Superconductivity in the Surface State of Noble Metal Gold and its Fermi Level Tuning by EuS Dielectric

The induced superconductivity (SC) in a robust and scalable quantum material with strong Rashba spin-orbit coupling is particularly attractive for generating topological superconductivity and Majorana bound states (MBS). Gold (111) thin film has been proposed as a promising candidate because of the large Rashba energy, the predicted topological nature, and the possibility for large-scale MBS device fabrications. We experimentally demonstrate two important steps towards achieving such a goal. We successfully show induced SC in the Shockley surface state (SS) of ultrathin Au(111) layers grown over epitaxial vanadium films, which is easily achievable on a wafer scale. The emergence of SC in the SS, which is physically separated from a bulk superconductor, is attained by indirect quasiparticle scattering processes instead of by conventional interfacial Andreev reflections. We further show the ability to tune the SS Fermi level ($E_F$) by interfacing SS with a high-$\kappa$ dielectric ferromagnetic insulator EuS. The shift of $E_F$ from $\sim 550$ to $\sim 34\mathrm{mV}$ in superconducting SS is an important step towards realizing MBS in this robust system.

Figure 1

(a) Large scale constant current STM topography image of 20 nm thick V film grown on a sapphire substrate. (Inset): Schematic layout of the heterostructure. (b) High resolution STM image confirming the V(110) surface [16]. (c) The large scale atomic terraces of 4 nm Au grown on V. (Inset): $dI/dV$ tunneling spectrum showing the edge of the surface energy band. (d) Atomic resolved STM image showing the hexagonal atomic lattice of Au(111).

Figure 2

(a) STM topography image of sub-ML EuS grown on a $\mathrm{Au}(111)/\mathrm{V}$ surface. A continuous EuS layer is obtained when the thickness is above 3 ML ($\sim 1\mathrm{nm}$), which causes difficulties for the STM scanning due to insulating EuS. The line scan profile (inset) shows the height of the EuS island is $\sim 3\AA$. (b) STS spectrum obtained on top of pristine Au(111) (red) and on Au(111) with 1 ML EuS island (blue), respectively. The bottom of the SS band shifts towards $E_F$ by $\sim 0.2\mathrm{eV}$. The tunneling spectra are normalized to the $dI/dV$ peaks, respectively. In each case, two $dI/dV$ scans (manually shifted on the vertical axis) are shown to demonstrate the reproducibility. (c) With 2.4 nm EuS grown on Au(111), $E_F$ is found to be only $\sim 34\mathrm{meV}$ above $E_{SS}$. Because 2.4 nm EuS is insulating, the $dI/dV$ spectra are measured in planar tunnel junction devices. The tunneling spectrum is normalized to the $dI/dV$ peak that corresponds to $E_{SS}$. The inset shows the $dI/dV$ spectrum in the full bias voltage range. The clear asymmetry (inset), showing the $dI/dV$ peak only at the negative bias voltage side, is consistent with our tunneling experiment setup and the band structure of Au(111), which are shown in (d).

Figure 3

(a) $dI/dV$ of the TJ shown in Fig. 2 inset (EuS thickness: 2.4 nm). Because $dI/dV$ depends on the DOS of the two layers (Au and Al) in the vicinity of the insulating barrier EuS, the $dI/dV$ gap reflects the induced SC in bulk Au(111). The doublet on the coherence peak is a signature of the SS gap. (b) A control sample showing that the doublet is not due to the MEF of EuS. The control TJs have one Al tunneling electrode coupled to EuS with decorated Au to tune the interface SOC (noted as $\mathrm{Al}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Au}/\mathrm{Al}/\mathrm{EuS}$). (c) $dI/dV$ of the same device as in (a) measured at $T=1.0\mathrm{K}$ (black curve). The multiple conductance peaks are well modeled (blue curve) by the S-I-S tunneling of the two band SC in Au (111) [Fig. 3]. (d) Schematics of the S-I-S tunneling model with Au having two induced SC gaps.

Figure 4

(a) Tunneling conductance of the planar TJ (schematic on the right) in the presence of a parallel magnetic field. Half of the tunneling peaks demonstrate contrasting responses to the magnetic field. (b) The enlarged data of each tunneling peak. For tunneling involving the bulk quasiparticles of Au, the conductance peak (red curves) is reduced by the applied magnetic field. For tunneling involving the SS quasiparticles of Au, the conductance peak (red curves) is enhanced by the applied magnetic field, which indicates the 2D nature of the quasiparticles in SS.