29 research outputs found
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.Comment: 13 pages, 3 figure
Atomically precise incorporation of BN doped rubicene into graphene nanoribbons
Substituting heteroatoms and non-benzenoid carbons into nanographene
structure offers an unique opportunity for atomic engineering of electronic
properties. Here we show the bottom-up synthesis of graphene nanoribbons (GNRs)
with embedded fused BN-doped rubicene components on a Au(111) surface using
on-surface chemistry. Structural and electronic properties of the BN-GNRs are
characterized by scanning tunneling microscopy (STM) and atomic force
microscopy (AFM) with CO-terminated tips supported by numerical calculations.
The periodic incorporation of BN heteroatoms in the GNR leads to an increase of
the electronic band gap as compared to its undoped counterpart. This opens
avenues for the rational design of semiconducting GNRs with optoelectronic
properties.Comment: 18 pages, 4 figure
Precise engineering of quantum dot array coupling through their barrier widths
Quantum dots are known to confine electrons within their structure. Whenever they periodically aggregate into arrays and cooperative interactions arise, novel quantum properties suitable for technological applications show up. Control over the potential barriers existing between neighboring quantum dots is therefore essential to alter their mutual crosstalk. Here we show that precise engineering of the barrier width can be experimentally achieved on surfaces by a single atom substitution in a haloaromatic compound, which in turn tunes the confinement properties through the degree of quantum dot intercoupling. We achieved this by generating self-assembled molecular nanoporous networks that confine the twodimensional electron gas present at the surface. Indeed, these extended arrays form up on bulk surface and thin silver films alike, maintaining their overall interdot coupling. These findings pave the way to reach full control over two-dimensional electron gases by means of self-assembled molecular networks
Electrospray deposition of organic molecules on bulk insulator surfaces
Large organic molecules are of important interest for organic-based devices such as hybrid photovoltaics or molecular electronics. Knowing their adsorption geometries and electronic structures allows to design and predict macroscopic device properties. Fundamental investigations in ultra-high vacuum (UHV) are thus mandatory to analyze and engineer processes in this prospects. With increasing size, complexity or chemical reactivity, depositing molecules by thermal evaporation becomes challenging. A recent way to deposit molecules in clean conditions is Electrospray Ionization (ESI). ESI keeps the possibility to work with large molecules, to introduce them in vacuum, and to deposit them on a large variety of surfaces. Here, ESI has been successfully applied to deposit triply fused porphyrin molecules on an insulating KBr(001) surface in UHV environment. Different deposition coverages have been obtained and characterization of the surface by in-situ atomic force microscopy working in the non-contact mode shows details of the molecular structures adsorbed on the surface. We show that UHV-ESI, can be performed on insulating surfaces in the sub-monolayer regime and to single molecules which opens the possibility to study a variety of complex molecules
Systematic measurement of pentacene assembled on Cu(111) by bimodal dynamic force microscopy at room temperature
A wide range of depositions of pentacene on a Cu(111) surface was studied by frequency modulation bimodal dynamic force microscopy at room temperature. The morphologies and stabilities of the pentacene films were systematically studied via detections of the vertical and lateral tip-sample interactions at sub-molecular-scale resolution. At low coverage, pentacene adsorbs flat on the surface where two phases of molecular assemblies were observed. At the boundary between the different phase domains, a large magnitude of the dissipation energy, arising from the high mobility of pentacene, was measured while no significant increase of the dissipation energy was observed at the domain boundary of different orientations in the same phase. By increasing the amount of the deposited molecules, the second layer with the flatly aligned molecule tilted with a longitudinal molecular axis was observed over a small area, but most of the molecules moved to a certain site and formed crystallites with bulk structure on top of the first flat layer. We found that the work function of the bulk surface was strongly affected by the first flat layer
Majorana Fermions in magnetic chains
Majorana fermions have recently garnered a great attention outside the field of particle physics, in condensed matter physics. In contrast to their particle physics counterparts, Majorana fermions are zero energy, chargeless, spinless, composite quasiparticles, residing at the boundaries of so-called topological superconductors. Furthermore, in opposition to any particles in the standard model, Majorana fermions in solid-state systems obey non-Abelian exchange statistics that make them attractive candidates for decoherence-free implementations of quantum computers. In this review, we report on the recent advances to realize synthetic topological superconductors supporting Majorana fermions with an emphasis on chains of magnetic impurities on the surface of superconductors. After outlining the theoretical underpinning responsible for the formation of Majorana fermions, we report on the subsequent experimental efforts to build topological superconductors and the resulting evidence in favor of Majorana fermions, focusing on scanning tunneling microscopy and the hunt for zero-bias peaks in the measured current. We conclude by summarizing the open questions in the field and propose possible experimental measurements to answer them
Stick-Slip Motion of ssDNA over Graphene
We have performed molecular dynamics simulations of nanomanipulation experiments on short single-stranded DNA chains elastically driven on a graphene surface. After a brief transient, reproducible stick–slip cycles are observed on chains made by 10 units of thymine, cytosine, adenine, and guanine. The cycles have the periodicity of the graphene substrate, and take place via an intermediate stage, appearing as a dip in the sawtooth variations of lateral force recorded while the chains are manipulated. Guanine presents remarkable differences from the other bases, since a lower number of nucleotides are prone to stick to the substrate in this case. Nevertheless, the magnitudes of static friction and lateral stiffness are similar for all chains (30 pN and 0.7 N/m per adsorbed nucleotide respectively)
Mechanical dissipation from charge and spin transitions in oxygen-deficient SrTiO3 surfaces
Bodies in relative motion separated by a gap of a few nanometers can
experience a tiny friction force. This non-contact dissipation can have various
origins and can be successfully measured by a sensitive pendulum atomic force
microscope tip oscillating laterally above the surface. Here, we report on the
observation of dissipation peaks at selected voltage-dependent tip-surface
distances for oxygen-deficient strontium titanate (SrTiO_3) surface at low
temperatures (T = 5K). The observed dissipation peaks are attributed to
tip-induced charge and spin state transitions in quantum-dot-like entities
formed by single oxygen vacancies (and clusters thereof, possibly through a
collective mechanism) at the SrTiO_3 surface, which in view of technological
and fundamental research relevance of the material opens important avenues for
further studies and applications
Organometallic Bonding in an Ullmann-Type On-Surface Chemical Reaction Studied by High-Resolution Atomic Force Microscopy
The on-surface Ullmann-type chemical reaction synthesizes polymers by linking carbons of adjacent molecules on solid surfaces. Although an organometallic compound is recently identified as the reaction intermediate, little is known about the detailed structure of the bonded organometallic species and its influence on the molecule and the reaction. Herein atomic force microscopy at low temperature is used to study the reaction with 3,9-diiododinaphtho[2,3-b:2′,3′-d]thiophene (I-DNT-VW), which is polymerized on Ag(111) in vacuum. Thermally sublimated I-DNT-VW picks up a Ag surface atom, forming a C[BOND]Ag bond at one end after removing an iodine. The C[BOND]Ag bond is usually short-lived, and a C[BOND]Ag[BOND]C organometallic bond immediately forms with an adjacent molecule. The existence of the bonded Ag atoms strongly affects the bending angle and adsorption height of the molecular unit. Density functional theory calculations reveal the bending mechanism, which reveals that charge from the terminus of the molecule is transferred via the Ag atom into the organometallic bond and strengths the local adsorption to the substrate. Such deformations vanish when the Ag atoms are removed by annealing and C[BOND]C bonds are established