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Register a new userSuperconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory
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Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{rm c}simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $kappa=lambda/xi=30(6)$ ($lambda$ is the magnetic penetration depth, $xi$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$Delta(T)$] reconstructed from $lambda^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2Delta/k_{rm B}T_{rm c}simeq 4.34$ was estimated thus implying that Bi-III stays within the strong coupling regime. The Density Functional Theory calculations suggest that superconductivity in Bi-III could be described within the Eliashberg approach with the characteristic phonon frequency $omega_{rm ln}simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by the Fermi surface nesting.
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The application of the muon-spin rotation/relaxation ($mu$SR) technique for studying type-I superconductivity is discussed. In the intermediate state, i.e. when a type-I superconducting sample with non-zero demagnetization factor $N$ is separated into normal state and Meissner state (superconducting) domains, the $mu$SR technique allows to determine with very high precision the value of the thermodynamic critical field $B_{rm c}$, as well as the volume of the sample in the normal and the superconducting state. Due to the microscopic nature of $mu$SR technique, the $B_{rm c}$ values are determined directly via measurements of the internal field inside the normal state domains. No assumptions or introduction of any type of measurement criteria are needed. Experiments performed on a classical type-I superconductor, a cylindrically shaped $beta-$Sn sample, allowed to reconstruct the full $B-T$ phase diagram. The zero-temperature value of the thermodynamic critical field $B_{rm c}(0)=30.578(6)$ mT and the transition temperature $T_{rm c}=3.717(3)$ K were determined and found to be in a good agreement with the literature data. An experimentally obtained demagnetization factor is in very good agreement with theoretical calculations of the demagnetization factor of a finite cylinder. The analysis of $B_{rm c}(T)$ dependence within the framework of the phenomenological $alpha-$model allow to obtain the value of the superconducting energy gap $Delta=0.59(1)$ meV, of the electronic specific heat $gamma_e=1.781(3)$ ${rm mJ}/{rm mol}; {rm K}^2$ and of the jump in the heat capacity ${Delta C(T_c)}/{gamma T_{rm c}}=1.55(2)$.
The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{rm c}simeq3.92$ K at pressure $psimeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{rm c}(0)simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{rm c}$ and the temperature evolution of $B_{rm c}$. The estimated value for the retardation (coupling) parameter $k_{rm B}T_{rm c}/omega_{rm ln} approx 0.07$ ($omega_{rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.
We present a first-principles approach to describe magnetic and superconducting systems and the phenomena of competition between these electronic effects. We develop a density functional theory: SpinSCDFT, by extending the Hohenberg-Kohn theorem and constructing the non-interacting Kohn- Sham system. An exchange-correlation functional for SpinSCDFT is derived from the Sham Schluter connection between the SpinSCDFT Kohn-Sham and a self-energy in Eliashberg approximation. The reference Eliashberg equations for superconductors in the presence of magnetism are also derived and discussed.
We present a muon spin rotation (muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we observe some interesting parallels with the electron doped SmFeAsO1-xFx 1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and arXiv:0807.4876]. In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below Tcapprox25K. To the contrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed AF order and others that are non magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et al., arXiv:0811.2224] and thus seems to be fairly common in these hole doped systems.
We extend the two leading methods for the emph{ab initio} computational descrip tion of phonon-mediated superconductors, namely Eliashberg theory and density fu nctional theory for superconductors (SCDFT), to include plasmonic effects. Furth ermore, we introduce a hybrid formalism in which the Eliashberg approximation fo r the electron-phonon coupling is combined with the SCDFT treatment of the dynam ically screened Coulomb interaction. The methods have been tested on a set of we ll-known conventional superconductors by studying how the plasmon contribution a ffects the phononic mechanism in determining the critical temperature (tc). Our simulations show that plasmonic SCDFT leads to a good agreement between predict ed and measured tcs, whereas Eliashberg theory considerably overestimates the plasmon-mediated pairing and, therefore, tc. The hybrid approach, on the other hand, gives results close to SCDFT and overall in excellent agreement with exper iments.
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