Neutron spectroscopic study of crystal field excitations inTb2Ti2O7andTb2Sn2

Abstract

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.

Double vibronic process in the quantum spin ice candidate Tb2Ti2O7 revealed by terahertz spectroscopy

“…As the temperature is lowered down to 6 K, an absorption peak develops around 0.42 THz (1.7 meV). This feature is consistent with the first excited CEF level previously reported by neutron and optical spectroscopic studies [25][26][27][28][29]. A much weaker and so far unreported peak is visible at 0.67 THz (2.78 meV).…”

“…It features a first excited doublet at a low energy of ∆ ≈ 1.5 meV above the ground state [25][26][27][28][29], compared to 25 meV for Ho 3+ in the prototype spin ice compound. Furthermore, below ∼20 K, this excited doublet has been shown to couple to a transverse acoustic phonon, forming a hybrid magneto-elastic excitation [30].…”

Quenched crystal-field disorder and magnetic liquid ground states inTb2Sn2−xTixO7

“…Well defined low energy CF excitations for the end members, x = 0 and x = 2, appear as sharp peaks in energy between 1-2 meV with weak dispersion, such that they display a minimum near |Q| ∼ 1.25Å −1 for both cases 13,[15][16][17] . However, as seen in Fig.…”

“…However, this is not the case for Tb 3+ in Tb 2 Ti 2 O 7 , wherein the J = 6 total angular momentum multiplet splits into (2J + 1) = 13 states, the lowest energy of which are a ground state doublet and an excited state doublet roughly 1.2 meV above it [13][14][15] . Tb 2 Ti 2 O 7 displays antiferromagnetic interactions with a Curie-Weiss constant of ∼ -19 K 15-17 , but no conventional long range order to below ∼ 0.1 K, even though it would be expected to order magnetically near ∼ 1 K, were its CF ground state cleanly separated in energy from its excited states 18,19 .…”

“…Again at low temperatures the relaxation rate rises gradually to a low temperature limit of ∼ 2 µsec −1 , showing a progressively smaller low temperature limit than either end member; again consistent with a disordered and dynamic ground state, and no hysteretic behavior, in the presence of strong B-site disorder. 13 . Remarkably, our neutron spectroscopy of the CF states in the presence of B-site disorder strongly suggests that doublet states present as extended continua in energy, while singlet states have their energy eigenvalues and lifetimes modified, but appear less strongly affected.…”