Two superconducting components with different symmetries in Nd1-xSrxNiO2 films

The pairing mechanism in cuprates remains as one of the most challenging issues in the field of condensed matter physics. The unique 3d9 electron orbital of the Cu2+ ionic states in cuprates is supposed to be the major player for the occurrence of superconductivity. Recently, superconductivity at about 9-15 K was discovered in infinite layer thin films of nickelate Nd1-xSrxNiO2 (x=0.1-0.2) which is believed to have the similar 3d9 orbital electrons. The key issue concerned here is about the superconducting gap function. Here we report the first set data of single particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted very well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Our results suggest that the newly found Ni-based superconductors play as close analogs to cuprates, and thus demonstrate the commonality of unconventional superconductivity.

exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Our results suggest that the newly found Ni-based superconductors play as close analogs to cuprates, and thus demonstrate the commonality of unconventional superconductivity.
The pairing mechanism of cuprate superconductors has been intensively studied for more than three decades, but the fundamental reason remains unresolved yet. A common feeling summarized from the past tremendous investigations tells that the 3d 9 orbital electrons of the Cu 2+ ionic state are crucial for the formation of superconductivity. Recently superconductivity with transition temperatures of about 9~15K was observed in infinite layer nickelate thin films of Nd1-xSrxNiO2 (x=0.1-0.2) which may share the similar 3d 9 orbital electrons as that in cuprates (1,2). This discovery has drawn enormous attention (3)(4)(5)(6)(7)(8)(9)(10)(11)(12) since it may provide deeper insight about the pairing mechanism of unconventional superconductivity in cuprates. Concerning the pairing mechanism, the core issue is to know the superconducting gap function which measures the pairing interaction of the two electrons of a Cooper pair.
One of the effective ways to detect the superconducting gap function is to measure the single particle tunneling spectrum in the superconducting state. In this paper, we report such investigations for the first time on the Nd1-xSrxNiO2 thin films. The results expose essential message about the superconducting pairing in this newly found superconducting system. The Nd1-xSrxNiO3 thin films with thickness of about 6 nm are deposited with reactive molecular beam epitaxy (MBE) technique which is different from pulsed laser deposition (PLD) method used in previous work (1)(2)(3). Superconductivity is achieved by annealing the sample placed in an evacuated quartz tube together with a pellet of CaH2. A similar procedure given in previous report (3) for the heat treatment is followed. The x-ray diffraction (XRD) data together with observation of superconductivity assures that the samples after post-annealing become Nd1-xSrxNiO2 (NSNO). Results reported here are obtained on samples with a nominal composition of x = 0.2. The details of synthesis and characterization of the samples will be published separately. Shown in Fig. 1A is a schematic plot of atomic structure of the NSNO. Fig.1B shows the temperature dependence of resistivity of the NSNO film measured by using a standard four-probe technique. One can see that the onset transition temperature is about 15.3 K, and zero resistivity is achieved at about 9.1 K. The rounded transition near the onset point tells that fluctuating superconductivity can occur at about 18 K. In Fig.1C,D, we show the topographic images of one film in a 2D and 3D manner, which is measured by the scanning tunneling microscope (STM). One can see that the surface is not atomically flat showing a roughness of about 1~2 nm. This large roughness may be induced by postannealing on the films. Details about characterization of the films are given in Supplementary Information Note 1.
We then measure the tunneling spectra at different positions on the surface of the film. Due to the large surface roughness, we cannot measure spectrum with a line-scan mode in a large area, however it can be done at different locations. It is found that the tunneling spectrum is not uniform across the sample, suggesting that the film after annealing is not in a perfect epi-texture state. However, from hundreds of the measured spectra, we find that they predominantly show two types of features. One type shows a fully gapped feature with the gap of about 2.35 meV. Typical data measured at 0.35 K are shown in Fig.2A. We fit the data with the Dynes model and get a quite nice fitting, as shown by the red curve. Details about the fitting are presented in the Supplementary Information Note 2. A slight anisotropy (about 15% weight of the differential conductivity) is added to the gap function in order to have a good fit. This indicates that at least one of the bands is fully gapped. We have conducted measurements at different positions in a small area and find that the spectra all show this type behavior, the results are shown in Fig.2B. Beside the coherence peaks at about 2.35 mV, two strong side peaks show up at about 5~6 mV. The global shape of the spectrum suggests that these side peaks may correspond to some bosonic modes. In Supplementary Fig.1 we present the data measured at about 1.5 K, one can see that, due to the thermal broadening effect, the bottom of the spectrum is elevated and the coherence peaks become rounded. Another type of spectrum shows a typical V-shape feature, which is shown in Fig.3A. By doing the Dynes model fitting, as displayed by the red curve, we find that the spectrum can be nicely fitted with a d-wave gap. The maximum gap obtained through the fitting is about 3.9 meV. This type of spectrum can also be measured at different positions of the sample, and some time we find that this type of feature exists everywhere in a small area. In Fig.3B, we put the two types of spectra together. On the V-shape spectrum, we can see a weak kinky point at the bias voltage of about 2.35 mV which corresponds very well to the value of the spectrum with full gap. Thus we believe that the two kinds of spectra with different gap symmetries correspond to the gaps on different Fermi surfaces. The spectra with this V-shape feature measured at 1.5 K are shown in Supplementary Fig.2. Again the bottom at zero bias is elevated and the coherence peaks are smeared. In Supplementary Fig.3 we show a spectrum in a wide energy scale (100mV). One can see that there is a clear asymmetric background beyond the gap. This feature was also observed in cuprates and was attributed to the strong correlation effect (13). This asymmetric can also be explained as due to the tunneling matrix problem (14) concerning the multiband feature in the present system. The topographic images shown in we need to do further phase-referenced quasi-particle interference experiments on single crystal samples when they are available (20,21), which has been conducted successfully in iron based superconductors (22,23) and cuprates (24). Clearly, more efforts are desired in order to pin down the assignment of the superconducting gaps on different Fermi pockets.
In summary, on Nd1-xSrxNiO2 thin films, we have found predominantly two superconducting components with distinct gap symmetries.  has coordinated the whole work.

Competing interests:
The Authors declare no competing interests.   (S1) Here f(ε) is the Fermi distribution function containing the information of temperature. We set the practical electronic temperature as 1 K which is a little higher than the nominal one displayed by thermometer. The scattering factor  denotes the inverse quasiparticle lifetime in unit of meV and represents the azimuth angle along the Fermi surface in the Brillouin zone. The best fittings to the data shown in Fig.2 and 3

Note 3. Control experiments on other samples with different morphologies
We have successfully repeated the primary results presented in Fig.2 and for B,C are Vset = 5 mV, Iset =100 pA.
Supplementary Figure 3 Tunneling spectra acquired within 100 mV at 1.5 K.
A V-shaped background indicates a bad metal behavior in the normal state of Nd1-xSrxNiO2. The suppression of spectral weight near zero energy is induced by the formation of superconducting gaps. The seemingly weak signature of the superconducting gap on this spectrum is due to two reasons. One is that we change the set-point conditions as Vset = 100 mV, Iset = 100 pA which sets a relatively longer distance between the tip and sample, experiencing an exponential decay of superconducting order parameter at the end of the tip.
Another is that we increase the AC oscillation amplitude to enhance the dI/dV signal while it inevitably smears out the low-energy details of the spectrum.