Probing variations of the Rashba spin-orbit coupling at the nanometer scale

The Rashba effect as an electrically tunable spin-orbit interaction is the base for a multitude of possible applications such as spin filters, spin transistors, and quantum computing using Majorana states in nanowires. Moreover, this interaction can determine the spin dephasing and antilocalization phenomena in two dimensions. 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 and the otherwise forbidden electron backscattering in topologically protected channels. 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 eV{\AA}). 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.Comment: A modified version will be published in Nature Physic

An ultrahigh-vacuum cryostat for simultaneous scanning tunneling microscopy and magneto-transport measurements down to 400mK

We present the design and calibration measurements of a scanning tunneling microscope setup in a 3He 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 one-story 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 microstructured 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 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 mueV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 mueV, implying that the system operates close to the physical noise limit.Comment: 12 pages, 11 figure

The STRIP instrument of the Large Scale Polarization Explorer: microwave eyes to map the Galactic polarized foregrounds

In this paper we discuss the latest developments of the STRIP instrument of the "Large Scale Polarization Explorer" (LSPE) experiment. LSPE is a novel project that combines ground-based (STRIP) and balloon-borne (SWIPE) polarization measurements of the microwave sky on large angular scales to attempt a detection of the "B-modes" of the Cosmic Microwave Background polarization. STRIP will observe approximately 25% of the Northern sky from the "Observatorio del Teide" in Tenerife, using an array of forty-nine coherent polarimeters at 43 GHz, coupled to a 1.5 m fully rotating crossed-Dragone telescope. A second frequency channel with six-elements at 95 GHz will be exploited as an atmospheric monitor. At present, most of the hardware of the STRIP instrument has been developed and tested at sub-system level. System-level characterization, starting in July 2018, will lead STRIP to be shipped and installed at the observation site within the end of the year. The on-site verification and calibration of the whole instrument will prepare STRIP for a 2-years campaign for the observation of the CMB polarization.Comment: 17 pages, 15 figures, proceedings of the SPIE Astronomical Telescopes + Instrumentation conference "Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX", on June 15th, 2018, Austin (TX

A compact all-UHV 3He-system for combined scanning probe and magnetotransport experiments down to 400 mK

This work presents the setup of a novel 400 mK-14 T-UHV scanning tunneling and atomic force microscopy system using complete UHV-technology also for the cryostat. This allows a more compact system exhibiting a height of only 2.9 m. Key performance indicators of the system are a holding time of 10 days, more than any other published 3He cryostat for SPM, a noise level of ~1pm in vertical displacement measured in open-loop configuration, an energy resolution compatible with an electron temperature of 450 mK, a voltage noise of below 60 μVrms and the ability to map the atomic structure on InSb(110) down to 500 fA. These excellent performance indicators are achieved mainly by introducing a thermal switch between the 1K-pot and the sorption pumped 3He area leading to an effective mechanical decoupling and by careful low-pass filtering all cables entering the cryostat. The whole system including standard UHV preparation and analysis tools has been designed in the thesis of Dr. rer. nat. Stefan Becker in cooperation with Cryovac Tieftemperatur Inc. which provided the cryostat design to fit into the foreseen acoustic shielding room. The room-temperature UHV-components have partly already been assembled. Some modifications to the room temperature system were performed during this work to ameliorate the handling as well as the functionality. The central part of this work is the improvement of the cryostat’s prototype as well as the finalization of the 400 mK-insert. Improvements of the cryostat include modifications on the tanks such as added shields to reduce thermal radiation and filling the liquid nitrogen tank with copper wool to reduce noise by bubbling. A major modification was the change to thermal switches for precooling the 3He, which improved the handling of the insert as well as the thermal coupling significantly. After completion of the system, tests of its scanning probe and magnetotransport abilities have been performed down to low temperatures leading to the performance indicators mentioned above

The STRIP instrument of the Large Scale Polarization Explorer:Microwave eyes to map the Galactic polarized foregrounds

\u3cp\u3eIn this paper we discuss the latest developments of the STRIP instrument of the Large Scale Polarization Explorer (LSPE) experiment. LSPE is a novel project that combines ground-based (STRIP) and balloon-borne (SWIPE) polarization measurements of the microwave sky on large angular scales to attempt a detection of the B-modes of the Cosmic Microwave Background polarization. STRIP will observe approximately 25% of the Northern sky from the Observatorio del Teide in Tenerife, using an array of forty-nine coherent polarimeters at 43 GHz, coupled to a 1.5 m fully rotating crossed-Dragone telescope. A second frequency channel with six-elements at 95 GHz will be exploited as an atmospheric monitor. At present, most of the hardware of the STRIP instrument has been developed and tested at sub-system level. System-level characterization, starting in July 2018, will lead STRIP to be shipped and installed at the observation site within the end of the year. The on-site verification and calibration of the whole instrument will prepare STRIP for a 2-years campaign for the observation of the CMB polarization.\u3c/p\u3