Type-I superconductivity of the layered silver oxide ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$

We report the zero-resistivity transition and the details of the magnetic transition of a layered silver oxide ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ single crystal, which provide definitive evidence of superconductivity in this compound. In the ac susceptibility of a monocrystal, we observed large supercooling, as well as positive peaks in the real part of the susceptibility indicating the reversibility of the magnetic process. These observations reveal that ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ is an oxide that shows type-I superconductivity. Evaluation of the superconducting parameters not only gives confirming evidence of type-I superconductivity, but also indicates that it is a dirty-limit superconductor. We also analyze supercooling to determine the upper limit of the Ginzburg-Landau parameter.

Figure 1

(Color online) Temperature dependence of the out-of-plane resistivity ${\rho }_c$ of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$ below 70 mK. The sweep rate was approximately $0.05\mathrm{mK}∕\min$. Hysteretic behavior at the transition is attributable to a residual magnetic field. The inset photo on the left shows the single crystal used for the measurements. The long axis of the stick corresponds to the $c$ axis. The inset on the right shows the crystal structure of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$. Red and blue spheres represent the silvers on the Kagome lattice and the chain, respectively.

Figure 2

(Color online) ac susceptibility of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$. (a) The result of a temperature sweep with a sweep rate of $0.2\mathrm{mK}∕\min$. The residual field $H_{\mathrm{res}}$ has been compensated in this sweep, yielding $T_{\mathrm{c}0}=52.4\mathrm{mK}$. (b) The results of field sweeps at several temperatures with a sweep rate of $24–47\mathrm{mOe}∕\min$. From the slight asymmetry of the data, the residual field is estimated as $H_{\mathrm{res}}=0.040\mathrm{Oe}$. The double peaks seen in 45 and 47 mK do not seem to be intrinsic, since they are not reproducible with other crystals.

Figure 3

Phase diagram of the superconducting phase of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$, determined from the field-sweep data of ac susceptibility. The residual field has been subtracted in the shield $H_{\mathrm{res}}=0.040\mathrm{Oe}$ from the raw data. The filled squares are the superconducting to normal transition field and should be equal to $H_{\mathrm{c}}$ (see text). The crosses are the supercooling field $H_{\mathrm{sc}}$ corresponding to the normal to superconducting transitions. The broken line is the result of fitting with $H_{\mathrm{c}}\left(T\right)=H_{\mathrm{c}0}[1-{(T∕T_{\mathrm{c}0})}^{\alpha }]$.

Figure 4

The ratio ${\kappa }_{\mathrm{sc}}\equiv H_{\mathrm{sc}}∕\left(1.695\sqrt{2}{H}_{\mathrm{c}}\right)$ of ${\mathrm{Ag}}_5{\mathrm{Pb}}_2{\mathrm{O}}_6$, which gives the upper limit of the GL parameter. The upturn of the graph near $T_{\mathrm{c}}$ is attributable to the size effect. The broken line is the result of linear fitting between $T=35$ and 47 mK, where the size effect is not significant.