Theoretical study of the role of the tip in enhancing the sensitivity of differential conductance tunneling spectroscopy on magnetic surfaces

Based on a simple model for spin-polarized scanning tunneling spectroscopy (SP-STS) we study how tip magnetization and electronic structure affects the differential conductance (dI/dV) tunneling spectrum of an Fe(001) surface. We take into account energy dependence of the vacuum decay of electron states, and tip electronic structure either using an ideal model or based on ab initio electronic structure calculation. In the STS approach, topographic and magnetic contributions to dI/dV can clearly be distinguished and analyzed separately. Our results suggest that the sensitivity of STS on a magnetic sample can be tuned and even enhanced by choosing the appropriate magnetic tip and bias setpoint, and the effect is governed by the effective spin-polarization.Comment: 22 pages manuscript, 4 figures; http://link.aps.org/doi/10.1103/PhysRevB.83.21441

Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

We propose an atom-superposition-based method for simulating spin-polarized scanning tunneling microscopy (SP-STM) from first principles. Our approach provides bias dependent STM images in high spatial resolution, with the capability of using either constant current or constant height modes of STM. In addition, topographic and magnetic contributions can clearly be distinguished, which are directly comparable to results of SP-STM experiments in the differential magnetic mode. Advantages of the proposed method are that it is computationally cheap, it is easy to parallelize, and it can employ the results of any ab initio electronic structure code. Its capabilities are illustrated for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer on Ag(111) in a noncollinear magnetic $120^{\circ}$ N\'eel state. We show evidence that the magnetic contrast is sensitive to the tip electronic structure, and this contrast can be reversed depending on the bias voltage.Comment: 28 pages manuscript, 1 table, 5 figure

Spin-correlations and magnetic structure in an Fe monolayer on 5d transition metal surfaces

We present a detailed first principles study on the magnetic structure of an Fe monolayer on different surfaces of 5d transition metals. We use the spin-cluster expansion technique to obtain parameters of a spin model, and predict the possible magnetic ground state of the studied systems by employing the mean field approach and in certain cases by spin dynamics calculations. We point out that the number of shells considered for the isotropic exchange interactions plays a crucial role in the determination of the magnetic ground state. In the case of Ta substrate we demonstrate that the out-of-plane relaxation of the Fe monolayer causes a transition from ferromagnetic to antiferromagnetic ground state. We examine the relative magnitude of nearest neighbour Dzyaloshinskii-Moriya (D) and isotropic (J) exchange interactions in order to get insight into the nature of magnetic pattern formations. For the Fe/Os(0001) system we calculate a very large D/J ratio, correspondingly, a spin spiral ground state. We find that, mainly through the leading isotropic exchange and Dzyaloshinskii-Moriya interactions, the inward layer relaxation substantially influences the magnetic ordering of the Fe monolayer. For the Fe/Re(0001) system characterized by large antiferromagnetic interactions we also determine the chirality of the $120^{\circ}$ N\'eel-type ground state.Comment: 15 pages, 8 figures, 2 table

Simulation of spin-polarized scanning tunneling spectroscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

We propose a computationally efficient atom-superposition-based method for simulating spin-polarized scanning tunneling spectroscopy (SP-STS) on complex magnetic surfaces based on the sample and tip electronic structures obtained from first principles. We go beyond the commonly used local density of states (LDOS) approximation for the differential conductance dI/dV. The capabilities of our approach are illustrated for a Cr monolayer on a Ag(111) surface in a noncollinear magnetic state. We find evidence that the simulated tunneling spectra and magnetic asymmetries are sensitive to the tip electronic structure, and we analyze the contributing terms. Related to SP-STS experiments, we show a way to simulate two-dimensional differential conductance maps and qualitatively correct effective spin-polarization maps on a constant current contour above a magnetic surface

Three-dimensional Wentzel-Kramers-Brillouin approach for the simulation of scanning tunneling microscopy and spectroscopy

We review the recently developed three-dimensional (3D) atom-superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS) based on ab initio electronic structure data. In the method, contributions from individual electron tunneling transitions between the tip apex atom and each of the sample surface atoms are summed up assuming the one-dimensional (1D) Wentzel-Kramers-Brillouin (WKB) approximation in all these transitions. This 3D WKB tunneling model is extremely suitable to simulate spin-polarized STM and STS on surfaces exhibiting a complex noncollinear magnetic structure, i.e., without a global spin quantization axis, at very low computational cost. The tip electronic structure from first principles can also be incorporated into the model, that is often assumed to be constant in energy in the vast majority of the related literature, which could lead to a misinterpretation of experimental findings. Using this approach, we highlight some of the electron tunneling features on a prototype frustrated hexagonal antiferromagnetic Cr monolayer on Ag(111) surface. We obtain useful theoretical insights into the simulated quantities that is expected to help the correct evaluation of experimental results. By extending the method to incorporate a simple orbital dependent electron tunneling transmission, we reinvestigate the bias voltage- and tip-dependent contrast inversion effect on the W(110) surface. STM images calculated using this orbital dependent model agree reasonably well with Tersoff-Hamann and Bardeen results. The computational efficiency of the model is remarkable as the k-point samplings of the surface and tip Brillouin zones do not affect the computational time, in contrast to the Bardeen method. In a certain case we obtain a relative computational time gain of 8500 compared to the Bardeen calculation, without the loss of quality. We discuss the advantages and limitations of the 3D WKB method, and show further ways to improve and extend it

