Superconducting density of states of nitridized Aluminum thin films

Abstract

Nitride-based superconductors represent a family of superconducting thin film materials displaying higher quality than their corresponding bare superconductor when used in devices for applications such as cosmic radiation sensing. In recent times, Niobium-based and Titanium-based nitrides were used to improve the quality of superconducting devices in quantum technology applications. Recently, nitridized Aluminum (NitrAl) has been found to display higher critical temperatures and enhanced resilience to magnetic fields compared to those of Al, making it a new interesting candidate for superconducting quantum circuit applications. However, the microscopic properties of NitrAl remain highly unexplored. Here we use Scanning Tunneling Microscope (STM) to measure the superconducting density of states of a thin film sample of nitridized-Aluminum (NitrAl), with a room temperature resistivity between pure Al and fully insulating aluminum nitride. We show that the in-gap density of states is zero up to about $\hbarω=250~\mathrm{μeV}$ and that there is a distribution of values of the superconducting gap around $Δ_0=360~\mathrm{μeV}$, close to the BCS expectation $Δ=1.76 k_{\mathrm{B}}T_{\mathrm{c}}$. We also find varying superconducting gap values at the nanometer scale, by approximately 10\%, when probing different regions of the sample. These results suggest a gap which is larger than the one of pure Al, and is spatially more homogeneous than the superconducting gap values often found in thin films. Our work demonstrates that STM is as a powerful tool to screen materials for quantum devices through the measurement of the spatial dependence of the superconducting density of states.

Figures (4)

• Figure 1: Superconducting tunneling conductance on NitrAl thin film. Data were taken with a bias voltage of $V=2.5 ~\mathrm{mV}$, a current of $I=2~\mathrm{nA}$ and at a temperature $T=100~\mathrm{mK}$. Upper left inset shows a schematic representation of the tunneling experiment showing the NitrAl thin film in grey, the Si layers on the wafer in green and orange and the STM tip in black. Lower left inset shows the resistive transition of the measured film (data are sample "G" of Ref. TorrasColoma2024). Right inset shows the tunneling conductance of NitrAl at a larger bias voltage range (taken starting at $V=70~\mathrm{mV}$, $I=3~\mathrm{nA}$ and at a temperature of $T=100~\mathrm{mK}$).
• Figure 2: Tunneling conductance of NitrAl film versus temperature. a Left panel shows the tunneling conductance vs bias voltage as a function of temperature (dots) and calculated conductance curves obtained after convoluting the DOS shown on the right panel with the derivative of the Fermi function at each temperature, as described in the text. Curves are shifted vertically for clarity. b Temperature dependence of the superconducting gap, obtained as described in the text. The solid line is the BCS temperature dependence. Left inset shows the skewed Gaussian distribution $\gamma_i(\Delta_i)$ with a peak at $\Delta_0=0.37~\mathrm{meV}$ used for the $T=100~\mathrm{mK}$ fit. Right inset shows the normalized DOS at zero energy. We find the error bars by performing reasonable fits to tunneling conductance curves with a finite density of states and using the standard deviation as the error.
• Figure 3: Spatial dependence of the superconducting density of states of NitrAl at zero magnetic field and at $100~\mathrm{mK}$. a STM topography on NitrAl thin films. b (c) Superconducting gap width map (Onset voltage for finite conductance map) corresponding to the area shown in a. d Tunneling conductance curves acquired on the colored dots marked in b. We mark the first and last curves with numbers "1" and "8". We mark the position of $0.3~\mathrm{mV}$ with grey dashed vertical lines.
• Figure 4: Spatial dependence of the superconducting density of states of NitrAl at a finite magnetic field and at $100~\mathrm{mK}$. a Tunneling conductance map obtained at zero bias, $200~\mathrm{mT}$, and $100~\mathrm{mK}$. b Tunneling conductance curves as a function of applied magnetic field, taken on the areas of a with blue color. c Tunneling conductance value at zero voltage as a function of applied magnetic field. Values obtained from curves in b. We find that superconductivity vanishes altogether at about $\sim 500~\mathrm{mT}$ in the areas we have studied.