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Low-temperature scanning tunneling spectroscopy is a key method to probe electronic and magnetic properties down to the atomic scale, but suffers from extreme vibrational sensitivity. This makes it challenging to employ closed-cycle cooling with its required pulse-type vibrational excitations, albeit this is mandatory to avoid helium losses for counteracting the continuously raising helium prices. Here, we describe a compact ultra-high vacuum scanning tunneling microscope (STM) system with an integrated primary pulse tube cooler (PTC) for closed-cycle operation. It achieves temperatures down to 1.5 K via a secondary Joule–Thomson stage and a z-noise down to 300 fmRMS in the STM junction for the frequency range of 0.1 Hz–5 kHz (feedback loop off). This is better than many STMs cooled by an external supply of liquid helium. The challenge to combine an effective vibrational decoupling from the PTC with sufficient thermal conduction is tackled by using a multipartite approach including the concept of bellows with minimal stiffness to decouple the PTC vibrationally from the STM and an optimized STM design with minimal vibrational transfer to the STM junction. As important benchmarks, we could reduce the voltage noise in the tunnel junction down to 120 μV and supply radio frequency excitations up to 40 GHz with amplitudes up to 10 mV in the junction via a close-by antenna. The development principally enables other secondary cooling stages such that it opens the perspective for a helium conserving operation of STMs across the whole interesting temperature range. [ABSTRACT FROM AUTHOR]

The contributions of surface science methods to discover and improve 3D topological insulator materials are reviewed herein, illustrated with examples from the authors' own work. In particular, it is demonstrated that spin‐polarized angular‐resolved photoelectron spectroscopy is instrumental to evidence the spin‐helical surface Dirac cone, to tune its Dirac point energy toward the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, procedures are introduced to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

GraphENe quantum dots (GQDs) are considered as promising alternatives to quantum dots in III–V semiconductors, e.g., for the use as spin qubits. However, this potENtial has not beEN realized in experimENts so far, calling for a more fundamENtal investigation of GQDs including a full control of the wave function properties during transport spectroscopy. Here, we review the recENt success in mapping wave functions of GQDs by scanning tunneling microscopy (STM). We show that GQDs on Ir(111) are softly confined rENdering the wave functions more regular. The wave function mapping is disturbed by a sp ‐ type surface state from Ir(111), which can be improved by Au intercalation or adequate Fourier analysis. Edge states are absENt on Ir(111), but reappear after intercalating Au and coupling the σ ‐ bonds of the zig ‐ zag edges to BN. Finally, we show how the goal of probing such wave functions during transport spectroscopy might be achieved. Therefore, we presENt transport spectroscopy of GQDs on SiO2 which are prepared in a STM compatible fashion. 3D view of STM image of graphENe supported by SiO2; Insets (from left to right): STM image of graphENe nano ‐ islands on Ir(111) and scanning tunneling spectroscopy images of the three low ENergy wave functions of a particular nano ‐ island. (© 2015 WILEY ‐ VCH Verlag GmbH &Co. KGaA, Weinheim) Part of Focus Issue on ”Carbononics” (Eds.: Pawel Hawrylak, Francois Peeters, Klaus Ensslin) Wave function mapping of graphENe quantum dots by scanning tunneling microscopy is reviewed covering confined states and edge states. Besides studies, where the graphENe is deposited on a metal, the authors opEN the perspective to map the wave functions of graphENe quantum dots on insulators, such that the results can be directly compared to transport spectroscopy. [ABSTRACT FROM AUTHOR]

Helical locking of spin and momentum and prohibited backscattering are the key properties of topologically protected states. They are expected to enable novel types of information processing by providing pure spin currents, or fault tolerant quantum computation by using the Majorana fermions at interfaces of topological states with superconductors. So far, the required helical conduction channels used to realize Majorana fermions are generated through the application of an axial magnetic field to conventional semiconductor nanowires. Avoiding the magnetic field enhances the possibilities for circuit design significantly. Here, we show that subnanometre-wide electron channels with natural helicity are present at surface step edges of the weak topological insulator Bi14Rh3I9 (ref. ). Scanning tunneling spectroscopy reveals the electron channels to be continuous in both energy and space within a large bandgap of 200 meV, evidencing its non-trivial topology. The absence of these channels in the closely related, but topologically trivial compound Bi13Pt3I7 corroborates the channels' topological nature. The backscatter-free electron channels are a direct consequence of Bi14Rh3I9's structure: a stack of two-dimensional topologically insulating, graphene-like planes separated by trivial insulators. We demonstrate that the surface of Bi14Rh3I9 can be engraved using an atomic force microscope, allowing networks of protected channels to be patterned with nanometre precision. [ABSTRACT FROM AUTHOR]

We describe a fully ultra-high vacuum compatible scanning tunneling microscope (STM) optimized for radio-frequency signals. It includes in-situ exchangeable tips adapted to high frequency cabling and a standard sample holder, which offer access to the whole range of samples typically investigated by STM. We demonstrate a time resolution of 120 ps 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. [ABSTRACT FROM AUTHOR]

A simple pathway to grow thin films of N-doped MgO (MgO:N), which has been found experimentally to be a ferromagnetic d0 insulator, is presented. It relies on the catalytic properties of a Mo(001) substrate using the growth of Mg in a mixed atmosphere of O2 and N2. Scanning tunneling spectroscopy reveals that the films are insulating and exhibit an N-induced state slightly below the conduction band minimum. [ABSTRACT FROM AUTHOR]