Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe

概要

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Magnetoresistivity measurements with fine tuning of the field direction on high quality single crystals of the ferromagnetic superconductor UCoGe show anomalous anisotropy of the upper critical field $H_{\text{c2}}$. $H_{\text{c2}}$ for $H\parallel \text{$b$-axis}$ ($H_{\text{c2}}^{b}$) in the orthorhombic crystal structure is strongly enhanced with decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The temperature dependence of $H_{\text{c2}}^{a}$ shows upward curvature with a low temperature value exceeding 30 T, while $H_{\text{c2}}^{c}$ at 0 K is very small (${\sim}0.6$ T). Contrary to conventional ferromagnets, the decrease of the Curie temperature with increasing field for $H\parallel \text{$b$-axis}$ marked by an enhancement of the effective mass of the conduction electrons appears to be the origin of the S-shaped $H_{\text{c2}}^{b}$ curve. These results indicate that the field-induced ferromagnetic instability or magnetic quantum criticality reinforces superconductivity.
2009-11-15