169 research outputs found
Imaging Pauli repulsion in scanning tunneling microscopy
A scanning tunneling microscope (STM) has been equipped with a nanoscale
force sensor and signal transducer composed of a single D2 molecule that is
confined in the STM junction. The uncalibrated sensor is used to obtain
ultra-high geometric image resolution of a complex organic molecule adsorbed on
a noble metal surface. By means of conductance-distance spectroscopy and
corresponding density functional calculations the mechanism of the
sensor/transducer is identified. It probes the short-range Pauli repulsion and
converts this signal into variations of the junction conductance.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Ab initio study of a mechanically gated molecule: From weak to strong correlation
The electronic spectrum of a chemically contacted molecule in the junction of
a scanning tunneling microscope can be modified by tip retraction. We analyze
this effect by a combination of density functional, many-body perturbation and
numerical renormalization group theory, taking into account both the
non-locality and the dynamics of electronic correlation. Our findings, in
particular the evolution from a broad quasiparticle resonance below to a narrow
Kondo resonance at the Fermi energy, correspond to the experimental
observations.Comment: 4 pages, 3 figure
Electrical transport through a mechanically gated molecular wire
A surface-adsorbed molecule is contacted with the tip of a scanning tunneling
microscope (STM) at a pre-defined atom. On tip retraction, the molecule is
peeled off the surface. During this experiment, a two-dimensional differential
conductance map is measured on the plane spanned by the bias voltage and the
tip-surface distance. The conductance map demonstrates that tip retraction
leads to mechanical gating of the molecular wire in the STM junction. The
experiments are compared with a detailed ab initio simulation. We find that
density functional theory (DFT) in the local density approximation (LDA)
describes the tip-molecule contact formation and the geometry of the molecular
junction throughout the peeling process with predictive power. However, a
DFT-LDA-based transport simulation following the non-equilibrium Green's
functions (NEGF) formalism fails to describe the behavior of the differential
conductance as found in experiment. Further analysis reveals that this failure
is due to the mean-field description of electron correlation in the local
density approximation. The results presented here are expected to be of general
validity and show that, for a wide range of common wire configurations,
simulations which go beyond the mean-field level are required to accurately
describe current conduction through molecules. Finally, the results of the
present study illustrate that well-controlled experiments and concurrent ab
initio transport simulations that systematically sample a large configuration
space of molecule-electrode couplings allow the unambiguous identification of
correlation signatures in experiment.Comment: 31 pages, 10 figure
Kondo effect by controlled cleavage of a single molecule contact
Conductance measurements of a molecular wire, contacted between an epitaxial
molecule-metal bond and the tip of a scanning tunneling microscope, are
reported. Controlled retraction of the tip gradually de-hybridizes the molecule
from the metal substrate. This tunes the wire into the Kondo regime in which
the renormalized molecular transport orbital serves as spin impurity at half
filling and the Kondo resonance opens up an additional transport channel.
Numerical renormalization group simulations suggest this type of behavior to be
generic for a common class of metal molecule bonds. The results demonstrate a
new approach to single-molecule experiments with atomic-scale contact control
and prepare the way for the ab initio simulation of many-body transport through
single-molecule junctions.Comment: Main text: 41 pages including references and captions, 9 figures.
Supplementary information: 5 pages including 2 figures New experimental and
theoretical data supporting initial claims are added. The paper has been
reworked from the letter format into a longer versio
Dynamical bi-stability of single-molecule junctions: A combined experimental/theoretical study of PTCDA on Ag(111)
The dynamics of a molecular junction consisting of a PTCDA molecule between
the tip of a scanning tunneling microscope and a Ag(111) surface have been
investigated experimentally and theoretically. Repeated switching of a PTCDA
molecule between two conductance states is studied by low-temperature scanning
tunneling microscopy for the first time, and is found to be dependent on the
tip-substrate distance and the applied bias. Using a minimal model Hamiltonian
approach combined with density-functional calculations, the switching is shown
to be related to the scattering of electrons tunneling through the junction,
which progressively excite the relevant chemical bond. Depending on the
direction in which the molecule switches, different molecular orbitals are
shown to dominate the transport and thus the vibrational heating process. This
in turn can dramatically affect the switching rate, leading to non-monotonic
behavior with respect to bias under certain conditions. In this work, rather
than simply assuming a constant density of states as in previous works, it was
modeled by Lorentzians. This allows for the successful description of this
non-monotonic behavior of the switching rate, thus demonstrating the importance
of modeling the density of states realistically.Comment: 20 pages, 6 figures, 1 tabl
Yorug’lik fotokatalitik faolligi yaxshilangan PANI/TIO2 kompozitsiyasi
Polianilin (PANI) ko’rinadigan yorug’lik va fotokatalitik samaradorlikni oshirish uchun kengaytirilgan fotoreaktivlikka erishish uchun TiO2 sirtini modifikatsiyalash uchun istiqbolli o’tkazuvchan polimerdir. Ushbu tadqiqotda biz ko’rinadigan yorug’lik nurlanishi ostida metilen ko’k (MB), bisfenol A (BPA) va bakteriofag MS2 ning fotokatalitik degradatsiyasi uchun ishlatilgan PANI ning turli vazn nisbatlariga ega PANI/TiO2 kompozitsiyasining sintezi haqida xabar beramiz. Kompozitning funktsional guruhlari, morfologiyasi va yorug’lik reaktsiyasi Furye-transform infraqizil spektroskopiya, dala-emissiya uzatuvchi elektron mikroskopiya va diffuz aks ettiruvchi UV- ko’rinadigan spektroskopiya bilan tavsiflangan. Og’irlik nisbati bo’yicha 4% PANI o’z ichiga olgan PANI/TiO2 kompozitsiyasi MB degradatsiyasi uchun eng mos edi va bu fotokatalizator takroriy foydalanishdan keyin ham (to’rt sikl) juda barqaror edi. BPA va bakteriofag MS2 ning PANI/TiO2 kompozitsiyasi bilan parchalanishi ko’rinadigan yorug’lik nurlanishida mos ravishda 360 daqiqada 80% va 120 daqiqada 96,2% ga yetdi. Shu sababli, ko’rinadigan yorug’lik fotokatalitik samaradorligi va barqarorligi yaxshilangan PANI/TiO2 kompozitsiyasi suvni ifloslantiruvchi moddalarning degradatsiyasi uchun keng qo’llanilishi mumkin
Quantum transport through STM-lifted single PTCDA molecules
Using a scanning tunneling microscope we have measured the quantum
conductance through a PTCDA molecule for different configurations of the
tip-molecule-surface junction. A peculiar conductance resonance arises at the
Fermi level for certain tip to surface distances. We have relaxed the molecular
junction coordinates and calculated transport by means of the Landauer/Keldysh
approach. The zero bias transmission calculated for fixed tip positions in
lateral dimensions but different tip substrate distances show a clear shift and
sharpening of the molecular chemisorption level on increasing the STM-surface
distance, in agreement with experiment.Comment: accepted for publication in Applied Physics
TIA1 Mutations in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia Promote Phase Separation and Alter Stress Granule Dynamics.
Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are age-related neurodegenerative disorders with shared genetic etiologies and overlapping clinical and pathological features. Here we studied a novel ALS/FTD family and identified the P362L mutation in the low-complexity domain (LCD) of T cell-restricted intracellular antigen-1 (TIA1). Subsequent genetic association analyses showed an increased burden of TIA1 LCD mutations in ALS patients compared to controls (p = 8.7 × 1
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