Pairing state in the ruthenocuprate superconductor $\mathrm{Ru}{\mathrm{Sr}}_2\mathrm{Gd}{\mathrm{Cu}}_2{\mathrm{O}}_8$: A point-contact Andreev reflection spectroscopy study

The results of point contact Andreev reflection spectroscopy on polycrystalline $\mathrm{Ru}{\mathrm{Sr}}_2\mathrm{Gd}{\mathrm{Cu}}_2{\mathrm{O}}_8$ pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified Blonder–Tinkham–Klapwijk model considering a $d$-wave symmetry of the superconducting order parameter. Remarkably low values of the energy gap $\Delta =(2.8\pm 0.2)\mathrm{meV}$ and of the $2\Delta ∕k_B{T}_c\simeq 2$ ratio are inferred. From the temperature evolution of the $dI∕dV$ vs $V$ characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the $\Delta$ vs $H$ evolution, a critical magnetic field $H_{c_2}\simeq 30\mathrm{T}$ is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the point contact junction, as a consequence of the granularity of the sample.

Figure 1

Conductance characteristics, at low temperatures, for different barriers $Z$ as obtained by the BTK model for a point contact junction between a normal metal and a $s$-wave [(a),(b),(c)], a $d$-swave [(d),(e),(f)] and an anisotropic $s$-wave superconductor [(g),(h),(i)].

Figure 2

(Color online) The $dI∕dV$ vs $V$ characteristics measured in different Ru-1212/Pt–Ir PC junctions at $4.2\mathrm{K}$. The experimental data (dots) are shown together with the best theoretical fittings (solid lines) obtained by a modified BTK model for a $d$-wave symmetry of the superconducting order parameter.

Figure 3

(Color online) Intergrain coupling effect in polycrystalline samples. The measured voltage $V_{\mathit{\text{measured}}}$ is the sum of two terms: $V_{\mathit{PC}}$, the voltage drops between tip and sample, the N/S point contact junction, and $V_J$, the voltage drops between two superconducting grains, forming the S/I/S Josephson junction.

Figure 4

(Color online) Temperature evolution of the conductance spectrum of Fig. 2c from $T=4.2\mathrm{K}$ to up the critical temperature (dots). The solid lines are the theoretical fittings obtained by a modified $d$-wave BTK model with the energy gap as only free parameter.

Figure 5

(Color online) Temperature dependence of the superconducting energy gap as inferred from the theoretical fittings shown in Fig. 4. The solid line is a guide for the eyes. The right hand scale refers to the temperature evolution of the measured height of the ZBCP normalized to the $4.2\mathrm{K}$ value.

Figure 6

(Color online) Magnetic field dependence of the normalized $dI∕dV$ vs $V$ characteristics at $T=4.2\mathrm{K}$ from $0\text{to}2\mathrm{T}$ (dots) for the spectra of Fig. 2b. The full lines are the corresponding theoretical fittings. In the inset the magnetic field dependence of the energy gap is reported.

Figure 7

Normalized conductance curves for the contact of Fig. 2e measured at $T=4.2\mathrm{K}$ in magnetic field up to $2.5\mathrm{T}$. When the field is switched off, the original spectra are recovered.