Superconductivity in the charge-density-wave state of the organic metal $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{K}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4$

The superconducting transition in the layered organic compound $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{K}\mathrm{Hg}{\left(\mathrm{SCN}\right)}_4$ has been studied in the two hydrostatic pressure regimes where a charge-density wave is either present or completely suppressed. Within the charge-density-wave state the experimental results reveal a network of weakly coupled superconducting regions. This is especially seen in a strong enhancement of the measured critical field and the corresponding positive curvature of its temperature dependence. Further, it is shown that on lowering the pressure into the density-wave state traces of a superconducting phase already start to appear at a much higher temperature.

Figure 1

Temperature sweeps of the interlayer resistance of sample 1 at different pressures. For clarity, each curve has a different resistance scale. At $P>2.5\mathrm{kbar}$, there are sharp transitions from the NM state to superconductivity. Within the CDW state, $P\lesssim 2.5\mathrm{kbar}$, the superconducting transitions are broadened and the zero-resistance temperature decreases.

Figure 2

Comparison of the in- and interplane resistances of sample 2 at pressures above (upper graph) and below (lower graph) the critical value $P_c=2.5\mathrm{kbar}$.

Figure 3

Temperature dependence of the interlayer resistance at ambient pressure for three different samples.

Figure 4

(Color online) Field dependence of the interlayer resistance of sample 1 at various constant temperatures for pressures above and below $P_c$.

Figure 5

(Color online) Critical fields and temperatures determined at pressures around the critical value $P_c=2.5\mathrm{kbar}$. Filled symbols are obtained from field sweeps, see Fig. 4, and open circles from temperature sweeps, see Fig. 7.

Figure 6

Critical fields at $P=2\mathrm{kbar}$ (solid symbols) and $P=3.5\mathrm{kbar}$ (open symbols) determined from the resistive transition in the field sweeps using different criteria. The inset shows how the criteria are chosen: the onset (squares), the inflection point (circles), and the end of the SC transition where the resistance is $\approx 10\%$ of the normal metallic value (triangles).

Figure 7

Temperature dependent interlayer resistance at different constant magnetic fields, at $P=2\mathrm{kbar}$.

Figure 8

Within the CDW state $(P=2\mathrm{kbar})$ the decrease of the interlayer resistance accelerates at much higher temperatures than in the NM state $(P=3\mathrm{kbar})$. This decrease strongly depends on the level of the applied current (a) and magnetic field perpendicular to the layers (b).

Figure 9

Proposed $P\text{−}T$ phase diagram. Filled symbols show the phase transitions between different states. Open symbols mark the onset and zero-resistance temperatures of the broadened main SC transition in the CDW state. The lines are guides for the eye.