Acoustoelectric Transport in Graphene

The acoustoelectric effect in graphene is studied in a graphene/lithium niobate hybrid system, which was prepared by transferring large area single-layer graphene grown on copper onto lithium niobate SAW devices. The transfer of momentum from the surface acoustic waves (SAWs), generated on the surface of the lithium niobate, to the carriers in graphene results in an attenuation and velocity shift of the wave, and gives rise to an acoustoelectric current. The acoustoelectric current, and the amplitude and velocity of the SAW are measured using a sourcemeter and oscilloscope, respectively. Macroscopic acoustoelectric current flowing over several hundred micrometers is demonstrated in graphene, which is measured to be directly proportional to the SAW intensity and frequency at room temperature. A relatively simple classical relaxation model, which describes the piezoelectric interaction between SAWs and the carriers in a two-dimensional electron system, is used to explain the experimental observations. The investigation of the acoustoelectric current as a function of temperature demonstrates the ability of SAWs of different wavelengths to probe graphene at different length scales. By tuning the conductivity of the graphene through the use of a top gate, voltage-controlled phase (velocity) shifters are demonstrated. The acoustoelectric current measured as a function of gate voltage demonstrates that an equal density of electrons and holes are transported at the charge neutrality point, reflecting the unique properties of graphene.University of Exete

Temperature dependence of the acoustoelectric current in graphene

PublishedThe acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave (SAW) intensity, and frequency. At high SAW frequencies, the measured acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions.Royal Societ

Tip regeneration and positional information in the slug of Dictyostelium discoideum

We show in this paper that in the case of the slug of the cellular slime mould Dictyostelium discoideum the time which it takes for a new tip to regenerate at a given level can be used as a measure of positional information at that level. Our basic experiment consists ofamputating slugs at various distances from the existing tip and thereby inducing the regeneration of a fresh tip; the time needed for regeneration is estimated by two independent methods. An identical operation, when performed in the anterior portion of a previously cut slug, tells us how this position-dependent regeneration time adjusts to a sudden change in the size of the slug. The reasons which lead us to conclude that tip regeneration times are in one-to-one correspondence with positional information are as follows, (i) Depending on their positions, the cells in a slug take different times to regenerate a new tip following amputation; (ii) regeneration times are scaled in relation to the total length of the slug and increase monotonically with the length cut off; and (iii) as judged by the regeneration time, cells can assess and remember their positions relative to the length of the slug. Our results highlight the importance of two rate processes. One might think of the slower process as being related to the setting up of a system of positional information in the slug, and the faster process as being a reflection of the kinetics of the positional value changing locally till it reaches the level appropriate to a tip

Data-driven Approach for Static Hedging of Exchange Traded Options

This paper presents a data-driven interpretable machine learning algorithm for semi-static hedging of Exchange Traded options, considering transaction costs with efficient run-time. Further, we provide empirical evidence on the performance of hedging longer-term National Stock Exchange (NSE) Index options using a self-replicating portfolio of shorter-term options and cash position, achieved by the automated algorithm, under different modeling assumptions and market conditions, including Covid period. We also systematically assess the model's performance using the Superior Predictive Ability (SPA) test by benchmarking against the static hedge proposed by Peter Carr and Liuren Wu and industry-standard dynamic hedging. We finally perform a thorough Profit and Loss (PnL) attribution analysis on the target option and hedge portfolios (dynamic and static) to discern the factors explaining the superior performance of static hedging.Comment: 42 pages, 11 figure

The scale-invariance of spatial patterning in a developing system

Regulating systems, that is, those which exhibit scale-invariant patterns in the adult, are supposed, to do so on account of interactions between cells during development. The nature of these interactions has to be such that the system of positional information ("map") in the embryo also regulates. To our knowledge, this supposition regarding a regulating map has not been subjected to a direct test in any embryonic system. Here we do so by means of a simple and novel criterion and use it to examine tip regeneration in the mulicellular stage (slug) ofDictyostelium discoideum. When anterior, tip-containing fragments of slugs are amputated, a new tip spontaneously regenerates at the cut surface of the (remaining) posterior fragment. The time needed for regeneration to occur depends on the relative size of the amputated fragment but is independent of the total size of the slug. We conclude from this finding that there is at least one system underlying positional information in the slug which regulates

Depletion of intrinsic expression of Interleukin-8 in prostate cancer cells causes cell cycle arrest, spontaneous apoptosis and increases the efficacy of chemotherapeutic drugs

