Dimensionality of superconductivity in the layered organic material ${\mathrm{EtMe}}_3\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ under pressure

We measured the ac magnetic susceptibility for the layered organic superconductor ${\mathrm{EtMe}}_3\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ under pressure with a dc magnetic field applied perpendicular to the ac field. We investigated the dc field dependence of the ac susceptibility in detail and concluded that the superconductivity in ${\mathrm{EtMe}}_3\mathrm{P}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ is an anisotropic three-dimensional superconductivity even at low temperatures, which contrasts with the large majority of other correlated electron layered superconductors such as high-$T_{\mathrm{c}}$ cuprate and $\kappa$-(${\mathrm{ET})}_{2}X$ systems.

Figure 1

(a) LC tank circuit to measure ac susceptibility. dc magnetic field $H_{\mathrm{dc}}$ was also applied perpendicular to the ac field $H_{\mathrm{ac}}$. (b) Schematic for the configuration of $H_{\mathrm{ac}}$ and $H_{\mathrm{dc}}$. The angle between the directions of the two-dimensional layers and $H_{\mathrm{dc}}$ is defined as $\theta$, which was varied by a rotation mechanism.

Figure 2

Temperature dependence of the resonance frequency $f$ of the LC tank circuit under $H_{\mathrm{dc}}=0$ T. The insets show the temperature dependence of $1/\eta (1-f_0^2/f^2)$, where $\eta$ is $\sim 0.033$ for sample no. 1, $\sim 0.027$ for sample no. 2, and $\sim 0.13$ for sample no. 3. Note that the values of $\eta$ have uncertainties of $\pm 50\%$. It is also noted that the sample thickness is $\sim 50\mu \mathrm{m}$ and thus the magnetic penetration effect is not ignorable.

Figure 3

(a) Temperature dependence of $1-f_0^2/f^2$ at various angles $\theta$ under $H_{\mathrm{dc}}=0.025$ and 0.1 T. (b–e) Angle dependence of $1-f_0^2/f^2$ under $H_{\mathrm{dc}}=0.025$ (squares) and 0.1 T (triangles), derived from the data in Fig. 3.

Figure 4

(a) Temperature dependence of $1-f_0^2/f^2$ at various $H_{\mathrm{dc}}$ at $\theta =0^{\circ }$. (b) Temperature dependence of $H_{\mathrm{c}2}^{\parallel }$ indicated by the arrows in Fig. 4.

Figure 5

(a) Temperature dependence of $1-f_0^2/f^2$ at various angles $\theta$ under $H_{\mathrm{dc}}=1$ T. (b) Angle dependence of the transition temperature $T_{\mathrm{c}}$, indicated by the arrows in Fig. 5. The solid and dashed lines are the fitted curves of the A3DSC and 2DSC models, respectively. The A3DSC model can fit the data very well, while the 2DSC model cannot.