Large Temperature Dependence of the Number of Carriers in Co-DopedBaFe2As2

Abstract

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe1−xCox)2As2 as a function of Co content and temperature. We show that there is a kdependent energy shift compared to density functional calculations, which is large at low Co contents and low temperatures and reduces the volume of hole and electron pockets by a factor 2. This k-shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations.PACS numbers: 79.60.-i, 71.18.-y, 71.30.-h Since the discovery of iron based superconductors, angle resolved photoelectron spectroscopy (ARPES) has given valuable information about their electronic structure [1,2]. In most cases, the measured spectra are in broad agreement with Density Functional Theory (DFT) calculations, after renormalization by a factor 2 to 3. This range of values is in good agreement with mass enhancements predicted by Dynamical Mean Field Theory (DMFT) [3,4]. More unusual is a shrinking of the hole and electron pocket sizes compared to DFT calculations, observed both by ARPES and de Haas-Van Alphen experiments [5]. In a Fermi liquid (FL) picture, this corresponds to a down shift of the hole bands compensated by an up shift of the electron ones, as sketched in Fig. 1(a). As it depends on k, we refer to this effect as kshift in the following. Its origin is so far unclear, but it seems associated with the strength of correlations. In P-substituted BaFe 2 As 2 , dHvA measurements reported that the maximum k-shift corresponds to the largest effective masses [6]. In Ru-substituted BaFe 2 As 2 , both the k-shift and the renormalization weaken at large doping content [7,8]. Ortenzi et al. showed that interband interactions mediated by low energy bosons, such as spin fluctuations, can lead to such a k-shift [9]. If this is indeed the case in pnictides, it contains important information about the interactions in these compounds and deserves a detailed study.We show here with ARPES in Co-doped BaFe 2 As 2 that the k-shift is suppressed for high Co content, when the hole pockets fill up. We reveal in addition a very strong temperature (T) dependence of the electronic structure. Up to 10% Co, the electron pockets are expanding with increasing T by an amount as large as 50%.A similar T dependence was reported recently for Rudoped BaFe 2 As 2 [10]. At higher Co content (30%), the electron pockets on the contrary shrink at high T. We show that this can be largely explained by thermal excitations in these narrow multiband systems. We present both simple models and calculations using the density of states (DOS) obtained by DFT within the Local Density Approximation (LDA) and by DMFT in order to simulate the effect of temperature within a...

ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

“…To match the data, it is however further necessary to shift up the bands by 0.1eV before renormalizing them. This corresponds to a "shrinking" of the electron pockets that we described in details before [23]. A concomitant downward shift of the hole bands allows charge conservation.…”

“…Band structure calculations were perfomed within the local density approximation (LDA), using the Wien2K package [34], with the experimental structures. The Co doping of 8% was treated in the virtual crystal approximation [23].…”

“…[12,13]. The general electronic structure has already been studied in some details by ARPES, both for Co8 [5,[22][23][24] and LiFeAs [10,25,26]. We add here more focused information on the separation of hole and electron bands of different orbital character and their linewidths.…”

“…In panel (b), the same information is given for electron pockets in Co8. We do not give results for the hole pockets in Co8, because they significantly overlap and are more difficult to resolve from each other [22,23]. In panel (c), we further show the temperature dependence of the lifetime at the Fermi level in Co8.…”

Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

“…To distinguish these two features experimentally, one could in principle use spin polarized ARPES or use the fact that while ∆ nem has a pronounced temperature dependence, λ SOC is expected to be nearly temperatureindependent. However, the electronic spectral-function's lifetime, manifested as a broadening of the ARPES data, is also strongly temperature-dependent 18,19 , making this procedure challenging.…”