Oxygen isotope effect on the vibrational modes ofLa1−xCaxMnO

Abstract

Raman-scattering experiments have been carried out on polycrystalline samples of La 0.65 Ca 0.35 Mn 16 O 3 and isotope-exchanged La 0.65 Ca 0.35 Mn 18 O 3 as a function of temperature. The most interesting feature in the spectra is a vibrational mode that occurs at a nominal frequency of 0 Ϸ230 cm Ϫ1 , which is assigned to an out-ofphase rotational mode that involves motion of the O͑1͒ atoms. The frequency of this phonon depends strongly on the Ca concentration and has been correlated with the strength of the Jahn-Teller lattice distortion as measured by the tolerance factor. As the temperature is lowered through the critical temperature, the phonon frequency 0 increases rather abruptly and continues to harden as the temperature is lowered further. In contrast, the linewidth ⌫ 0 decreases abruptly when the sample passes through the transition from the paramagnetic insulating phase to the more metallic ferromagnetic phase. The observed temperature dependences of the frequency and linewidth are well described by a model that incorporates a double-exchange mechanism in the presence of an electron-phonon interaction. The agreement between experiment and theory suggests that the temperature dependence of the phonon is determined primarily by the spin alignment associated with the double-exchange mechanism. The results for both 0 and ⌫ 0 imply that there are no significant structural modifications associated with the transition to the ferromagnetic phase. Finally, the linewidth ⌫ 0 in the isotopeexchanged sample ( 18 O) is smaller than in the 16 O sample, an observation that is consistent with the smaller electron-phonon-coupling constant in the 18 O compound.

Raman spectra in epitaxial thin films ofLa1−xCaxMnO3(Xiong1, Chen2, Wang3 et al. 2004Phys. Rev. B502232View full textAdd to dashboardCite

“…While a tensile strain affects the lattice strongly. These sharp peaks in the 400 to 500 cm −1 range always appear together in the Raman spectra of LCMO 37,41,43,45,46 . Based on their similar behavior with the temperature and substrates in our case and the earlier reports in films and bulk samples 37,41 , we attribute these narrow peaks to the same symmetry.…”

“…The peak of ω 1 sharpens slightly below 220 K. As shown in the inset of Fig.2a, the magnetic phase transition can be observed clearly, the Curie Temperature (T C ) is about 220 K. The sharpening of ω 1 peak has been reported earlier by Podobedov et al 38 and attributed to the spin-lattice coupling in the spin-ordered FM state. Recently, another explanation of this narrowing of ω 1 , could be the increase of long range structural coherence due to the reduction of the dynamic JT distortion 37,45 .…”

Raman scattering in charge-ordered Pr0.63Ca0.37MnO3: Anomalous temperature dependence of linewidth

“…Recent experiments on LaMnO 3 have revealed the presence of high-energy modes (∼ 1000cm −1 ) which have been attributed to the orbital excitations [17]. However, doped manganites being pseudo-cubic, Raman lines are extremely weak and broad [9][10][11][12][13]. The number of modes observed in doped manganites are very few; e.g., in La 0.7 Ca 0.3 MnO 3 there are five modes observed at low temperature at 70, 133, 235, 435, and 670cm −1 [11].…”

“…There are many Raman studies on manganites in recent times covering both the vibrational [9][10][11][12][13] and electronic Raman scattering [14] across the metal-insulator transition. In a recent work on polycrystalline Pr 0.65 Ca 0.35 MnO 3 [15], temperature dependence of the modes at 475 and 610cm −1 was studied and the splitting of these modes below T CO was attributed to the folding of the Brillouin zone in the CO state.…”

Isotope effect on Raman spectra of polycrystalline La0.67Ca0.33MnO3

“…Recently, it was reported that a bottleneck spin relaxation takes place from the exchange-coupled M n 4C ion via the M n 3C Jahn-Teller ions to the lattice. 18 Recently, Irwin et al 19 reported on an oxygen isotope effect for the vibration modes. They discussed mainly the blue shift of the out-ofphase rotational mode (230 cm 1 ) and explained the shift by a mode that incorporates a double-exchange mechanism in the presence of an electron-phonon interaction.…”