Cluster structure of superconducting phase and the nature of peaks in the doping dependences of the London penetration depth in iron pnictides

Abstract

The mechanism of formation of dome-like phase diagrams and the features of magnetic field penetration in iron pnictides with hetero-and isovalent doping are considered within the framework of the previously proposed model, which assumes the local character of doping and the cluster structure of resulted superconducting phase. It is shown that the proposed model, despite its simplicity and neglect of the features of the electronic structure, makes it possible not only to accurately calculate the positions of superconducting domes on the phase diagrams of specific HTSC compounds, but also to explain the nature and position of sharp peaks in the London penetration depth depending on the doping level.

Local transformation of the electronic structure and generation of free carriers in cuprates and ferropnictides under heterovalent and isovalent doping

“…This approach, however, fails to agree with many experiments where spatial variations of local charge density and superconducting order parameter are observed in doped specimens [3][4][5][6]. What is more, many experiments demonstrate that doped carriers in both cuprates and ferropnictides are strictly localized in the nearest vicinity of the dopant [7][8][9][10][11].Previously, we proposed a unified mechanism of local iso-and heterovalent (hole and electron) doping, which is implemented in superconducting cuprates and ferropnictides [12,13].The proposed mechanism assumes the local nature of the transformation of their band structure under doping, which leads to the formation of inhomogeneous cluster structure of the superconducting phase [13], the parameters of which are determined by the concentration of the dopant and crystal structure of the undoped compound.We have shown that basing only on the knowledge of the crystal structure and type of dopant, it is possible: 1) to accurately determine the positions of superconducting domes in the…”

“…Therefore, despite the close relationship between Ba(Fe1-xNix)2As2 and Ba(Fe1-xCox)2As2, the geometry of their trion complexes is significantly different (Fig. 2 e, f), which causes the difference in the phase diagrams [13].…”

“…This, in our opinion, is the essence of heterovalent doping, which is accompanied with the formation of trion complexes. The geometry of such a complex can be determined for each compound, proceeding only from the knowledge of dopant position and surrounding symmetry [12,13]. Figure 2 shows examples of trion complexes in some cuprates and ferropnictides.…”

“…To date, a list of high-temperature superconductor compounds includes mainly representatives of two classes: cuprates and ferropnictides. In an undoped (stoichiometric) state, phase diagrams of HTSC compounds [12], 2) to explain the nature and the positions of sharp peaks in the London penetration depth as well as the positions of anomalies in the anisotropy of resistance depending on the level heterovalent or isovalent doping [13], 3) to understand the nature of "magic" concentration values corresponding to abrupt changes in superconducting properties (1/8 anomaly, etc.) [12].…”

“…Previously, we proposed a unified mechanism of local iso-and heterovalent (hole and electron) doping, which is implemented in superconducting cuprates and ferropnictides [12,13].…”