SFB 616

 

Pictures SFB616
  
  

 Project C6:
Electronic energy dissipation in thin metal films


 The central goal of our project is to investigate the local mobility of conduction electrons in geometrically confined metallic structures such as thin films and islands. By using eddy current microscopy two major advantages are combined: a rather high spatial resolution can be achieved and contacting is not necessary. The experiments will be performed on epititaxial metallic thin films or islands prepared on insulating substrates. These metallic structures exhibit different types and densities of defects so that the scattering processes associated with these defects can be identified and quantitatively characterized. The influence of lateral confinement, size and height will also be studied.

 

 

 Eddy current microscopy [1] is a spin-off of the conventional dynamic force microscopy. If a magnetic tip oscillates above the sample eddy currents are induced within conducting materials (Joule energy dissipation). In a sample with finite resistivity a damping of the oscillation occurs. Any local variation (resolution comparable with MFM) in the electronic conductivity can thus be detected in the damping signal.

 

Fig. 1.: Phase image of the gold pattern obtained at two different tip-sample distances of 40~nm (upper panel) and 100~nm (lower panel), respectively. The mean phase shift is 1.0 and 0.3 respectively. Image size: 1.5 x 0.75 µm2.

 In order to determine the origin of the signal in in eddy current microscopy we used especially prepared gold nano patterns embedded in a non-conducting polycarbonate matrix and measured the distance dependence of the phase signal. Our data clearly shows that the interacting forces are long ranged and herefore are likely due to the electromagnetic interaction between the magnetic tip and the conducting parts of the surface. (Fig.1).

 

Fig. 2.: Topography image of chains of nanodots on CaF2(111) created by irridation with 93 MeV Xe23+ ions. The inset shows a linescan along the arrow, which also depicts the direction of the incoming beam. The height of the nanodots is in the range from 6 to 15 nm. Frame size: 2.5x2.5 µm2. Dissipation image of the CaF2 sample acquierd under UHV conditions using a magnetic tip. Frame size 215x215 nm2. Dissipation is about 0.5 eV/cycle when scanning over the non-irridiated areas,
and 2 eV/cycle above the hillocks.

 In another experiment CaF2(111) single crystal  surfaces have been irradiated with fast  heavy ions under oblique angles resulting in chains of nanosized hillocks. In order to characterize these nanodots with respect to their conductivity we have applied eddy current microscopy. Measurements in ultra high vacuum as well as under ambient conditions reveal a clearly enhanced electromagnetic interaction between the magnetic tip and the nanodots. The dissipated energy per cycle is comparable to the value found for metals, indicating that the interaction of the ion with the target material leads to the creation of metallic Ca nanodots on the surface.

 

Typical flooded CaF2-images. a) non-irradiated surface b) irradiated under perpendicular incidence c) irradiated under glancing angle d) irradiated surface imaged under UHV conditions. White dots dot are ion induced protrusions.

 Note, that the patches do not occur in the Dissipation image, because they are adsorbates. We investigated this in detail on freshly cleaved CaF2(111) crystals under ambient coniditions. Within several minutes after cleavage subtle protrusions occur which grow with time. Ion irradiated are show an enhanced susceptibility for adsorbtion.

 

 One of the system that we are going to study in UHV is Ag/Si(111). Silver does not form a silicide and epitaxial islands can be prepared with good control over the type and density of defects. Both have been studied by STM, LEED and LEEM [e.g. 2,3]. Conventional resistivity measurements show a strong dependence on several parameters such as film thickness, roughness or adsorption of CO. In addition to experiments at variable temperatures, all these paramteres can be used to characterize dissipation processes of conduction electrons in metallic systems.

 

 This project is in close collaborations with projects C2, C4, C7 and A6.


 
  Poster:

Internationaler Workshop 2008


Internationaler
Workshop 2008

C6 Roll et al.
PDF (0.6 MB)

SFB616 Remagen 2007


SFB616 Remagen 2007
C6 Roll et al.
 PDF (1.1 MB)

 

 
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