Magnetic susceptibility and resistive transitions of superconducting (TMTSF)Cl2O4: Critical magnetic fields

We report measurements of the ac magnetic susceptibility and dc resistive transition in the zero-pressure organic superconductor di-tetramethylselenafulvalene perchlorate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$]. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [$H_{\mathrm{cl}}\mathrm{}\left(0\right)<1\mathrm{}$ Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, $T_c=1.0\mathrm{} \mathrm{and} 0.65$ K, respectively. Magnetic-field-induced transitions, measured both inductively and resistively, are broad and characteristically different. Critical magnetic fields measured inductively are moderately anisotropic ($\frac{H_{c2\mathrm{}}^{\parallel }}{H_{c2\mathrm{}}^{\perp }}\approx 3.2$) and relatively small in magnitude [$H_{c2\mathrm{}}^{\perp }\mathrm{}\left(0\right)\mathrm{}\approx 150$ Oe]. Measured resistively, the critical magnetic fields are highly anisotropic ($\frac{H_{c2\mathrm{}}^{\parallel }}{H_{c2\mathrm{}}^{\perp }}\approx 20$) and large in magnitude [$H_{c2\mathrm{}}^{\perp }\mathrm{}\left(0\right)\mathrm{}\approx 1500$ Oe]. Results suggest that ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$Cl${\mathrm{O}}_4$ is a quasi-one-dimensional or quasi-two-dimensional superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields.