223 research outputs found
Electronic and magnetic properties of zigzag graphene nanoribbons on the (111) surface of Cu, Ag and Au
We have carried out an ab initio study of the structural, electronic and
magnetic properties of zigzag graphene nanoribbons on Cu(111), Ag(111) and
Au(111). Both, H-free and H-terminated nanoribbons are considered revealing
that the nanoribbons invariably possess edge states when deposited on these
surfaces. In spite of this, they do not exhibit a significant magnetization at
the edge, with the exception of H-terminated nanoribbons on Au(111), whose
zero-temperature magnetic properties are comparable to those of free-standing
nanoribbons. These results are explained by the different hybridization between
the graphene 2p orbitals and those of the substrates and, for some models, by
the sizable charge transfer between the surface and the nanoribbon.
Interestingly, H-free nanoribbons on Au(111) and Ag(111) exhibit two main peaks
in the local density of states around the Fermi energy, which originate from
different states and, thus, do not indicate edge magnetism.Comment: 5pages, 3figure
Versatile scanning tunneling microscopy with 120ps time resolution
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.Comment: 4 pages, 4 figure
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
One-dimensional Si chains embedded in Pt(111)and protected by a hexagonal boron-nitride monolayer
Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at
300K leads to a network of one-dimensional Si chains. On the bare Pt(111)
surface, the chains, embedded into the Pt surface, are orientated along the
-direction. They disappear within a few hours in ultrahigh vacuum due to
the presence of residual gas. Exposing the chains to different gases
deliberately reveals that CO is largely responsible for the disappearance of
the chains. The chains can be stabilized by a monolayer of hexagonal boron
nitride, which is deposited prior to the Si deposition. The resulting Si chains
are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface
and survive even an exposure to air for 10 minutes.Comment: 8 pages, 4 Figure
Error estimation of closed-form solution for annual rate of structural collapse
With the increasing emphasis of performance-based earthquake engineering (PBEE) in
the engineering community, several investigations have been presented outlining simplified
approaches suitable for performance-based seismic design (PBSD). Central to most of these
PBSD approaches is the use of closed-form analytical solutions to the probabilistic integral
equations representing the rate of exceedance of key performance measures. Situations where
such closed-form solutions are not appropriate primarily relate to the problem of extrapolation
outside of the region in which parameters of the closed-form solution are fit. This study
presents a critical review of the closed form solution for the annual rate of structural collapse.
The closed form solution requires the assumptions of lognormality of the collapse fragility
and power model form of the ground motion hazard, of which the latter is more significant
regarding the error of the closed-form solution. Via a parametric study, the key variables
contributing to the error between the closed-form solution and solution via numerical
integration are illustrated. As these key variables can not be easily measured it casts doubt on
the use of such closed-form solutions in future PBSD, especially considering the simple and
efficient nature of using direct numerical integration to obtain the solution
Vermessung elektronischer Wellenfunktionen in Halbleitern: Von Quantenpunkten zu komplexeren Systemen
Mit Hilfe der Rastertunnelspektroskopie bei tiefen Temperaturen ist die lokale elektronische Zustandsdichte des III-V-Halbleiters InAs in verschiedenen Dimensionen vermessen worden. Dabei wurde neben der Dimension auch das externe Magnetfeld und die Unordnung als thermodynamische Parameter variiert. In nulldimensionalen Quantenpunkten konnten einzelne Wellenfunktionen abgebildet werden. In höherdimensionalen Systemen wurden stets mehrere Wellenfunktionen gleichzeitig abgebildet. Unterschiedliche Effekte wie Perkolation, schwache Lokalisierung, Driftzustände im Magnetfeld oder partielle Lokalisierung im extremen Quantenlimes konnten im Realraum dargestellt werden
Інтегральне числення функції однієї змінної. Матеріали методичного забезпечення поглибленого вивчення розділу студентами технічних спеціальностей
Подано основні теоретичні відомості до розділу «Інтегральне числення функції однієї змінної», наведено розв’язання задач підвищеної складності та приклади для самостійного розгляду
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 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
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