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TT: Tiefe Temperaturen

TT 22: Postersitzung III: Syst. korr. Elektr.: Theorie II (1-15), Metall-Isol.- und Phasenüberg
änge (16-33), SL: Massivmat., Bandl., Pinning, Vortexdyn., Transport, Korngr. (34-43), Niedrigdim. Syst., Magnetotransport (44-63)

TT 22.33: Poster

Donnerstag, 14. März 2002, 14:00–17:30, A

Giant softening of high frequency phonon modes in LaMnO_3+δ — •K.-Y. Choi¹, P. Lemmens^1,2, Yu. Pashkevich³, V. Gnezdilov⁴, G. Güntherodt¹, A. Eremenko⁴, S. Barilo⁵, S. Shiryaev⁵, and A. Soldatov⁵ — ¹2. Physikalisches Institut, RWTH Aachen, 52056 Aachen, Germany — ²IMNF, TU Braunschweig, D-38106 Braunschweig, Germany — ³Donetsk Phystech NASU, 83114 Donetsk, Ukraine — ⁴Inst. for Low Temp. Physics, NASU, 61164 Kharkov, Ukraine — ⁵Inst. of Physics of Sol. & Semicond., 220072 Minsk, Belarus

High quality single crystals of LaMnO_3+δ (δ =0.085, 0.092, 0.097, 0.125) have been investigated by Raman light scattering in the 5-300 K temperature range. All samples possess ferromagnetic ordering with Curie temperatures T_c =148, 178, 186, and 248 K with increasing δ. The formation of a ferromagnetic insulating state at T_co below T_c was observed as a sharp upturn in resistivity and the appearance of fine structures in the Raman spectra. A strong softening of two high frequency phonon modes at 495 and 620 cm⁻¹ has been observed between T_c and 5 K. The largest shifts at 5K reach up to 30 cm⁻¹ for both modes in the rhombohedral sample (δ=0.125). In all samples these modes do not demonstrate any conspicuous temperature dependencies above T_c. A magnetic field leads to a more pronounced T-dependence of the phonons attributed to a supression of orbital fluctuations.
We acknowledge support by DFG SPP1073, NATO and INTAS.