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Register a new userDo Transition Metal Substitutions Dope Carriers in Iron Based Superconductors?
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We investigate the currently debated issue concerning whether transition metal substitutions dope carriers in iron based superconductors. From first-principles calculations of the configuration-averaged spectral function of BaFe$_2$As$_2$ with disordered Co/Zn substitutions of Fe, important doping effects are found beyond merely changing the carrier density. While the chemical potential shifts suggest doping of a large amount of carriers, a reduction of the coherent carrier density is found due to the loss of spectral weight. Therefore, none of the change in the Fermi surface, density of states, or charge distribution can be solely used for counting doped coherent carriers, let alone presenting the full effects of the disordered substitutions. Our study highlights the necessity of including disorder effects in the studies of doped materials in general.
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Following the discovery of the potentially very high temperature superconductivity in monolayer FeSe we investigate the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore we find that the disorder induced broadening causes an effective hole doping. The surprising net result is that in terms of the band structure Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measured by angle resolved photoemission spectroscopy.
The experimental transport scattering rate was determined for a wide range of optimally doped transition metal-substituted FeAs-based compounds with the ThCr2Si2 (122) crystal structure. The maximum transition temperature Tc for several Ba-, Sr-, and Ca-based 122 systems follows a universal rate of suppression with increasing scattering rate indicative of a common pair-breaking mechanism. Extraction of standard pair-breaking parameters puts a limit of sim26 K on the maximum Tc for all transition metal-substituted 122 systems, in agreement with experimental observations, and sets a critical scattering rate of 1.5x10^14 s^-1 for the suppression of the superconducting phase. The observed critical scattering rate is much weaker than that expected for a sign-changing order parameter, providing important constraints on the nature of the superconducting gap in the 122 family of iron-based superconductors.
Based on a two-orbital honeycomb lattice model and random phase approximation, we investigate the pairing symmetry of the Ni-based transition-metal trichalcogenide. We find that an I-wave (A2g) state and a chiral d-wave state are dominant and nearly degenerate for typical electron and hole dopings. These two states carry nontrivial topological properties, which are manifested by the presence of chiral edge states in the d+id-wave state and dispersionless Andreev bound state at zero energy in the I-wave state. Ni-based transition-metal trichalcogenides provide us a new platform to study the exotic phenomena emerged from electron-electron correlation effects.
Iron-based superconducting layered compounds have the second highest transition temperature after cuprate superconductors. Their discovery is a milestone in the history of high-temperature superconductivity and will have profound implications for high-temperature superconducting mechanism as well as industrial applications. Raman scattering has been extensively applied to correlated electron systems including the new superconductors due to its unique ability to probe multiple primary excitations and their coupling. In this review, we will give a brief summary of the existing Raman experiments in the iron-based materials and their implication for pairing mechanism in particular. And we will also address some open issues from the experiments.
We examine the relevance of several major material-dependent parameters to the magnetic softness in iron-base superconductors by first-principles electronic structure analysis of their parent compounds. The results are explained in the spin-fermion model where localized spins and orbitally degenerate itinerant electrons coexist and are coupled by Hunds rule coupling. We found that the difference in the strength of the Hunds rule coupling term is the major material-dependent microscopic parameter for determining the ground-state spin pattern. The magnetic softness in iron-based superconductors is essentially driven by the competition between the double-exchange ferromagnetism and the superexchange antiferromagnetism.
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