Construction of a 390 mK-14 T scanning tunnelling microscope used for investigations of the random Rashba effect at the nanometre scale

  • Aufbau eines 390mK-14T Rastertunnelmikroskops verwendet für Untersuchungen des variablen Rashba-Effekts auf der Nanometerskala

Bindel, Jan Raphael; Morgenstern, Markus; Schäpers, Thomas (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

This thesis describes the design and performance of a newly developed low-temperature ultra-high-vacuum scanning tunneling microscope (LT-UHVSTM) system operating at T = 390 mK and magnetic fields up to 14 T. The system’s performance is evaluated and compared with that of existing systems. Its outstanding mechanical stability δz = 0.7 pm at a bandwidth of 700 Hz is presented using a method that allows for comparison with other systems. Atomic resolution for stabilisation currents down to Istab = 0.5 pA and for magnetic fields up to B = 14 T is achieved. An electron temperature of Tel = 430 mK at one electrode and Tel = 520 mK at the other electrode of the tunnelling junction is ascertained through evaluation of superconducting tunnelling gaps and Shubnikov-de Haas oscillation, using the system magneto-transport capabilities. The Josephson peak in a superconductor-superconductor junction is used to ascertain a bias voltage noise of the tunneling junction of ∆Vbias = 16 µV at a bandwidth of ~700 Hz. The long holding time thold = 10.5 days at 400 mK, the virtually infinite holding time at T = 9 K, the spacious optical access to tip and sample at 25 K inside the UHV-cryostat, and the attached UHV-analysis and preparation chambers with a load lock, constitute a versatile system. In the second part of this thesis a newly developed technique is used to map the variations of the Rashba spin-orbit coupling in a Cs/p-InSb sample down to the nanometre scale, limited by the magnetic length lB ≈ 10 nm at B = 6 T. This reveals a Rashba parameter of αR(R) = 1.2 eVÅ, with root mean square variations of δαR = 0.15 eVÅ. We find a correlation between the Rashba parameter variations and the local potential. Using an analytical and a simple numeric model, this correlation and the Rashba fluctuations are attributed to a locally changing electric field, resulting from the randomly distributed dopants. The spin dephasing length, which is determined by the Rashba fluctuation, is estimated to be lSpin = 250 nm. These experimental findings suggest that the performance of the most advanced spin transistors is likely dominated by variations of the Rashba parameter. Furthermore, the nodal structures of the zeroth and the first Landau level (LL0 & LL1) wave functions are spatially resolved within this sample system. The drift states, which probe the potential disorder, develop with increasing energy for LL0 from a Gaussian-like distribution into a ring structure growing in diameter and for LL1 from a ring into a growing double-ring structure.