Probing absolute spin polarization at the nanoscale.

Probing absolute values of spin polarization at the nanoscale offers insight into the fundamental mechanisms of spin-dependent transport. Employing the Zeeman splitting in superconducting tips (Meservey-Tedrow-Fulde effect), we introduce a novel spin-polarized scanning tunneling microscopy that combines the probing capability of the absolute values of spin polarization with precise control at the atomic scale. We utilize our novel approach to measure the locally resolved spin polarization of magnetic Co nanoislands on Cu(111). We find that the spin polarization is enhanced by 65% when increasing the width of the tunnel barrier by only 2.3 Å due to the different decay of the electron orbitals into vacuum.

A 10 mK scanning tunneling microscope operating in ultra high vacuum and high magnetic fields.

We present design and performance of a scanning tunneling microscope (STM) that operates at temperatures down to 10 mK providing ultimate energy resolution on the atomic scale. The STM is attached to a dilution refrigerator with direct access to an ultra high vacuum chamber allowing in situ sample preparation. High magnetic fields of up to 14 T perpendicular and up to 0.5 T parallel to the sample surface can be applied. Temperature sensors mounted directly at the tip and sample position verified the base temperature within a small error margin. Using a superconducting Al tip and a metallic Cu(111) sample, we determined an effective temperature of 38 ± 1 mK from the thermal broadening observed in the tunneling spectra. This results in an upper limit for the energy resolution of ΔE = 3.5 kBT = 11.4 ± 0.3 µeV. The stability between tip and sample is 4 pm at a temperature of 15 mK as demonstrated by topography measurements on a Cu(111) surface.

Miniature active damping stage for scanning probe applications in ultra high vacuum.

Scanning probe microscope (SPM) experiments demand a low vibration level to minimize the external influence on the measured signal. We present a miniature six-degree of freedom active damping stage based on a Gough-Stewart platform (hexapod) which is positioned in ultra high vacuum as close to the SPM as possible. In this way, vibrations originating from the experimental setup can be effectively reduced providing a quiet environment for the SPM. In addition, the hexapod provides a rigid reference point, which facilitates wiring as well as sample transfer. We outline the main working principle and show that for scanning tunneling microscopy (STM) measurements of a Si(111) 7 × 7 surface, the hexapod significantly improves the stability and quality of the topographic images.

Design criteria for scanning tunneling microscopes to reduce the response to external mechanical disturbances.

We present a simple one-dimensional model to find design criteria for a scanning tunneling microscope (STM) minimizing the response of the tip-sample distance to external mechanical disturbances. The underlying concept-achieving a response that is in phase and same amplitude-goes beyond the conventional approach to construct the STM as stiff as possible. It introduces optimization conditions relating the resonance frequencies of the different components to the STM assembly, which can be implemented accordingly during the STM design process. In this way an improvement in the response to external disturbances of several orders of magnitude can be achieved. Calculations for three typical STM designs are presented along with the corresponding optimization criteria. For one of the designs an improvement in performance has been experimentally verified. The results can also be extended to other scanning probe techniques.