Nernst effect in single crystals of the pnictide superconductor CaFe1.92Co0.08As2 and parent compound CaFe2As2

We report a combined study of the Nernst coefficient (v), Hall effect and thermoelectric power of CaFe2As2 and CaFe1.92Co0.08As2 single crystals. The absolute value of v in both samples is enhanced, probably due to ambipolar flow of electron- and hole-like quasiparticles. The onset of spin-density-wave order in CaFe2As2 causes further rapid rise of the Nernst coefficient. On the contrary, in the CaFe1.92Co0.08As2 crystal we do not see any feature of v(T), which could be clearly associated with SDW fluctuations. In this Co-doped sample there is also no noticeable increase in of v in the vicinity of superconducting transition, despite the expectation of such due to vortex movement.Comment: 6 pages, 3 figures; (v2) slight changes and clarifications; to appear in Phys. Rev.

Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $\tau(\theta)$ in the mixed state of LaO$_{0.9}$F$_{0.1}$FeAs single crystals as a function of temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter $\gamma=(m_c/m_{ab})^{1/2}$ from $\tau(\theta)$. We show that $\gamma$ depends strongly on both $T$ and $H$, reaching a maximum value of $\sim$ 10 followed by a decrease towards values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth $\lambda_{ab}$ and $\gamma$ suggest the onset of suppression of a superconducing gap at $H \approx H_{c2}/3$.Comment: 7 pages, 7 figure

Re-entrance of resistivity due to the interplay of superconductivity and magnetism in \ce{Eu_{0.73}Ca_{0.27}(Fe_{0.87}Co_{0.13})2As2}

By simultaneous Co and Ca-doping we were able to obtain an \ce{EuFe2As2}-based compound with superconductivity appearing above the antiferromagnetic order of \ce{Eu^{2+}} magnetic moments. However, as soon as the antiferromagnetic order appears a re-entrance behavior is observed \textemdash{} instead of zero resistivity and diamagnetic signal down to the temperature of \unit[2]{K}. By investigating magnetization, ac susceptibility and electrical transport properties of \ce{Eu_{0.73}Ca_{0.27}(Fe_{0.87}Co_{0.13})2As2} and comparing them to previously studied M\"ossbauer effect and neutron scattering measurements of this and similar compounds an explanation of such behavior is proposed.Comment: Main paper of 9 pages with 9 figures with Supplementary Material

3D-xy critical properties of YBa2Cu4O8 and magnetic field induced 3D to 1D crossover

We present reversible magnetization data of a YBa2Cu4O8 single crystal and analyze the evidence for 3D-xy critical behavior and a magnetic field induced 3D to 1D crossover. Remarkable consistency with these phenomena is observed in agreement with a magnetic field induced finite size effect, whereupon the correlation length transverse to the applied magnetic field cannot grow beyond the limiting magnetic length scale L_H. By applying the appropriate scaling form we obtain the zero-field critical temperature, the 3D to 1D crossover, the vortex melting line and the universal ratios of the related scaling variables. Accordingly there is no continuous phase transition in the (H,T)-plane along the H_c2-lines as predicted by the mean-field treatment.Comment: 8 pages, 4 figure