Very low temperature AFM-STM combined microscopy

Staff: Clemens Winkelmann, Hervé Courtois

Beyond the imaging of a metallic surface, the scanning tunneling microscopy (STM) enables the spectroscopy of the local density of electronic states at the nanometer scale. We developed a scanning tunneling microscope operating at very low temperature (60 mK) in a dilution refrigerator, with the objective to perform local spectroscopy studies of phase-coherent quantum structures with the best energy resolution. The comparison of the test spectroscopy performed on a plain superconducting Nb film with a thermally smeared BCS spectra provides an estimated energy resolution down to 30 µeV (or 170 mK).

Test spectroscopy on a Nb layer.

Spectroscopy of a Nb layer featuring an excellent energy resolution.

We recently built a combined AFM-STM microscope operating at very low temperature. This new microscope is most useful to perform local spectroscopy measurements on partially insulating samples, like a superconducting wire patterned on an isulating substrate.

Very low temperature AFM-STM microscope

Very low temperature AFM-STM microscope.

The figure below displays an AFM image of a submicron Nb wire realized by e-beam lithography on a sapphire substrate. The tunneling spectra have been acquired while scanning the surface in the STM mode, along the arrow drawn on the AFM image.

AFM topography and tunneling spectroscopy on a Nb wire.

In collaboration with E. Bustarret et al at Institut Néel, we have studied the local superconductivity in highly doped semiconductors : silicon and polycrystalline diamond. In silicon, we have performed the very first tunnel spectroscopy of this new superconductor. We could demonstrate the BCS nature of the superconducting coupling in this material. In polycrystalline, diamond, we have the strong correlation between the granular microstructure and he local superconductivity strength.

Publications:

[1] "Combined Scanning Force Microscopy and Scanning Tunneling Spectroscopy of an electronic nano-circuit at very low temperature", J. Senzier, P. S. Luo and H. Courtois, Appl. Phys. Lett. 90, 043114 (2007).
[2] "Spatially correlated microstructure and superconductivity in polycrystalline boron-doped diamond", F. Dahlem, P. Achatz, O. A. Williams, D. Araujo, E. Bustarret, and H. Courtois, Physical Review B 82, 033306 (2010).
[3] "Sub-Kelvin tunneling spectroscopy showing Bardeen-Cooper-Schrieffer superconductivity in heavily boron-doped silicon epilayers", F. Dahlem, T. Kociniewski, C. Marcenat, A. Grockowiak, L. Pascal, P. Achatz, J. Boulmer, D. Debarre, T. Klein, E. Bustarret, H. Courtois, Physical Review B Rapid Communication 82, 140505(R) (2010).
[4] "A subKelvin scanning probe microscope for the electronic spectroscopy of an individual nano-device", T. Quaglio, F. Dahlem, S. Martin, A. Gérardin, C. Winkelmann and H. Courtois, Review of Scientific Instruments 83, 123702 (2012).



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