Tunnelling spectroscopy of the interface between Sr2RuO4 and a single ru micro-inclusion in eutectic crystals

The understanding of the zero bias conductance peak (ZBCP) in the tunnelling spectra of superconductor/normal metal (S/N) junctions involving d-wave cuprate superconductors has been important in the determination of the phase structure of the superconducting order parameter. In this context, the involvement of a p-wave superconductor such as Sr2RuO4 2) in tunnelling studies is indeed of great importance. We have recently succeeded in fabricating devices that enable S/N junctions forming at interfaces between Sr2RuO4 and Ru micro-inclusions in eutectic crystals to be investigated. We have observed a ZBCP and have interpreted it as due to the Andreev bound state, commonly seen in unconventional superconductors. Also we have proposed that the onset of the ZBCP may be used to delineate the phase boundary for the onset of a time reversal symmetry broken (TRSB) state within the superconducting state, which does not always coincide with the onset of the superconducting state. However, these measurements always involved two interfaces between Sr2RuO4 and Ru. In the present study, we have extended the previous measurements to obtain a deeper insight into the properties of a single interface between Sr2RuO4 and Ru. The layered peroviskite Sr2RuO4, recognised as one of the strongest candidates for a spin-triplet superconductor, has attracted great research interest in spite of its relatively low Tc of 1.5K. 4) Taken together several experiments, such as NMR and muon spin relaxation, the basic form of the vector order parameter is constrained to be dðkÞ 1⁄4 z 0ðkx þ ikyÞ, corresponding to a TRSB state (the chiral state). Although this basic form is too simplified to explain other existing experimental results, the incorporation of band-dependent gap structure and strong in-plane anisotropy of the superconducting gap allows the existing experimental results to be reconciled. Interesting aspects of Sr2RuO4 include superconductivity in eutectic systems such as Sr2RuO4–Ru 8) and Sr2RuO4– Sr3Ru2O7. 9) In Sr2RuO4–Ru, superconductivity with an enhanced Tc, called the 3-K phase, is known to occur within Sr2RuO4 at interfaces between Sr2RuO4 and Ru microinclusions. Besides this, these eutectic systems, as a consequence of the eutectic solidification, contain natural interfaces with the spin-triplet superconductor Sr2RuO4. Under certain circumstances, such interfaces may be regarded as S/N junctions. In fact, Mao et al. performed break-junction experiments on Sr2RuO4–Ru eutectic and observed a ZBCP characteristic of unconventional superconductivity. Recently, as mentioned earlier, we have also used interfaces in eutectic Sr2RuO4–Ru to obtain well-defined S/N junctions illustrated in Fig. 1, using a micro-fabrication technique. Details of the fabrication and the crystal growth are described in refs. 3 and 8. As depicted in Fig. 1, each S/N junction possesses a Ti/Au electrode directly attached to the Ru micro-inclusion through 2 3 mm rectangular contact windows. The SiO2 film is an insulating layer deposited on the ab-plane of Sr2RuO4. Achieving good electrical contacts is possible practically only on Ru inclusions because of a non-superconducting layer with a high resistivity forming on the ab-plane surface of Sr2RuO4. Consequently, actual measurement current is injected via two electrodes on Ru inclusions. Therefore, the two Sr2RuO4/Ru junctions in series are inevitably involved for the measurement. Besides, this measurement configuration leads to the resistances of the S/N junctions, Sr2RuO4, Ru, and Ti/Au electrodes in series all contributing to the whole resistance measured. However, the resistance measured is dominated by that of the Sr2RuO4/Ru junctions because the resistance measured, typically on the order of 1 , is much larger than the resistance of the other components. We use the identical devices also in the present study. A lock-in technique was employed to measure the differential conductance. Similarly to work in ref. 10, we observed a clear ZBCP in the previous study in ref. 3. The ZBCP is attributed to a sign change in the order parameter on the Fermi surface, characteristic of unconventional superconductivity. Figure 2 shows two spectra of different devices at 0.6 K. The upper trace was obtained using the same device as used in the previous study, and shows a sharp ZBCP in addition to a broad peak. By contrast, the lower trace appears to be similar but without a sharp ZBCP. Such S/N junctions may be phenomenologically modelled using a steep barrier at the interface, assuming a -function potential with its strength Z. With increasing Z, the tunnelling limit, where the resultant spectra represent the density of states at the interface, is approached. In this context, the overall similarity of the two spectra and the moderate closeness of the normal conductance values may suggest the proximity of interface conditions such as the barrier height (probably rather high Z), whereas the origin of the difference between the two spectra is unclear. Based on this assumption, these observations perhaps signal that the spectrum depends on the (effective) direction of the interface relative to the crystallographical axes. In fact, S/N junctions including high Tc cuprates with d-wave pairing show strong dependence of the tunnelling spectrum on the direction of the interface. It is well established that the height of ZBCP takes a maximum (minimum) for 1⁄4 =8 ( 1⁄4 0), where is the angle Journal of the Physical Society of Japan Vol. 75, No. 12, December, 2006, 125001 #2006 The Physical Society of Japan