Visualizing superconductivity in FeSe nanoflakes on ${\mathrm{SrTiO}}_3$ by scanning tunneling microscopy

Scanning tunneling microscopy and spectroscopy have been employed to investigate the superconductivity in single unit-cell FeSe nanoflakes on ${\mathrm{SrTiO}}_3$ substrates. We find that the differential conductance $dI/dV$ spectra are spatially nonuniform and fluctuate within the flakes as their area is reduced to below $\sim 150{\mathrm{nm}}^2$. An enhancement in the superconductivity-related gap size as large as $25\%$ is observed. The superconductivity behavior disappears when the FeSe nanoflakes reduce to $\sim 40{\mathrm{nm}}^2$. Compared to a previous report [Wang et al., Chin. Phys. Lett. 29, 037402 (2012)], the gap is asymmetric relative to the Fermi energy $E_{\mathrm{F}}$. All the features, particularly the fluctuating gap and quenched superconductivity, could be accounted for by quantum size effects. Our study helps to understand nanoscale superconductivity in low-dimensional systems.

Figure 1

(a) STM topographic image $(V_{\mathrm{s}}=4$ V, $I=20$ pA, $100\times 100$ nm) of single UC FeSe films grown on a ${\mathrm{SrTiO}}_3$ substrate. The dark trenches are primarily caused by missing atoms. (b)–(e) STM topographies $(I=30$ pA, $10\times 10$ nm) in the vicinity of an isolated rectangular FeSe nanoflake, and (f) cross-section height profiles along the straight lines in (b)–(e) at different sample voltages $V_{\mathrm{s}}$. The bright spots in (b)–(e) correspond to the topmost Se atoms.

Figure 2

Comparison of $dI/dV$ spectra acquired on single UC FeSe nanoflakes with different area $A$. (a) Typical differential conductance $dI/dV$ spectrum in a large FeSe nanoflake $(A>900$ ${\mathrm{nm}}^2)$. Two vertical gray lines indicate the energy positions of superconducting coherence peaks, with their separation defined as $2\Delta$. Tunneling gap is stabilized at $V_s$ = 50 mV and $I=95$ pA. (b) Normalized $dI/dV$ spectra taken at equal separations $(\sim 0.8$ nm) along a line of FeSe nanoflakes with $A=62$ ${\mathrm{nm}}^2$. Colored curves indicate the asymmetry of $dI/dV$ spectra at various positions, signaling great spatial inhomogeneity. Here, we calculate $2\Delta$ from the two strongest coherence peaks below and above $E_{\mathrm{F}}$ (not marked), respectively, in line with (a). The fine structures beyond the gaps may originate from QSE-resulted discrete energy levels.

Figure 3

(a) Plot of gap magnitude $\Delta$, and (b) gap asymmetry $\delta$ = ${\Delta }_{+}-{\Delta }_{-}$ vs FeSe nanoflake area $A$, with ${\Delta }_{+}$ and ${\Delta }_{-}$ indicating the deviations of the two coherence peaks above and below $E_{\mathrm{F}}$ from $E_{\mathrm{F}}$, respectively. Gray dotted lines are guides to the eye.

Figure 4

(a), (b) $dI/dV$ spectra on extremely small FeSe nanoflakes $(A<40$ ${\mathrm{nm}}^2)$, which show distinct differences from those in larger FeSe nanoflakes $(A>40$ ${\mathrm{nm}}^2)$. (c) Plot of HOQWS-LUQWS separation ${\Delta }_{\mathrm{QWS}}$ as a function of FeSe nanoflake area $A$.