Unusual pressure evolution of the Meissner and Josephson effects in the heavy-fermion superconductor ${\mathbf{UPt}}_3$

The Josephson effect between a single-crystal ${\mathrm{UPt}}_3$ and a conventional superconductor Al has been investigated under pressure for the junction on the ${\mathrm{UPt}}_3$ surface perpendicular to the hexagonal $c\left[0001\right]$ axis. Simultaneously measured magnetization has revealed that the Meissner fraction approaches a minimum value of $\sim 1\%$ at the critical pressure of $P_{\mathrm{c}}\sim 0.4\mathrm{GPa}$. The critical temperature $T_{\mathrm{c}}$ and the temperature $T_{\mathrm{J}}$ where a measurable Josephson critical current $I_{\mathrm{c}}$ first appears decrease with increasing pressure; the decreasing rate of $|d{T}_{\mathrm{c}}/dP|$ is almost constant, whereas $|d{T}_{\mathrm{J}}/dP|$ shows an abrupt increase above $P_{\mathrm{c}}$, indicating that the Josephson effect is suppressed above $P_{\mathrm{c}}$. These results, together with the decrease in penetration depth $\lambda$ derived from the magnetic-field dependence of $I_{\mathrm{c}}$, suggest that the superconducting phase is modified above $P_{\mathrm{c}}$ by the disappearance of the symmetry-breaking antiferromagnetic order.

Figure 1

Schematics of the experimental setup: (a) the Josephson junction and (b) the coils.

Figure 2

Temperature dependence of dc susceptibility under various pressures for (a) zero-field-cooled (ZFC) and (b) field-cooled (FC) processes. The inset: pressure dependence of Meissner fraction ($={\chi }_{\mathrm{FC}}/{\chi }_{\mathrm{ZFC}}$) at the temperature lower than $T_{\mathrm{c}}$ onset by 0.1 K (closed circle), the range of the increase extrapolated to 0 K using a differential coefficient at that temperature (arrow) and superconducting transition temperature $T_{\mathrm{c}}$ (open circle).

Figure 3

(a) Temperature dependence of the Josephson critical current under various pressures for the $c$-axis junction. The inset: typical $I-V$ characteristics at 0.53 GPa.

Figure 4

(a)–(c) Magnetic-field dependence of the Josephson critical current under various pressures for the $c$-axis junction. The curves in the figures indicate fits using the Fraunhofer diffraction pattern. (d) Pressure dependence of maximum critical current $I_{\mathrm{c}0}$ and magnetic penetration depth $\lambda$ at 0 K obtained by fitting.

Figure 5

Phase diagram of ${\mathrm{UPt}}_3$ under pressure. The closed circles and closed triangles indicate $T_{\mathrm{c}}$ and $T_{\mathrm{J}}$, respectively. The open circles indicate the double-transition temperature at ambient pressure obtained for the ${\mathrm{UPt}}_3-\mathrm{Nb}$ junction [23]. The straight lines indicate the phase boundaries. The dashed line shows the possible BC phase boundary drawn using the reported $dT/dP$ value [15]. The inset: pressure dependence of $T_{\mathrm{J}}/T_{\mathrm{c}}$.