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Register a new userA Large Iron Isotope Effect in SmFeAsO1-xFx and Ba1-xKxFe2As2
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The recent discovery of superconductivity in oxypnictides with the critical temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. However, superconductivity and magnetism in pnictide superconductors show a strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to 0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows the same effect on TSDW as TCc These results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity.
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We report the results of a systematic investigation of the phase diagram of the iron-based superconductor, Ba1-xKxFe2As2, from x = 0 to x = 1.0 using high resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of Deltax <~ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1-xKxFe2As2 crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with space group Fmmm and a striped antiferromagnet (AF) with space group Fcmmm. The transition temperature falls from a maximum of 139K in the undoped compound to 0K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3) and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 falling to 3 K at x = 1.0. We discuss reasons for the suppression of the spin-density-wave order and the electron-hole asymmetry in the phase diagram.
We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1-xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transi- tion temperature with hole doping coincides with the magnetic excitations splitting into two incom- mensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s+- symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.
The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.
The role of electron-phonon interactions in iron-based superconductor is currently under debate with conflicting experimental reports on the isotope effect. To address this important issue, we employ the renormalization-group method to investigate the competition between electron-electron and electron-phonon interactions in these materials. The renormalization-group analysis shows that the ground state is a phonon-dressed unconventional superconductor: the dominant electronic interactions account for pairing mechanism while electron-phonon interactions are subdominant. Because of the phonon dressing, the isotope effect of the critical temperature can be normal or reversed, depending on whether the retarded intra- or inter-band interactions are altered upon isotope substitutions. The connection between the anomalous isotope effect and the unconventional pairing symmetry is discussed at the end.
The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00, 0.27) have been studied by inelastic X-ray scattering measurement at room temperature. K doping induces the softening and broadening of phonon modes in the energy range E = 10-15 meV. Analysis with a Born-von Karman force-constant model indicates that the softening results from reduced interatomic force constants around (Ba,K) sites following the displacement of divalent Ba by monovalent K. The phonon broadening may be explained by the local distortions induced by the K substitution. Extra phonon modes are observed around the wave vector q = (0.5,0,0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either from the local disorder induced by K doping or from electron-phonon coupling.
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