Phonon and crystal field excitations in geometrically frustrated rare earth titanates

Abstract

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated ͑295-4 K͒. The resulting full phononic characterization of the R 2 Ti 2 O 7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb 2 Ti 2 O 7 in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb 3+ sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13 cm −1 is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb 2 Ti 2 O 7 . The additional crystal field information on Tb 2 Ti 2 O 7 adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.

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].…”

Temperature-dependent Raman and x-ray studies of the spin-ice pyrochloreDy2Ti2O7and nonmagnetic pyrochlore

“…Some of the previous studies [23,24,25] labeled the mode M9(N9) as A 1g + F 2g , and overlooked the weak mode near 450 cm −1 . We and Lummen et al [16] have observed a mode near 450 cm −1 in several of the pyrochlore titanates. This mode has sometimes been assigned to T iO 2 as an impurity phase in the pyrochlore sample [26].…”

“…We note that Lummen et al [16] and Maczka et al [17] also see this "new" mode in their low-temperature Raman spectra of Dy 2 T i 2 O 7 . Lummen et al tentatively assigned the "new" mode near 300 cm −1 to the first excited CF level after the crystal field splitting estimation due to Rosenkranz et al [28].…”

“…Recently, vibrational properties of some titanate pyrochlores (including the spin-ice Dy 2 T i 2 O 7 ) have been studied using infrared reflectivity [15] and Raman spectroscopy [16,17] up to room temperature. In the work by Maczka et al [17] on R 2 T i 2 O 7 (R = Er, Gd and Dy), Raman studies of Gd 2 T i 2 O 7 and Er 2 T i 2 O 7 were carried out to determine the temperature dependence of Raman mode frequencies and line-widths.…”