Low-temperature and high-pressure Raman and x-ray studies of pyrochlore Tb2Ti2O7 : Phonon anomalies and possible phase transition (original) (raw)
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Journal of Raman Spectroscopy, 2011
Anomalous temperature dependence of Raman phonon wavenumbers attributed to phonon-phonon anharmonic interactions has been studied in two different families of pyrochlore titanates. We bring out the role of the ionic size of titanium and the inherent vacancies of pyrochlore in these anomalies by studying the effect of replacement of Ti 4 + by Zr 4 + in Sm 2 Ti 2 O 7 and by stuffing Ho 3 + in place of Ti 4 + in Ho 2 Ti 2 O 7 with appropriate oxygen stoichiometry. Our results show that an increase in the concentration of the larger ion, i.e. Zr 4 + or Ho 3 + , reduces the phonon anomalies, thus implying a decrease in the phonon-phonon anharmonic interactions. In addition, we find signatures of coupling between a phonon and crystal field transition in Sm 2 Ti 2 O 7 , manifested as an unusual increase in the phonon intensity with increasing temperature.
We revisit the assignment of Raman phonons of rare-earth titanates by performing Raman measurements on single crystals of O 18 isotope-rich spin ice Dy 2 Ti 2 O 18 7 and nonmagnetic Lu 2 Ti 2 O 18 7 pyrochlores and compare the results with their O 16 counterparts. We show that the low-wavenumber Raman modes below 250 cm À1 are not due to oxygen vibrations. A mode near 200 cm À1 , commonly assigned as F 2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorderinduced Raman active mode involving Ti 4+ vibrations. Moreover, we address here the origin of the 'new' Raman mode, observed below T C~1 10 K in Dy 2 Ti 2 O 7 , through a simultaneous pressure-dependent and temperature-dependent Raman study. Our study confirms the 'new' mode to be a phonon mode. We find that dT C /dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed.
Temperature-dependent studies of the geometrically frustrated pyrochloresHo2Ti2O7andDy2Ti2O7
Physical Review B, 2009
A temperature-dependent Raman study of Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 single crystals was performed in the 5-873 K temperature range. Polarized spectra allowed us to establish the symmetries of the observed bands and revise the mode assignment made in previous works. Our studies revealed also two additional bands near 287 and 300 cm −1 for Dy 2 Ti 2 O 7 , which can be assigned to crystal field transitions in Dy 3+ ions. Temperature dependent Raman studies showed large increase of linewidths. These changes have been analyzed in terms of strong third-order phonon-phonon anharmonic interactions. The Raman spectra also showed anomalous softening of the majority of phonon modes upon cooling in the whole temperature range studied. In contrast to this behavior, the F 2g ϳ310 cm −1 and E g ϳ330 cm −1 phonon modes showed hardening upon cooling down to about ͑100-120 K͒ and then anomalous softening below this temperature. This anomalous behavior of phonon wave numbers has been attributed to the increase in octahedral distortion upon cooling.
Physical Review B, 2011
O 7 using femtosecond laser pulses (65 fs, 1.57 eV) in degenerate time-resolved transmission experiments as a function of temperature from 4 to 296 K. At 4 K, two coherent phonons are observed at ∼5.3 THz (5.0 THz) and ∼9.3 THz (9.4 THz) for Dy 2 Ti 2 O 7 (Gd 2 Ti 2 O 7 ), whereas three coherent phonons are generated at ∼5.0, 8.6, and 9.7 THz for Tb 2 Ti 2 O 7 . In the case of spin-ice Dy 2 Ti 2 O 7 , a clear discontinuity is observed in the linewidths of both the coherent phonons as well as in the phase of lower-energy coherent phonon mode, indicating a subtle structural change at 110 K. Another important observation is a phase difference of π between the modes in all the samples, thus suggesting that the driving forces behind the generation of these modes could be different in nature, unlike a purely impulsive or displacive mechanism.
eprint arXiv: 0812.0026, 2008
We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound Dy 2 T i 2 O 7 and its non-magnetic analogue Lu 2 T i 2 O 7 . Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy 2 T i 2 O 7 , appearing below a temperature of T c =110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O 18 -isotope substituted Dy 2 T i 2 O 7 confirm the phonon origin of the "new" mode. These findings, absent in Lu 2 T i 2 O 7 , suggest that the room temperature cubic lattice of the pyrochlore Dy 2 T i 2 O 7 undergoes a "subtle" structural transformation near T c . We find anomalous red-shift of some of the phonon modes in both the Dy 2 T i 2 O 7 and the Lu 2 T i 2 O 7 as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.
Physical Review B, 2008
We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound Dy2Ti2O7Dy_2Ti_2O_7Dy2Ti2O7 and its non-magnetic analogue Lu2Ti2O7Lu_2Ti_2O_7Lu2Ti2O7. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy2Ti2O7Dy_2Ti_2O_7Dy2Ti2O7, appearing below a temperature of Tc=T_c=Tc=110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O$^{18}$-isotope substituted Dy2Ti2O7Dy_2Ti_2O_7Dy2Ti2O7 confirm the phonon origin of the "new" mode. These findings, absent in Lu2Ti2O7Lu_2Ti_2O_7Lu2Ti2O7, suggest that the room temperature cubic lattice of the pyrochlore Dy2Ti2O7Dy_2Ti_2O_7Dy2Ti2O7 undergoes a "subtle" structural transformation near TcT_cTc. We find anomalous \textit{red-shift} of some of the phonon modes in both the Dy2Ti2O_7Dy_2Ti_2O_7Dy2Ti2O7 and the Lu2Ti2O7Lu_2Ti_2O_7Lu_2Ti_2O_7 as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.
Structural Fluctuations in the Spin-Liquid State of Tb2Ti2O7
Physical Review Letters, 2007
High resolution X-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin liquid, or cooperative paramagnetic state, at temperatures below ∼ 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd3m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at ∼ 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures.
Phonon and crystal field excitations in geometrically frustrated rare earth titanates
Physical Review B, 2008
The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations of the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R2Ti2O7 pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb2Ti2O7 in Raman spectroscopy for the first time, 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 Tb2Ti2O7. These findings call for a careful reassessment of the low temperature structure of Tb2Ti2O7, which may serve to improve its theoretical understanding.
Phonon anharmonicity in bulk Td-MoTe2
Applied Physics Letters, 2016
We examine anharmonic contributions to the optical phonon modes in bulk T d-MoTe 2 through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Grüneisen model. However, below 100 K we observe nonlinear temperaturedependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near T ≈ 250 K that correlate well with the T d →1T ′ structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk T d-MoTe 2 , an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals.