Marcin Kisiel

Probing atomic structure and Majorana wavefunctions in mono-atomic Fe chains on superconducting Pb surface

Motivated by the striking promise of quantum computation, Majorana bound states (MBSs) in solid-state systems have attracted wide attention in recent years. In particular, the wavefunction localization of MBSs is a key feature and crucial for their future implementation as qubits. Here, we investigate the spatial and electronic characteristics of topological superconducting chains of iron atoms on the surface of Pb(110) by combining scanning tunneling microscopy (STM) and atomic force microscopy (AFM). We demonstrate that the Fe chains are mono-atomic, structured in a linear fashion, and exhibit zero-bias conductance peaks at their ends which we interprete as signature for a Majorana bound state. Spatially resolved conductance maps of the atomic chains reveal that the MBSs are well localized at the chain ends (below 25 nm), with two localization lengths as predicted by theory. Our observation lends strong support to use MBSs in Fe chains as qubits for quantum computing devices.

Suppression of electronic friction on Nb films in the superconducting state

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient Γ is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Γ is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory, meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. Γ is found to be proportional to d-1 and V2 in the metallic state, whereas Γ∼d-4 and Γ∼V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

Pure hydrogen low-temperature plasma exposure of HOPG and graphene: Graphane formation?

Summary Single- and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to a pure hydrogen low-temperature plasma (LTP). Characterizations include various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence-band structure, which suggests double-sided hydrogenation. With the scanning tunneling microscopy technique, various atomic-scale charge-density patterns were observed, which may be associated with different C–H conformers. Hydrogen-LTP-exposed graphene on SiO2 has a Raman spectrum in which the D peak to G peak ratio is over 4, associated with hydrogenation on both sides. A very low defect density was observed in the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen-LTP-exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing at over 1000 °C.

Giant frictional dissipation peaks and charge-density-wave slips at the NbSe2 surface

Understanding nanoscale friction and dissipation is central to nanotechnology. The recent detection of the electronic-friction drop caused by the onset of superconductivity in Nb by means of an ultrasensitive non-contact pendulum atomic force microscope (AFM) raised hopes that a wider variety of mechanical-dissipation mechanisms become accessible. Here, we report a multiplet of AFM dissipation peaks arising a few nanometres above the surface of NbSe2--a layered compound exhibiting an incommensurate charge-density wave (CDW). Each peak appears at a well-defined tip-surface interaction force of the order of a nanonewton, and persists up to 70 K, where the short-range order of CDWs is known to disappear. Comparison of the measurements with a theoretical model suggests that the peaks are associated with local, tip-induced 2π phase slips of the CDW, and that dissipation maxima arise from hysteretic behaviour of the CDW phase as the tip oscillates at specific distances where sharp local slips occur.

Low temperature ultrahigh vacuum noncontact atomic force microscope in the pendulum geometry

A noncontact atomic force microscope (nc-AFM) operating in magnetic fields up to ±7 T and liquid helium temperatures is presented in this article. In many common AFM experiments the cantilever is mounted parallel to the sample surface, while in our system the cantilever is assembled perpendicular to it; the so called pendulum mode of AFM operation. In this mode measurements employing very soft and, therefore, ultrasensitive cantilevers can be performed. The ultrahigh vacuum conditions allow to prepare and transfer cantilevers and samples in a requested manner avoiding surface contamination. We demonstrate the possibility of nc-AFM and Kelvin force probe microscopy imaging in the pendulum mode. Ultrasensitive experiments on small spin ensembles are presented as well.

Hydrogen plasma microlithography of graphene supported on a Si/SiO2 substrate

In this work, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of single layer graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.

