The Rashba effect as an electrically tunable spin-orbit interaction 1 is the base for a multitude of possible applications 2-4 such as spin filters 3 , spin transistors 5,6 , and quantum computing using Majorana states in nanowires 7,8 . Moreover, this interaction can determine the spin dephasing 9 and antilocalization phenomena in two dimensions. 10 However, the real space pattern of the Rashba parameter has never been probed, albeit it critically influences, e.g., the more robust spin transistors using the spin helix state 6,11,12 and the otherwise forbidden electron backscattering in topologically protected channels. 13,14 Here, we map this pattern down to nanometer length scales by measuring the spin splitting of the lowest Landau level using scanning tunnelling spectroscopy. We reveal strong fluctuations correlated with the local electrostatic potential for an InSb inversion layer with a large Rashba coefficient (~1 eVÅ).The novel type of Rashba field mapping enables a more comprehensive understanding of the critical fluctuations, which might be decisive towards robust semiconductor-based spintronic devices.

We describe a fully ultra-high vacuum compatible scanning tunneling microscope (STM) optimized for radiofrequency signals. It includes in-situ exchangeable tips adapted to high frequency cabling and a standard sample holder, which offers access to the whole range of samples typically investigated by STM. We demonstrate a time resolution of 120 ps by using the nonlinear I(V )-characteristic of the surface of highly oriented pyrolithic graphite. We provide atomically resolved images in pulse mode related to a spatially varying nonlinearity of the local density of states of the sample, thus, demonstrating the possible spatial resolution of the instrument in pulse mode. Analysis of the noise reveals that changes in the tunneling junction of 50 pA are dynamically detectable at 120 ps time resolution.

We present the design and calibration measurements of a scanning tunneling microscope setup in a 3 He ultrahigh-vacuum cryostat operating at 400 mK with a hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for assembly, the cryostat fits in a onestory lab building. The microscope features optical access, an xy table, in-situ tip and sample exchange, and enough contacts to facilitate atomic force microscopy in tuning fork operation and simultaneous magneto-transport measurements on the sample. Hence, it enables scanning tunneling spectroscopy on microstrucured samples which are tuned into preselected transport regimes. A superconducting magnet provides a perpendicular field of up to 14 T. The vertical noise of the scanning tunneling microscope amounts to 1 pm rms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 µeV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 µeV, implying that the system operates close to the physical noise limit.

The inversion layer of p-InSb(110) obtained by Cs adsorption of 1.8% of a monolayer is used to probe the Landau level wave functions within smooth potential valleys by scanning tunneling spectroscopy at 14 T. The nodal structure becomes apparent as a double peak structure of each spin polarized first Landau level, while the zeroth Landau level exhibits a single peak per spin level only. The real space data show single rings of the valley-confined drift states for the zeroth Landau level and double rings for the first Landau level. The result is reproduced by a recursive Green function algorithm using the potential landscape obtained experimentally. We show that the result is generic by comparing the local density of states from the Green function algorithm with results from a well-controlled analytic model based on the guiding center approach.

We present a mask aligner driven by three piezomotors which guides and aligns a SiN shadow mask under capacitive control towards a sample surface. The three capacitors for read out are located at the backside of the thin mask such that the mask can be placed at a μm distance from the sample surface, while keeping it parallel to the surface, without touching the sample by the mask a priori. Samples and masks can be exchanged in-situ and the mask can additionally be displaced parallel to the surface. We demonstrate an edge sharpness of the deposited structures below 100 nm, which is likely limited by the diffusion of the deposited Au on Si(111).