Competitive and cooperative electronic states in Ba(Fe1−xTx)2As2 with T = Co, Ni, Cr (original) (raw)
Related papers
Nano letters, 2017
Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe1-xCox)2As2 crystals with x=0.06, with Tc = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.
Hole-doped BaFe2-xCrxAs2 Crystals: A Case of Non-superconductivity
We investigate the physical properties and electronic structure upon Cr-doping in the iron arsenide layers of BaFe 2 As 2 . This form of hole-doping leads to suppression of the magnetic/structural phase transition in BaFe 2-x Cr x As 2 for x > 0, but does not lead to superconductivity. For various x values, temperature dependence of the resistivity, specific heat, magnetic susceptibility, Hall coefficient, and single crystal x-ray diffraction data are presented. The materials show signatures of approaching a ferromagnetic state with x, including a metamagnetic transition for x as little as 0.36, an enhanced magnetic susceptibility, and a large Sommerfeld coefficient. Such results reflect renormalization due to spin fluctuations and they are supported by density functional calculations at x = 1.
Electronic phase diagram of disordered Co doped BaFe 2 As 2−δ
Superconductor Science and Technology, 2013
Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1−xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1−xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.
Neutron scattering study of the interplay between structure and magnetism inBa(Fe1−xCox)2As2
Physical Review B, 2009
Single crystal neutron diffraction is used to investigate the magnetic and structural phase diagram of the electron doped superconductor Ba(Fe1−xCox)2As2. Heat capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in BaFe2As2 into two distinct transitions. For x=0.025, we find that the upper transition is between the high-temperature tetragonal and low-temperature orthorhombic structures with (TTO = 99 ± 0.5 K) and the antiferromagnetic transition occurs at TAF = 93 ± 0.5 K. We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at x ≈ 0.055. However, there is a region of coexistence of antiferromagnetism and superconductivity. The effect of the antiferromagnetic transition can be seen in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction. We infer from this that there is strong coupling between the antiferromagnetism and the crystal lattice.
Physical Review B, 2011
We present neutron diffraction measurements on single crystal samples of non-superconducting Ba(Fe1−xCrx)2As2 as a function of Cr-doping for 0≤x≤0.47. The average SDW moment is independent of concentration for x≤0.2 and decreases rapidly for x≥0.3. For concentrations in excess of 30% chromium, we find a new G-type antiferromagnetic phase which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured and competition between these ordered states and superconductivity naturally explains the absence of superconductivity in the Cr-doped materials.
Physical Review B, 2019
Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the Ba(Fe1-xCox)2As2 family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and Ba(Fe1-xCox)2As2 is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap ∆e(x,φ) has an intermediate value between the dominant inner Δ2h(x) and outer Δ1h(x) hole gaps. Close to the AFM transition the electron gap ∆e(x, φ) increases sharply and becomes closer in magnitude to the dominant inner hole gap Δ2h(x). The same two-gap state with close electron and inner hole gaps Δ2h(x) ≈ ∆e(x, φ) is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap ∆e(x, φ) is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap ∆e(x, φ), the doping dependence of the hole gaps Δ1,2h(x) and the critical temperature Tc(x), both before and after the AFM transition, are associated with a change of the density of states γnh(x) and the intraband electron-phonon interaction in the hole bands. The non-phonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the Ba(Fe1-xCox)2As2 family. The high-Tc iron-based superconductors (FeSCs) are multiband quasi-two-dimensional compounds with strongly anisotropic Fermi surface and low carrier density in the hole-like and electron-like bands [1]. The Fermi surface (FS) of these compounds consists of hole-like (h) pockets at the Γ point and electron-like (e) pockets centered at the X = (π, 0) and Y = (0, π) points of the Brillouin zone. Compared to strongly correlated high-Tc cuprates, which are similar in their basic characteristics, electron-electron correlations in FeSCs are not large (see, for example, reviews [2, 3]).The parent orthorhombic (Ort) Fe-based compounds are antiferromagnetic (AFM) metals of spin-density wave (SDW) type with the magnetic ordering vectors Q = (π, 0), (0, π). Unlike dielectric parent high-Tc cuprates, they have free electronic states at the FS that are not associated with magnetism but can, in principle, be involved in superconducting (SC) pairing. The electronic structure of these compounds is very sensitive to small changes in doping, pressure, and degree of disorder. When in parent compounds the magnetic atoms Fe (3d 6) in the a-b plane are replaced by atoms with larger number of d electrons (electron doping) or the non-magnetic atoms out of this plane are replaced by atoms with smaller valence (hole doping), antiferromagnetism is gradually suppressed which leads to the onset of superconductivity. In this regime, the AFM and SC gaps coexist at the Fermi surface
BaT2As2 single crystals (T=Fe, Co, Ni) and superconductivity upon Co-doping
Physica C: Superconductivity, 2009
The crystal structure and physical properties of BaFe 2 As 2 , BaCo 2 As 2 , and BaNi 2 As 2 single crystals are surveyed. BaFe 2 As 2 gives a magnetic and structural transition at T N = 132(1) K, BaCo 2 As 2 is a paramagnetic metal, while BaNi 2 As 2 has a structural phase transition at T 0 = 131 K, followed by superconductivity below T c = 0.69 K. The bulk superconductivity in Codoped BaFe 2 As 2 below T c = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe 1.84 Co 0.16 As 2 indicate the inter-band scattering due to Co is weak. 74.25.Ha, 74.25.Jb
Probing Local Variations of Superconductivity on the Surface of Ba (Fe1-xCox) 2As2 Single Crystals
2010
The spatially resolved electrical transport properties have been studied on the surface of optimally-doped superconducting Ba(Fe 1-x Co x ) 2 As 2 single crystal by using a four-probe scanning tunneling microscopy. While some non-uniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at T C = 22.1 K has been observed with a transition width ΔT C = 0.2 K. In the non-uniform contrast region, T C is found to vary between 19.6 and 22.2 K with ΔT C from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the non-* Corresponding author. apli@ornl.gov 2 uniform region. Thus the variations of superconductivity are associated with local compositional change.