STM Performance

In this section the topographic and spectroscopic capabilities of the new STM system are presented. A reference sample to test the topographic STM stability and spectroscopy is the Au(111) surface which has been imaged in Fig. 3.13a. The monatomic step appears sharp and the 22 x √

3 “herring-bone” reconstruction of the surface with alternating fcc and hcp stacking regions (see also inset) is well resolved. From that image one can derive that the topographic noise level is below 3 pm. The dI/dV-spectrum plotted in Fig. 3.13b yields the onset of the Shockley type surface state at -0.51 eV in good agreement with low-temperature STS [27] and photoemission [28]

3.6. STM PERFORMANCE 29

(a) (b)

Figure 3.13: (a) 80×80 nm²: STM image of the Au(111) with the 22×√ 3

“herringbone” reconstruction. (b) dI/dV-spectrum showing the onset of the Shockley type surface state at -0.51 eV.

results.

The Au(100) surface is an other low index face of a gold single crystal.

It exhibits a quasi-hexagonal surface reconstruction that will be discussed in more detail in Ch. 4 in the context of its electronic properties. Besides the reconstruction lines also steps and boundaries separating two equivalent do-mains of the reconstruction are visible in Fig. 3.14. On can see that impurity atoms have nucleated at step edges and domain walls. The Au(100) surface was used to demonstrate the high spatial resolution of the new STM system also in spectroscopical measurements. For that purpose a minute amount of Co atoms has been evaporated on the surface with a commercial electron beam evaporator at a substrate temperature of 200 K. Immediately after the preparation the sample was transferred back to the STM. By this procedure single Co atoms remain isolated on the surface as shown in Fig. 3.15a. Their preferential adsorption site is on top of the reconstruction lines and at the crossing points of two reconstruction lines. The local electronic structure of the adatom is revealed by Scanning Tunneling Spectroscopy (STS).

In Fig. 3.15b adI/dV spectrum acquired on top of a Co adatom is plotted in direct comparison with an “off-spectrum‘” on the bare Au(100) surface.

While the “off-spectrum” is flat, a strong feature around the Fermi energy is apparent in the spectrum on the Co adatom. It can be fitted by a Fano line

Figure 3.14: 90×90 nm²: STM image of the bare Au(100) sur-face. The quasi-hexagonal recon-struction is apparent. Impurity atoms have nucleated at step edges and domain walls.

shape. This line shape is characteristic for the Kondo effect which is due to a resonant coupling of the adsorbate spin with the substrate electrons [29].

The fit result is a Kondo temperature TK = 286K. It is significantly larger than the various T_Ks measured on different noble metal surfaces. Further measurements are necessary to reproduce the results and to clarify if this large value might be due to the influence of the reconstruction. This example in Fig. 3.15b proves that the STM is mechanically stable enough to record dI/dV point spectra on top of single atoms.

BCS - Gap

The results on different gold surfaces have been obtained at temperatures around 9 K. During these experiments the radiation shield was not in place (see Ch. 3.3.1). An appropriate way to demonstrate both, very low tem-peratures and high spectroscopical resolution is to resolve the BCS-gap of a superconductor. According to the theory of Bardeen, Cooper and Schrieffer (BCS-theory) [30], an energy gap around E_F opens in the density of states when a material becomes superconducting below the transition temperature Tc:

%(E)∝ |E|

√E²−∆² |E|>∆, (3.2)

%(E) = 0 |E|<∆, (3.3)

3.6. STM PERFORMANCE 31

(a) (b)

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 3

Figure 3.15: (a) 20×20 nm²: Single Co atoms on the Au(100) surface. The preferential adsorption site is on top of the reconstruction lines and their crossing points. (b): Spectrum on top of a single Co atoms and off spectrum.

where theT = 0Kgap energy ∆ is related to the critical superconducting temperature T_c by ∆ = 1.764k_BT_c. Therefore, a Pb crystal with a critical temperature of T_c = 7.2K has a gap energy ∆ = 1.3 meV. Because in STS the gap is measured symmetrically around E_F the resulting width is 2∆. At finite temperatures the gap gets smeared out and a nonzero signal remains also in the gap region.

From the experimental point of view, also the amplitude of the Lock-In signal, radio frequency (RF) noise and drift effects during the measurements are factors that contribute to a broadening of the observed gap feature.

A Pb(111) single crystal was used to demonstrate the capabilities of the STM regarding low temperatures and high resolution spectroscopy. It was prepared by several cycles of sputtering and annealing at 450 K. Two tunnel-ing spectra recorded on the Pb(111) surface at 4.3 K and 2.6 K are plotted in Fig. 3.16. For both spectra a gap feature symmetrical around E_F is evi-dent. At 2.6 K this feature is more pronounced and the gap is deeper. The different vertical position of the maxima in the dI/dV signal is due to the slightly different tunneling contact impedances of 8.7 MΩ and 10 MΩ of the red and the black curve, respectively. The width of the gaps is 6.8 meV for the spectrum at 4.3 K and 5.6 meV for the measurement at 2.6 K.

The observed gap feature is explained by assuming a tip which is covered

-15 -10 -5 0 5 10 15

Figure 3.16: BCS-gap of Pb mea-sured with a superconducting Pb tip at 4.3 K and 2.6 K.

with Pb and thus superconducting. In this case one expects the spectra of a superconductor-insulator-superconductor (SIS) junction. Such spectra show maxima at ∆_s(T_s)−∆_t(T_t) and ∆_s(T_s) + ∆_t(T_t), where ∆_s(T_s) and ∆_t(T_t) are the temperature dependent energy gaps of sample and tip, respectively.

The signal of the maximum inside the BCS-gap is still covered by the residual noise of the experiment.

It is reasonable to expect a superconducting Pb tip, because the tip is generally dipped into the sample several times to obtain optimal imaging conditions. Furthermore it is well known that Pb tends to jump to the tip and form a neck contacting tip and sample [31]. When approaching to the Au(100) surface the first time after the experiments on the Pb(111) surface the tip lost a large amount of material polluting the surface in the range of a micrometer, which is an other indication for a Pb coated tip. The measured superconducting gap spectrum proves that the spectral resolution of the STM system is below 1 meV.