Atomic Scale Friction Phenomena

Friction has long been the subject of research: the empirical da Vinci-Amontons friction laws have been common knowledge for centuries. Macroscopic experiments performed by the school of Bowden and Tabor revealed that macroscopic friction can be related to the collective action of small asperities. Over the last 25 years, experiments performed with the atomic force microscope have provided new insights into the physics of single asperities sliding over surfaces. This development, together with the results from complementary experiments using surface force apparatus and the quartz microbalance, have led to the new field of nanotribology. At the same time, increasing computing power has permitted the simulation of processes that occur during sliding contact involving several hundreds of atoms. It has become clear that atomic processes cannot be neglected when interpreting nanotribology experiments. Even on well-defined surfaces, experiments have revealed that atomic structure is directly linked to friction force. This chapter will describe friction force microscopy experiments that reveal, more or less directly, atomic processes during sliding contact. We will begin by introducing friction force microscopy, including the calibration of cantilever force sensors and special aspects of the ultrahigh vacuum environment. The empirical Prandtl-Tomlinson model often used to describe atomic stick-slip results is therefore presented in detail. We review experimental results regarding atomic friction, including thermal activation, velocity dependence and temperature dependence. The geometry of the contact is crucial to the interpretation of experimental results, such as the calculation of the lateral contact stiffness. The onset of wear on the atomic scale has recently been studied experimentally and it is described here. The chapter ends with a discussion of recent experiments aimed to detect the dissipative forces acting when a sharp tip is moved parallel and very close to a solid surface without being in contact with it, or when small entities such as single polymer chains, graphene nanoribbons or large organic molecules are manipulated.

Recent highlights in nanoscale and mesoscale friction

Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this “hot” research field is leading to new technological advances in the area of engineering and materials science.

Antibacterial effects of bio-inspired nanostructured materials

ABSTRACT Several properties of bio-inspired surfaces like chemical composition, surface topography, surface hydrophilicity and even surface charge could influence bacterial adhesion to implant materials. Therefore, a nanostructured surface is being investigated to avoid bacterial colonization by their physico-mechanical and chemical aspects. Both smooth and rough-surfaced titanium (PT, SLA) and zirconia (M and ZLA) surfaces were used as controls. Titanium SLA was modified by two-step-etching to create nanostructured surface. Antibacterial properties of the materials were tested by adhesion of Porphyromonas gingivalis (ATCC 33277). The vitality of bacteria was assessed by Live/Dead BacLight™ Bacterial Viability Kit or by conventional culturing on Columbia blood agar. Conventional culturing revealed reduction of bacteria on nanostructured titanium (5.27±0.8 x 104 CFU/mm2) in comparison to rough-surfaced control materials (ZLA 6.16±4.86 x 104 and SLA 1.53±0.75 x 105 CFU/mm2). However, smooth-surfaced control materials (M 2.25±0.84 x 104 and PT 6.63±5.77 x 103 CFU/mm2) showed similar results to the nanostructured material. Live/dead staining demonstrated the antimicrobial efficacy of the nanostructured material revealing reduction of vital bacteria population up to 70%. This effect was not observed on the control materials (bacterial vitality ≥95%). In conclusion, nanostructured titanium surface shows a reduction of vital bacteria. Therefore, bio-inspired nanostructures can modify the bacteria–titanium interaction.

Dissipation at large Separations

When two macroscopic bodies slide in contact, energy is dissipated due to friction. Sometimes it is desired, like in case brakes in the bicycle, sometimes unwelcome–when you ask yourself why your automated coffee machine broke for the third time. In nanoscale, a tiny friction force is present when bodies in relative motion are separated by few nanometer gap. This non-contact form of friction might be successfully measured by highly sensitive cantilever oscillating like a tiny pendulum over the surface. The elusive non-contact friction might arise due to vdW interaction, which is mediated by the long-range electromagnetic field or in many cases by fluctuations of static surface charges arising from material inhomogeneities. The huge dissipation might also orginate from hysteretic switching of the studied material under the external action of the oscillating probe. In this chapter several experiments reporting on non-contact friction are discussed. First the Joule dissipation channel is discussed. Next we report on non-contact friction measurement over metal–superconductor transition, which allows to distinguish between phononic and electronic contribution to friction. The non-contact friction due to switching of the charge density wave is discused in the last part of this chapter.