Element-selective structural visualization for the oxygen-induced surface of $\mathrm{Nb}\left(110\right)$

The highest superconducting transition temperature in Nb among elemental materials facilitates applications of superconducting junctions. Nonetheless, its surface forms self-organized oxide structures, which hinders the preparation of well-defined interfaces. We investigated the atomic structure of oxygen-induced $\mathrm{Nb}\left(110\right)$ surfaces, aiming to define the substrates for such interfaces. The atomic force microscopy and scanning tunneling microscopy measurements visualized two rows of apparently low-lying Nb and three rows of O between Nb chains. An optimized model structure is proposed based on density-functional theory calculations. Analysis of the Bader charge and the density of states clarified how the atoms appear in the microscopies.

Figure 1

Constant-current STM topographic images taken at temperature $T=5\mathrm{K}$. (a) A large field-of-view image. The sample bias voltage $V_{\mathrm{s}}=-10\mathrm{mV}$, current setpoint $I_{\mathrm{s}}=-10\mathrm{pA}$, and oscillation amplitude $A=90\mathrm{pm}$. (b) A magnified image with resolution of the ${\mathrm{Nb}}^{*}$ chain atoms. $V_{\mathrm{s}}=-10\mathrm{mV}, I_{\mathrm{s}}=-100\mathrm{pA}, A=0\mathrm{pm}$.

Figure 2

Constant-frequency-shift AFM topographic images taken at $T\sim 300\mathrm{K}$. (a) ${\mathrm{Nb}}^{*}$ chain structure in Nb mode imaging. $V_{\mathrm{s}}=300\mathrm{mV}$, the frequency-shift setpoint $\mathrm{\Delta }{f}_{\mathrm{s}}=-1.7\mathrm{Hz}, A=20\mathrm{nm}$. (b) A magnified view with atomic resolution to apparently low-lying Nb atoms. $V_{\mathrm{s}}=100\mathrm{mV}, \mathrm{\Delta }{f}_{\mathrm{s}}=-8.0\mathrm{Hz}, A=20\mathrm{nm}$. (c) A line profile of (b). (d) A quasiperiodic structure in O-mode imaging. $V_{\mathrm{s}}=900\mathrm{mV}, \mathrm{\Delta }{f}_{\mathrm{s}}=-217\mathrm{Hz}, A=5\mathrm{nm}$. (e) A magnified view, where three O rows are resolved. $V_{\mathrm{s}}=900\mathrm{mV}, \mathrm{\Delta }{f}_{\mathrm{s}}=-227\mathrm{Hz}, A=5\mathrm{nm}$. (f) A line profile of (e). The bias voltages are adjusted to compensate for the contact potential difference between the tips and the samples.

Figure 3

Correspondence of AFM and STM visualized atomic positions. (a) A frequency-shift image during a constant-height scan. The image is low-pass Fourier filtered. (b) Simultaneously obtained current image with Fourier low-pass filter. (c) A raw frequency-shift image at a constant-height scan. (d) Simultaneously obtained current image with low-pass Fourier filter. Yellow lines highlight the positions of ${\mathrm{Nb}}^{*}$ chains in current images, and the same pixels are also marked in the frequency-shift images. For (a)–(d), $V_{\mathrm{s}}\sim 1\mu \mathrm{V}, A=90\mathrm{pm}, T=5\mathrm{K}$. (e) Constant-frequency-shift AFM topographic image. $V_{\mathrm{s}}=-200\mathrm{mV}, \mathrm{\Delta }{f}_{\mathrm{s}}=-5.1\mathrm{Hz}, A=20\mathrm{nm}, T\sim 300\mathrm{K}$. (f) Constant-current topographic image in the same field of view. $V_{\mathrm{s}}=-2000\mathrm{mV}, I_{\mathrm{s}}=-30\mathrm{pA}, A=20\mathrm{nm}, T\sim 300\mathrm{K}$. Impurities are marked with yellow arrows for references to the position.

Figure 4

A DFT-optimized model structure. (a) Top view. The second Nb layer is depicted only on the left-hand-side column. (b) Side view. Blue, ${\mathrm{Nb}}^{*}$ chain atoms; green, apparently low-positioned Nb atoms; gray, bulk Nb atoms; red/orange, O atoms. The parallelogram in gray depicts the cell for the calculation.

Figure 5

Comparison of the experimental observations and the model. (a) STM topographic image [Fig. 1]. (b) Simulated STM image for $V_{\mathrm{s}}=-10\mathrm{mV}$. (c) AFM topographic image in Nb mode [Fig. 2]. (d) AFM topographic image in O mode [Fig. 2]. (e), (f) AFM line profiles [Figs. 2 and 2]. Images in (c) and (d) are processed with Fourier low-pass filters and affine corrections. The $z$ direction of the AFM line profiles are scaled by factors of (e) 5 and (f) 15.

Figure 6

DOS calculated for the present model. (a) Effect of the surface oxidization. (b) Contribution from the surface Nb and O atoms.

Figure 7

(a) A measured $\mathrm{d}I/\mathrm{d}V$ curve. $V_{\mathrm{s}}=-1\mathrm{V}, I_{\mathrm{s}}=-100\mathrm{pA}$, bias modulation amplitude $V_{\mathrm{mod}}=25\mathrm{mV}$, bias modulation frequency $f_{\mathrm{bias}}=617.3\mathrm{Hz}, A=0\mathrm{pm}$, and $T=5\mathrm{K}$. (b) A simulated conductance curve at $400\mathrm{pm}$ above ${\mathrm{Nb}}^{*}$.

Figure 8

DFT-optimized structural models from different initial states. The total energy for the model in Fig. 4 is $-458.19\mathrm{eV}$. The total energies are (a) $-458.07\mathrm{eV}$, (b) $-458.16\mathrm{eV}$, and (c) $-458.11\mathrm{eV}$.

Figure 9

A $\mathrm{d}I/\mathrm{d}V$ spectrum showing the superconducting gap observed on the oxide surface. $V_{\mathrm{s}}=-10\mathrm{mV}, I_{\mathrm{s}}=-1\mathrm{nA}, V_{\mathrm{mod}}=0.1\mathrm{mV}, f_{\mathrm{bias}}=617.3\mathrm{Hz}, A=0\mathrm{pm}, T=5\mathrm{K}$, and external magnetic field ${\mu }_{0}H=50\mathrm{mT}$.