Isotope effect on the transition temperature TcT_c in Fe-based superconductors: the current status

The results of the Fe isotope effect (Fe-IE) on the transition temperature $T_c$ obtained up to date in various Fe-based high temperature superconductors are summarized and reanalyzed by following the approach developed in [Phys. Rev. B 82, 212505 (2010)]. It is demonstrated that the very controversial results for Fe-IE on $T_c$ are caused by small structural changes occurring simultaneously with the Fe isotope exchange. The Fe-IE exponent on $T_c$ [$\alpha_{\rm Fe}=-(\Delta T_c/T_c)/(\Delta M/M)$, $M$ is the isotope mass] needs to be decomposed into two components with the one related to the structural changes ($\alpha_{\rm Fe}^{\rm str}$) and the genuine (intrinsic) one ($\alpha_{\rm Fe}^{\rm int}$). The validity of such decomposition is further confirmed by the fact that $\alpha_{\rm Fe}^{\rm int}$ coincides with the Fe-IE exponent on the characteristic phonon frequencies $\alpha_{\rm Fe}^{\rm ph}$ as is reported in recent EXAFS and Raman experiments.Comment: 7 pages, 4 figures. The paper is partially based on the results published in [New J. Phys. 12, 073024 (2010) = arXiv:1002.2510] and [Phys. Rev. B 82, 212505 (2010) = arXiv:1008.4540

Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2

Polycrystalline samples and submillimeter size single crystals of Na-doped Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments showed that the Na content depends not only on the pressure during the synthesis but also on the synthesis temperature and time. From a comparison of the Na-CCOC data with those of structurally related La214 cuprate superconductors we concluded that chlorine at the apical site is less effective that oxygen in supplying charge carriers to the CuO2 plans. As a result, the coupling between the CuO2 planes is weakened, the transition temperature Tc is reduced and the anisotropic nature is enhanced.Comment: 7 pages, 7 figures, 1 table, presenthed at the Eucas 2007 conference. Accepted for "Journal of Physics: Conference Series (JPCS)" 2008 and European News Forum, Issue 3 (2008

Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe_1-x

The Fe isotope effect (Fe-IE) on the transition temperature T_c and the crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing \simeq 92% of ^{56}Fe) by its lighter ^{54}Fe isotope leads to a shift of T_c of 0.22(5)K corresponding to an Fe-IE exponent of \alpha_Fe=0.81(15). Simultaneously, a small structural change with isotope substitution is observed by NDP which may contribute to the total Fe isotope shift of T_c.Comment: 4 pages, 3 figure

Correlation between the transition temperature and the superfluid density in BCS superconductor NbB_2+x

The results of the muon-spin rotation experiments on BCS superconductors NbB_2+x (x = 0.2, 0.34) are reported. Both samples, studied in the present work, exhibit rather broad transitions to the superconducting state, suggesting a distribution of the volume fractions with different transition temperatures (T_c)'s. By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (\lambda_0^{-2}) on T_c was reconstructed for temperatures in the range 1.5K<T_c<8.0K. \lambda_0^{-2} was found to obey the power law dependence \lambda_0^{-2}\propto T_c^{3.1(1)} which appears to be common for some families of BCS superconductors as, {\it e.g.}, Al doped MgB_2 and high-temperature cuprate superconductors as underdoped YBa_2Cu_3O_{7-\delta}.Comment: 9 pages, 7 figures. Accepted for publication in Phys. Rev.

Nodal-to-nodeless superconducting order parameter in LaFeAs1−x_{1-x}Px_xO synthesized under high pressure

Similar to chemical doping, pressure produces and stabilizes new phases of known materials, whose properties may differ greatly from those of their standard counterparts. Here, by considering a series of LaFeAs$_{1-x}$P$_x$O iron-pnictides synthesized under high-pressure high-temperature conditions, we investigate the simultaneous effects of pressure and isoelectronic doping in the 1111 family. Results of numerous macro- and microscopic technique measurements, unambiguously show a radically different phase diagram for the pressure-grown materials, characterized by the lack of magnetic order and the persistence of superconductivity across the whole $0.3 \leq x \leq 0.7$ doping range. This unexpected scenario is accompanied by a branching in the electronic properties across $x = 0.5$, involving both the normal and superconducting phases. Most notably, the superconducting order parameter evolves from nodal (for $x < 0.5$) to nodeless (for $x \geq 0.5$), in clear contrast to other 1111 and 122 iron-based materials grown under ambient-pressure conditions.Comment: 9 pages, 7 figures, Suppl. materia

Universal observation of multiple order parameters in cuprate superconductors

The temperature dependence of the London penetration depth \lambda was measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (\lambda_a and \lambda_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (\lambda_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of s+d-wave symmetry whereas it is exclusively s-wave along the c-direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.Comment: 5 pages, 3 figure