Anomalous In-Plane Anisotropy of the Onset of Superconductivity in $(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$

We report the magnetic-field amplitude and field-angle dependence of the superconducting onset temperature $T_c^{\mathrm{onset}}$ of the organic superconductor $(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$ in magnetic fields $\mathit{H}$ accurately aligned to the conductive $a{b}^{\prime }$ plane. We revealed that the rapid increase of the onset fields at low temperatures occurs both for $\mathit{H}\parallel b^{\prime }$ and $\mathit{H}\parallel a$, irrespective of the carrier confinement. Moreover, in the vicinity of the Pauli-limiting field, we report a shift of a principal axis of the in-plane field-angle dependence of $T_c^{\mathrm{onset}}$. This feature may be related to an occurrence of Fulde-Ferrell-Larkin-Ovchinnikov phases.

Figure 1

(a) Temperature dependence of $R_{c^{*}}$ in zero field. The directions of the orthogonal axes are illustrated in the inset. (b) Conductance difference $\Delta \sigma$ under an in-plane 50-kOe field applied parallel to the $b^{\prime }$ axis. Resistances for $H_{c^{*}}=0\mathrm{kOe}$ (blue curve) and $H_{c^{*}}=0.5\mathrm{kOe}$ (light blue curve) are plotted against the right vertical axis. (c) Temperature dependence of $\Delta \sigma$ with the in-plane field applied parallel to the $a$ axis and $H_{c^{*}}=1.0\mathrm{kOe}$. Some data are shifted vertically for clarity.

Figure 2

Phase diagrams for $\mathit{H}\parallel a$ (■) and $\mathit{H}\parallel b^{\prime }$ (●). The phase diagram for $\mathit{H}\parallel c^{*}$ is shown in the inset. The broken lines indicate the initial slopes of each curves.

Figure 3

Polar plots of $T_c^{\mathrm{onset}}(\varphi )$. The points for $|\varphi |>90°$ are from the same data as those for $|\varphi |\le 90°$. They are plotted in order to symmetrize the figure. Error bars are omitted for the sake of clarity. Purple crosses indicate that no SC anomaly in $\Delta \sigma (T)$ was observed above 80 mK. Solid red lines indicate the direction of the new principal axis $\mathit{X}$.

Figure 4

Field dependence of the relative difference between $T_c^{\mathrm{onset}}(+45°)$ and $T_c^{\mathrm{onset}}(-45°)$.