattering below ∼ 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin-stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265mK, which we discuss in the context of recent mean field theoretical analysis.