Evidence for anisotropic triplet superconductor order parameter in half-metallic ferromagnetic La${}_{0.7}$Ca${}_{0.3}$Mn${}_3$O proximity coupled to superconducting Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_4$

Scanning tunneling spectroscopy measurements performed on La${}_{0.7}$Ca${}_{0.3}$Mn${}_3$O (LCMO) films epitaxially grown on Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_4$ (PCCO) reveal localized penetration of superconductivity into the LCMO up to distances much larger than is possible for Cooper pairs in a singlet spin state to exist. This long-range proximity effect is manifested in the tunneling spectra as gaps and, less abundantly, as zero-bias conductance peaks (ZBCPs). Since ZBCPs were not found on the bare PCCO films, their appearance is attributed to an anisotropic ($p$ wave or $d$ wave) triplet-pairing superconductor order parameter induced in the LCMO.

Figure 1

The resistance vs temperature of an LCMO(15 nm)/PCCO(200 nm) bilayer showing a change in slope at ∼240 K, which approximately coincides with the Curie temperature of LCMO and the critical superconducting temperature of PCCO of 17 K. The inset shows the field dependence of the magnetization of the bilayer at 50 K.

Figure 2

Tunneling spectrum acquired on a bare PCCO film at 4.2 K (solid blue curve). The fit (open circle green curve) was computed for $c$-axis tunneling into a $d$-wave superconductor including finite lifetime broadening effects. Here, Δ is the gap, Γ is the Dynes broadening parameter, and $Z$ is the barrier strength parameter (as described in the main text). No zero-bias conductance peaks were observed on the bare PCCO film, although many regions were scanned. The inset shows a typical topographic image of the bare PCCO film, 200 nm thick, manifesting its crystalline structure; scale bar is 300 nm.

Figure 3

Scanning tunneling spectroscopy data measured at 4.2 K on LCMO(15 nm)/PCCO(200 nm) sample. (a) Representative tunneling spectrum acquired within the marked gapped region in the STM topography image shown in the inset. (b) Tunneling spectra acquired along the line indicated by the arrow on the topographic map in (a). All spectra data in the marked ∼100 × 100 nm${}^2$ region showed induced superconducting-like gaps similar to (a), indicative of a proximity effect. In the close environment of the gapped region, the spectra were metalliclike.

Figure 4

Tunneling spectra at 4.2 K showing ZBCPs. (a) Curve (1) is an average over normalized tunneling spectra acquired on a 10-nm LCMO/PCCO sample, while curve (2) was acquired on a 25-nm LCMO sample. (b) Tunneling spectra measured on a 25-nm LCMO/PCCO bilayer showing ZBCPs embedded in sharp gaplike features. The inset of (a) shows part of the spectra used to calculate curve (2). The spectra were measured over a region indicated by the arrow in the STM image presented in the inset of (b); scale bar is 50 nm.