Low-energy interband transitions in the infrared response of Ba(Fe1−xCox

Abstract

We studied the doping and temperature (T ) dependence of the infrared (IR) response of Ba(Fe 1-x Co x ) 2 As 2 single crystals. We show that a weak band around 1000 cm −1 , that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm −1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.

Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in UnderdopedBa1−xKxFe

“…2(b). In the paramagnetic state at T > T N1 and in the o-AF state at T N1 > T > T N2 the spectra are similar as previously reported for underdoped and undoped Ba 1−x K x Fe 2 As 2 [22][23][24][25][26]. Above T N1 , they display a Drude peak at low frequencies due to the itinerant carriers, and a pronounced tail to higher frequencies with an upturn toward a maximum around 5000 cm −1 , that arise from inelastic scattering and low-energy interband transitions, respectively.…”

“…The well-known features due to the SDW in the o-AF state are a strong spectral weight (SW) reduction and narrowing of the Drude-peak, a partial, gap-like suppression of σ 1 between 100 and 500 cm −1 and the formation of a band around 750 cm −1 that corresponds to the SDW pair-breaking peak [22][23][24][25]. These characteristic features also manifest them- that were previously also observed in underdoped Ba(Fe 1−x Co x ) 2 As 2 [25] and predicted [5].…”

Electron–boson spectral density function of correlated multiband systems obtained from optical data: Ba0.6K0.4Fe2As2and LiFeAs

“…Our impurity scattering rate of LiFeAs is consistent with a previous reported value of the same system [24] and indicates that the LiFeAs crystal may contain small amount of impurities. It is worth noting that two different approaches (one is with two Drude modes [33,35,36,37,38,41,42] and the other ours with two extended Drude modes) may give different fitting parameters, particularly the impurity scattering rates. Our approach explicitly includes the inelastic scattering between charge carriers in the model calculations.…”

“…And then we performed a Kramers-Kronig analysis independently to get the optical constants including the optical conductivity from the reflectance spectra. It has been known that the iron-pnictide materials including our two systems have low-energy interband transitions down to ∼20 meV energy levels [33,34,35,36,37,38]. We need to remove the interband transitions before application of the method since we apply our method to the optical data up to 100 meV.…”

“…However, the shape and intensity of the low-energy interband transition in the optical conductivity are not well-established yet. Therefore, we surveyed optical data published of four related pnictide systems (Co-, Ni-, and Kdoped BaFe 2 As 2 , and LiFeAs) [35,36,37,38]. We separated the interband transition components and averaged them; the averaged interband transition is displayed in dashed orange lines in Fig.…”