Competitive and cooperative electronic states in Ba(Fe1\u2212xTx)2As2 with T\u2009=\u2009Co, Ni, Cr

Qiang Zou,Zheng Gai,Zhiming Wu,Mingming Fu,Athena S Sefat,Li Li,David S Parker

doi:10.1038/s41535-021-00385-8

Qiang Zou, Zheng Gai + Show 5 more

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https://doi.org/10.1038/s41535-021-00385-8

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Abstract

The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.

Highlights

The interplay between magnetism and superconductivity (SC) is one of the fundamental topics for understanding the mechanism of superconductivity of unconventional superconductors in the cuprates and iron-based superconductors (FeSCs)[1,2] since SC appears near antiferromagnetic (AF) order
crystal. Since Ni (Cr)-122 has a G-AFM ground state (TN ~100 K), while optimally doped Co-122 is in the SC state (Tc ~22 K)[24]; and the composition of the Ni-122 was chosen to locate it in the coexistence range of SC and AFM orders (Tc ~19 K, TN ~45 K)
The L-shape-like state has been identified as a spin density wave spectrum with the signature features of a large asymmetry with respect to the Fermi level and a large residual DOS at EF20,23

Summary

Introduction

The interplay between magnetism and superconductivity (SC) is one of the fundamental topics for understanding the mechanism of superconductivity of unconventional superconductors in the cuprates and iron-based superconductors (FeSCs)[1,2] since SC appears near antiferromagnetic (AF) order. Experiments demonstrated that the superconducting pairing state is the unconventional dx2–y2, which is believed induced by spin fluctuations[3,4,5]. Magnetism in the FeSCs plays an important role in the electron pairing mechanism[2,4]. In the FeSC it is more complicated and controversial since Fe 3d orbitals form multiple Fermi surfaces (FS), while in cuprates only a single Cu d-band crosses the FS6–9. It is intriguing that crystals with electron-doping to Fe site (with Ni and Co) are superconducting while crystals with hole-dopants (with Cr) are not[14]

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