Large Temperature Dependence of the Number of Carriers in Co-Doped BaFe 2 As 2
Résumé
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 k-dependent 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 k-shift 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 Ru-doped 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 exci-tations 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 FL framework. These simulations allow to account for about half the experimental shift. Part of the expansion of the electron pockets could also be due to the weakening of the k-shift, as proposed in [11] for the model of interband interactions , or to an evolution of correlation effects. Single crystals of Ba(Fe 1−x Co x) 2 As 2 were grown using a FeAs self-flux method and studied in detail by transport measurements [12]. ARPES experiments were carried out at the CASSIOPEE beamline at the SOLEIL synchrotron, with a Scienta R4000 analyzer, an angular resolution of 0.2 • and an energy resolution of 15 meV. All measurements were perfomed at 34 eV, except otherwise mentioned. DFT band structure calculations were performed using the Wien2k package [13] at the experimental structure of BaFe 2 As 2. To model Co doping, we used the virtual crystal approximation (VCA), where Co induces a global rigid shift of the entire band structure. For the LDA+DMFT calculation, we used the technology of ref [4], which fully takes into account dynamical screening effects from first principles. Recently, we have shown that the electron pocket around X is an ellipse formed by a " deep " electron band of d xy symmetry along the long axis and a " shallow " electron band of d xz /d yz symmetry along the short axis [15].
Domaines
Physique [physics]
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