317 research outputs found
Bistable molecular conductors with a field-switchable dipole group
A class of bistable "stator-rotor" molecules is proposed, where a stationary
bridge (stator) connects the two electrodes and facilitates electron transport
between them. The rotor part, which has a large dipole moment, is attached to
an atom of the stator via a single sigma bond. Hydrogen bonds formed between
the rotor and stator make the symmetric orientation of the dipole unstable. The
rotor has two potential minima with equal energy for rotation about the sigma
bond. The dipole orientation, which determines the conduction state of the
molecule, can be switched by an external electric field that changes the
relative energy of the two potential minima. Both orientation of the rotor
correspond to asymmetric current-voltage characteristics that are the reverse
of each other, so they are distinguishable electrically. Such bistable
stator-rotor molecules could potentially be used as parts of molecular
electronic devices.Comment: 8 pages, 7 figure
Antiresonances in Molecular Wires
We present analytic and numerical studies based on Landauer theory of
conductance antiresonances of molecular wires. Our analytic treatment is a
solution of the Lippmann-Schwinger equation for the wire that includes the
effects of the non-orthogonality of the atomic orbitals on different atoms
exactly. The problem of non-orthogonality is treated by solving the transport
problem in a new Hilbert space which is spanned by an orthogonal basis. An
expression is derived for the energies at which antiresonances should occur for
a molecular wire connected to a pair of single-channel 1D leads. From this
expression we identify two distinct mechanisms that give rise to antiresonances
under different circumstances. The exact treatment of non-orthogonality in the
theory is found to be necessary to obtain reliable results. Our numerical
simulations extend this work to multichannel leads and to molecular wires
connected to 3D metallic nanocontacts. They demonstrate that our analytic
results also provide a good description of these more complicated systems
provided that certain well-defined conditions are met. These calculations
suggest that antiresonances should be experimentally observable in the
differential conductance of molecular wires of certain types.Comment: 22 pages, 5 figure
Electron Standing Wave Formation in Atomic Wires
Using the Landauer formulation of transport theory and tight binding models
of the electronic structure, we study electron transport through atomic wires
that form 1D constrictions between pairs of metallic nano-contacts. Our results
are interpreted in terms of electron standing waves formed in the atomic wires
due to interference of electron waves reflected at the ends of the atomic
constrictions. We explore the influence of the chemistry of the atomic
wire-metal contact interfaces on these standing waves and the associated
transport resonances by considering two types of atomic wires: gold wires
attached to gold contacts and carbon wires attached to gold contacts. We find
that the conductance of the gold wires is roughly for the
wire lengths studied, in agreement with experiments. By contrast, for the
carbon wires the conductance is found to oscillate strongly as the number of
atoms in the wire varies, the odd numbered chains being more conductive than
the even numbered ones, in agreement with previous theoretical work that was
based on a different model of the carbon wire and metal contacts.Comment: 14 pages, includes 6 figure
State Orthogonalization by Building a Hilbert Space: A New Approach to Electronic Quantum Transport in Molecular Wires
Quantum descriptions of many complex systems are formulated most naturally in
bases of states that are not mutually orthogonal. We introduce a general and
powerful yet simple approach that facilitates solving such models exactly by
embedding the non-orthogonal states in a new Hilbert space in which they are by
definition mutually orthogonal. This novel approach is applied to electronic
transport in molecular quantum wires and is used to predict conductance
antiresonances of a new type that arise solely out of the non-orthogonality of
the local orbitals on different sites of the wire.Comment: 4 pages 1 figur
Coherent electron-phonon coupling and polaron-like transport in molecular wires
We present a technique to calculate the transport properties through
one-dimensional models of molecular wires. The calculations include inelastic
electron scattering due to electron-lattice interaction. The coupling between
the electron and the lattice is crucial to determine the transport properties
in one-dimensional systems subject to Peierls transition since it drives the
transition itself. The electron-phonon coupling is treated as a quantum
coherent process, in the sense that no random dephasing due to electron-phonon
interactions is introduced in the scattering wave functions. We show that
charge carrier injection, even in the tunneling regime, induces lattice
distortions localized around the tunneling electron. The transport in the
molecular wire is due to polaron-like propagation. We show typical examples of
the lattice distortions induced by charge injection into the wire. In the
tunneling regime, the electron transmission is strongly enhanced in comparison
with the case of elastic scattering through the undistorted molecular wire. We
also show that although lattice fluctuations modify the electron transmission
through the wire, the modifications are qualitatively different from those
obtained by the quantum electron-phonon inelastic scattering technique. Our
results should hold in principle for other one-dimensional atomic-scale wires
subject to Peierls transitions.Comment: 21 pages, 8 figures, accepted for publication in Phys. Rev. B (to
appear march 2001
Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions with Eutectic Ga-In (EGaIn) Top-Contacts
This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga2O3 as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10-1 A/cm2 at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.
