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How interactions modify the number of electrons in iron based superconductors


In 2008, a new family of superconductors was discovered, based on Fe squared lattices. This attracted a lot of attention from many physicists because the temperature for superconductivity can be quite high (56K), which was unexpected from a theoretical point of view. This discovery raises fundamental questions about the microscopic nature of superconductivity and particularly about the relationship between superconductivity and magnetism. Analogies were immediately made with the other family of high-temperature superconductors, the cuprates. In these systems, superconductivity (at up to 150K !), discovered more than 20 years ago, has still not been well described from a theoretical point of view. The complexity of the problem is based on the fact that the electrons interact strongly with each other, i.e. they are correlated, which is difficult to describe theoretically. The role and nature of correlations in iron superconductors have naturally became key points in these studies.

 

One of the key differences between the two families is that in cuprates, there is one electron per site, which occupies one band (each « band » of a metal can be occupied by 2 electrons and have specific symmetry properties). Correlations make it difficult for an electron to jump onto the neighboring site if it is already occupied. In iron superconductors, there are 6 electrons per site, divided between five different bands. Each of these electrons has slightly different properties depending on which band it is located. How they all interact together soon proved to be of great interest in the study of these new materials.

 

At the LPS, we have been studying these solids for 5 years by performing angle-resolved photoemission spectroscopy at the CASSIOPEE beamline of the SOLEIL synchrotrins with samples synthesized at the CEA-SPEC. This technique allows to observe each of these "types" of different electrons and how they move in the metal. Recently, we have evidenced a new form of correlation directly related to the existence of these different "types" of electrons.

 

Figure 1a represents the energy dispersion of 3 of these bands. Only states under the Fermi level EF (shaded areas) are occupied by electrons. In normal cases, for solids with a single band, the position of the Fermi level is determined strictly by the total number of electrons. Here, one can in theory move the bands as shown in the figure, while maintaining the total number of electrons. Our studies show that such movements do occur and can change greatly depending on the material properties (here controlled by Co-doping, see Fig. 1b) and temperature (Fig. 1c). As a result, the number of electrons, which is usually independent of the temperature in a conventional metal, varies here by a factor of 2 at low and high temperatures in the most "correlated" area where magnetism and superconductivity occur !

 

The nature of the correlations causing these band shifts is still poorly understood. They may reflect the presence of strong magnetic fluctuations (see Ortenzi et al. PRL 103, 046404 (2009)). Our data on their evolution with temperature and doping should allow better models to be made.

 


(a) Diagram of the different bands of Fe superconductors for a "Γ X" direction of reciprocal space. The shaded areas represent the occupied parts of the electronic structure. The empty area around Γ defines a pocket of "holes" and the part occupied around X an electron pocket. The arrows indicate the movement of bands that preserve the total number of electrons (by following the arrows, there will be both less holes in Γ and less electrons in X). (b) Phase diagram of Ba(Fe1-xCox)2As2 compounds that make it possible to pass from a magnetic phase (AF) to a superconducting phase (SC) to a "normal" metal. The number of electrons in X determined by ARPES at low temperature is represented by symbols. At low doping levels, it is much smaller than in the calculation (black line), which suggests the movement of bands as shown in (a). (c) Variations in the number of electrons in X as a function of temperature for different x levels of Co.

 

Reference :

 

Large temperature dependence of the number of carriers in Co-doped BaFe2As2” V. Brouet, Ping-Hui Lin, Y. Texier, J. Bobroff, A. Taleb-Ibrahimi, P. Le Fèvre, F. Bertran, M. Casula, P. Werner, S. Biermann, F. Rullier-Albenque, A. Forget, and D. Colson, Physical Review Letters 110, 167002 (2013)

 

Contact :

 

Véronique Brouet : (veronique.brouet@u-psud.fr)