Phonons and Magnetic Excitations in Mott-Insulator LaTiO$_3$

We report on the optical properties of undoped single crystal LaMnO_3, the parent compound of the colossal magneto-resistive manganites. Near-Normal incidence reflectance measurements are reported in the frequency range of 20-50,000 cm-1 and in the temperature range 10-300 K. The optical conductivity, s_1(w), is derived by performing a Kramers-Kronig analysis of the reflectance data. The far-infrared spectrum of s_1(w) displays the infrared active optical phonons. We observe ...

We calculated the infrared conductivity spectrum of orbitally ordered LaMnO$_3$ in phonon frequency and overtone frequency ranges. We considered orbital exchange, Jahn-Teller electron-phonon coupling, and phonon--phonon coupling. The fundamental excitation of the phonon-coupled orbiton was only Raman active, not infrared active, while its overtone modes were both Raman and infrared active. Our calculations reproduced the small peaks near 1300 cm$^{-1}$ observed both in Raman ...

Polarized Raman scattering and infrared reflection spectra of hexagonal HoMnO$_3$ single crystals in the temperature range 10-300 K are reported. Group-theoretical analysis is performed and scattering selection rules for the second order scattering processes are presented. Based on the results of lattice dynamics calculations, performed within the shell model, the observed lines in the spectra are assigned to definite lattice vibrations. The magnetic ordering of Mn ions, whic...

The Raman-active phonons in perovskite-like LaMn1-xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0) were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly more phonon lines than allowed for the rhombohedral LaCoO3 structure were observed in the spectra.

We report results of an investigation of the temperature dependence of the magnon and phonon frequencies in NiO. A combination of Brillouin - Mandelstam and Raman spectroscopies allowed us to elucidate the evolution of the phonon and magnon spectral signatures from the Brillouin zone center (GHz range) to the second-order peaks from the zone boundary (THz range). The temperature-dependent behavior of the magnon and phonon bands in the NiO spectrum indicates the presence of an...

We present optical measurements of phononic excitations in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) and La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin films covering the full temperature range from the metallic ferromagnetic to the insulating paramagnetic phase. All eight phonons expected for the R$\bar{3}$c symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma symmetry in LCMO have been determined. Close to the ferromagnetic-to-paramagnetic transition both compounds rev...

We revisit the theory of magnetic Raman scattering in Mott insulators with strong spin-orbit coupling, with a major focus on Kitaev materials. We show that Kitaev materials with bond-anisotropic interactions are generally expected to show both one- and two-magnon responses. It is further shown that, in order to obtain the correct leading contributions to the Raman vertex operator $\mc{R}$, one must take into account the precise, photon-assisted microscopic hopping processes o...

We report an infrared reflection spectroscopy study of La$_{1/2}$Ca$_{1/2}$MnO$_3$ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T$_C$=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops be...

Raman scattering experiments have been carried out on single crystals of Nd$_{0.5}$Sr$_{0.5}$MnO$_3$ as a function of temperature in the range of 320-50 K, covering the paramagnetic insulator-ferromagnetc metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We unde...

Millimeter size single crystals of double perovskite La2NiMnO6 (LNMO) were grown by the High Temperature Solution Growth Method. The magnetic measurements and polarized Raman spectra between 5 and 350K provide evidence that the crystals have high degree of Ni(2+)/Mn(4+)ordering, small amount of lattice defects and highest Tc = 288K reported so far for ferromagnetic double perovskites. At a microscopic level the crystals are heavily twinned and the effect of twinning on the Ra...