Graphene quantum dots probed by scanning tunneling spectroscopy and transport spectroscopy after local anodic oxidation

Graphene quantum dots are considered as promising alternatives to quantum dots in III-V semiconductors, e.g., for the use as spin qubits due to their consistency made of light atoms including spin-free nuclei which both imply relatively long spin decoherene times. However, this potential has not been realized in experiments so far, most likely, due to a missing control of the edge configurations of the quantum dots. Thus, a more fundamental investigation of Graphene quantum dots appears to be necessary including a full control of the wave function properties most favorably during transport spectroscopy measurements. Here, we review the recent success in mapping wave functions of graphene quantum dots supported by metals, in particular Ir(111), and show how the goal of probing such wave functions on insulating supports during transport spectroscopy might be achieved.Comment: 14 pages, review articl

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

Mask aligner for ultrahigh vacuum with capacitive distance control

We present a mask aligner driven by three piezo motors 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 in $\mu$m distance from the sample surface, while keeping it parallel to the surface. 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).Comment: 5 pages, 3 figure

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

Evidence for topological band inversion of the phase change material Ge2Sb2Te5

We present an angle-resolved photoemission study of a ternary phase change material, namely Ge2Sb2Te5, epitaxially grown on Si(111) in the metastable cubic phase. The observed upper bulk valence band shows a minimum at Gamma-bar being 0.3 eV below the Fermi level E_F and a circular Fermi contour around Gamma-bar with a dispersing diameter of 0.27-0.36 Anstroms^-1. This is in agreement with density functional theory calculations of the Petrov stacking sequence in the cubic phase which exhibits a topological surface state. The topologically trivial cubic KH stacking shows a valence band maximum at Gamma in line with all previous calculations of the hexagonal stable phase exhibiting the valence band maximum at Gamma for a trivial Z_2 topological invariant nu_0 and away from Gamma for non-trivial nu_0. Scanning tunneling spectroscopy exhibits a band gap of 0.4 eV around E_F

Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise

We characterize InSb quantum dots induced by bottom finger gates within a nanowire that is grown via the vapor-liquid-solid process. The gates are separated from the nanowire by an exfoliated 35\,nm thin hexagonal BN flake. We probe the Coulomb diamonds of the gate induced quantum dot exhibiting charging energies of $\sim 2.5\,\mathrm{meV}$ and orbital excitation energies up to $0.3\,\mathrm{meV}$. The gate hysteresis for sweeps covering 5 Coulomb diamonds reveals an energy hysteresis of only $60\mathrm{\mu eV}$ between upwards and downwards sweeps. Charge noise is studied via long-term measurements at the slope of a Coulomb peak revealing potential fluctuations of $\sim 1\,\mu \mathrm{eV}/\mathrm{\sqrt{Hz}}$ at 1\,Hz. This makes h-BN the dielectric with the currently lowest gate hysteresis and lowest low-frequency potential fluctuations reported for low-gap III-V nanowires. The extracted values are similar to state-of-the art quantum dots within Si/SiGe and Si/SiO${_2}$ systems

Ambipolar Heating of Magnetars

Magnetars, neutron stars thought to be with ultra-strong magnetic fields of $10^{14 - 15}$ G, are observed to be much hotter than ordinary pulsars with $\sim 10^{12}$ G, and additional heating sources are required. One possibility is heating by the ambipolar diffusion in the stellar core. This scenario is examined by calculating the models using the relativistic thermal evolutionary code without making the isothermal approximation. The results show that this scenario can be consistent with most of the observed magnetar temperature data.Comment: 12 pages, 4 figures, 1 table. Accepted by the Astrophysical Journal on February 9, 202

Вища геодезія. Методичні рекомендації до виконання курсового проекту бакалаврами денної та заочної форм навчання спеціальності 193 Геодезія та землеустрій

Наведено мету, завдання та структуру курсового проекту, нормативні вимоги до геодезичних мереж згущення. Дано рекомендації з проектування, наведено типові схеми мережі, способи попереднього розрахунку точності елементів запроектованої мережі наближеним способом та із застосуванням програмного забезпечення

Tuning the pseudospin polarization of graphene by a pseudo-magnetic field

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudo-magnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudo-magnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudo-magnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene's pseudospin due to a strain induced pseudo-magnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO$_2$ support, as visible by an increased slope of the $I(z)$ curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudo-magnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudo-magnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics.Comment: Revised manuscript: streamlined the abstract and introduction, added methods to supplement, Nano Letters, 201