Anomalous dependence ofc-axis polarized FeB1gphonon mode with Fe and Se concentrations in Fe1

Abstract

We report on an investigation of the lattice dynamical properties in a range of Fe1+yTe1−xSex compounds, with special emphasis on the c-axis polarized vibration of Fe with B1g symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se x and excess Fe y concentrations. In parent compound Fe1+yTe, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature TN . The situation smoothly evolves toward a regular anharmonic behavior as Te is substituted for Se and long range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B1g phonon at low temperatures. We performed Density Functional Theory ab initio calculations within the local density approximation to calculate the phonon frequencies including magnetic polarization and Fe non-stoichiometry in the virtual crystal approximation. We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low energy magnetic fluctuations that can not be treated accurately within these approaches. As recently revealed by neutron scattering and muon spin rotation studies, these phenomena occur in the temperature range where anomalous decay of the B1g phonon is observed and suggests a peculiar coupling of this mode with local moments and spin fluctuations in Fe1+yTe1−xSex.

Superconductivity in Fe-chalcogenides

“…al. [74]. Except in pure FeTe, all Te doped samples show hardening of both A 1g and B 1g modes as the temperature is decreased.…”

“…The temperature-dependent phonon hardening and alternation in the electronic structure can also be identified via other spectral features in the Raman spectrum [73,74]. Fig.…”

“…al. [71], phonon dynamics in FeSe 1-x Te x were examined by several groups using Raman spectroscopy [72][73][74][75][76]. Fig.…”

Phonon spectra in the parent superconducting iron-tuned telluride Fe1+xTe from inelastic neutron scattering and ab initio calculations

“…Consequently, a better understanding of the magneto-elastic coupling mechanisms and properties of Fe1+xTe imposes to investigate the lattice dynamics side. In this context, Raman and infrared spectroscopies were used to map out the frequency shifts, and to explore the related vibrational features [28][29][30][31][32][33]. However, these techniques are subject to selection rules and are restricted only to the zone-centre region of the Brillouin zone.…”

“…However, these techniques are subject to selection rules and are restricted only to the zone-centre region of the Brillouin zone. On the computational side, DFT calculations of the lattice dynamics of iron telluride, at the -point, were also reported [28][29][30][31][32][33]. However, these were found to often disagree with each other, and with the available -point experiments, when considering fully relaxed structures (both lattice parameters and atomic coordinates), so that a reasonable comparison with experimental data was performed using partially relaxed geometries (relaxing only atomic coordinate and fixing the lattice parameters at their experimentally-refined values).…”