The mechanisms of multi-component paired-pulse facilitation of neurotransmitter release at the frog neuromuscular junction

We have studied the mechanisms of paired-pulse facilitation (PPF) of neurotransmitter release in isolated nerve-muscle preparations of the frog cutaneous pectoris muscle. In normal extracellular Ca2+ concentration ([Ca2+]o, 1.8 mM), as the interpulse interval was increased from 5 to 500 ms, PPF decayed as a sum of two exponential components: a larger but shorter first component (F1) and a smaller but more prolonged second component (F2). In low [Ca2+]o (0.5 mM), both F1 and F2 increased, and a third "early" component (Fe) appeared whose amplitude was larger and whose duration was shorter than F1 or F2. In the presence of the "fast" Ca2+ buffer BAPTA-AM, Fe disappeared, whereas F1 and F2 decreased in amplitude and duration. In contrast, the "slow" Ca2+ buffer EGTA-AM caused a decrease of Fe and reduction or complete blockade of F2, without any changes of F1. In solutions containing Sr2+ (1 mM), the magnitude of Fe was decreased, F1 was significantly reduced and shortened, but F2 was unaffected. Application of the calmodulin inhibitor W-7 (10 μM) at normal [Ca2+]o produced a marked decrease of F2, and at low [Ca2+]o, a complete blockade of Fe. These results suggest that PPF at frog motor nerve terminals is mediated by several specific for different PPF components intraterminal Ca2+ binding sites, which trigger neurotransmitter release. These sites have a higher affinity for Ca2+ ions and are located farther from the release-controlling Ca2+ channels than the Ca 2+ sensor that mediates phasic release. © 2009 Springer-Verlag

Retrogradely transported siRNA silences human mutant SOD1 in spinal cord motor neurons

The transgenic mouse model of familial amyotrophic lateral sclerosis (ALS) expressing human mutant (G93A) copper/zinc superoxide dismutase (SOD1) is an attractive model for studying the therapeutic effects of RNA interference (RNAi) because of the specific silencing of the mutant gene expression. We studied small interfering RNA (siRNA)-mediated down-regulation of human mutant G93A SOD1 gene in lumbar spinal cord of ALS mice. siRNA was applied onto the proximal nerve stump of severed sciatic nerves. One day after surgery the lumbar spinal cords were processed for RT-PCR examination. Treatment with specific siRNA resulted in 48% decrease in human SOD1 mRNA levels in lumbar spinal cord, but had no effect on the abundance of mouse ChAT and SNAP25 mRNAs which were used as randomly selected internal controls, the mark of a specific silencing of SOD1. Our findings demonstrate for the first time that siRNA, targeting mutant human SOD1 mRNA, is taken up by the sciatic nerve, retrogradely transported to the perikarya of motor neurons, and inhibits mutant SOD1 mRNA in G93A transgenic ALS mice. © 2009 Springer-Verlag

Alzheimer's β-amyloid-induced depolarization of skeletal muscle fibers: Implications for motor dysfunctions in dementia

Numerous findings obtained over the last decades suggest that accumulation of β-amyloid peptide (βAP) plays the central role in the pathogenesis of Alzheimer's disease. It is well established that βAP has wide range of toxic effects on neurons in vitro and in vivo, however the influence of βAP in the periphery and on various other types of excitable tissues, eg. skeletal muscle cells, is almost unknown despite the many non-cognitive and other extra-neuronal symptoms associated with Alzheimer's dementia. Here we utilized conventional electrophysiological technique to investigate the effects and mechanisms of βAP action on the resting membrane potential of frog skeletal muscle fibers. βAP in the range of concentrations from 10-6 to 10-8M produced slow, significant, reversible depolarization of muscle fiber membranes. The impact developed and was washed out faster at higher concentrations of βAP (10-6 - 10-7M). The effect of βAP was completely absent when applied in Na+-free Tris + solutions. βAP-mediated depolarization was also prevented by tetrodotoxin (10-5M) pre-treatment and rescued by tetrodotoxin after-treatment. These findings suggest that βAP-induced depolarization of skeletal muscle plasma membranes can significantly disturb the functioning of skeletal muscles and therefore contribute to motor dysfunction observed in Alzheimer's disease and other disorders associated with βAP accumulation. Copyright © 2009 S. Karger AG

Biological insight, high-throughput datasets and the nature of neuro-degenerative disorders

Life sciences are experiencing a historical shift towards a quantitative, data-rich regime. This transition has been associated with the advent of bio-informatics: mathematicians, physicists, computer scientists and statisticians are now commonplace in the field, working on the analysis of ever larger data-sets. An open question regarding what should drive scientific progress in this new era remains: will biological insight become increasingly irrelevant in a world of hypothesis-free, unbiased data analysis? This piece offers a different perspective, pin-pointing that biological thought is more-than-ever relevant in a data-rich setting. Some of the novel highthroughput information being acquired in the field of neuro-degenerative disorders is highlighted here. As but one example of how theory and experiment can interact in this new reality, our efforts in developing an idiopathic neuro-degenerative disease hematopoietic stemcell ageing theory are described. © 2013 Bentham Science Publishers