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Register a new userAbsence of an appreciable iron isotope effect on the transition temperature of the optimally doped SmFeAsO_{1-y} superconductor
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Parasharam Shirage Dr.
Publication date
2010
fields
Physics
and research's language is
English
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We report the iron (Fe) isotope effect on the transition temperature (Tc) in the oxygen-deficient SmFeAsO_{1-y}, a 50 K-class Fe-based superconductor. For the optimally-doped samples with Tc = 54 K, change of the Fe average atomic mass (MFe) causes a negligibly small shift in Tc, with the Fe isotope coefficient (alphaFe) as small as -0.024 pm 0.015. This result contrasts with the finite, inverse isotope shift observed in optimally-doped (Ba,K)Fe2As2, indicating that the contribution of the electron-phonon interaction markedly differs between these two Fe-based high-Tc superconductors.
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We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with various oxygen concentents, and examined pure doping ($p$) dependance of isotope effect on superconducting transition temperature. With decreasing oxygen contents, the isotope exponent $alpha$ monotonously increases without any anomaly around $p = 1/8$. The monotonous increase in $alpha$ indicates that phonons are involved in the mechanism which causes the monotonous $T_c$ suppression with underdoping.
We report the sulfur isotope effect on transition temperature in a BiS2-based superconductor Bi4O4S3. Polycrystalline samples of Bi4O4S3 were prepared using 32S and 34S isotope chemicals. From magnetization analyses, the isotope exponent (aS) was estimated as -0.1 < aS < 0.1. Although the Tc estimated from electrical resistivity was scattered as compared to those estimated from the magnetization, we observed no clear correlation between Tc and the isotope mass. The present results suggest that unconventional paring states are essential in Bi4O4S3.
The spin dynamics of an optimally doped YBa2Cu3O7 (Tc = 93 K) crystal array have been investigated in a wide range of momentum and energy (Q - E) space using the time-of-flight neutron scattering method. Incommensurate spin modulation in Q is a characteristic feature, as it is in the under-doped YBa2Cu3O6.7 with a different incommensurability. A linear relationship between the incommensurability and Tc is proposed. Along with the discovery of the same incommensurability in under-doped La2-ySryCuO4, it may be a generic characteristic of the high-Tc oxide superconductor.
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.
An inelastic neutron scattering experiment has been performed in the high-temperature superconductor $rm YBa_2Cu_3O_{6.89}$ to search for an oxygen-isotope shift of the well-known magnetic resonance mode at 41 meV. Contrary to a recent prediction (I. Eremin, {it et al.}, Phys. Rev. B {bf 69}, 094517 (2004)), a negligible shift (at best $leq$ +0.2 meV) of the resonance energy is observed upon oxygen isotope substitution ($^{16}$O$to^{18}$O). This suggests a negligible spin-phonon interaction in the high-$T_c$ cuprates at optimal doping.
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