Crystal field excitations in the Raman spectra of FeSe1−x

Raman spectra of polycrystalline FeSe1−x(x∼0.03) with a superconducting transition temperature of 9 K have been measured from room temperature down to 5 K. A mode at about 185 cm−1−1 is observed throughout the whole temperature range. It is assigned as the B1g phonon mode. This phonon mode hardens with decreasing temperature, but undergoes no noticeable change across both the structural phase transition and the superconducting transition, similar to the Raman phonon modes observed in the 1111 and 122 systems. Surprisingly, we also observed several additional Raman modes between 200 and 700 cm−1, which we assigned as due to the crystal field excitations of the 3d electrons in Fe2+ ions. These modes undergo abrupt change near the structural phase transition around 100 K, but do not exhibit much change across the superconducting transition.