1 Introduction Global energy demand is predicted to exceed 30 TW by 2050, about double the present value [1]. This predicament, known as the TeraWatt challenge, and concern over anthropogenic climate change, resource availability ("peak energy") and energy security have all increased the interest in renewable energy (hydroelectric, wind, solar, geothermal and biomass) [2]. One of the most promising renewable energy technologies is solar photovoltaics (PV) which convert sunlight directly into electrical energy. Although the resource potential of PV is enormous, it currently constitutes a small fraction (<1%) of global energy supply [2]. One of the main factors limiting the widespread adoption of PV is its low energy density, low efficiency, and relatively high cost in comparison to other energy technologies. This means that current PV technology can only compete in areas of high insolation or by government incentives such as feed-in tariff programs.One of the most relevant metrics for PV devices is the power conversion efficiency (PCE); that is, the efficiency with which sunlight can be converted to electrical power. A significant effort in PV research today aims to improve PCE while simultaneously reducing (or, at least, not significantly impacting) production cost. The vast majority of

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess related, but different ground states, with Tb2Sn2O7 displaying "soft" spin ice order below TN ∼ 0.87 K, while Tb2Ti2O7 enters a hybrid, glassy spin ice state below Tg ∼ 0.2 K. Our neutron measurements, performed at T = 1.5 K and 30 K, probe the crystal field states associated with the J = 6 states of Tb 3+ within the appropriate F d3m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb 3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of ∼ 2 increase in the crystal field bandwidth of the 2J + 1 = 13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous measurements on crystal field states in Tb2Sn2O7, wherein the ground-state doublet corresponds primarily to mJ = |±5 and the first excited state doublet to mJ = |±4 . In contrast, our results on Tb2Ti2O7 differ markedly from earlier studies, showing that the ground-state doublet corresponds to a significant mixture of mJ = |±5 , |∓4 , and |±2 , while the first excited state doublet corresponds to a mixture of mJ = |±4 , |∓5 , and |±1 . We discuss these results in the context of proposed mechanisms for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50 mK.

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Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb2B2O7 with B=Ti and Sn display a novel magnetic ground state in the presence of strong B-site disorder, characterized by a low susceptibility and strong spin fluctuations to temperatures below 0.1 K. These materials have been studied using ac-susceptibility and µSR techniques to very low temperatures, and time-offlight inelastic neutron scattering techniques to 1.5 K. Remarkably, neutron spectroscopy of the Tb 3+ crystal field levels appropriate to at high B-site mixing (0.5 < x < 1.5 in Tb2Sn2−xTixO7) reveal that the doublet ground and first excited states present as continua in energy, while transitions to singlet excited states at higher energies simply interpolate between those of the end members of the solid solution. The resulting ground state suggests an extreme version of a random-anisotropy magnet, with many local moments and anisotropies, depending on the precise local configuration of the six B sites neighboring each magnetic Tb 3+ ion.

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