Abstract Background The progression of all cancers is characterized by increased-cell proliferation and decreased-apoptosis. The androgen-independent prostate cancer (AIPC) is the terminal stage of the disease. Many chemokines and cytokines are suspects to cause this increased tumor cell survival that ultimately leads to resistance to therapy and demise of the host. The AIPC cells, but not androgen-responsive cells, constitutively express abundant amount of the pro-inflammatory chemokine, Interleukin-8 (IL-8). The mechanism of IL-8 mediated survival and therapeutic resistance in AIPC cells is unclear at present. The purpose of this report is to show the pervasive role of IL-8 in malignant progression of androgen-independent prostate cancer (AIPC) and to provide a potential new therapeutic avenue, using RNA interference. Results The functional consequence of IL-8 depletion in AIPC cells was investigated by RNA interference in two IL-8 secreting AIPC cell lines, PC-3 and DU145. The non-IL-8 secreting LNCaP and LAPC-4 cells served as controls. Cells were transfected with RISC-free siRNA (control) or validated-pool of IL-8 siRNA. Transfection with 50 nM IL-8 siRNA caused >95% depletion of IL-8 mRNA and >92% decrease in IL-8 protein. This reduction in IL-8 led to cell cycle arrest at G1/S boundary and decreases in cell cycle-regulated proteins: Cyclin D1 and Cyclin B1 (both decreased >50%) and inhibition of ERK1/2 activity by >50%. Further, the spontaneous apoptosis was increased by >43% in IL-8 depleted cells, evidenced by increases in caspase-9 activation and cleaved-PARP. IL-8 depletion caused significant decreases in anti-apoptotic proteins, BCL-2, BCL-xL due to decrease in both mRNA and post-translational stability, and increased levels of pro-apoptotic BAX and BAD proteins. More significantly, depletion of intracellular IL-8 increased the cytotoxic activity of multiple chemotherapeutic drugs. Specifically, the cytotoxicity of Docetaxel, Staurosporine and Rapamycin increased significantly (>40% at IC50 dose) in IL-8 depleted cells as compared to that in C-siRNA transfected cells. Conclusion These results show the pervasive role of IL-8 in promoting tumor cell survival, and resistance to cytotoxic drugs, regardless of the cytotoxic mechanism of antiproliferative drugs, and point to potential therapeutic significance of IL-8 depletion in men with AIPC.</p

Neural Networks for Portfolio-Level Risk Management: Portfolio Compression, Static Hedging, Counterparty Credit Risk Exposures and Impact on Capital Requirement

In this paper, we present an artificial neural network framework for portfolio compression of a large portfolio of European options with varying maturities (target portfolio) by a significantly smaller portfolio of European options with shorter or same maturity (compressed portfolio), which also represents a self-replicating static hedge portfolio of the target portfolio. For the proposed machine learning architecture, which is consummately interpretable by choice of design, we also define the algorithm to learn model parameters by providing a parameter initialisation technique and leveraging the optimisation methodology proposed in Lokeshwar and Jain (2024), which was initially introduced to price Bermudan options. We demonstrate the convergence of errors and the iterative evolution of neural network parameters over the course of optimization process, using selected target portfolio samples for illustration. We demonstrate through numerical examples that the Exposure distributions and Exposure profiles (Expected Exposure and Potential Future Exposure) of the target portfolio and compressed portfolio align closely across future risk horizons under risk-neutral and real-world scenarios. Additionally, we benchmark the target portfolio's Financial Greeks (Delta, Gamma, and Vega) against the compressed portfolio at future time horizons across different market scenarios generated by Monte-Carlo simulations. Finally, we compare the regulatory capital requirement under the standardised approach for counterparty credit risk of the target portfolio against the compressed portfolio and highlight that the capital requirement for the compact portfolio substantially reduces.Comment: 21 Pages, 8 Figures, 2 Table

Acoustoelectric photoresponse in graphene

Journal ArticleThe acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450nm) and red (735nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution

Macroscopic acoustoelectric charge transport in graphene

Copyright © 2013 AIP PublishingWe demonstrate macroscopic acoustoelectric transport in graphene, transferred onto piezoelectric lithium niobate substrates, between electrodes up to 500 μm apart. Using double finger interdigital transducers we have characterised the acoustoelectric current as a function of both surface acoustic wave intensity and frequency. The results are consistent with a relatively simple classical relaxation model, in which the acoustoelectric current is proportional to both the surface acoustic wave intensity and the attenuation of the wave caused by the charge transport.Royal Societ