Electron Transport Properties of Single-Molecule-Bearing Multiple Redox Levels Studied by EC-STM/STS
Multielectron systems as possible components of molecular electronics devices are attracting compelling experimental and theoretical interest. Here we studied by electrochemical scanning tunneling techniques (EC-STMicroscopy and EC-STSpectroscopy) the electron transport properties of a redox molecule endowed with two redox levels, namely, the hydroquinone/quinone (H2Q/Q) couple. By forming self-assembled monolayers on Au(111) of oligo-phenylene-vinylene (OPV) derivatized H2Q/Q moieties, we were able to explore the features of the tunneling current/overpotential relation in the EC-STS setup. The behavior of the tunneling current sheds light onto the mechanism of electron transport involving the redox levels of the H2Q/Q redox pair coupled to tip and substrate electrodes
Efficacy of NS-018, a potent and selective JAK2/Src inhibitor, in primary cells and mouse models of myeloproliferative neoplasms
Aberrant activation of Janus kinase 2 (JAK2) caused by somatic mutation of JAK2 (JAK2V617F) or the thrombopoietin receptor (MPLW515L) plays an essential role in the pathogenesis of myeloproliferative neoplasms (MPNs), suggesting that inhibition of aberrant JAK2 activation would have a therapeutic benefit. Our novel JAK2 inhibitor, NS-018, was highly active against JAK2 with a 50% inhibition (IC50) of <1 n, and had 30–50-fold greater selectivity for JAK2 over other JAK-family kinases, such as JAK1, JAK3 and tyrosine kinase 2. In addition to JAK2, NS-018 inhibited Src-family kinases. NS-018 showed potent antiproliferative activity against cell lines expressing a constitutively activated JAK2 (the JAK2V617F or MPLW515L mutations or the TEL–JAK2 fusion gene; IC50=11–120 n), but showed only minimal cytotoxicity against most other hematopoietic cell lines without a constitutively activated JAK2. Furthermore, NS-018 preferentially suppressed in vitro erythropoietin-independent endogenous colony formation from polycythemia vera patients. NS-018 also markedly reduced splenomegaly and prolonged the survival of mice inoculated with Ba/F3 cells harboring JAK2V617F. In addition, NS-018 significantly reduced leukocytosis, hepatosplenomegaly and extramedullary hematopoiesis, improved nutritional status, and prolonged survival in JAK2V617F transgenic mice. These results suggest that NS-018 will be a promising candidate for the treatment of MPNs
Molecular measurement of BCR-ABL transcript variations in chronic myeloid leukemia patients in cytogenetic remission
<p>Abstract</p> <p>Background</p> <p>The monitoring of <it>BCR-ABL </it>transcript levels by real-time quantitative polymerase chain reaction (RT-qPCR) has become important to assess minimal residual disease (MRD) and standard of care in the treatment of chronic myeloid leukemia (CML). In this study, we performed a prospective, sequential analysis using RT-qPCR monitoring of <it>BCR-ABL </it>gene rearrangements in blood samples from 91 CML patients in chronic phase (CP) who achieved complete cytogenetic remission (CCyR) and major molecular remission (MMR) throughout imatinib treatment.</p> <p>Methods</p> <p>The absolute level of <it>BCR-ABL </it>transcript from peripheral blood was serially measured every 4 to 12 weeks by RT-qPCR. Only level variations > 0.5%, according to the international scale, was considered positive. Sequential cytogenetic analysis was also performed in bone marrow samples from all patients using standard protocols.</p> <p>Results</p> <p>Based on sequential analysis of <it>BCR-ABL </it>transcripts, the 91 patients were divided into three categories: (A) 57 (62.6%) had no variation on sequential analysis; (B) 30 (32.9%) had a single positive variation result obtained in a single sample; and (C) 4 (4.39%) had variations of <it>BCR-ABL </it>transcripts in at least two consecutive samples. Of the 34 patients who had elevated levels of transcripts (group B and C), 19 (55.8%) had a < 1% of <it>BCR-ABL/BCR </it>ratio, 13 (38.2%) patients had a 1% to 10% increase and 2 patients had a >10% increase of RT-qPCR. The last two patients had lost a CCyR, and none of them showed mutations in the <it>ABL </it>gene. Transient cytogenetic alterations in Ph-negative cells were observed in five (5.5%) patients, and none of whom lost CCyR.</p> <p>Conclusions</p> <p>Despite an increase levels of <it>BCR-ABL/BCR </it>ratio variations by RT-qPCR, the majority of CML patients with MMR remained in CCyR. Thus, such single variations should neither be considered predictive of subsequent failure and nor an indication for altering imatinib dose or switching to second generation therapy. Changing of imatinib on the basis of <it>BCR-ABL/BCR</it>% sustained increase and mutational studies is a prudent approach for preserving other therapeutic options in imatinib-resistant patients.</p
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