| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 102 === We present spectroscopic ellipsometry and Raman-scattering measurements of superconducting Fe(Se,Te) materials. The c-axis oriented FeSe thin films with thickness of 300 nm were deposited on the MgO substrates using pulsed laser deposition. Its superconducting transition temperature is about 7.6 K. FeSe and Fe1.05Se0.447Te0.553 single crystals with (001) orientation show the superconducting transition temperatures at about 8 K and 12.5 K.
First, the absorption spectrum of FeSe single crystal determined from spectroscopic ellipsometry analysis shows four bands at about 1.7, 2.7, 3.9, and 6.2 eV. The optical absorption near 1.7 eV is due to on-site Fe2+ d-d excitations. Other high energy peaks above 2 eV are associated with the charge-transfer transitions between Se 4p and Fe 3d states. Corresponding FeSe single crystal, FeSe thin films was observed 1.7 eV and 6.2 eV absorption peak, and Fe1.05Se0.447Te0.553 single crystal was observed with 1.7, 2.7, and 6.2 eV absorption peak, and the electronic transitions with the same behavior corresponding to FeSe single crystals.
Second, room-temperature Raman-scattering spectrum of FeSe thin film exhibits four phonon modes at about 107, 180, 192, and 238 cm-1, displaying symmetries of Eg(1), A1g, B1g, and Eg(2), respectively. Furthermore, the B1g excitation peak near 2300 cm-1 is associated with two-magnon scattering. Additionally, with increasing temperature, A1g and B1g phonon modes disappear above 475 K. At 550 K, Raman-scattering spectrum transfers to that of α-Fe2O3. In contrast to high temperature Raman-scattering spectra, A1g and B1g phonon modes exhibit anomalies with decreasing temperature in frequency at 220, 100, and 8 K that coincide with the onset of short-range charge and/or orbital orders, structural and superconducting phase transitions. Notably, a very large hardening (~ 8 cm-1) of the B1g phonon mode is possibly due to spin-phonon coupling. With decreasing temperature, Fe2+ ions change from the high spin state into low spin state.
Finally, temperature-dependent Raman-scattering spectra of Fe1.05Se0.447Te0.553 single crystal show similar phonon anomalies at 240, 90, and 12.5 K. Interestingly, no large hardening of the B1g phonon mode can be seen, suggesting the Te doping disrupts the spin state transitions in Fe2+ ions.
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