Quasi-2D surface behavior of aged β-FeSe single crystals

In this work we report on the β-FeSe system and its dependence with aging. On freshly samples two vibrational modes attributed to A_1g (Se) and B_1g (Fe) are observed on its Raman spectra. After tens of days in ambient conditions, a new peak appears. We propose that this new peak is due to a loss of symmetry in the first sheets of the system, which occurs with aging, that make this new peak Raman active. This hypothesis is supported by Group theory calculations which predicts three optical modes near the center of the Brillouin zone, one attributed to Se, other to Fe and the last one a combination of both kind of atoms, all Raman active in the new symmetry group. Also we measured an enhancement of the critical temperature of the system in a sample with an aged face. β-FeSe films are known to have higher critical temperature than bulk single crystals.