Role of Many-particle excitations in Coulomb Blockaded Transport

We discuss the role of electron-electron and electron-phonon correlations in current flow in the Coulomb Blockade regime, focusing specifically on nontrivial signatures arising from the break-down of mean-field theory. By solving transport equations directly in Fock space, we show that electron-electron interactions manifest as gateable excitations experimentally observed in the current-voltage characteristic. While these excitations might merge into an incoherent sum that allows occasional simplifications, a clear separation of excitations into slow `traps' and fast `channels' can lead to further novelties such as negative differential resistance, hysteresis and random telegraph signals. Analogous novelties for electron-phonon correlation include the breakdown of commonly anticipated Stokes-antiStokes intensities, and an anomalous decrease in phonon population upon heating due to reabsorption of emitted phonons.Comment: 14 pages 10 figures, Invited article for the special issue on "Conductivity of single molecules and supramolecular architectures", IOP Journal of Physics Condensed matte

Performance analysis of an interacting quantum dot thermoelectric system

We analyze the nanocaloritronic performance of an interacting quantum dot that is subject to an applied bias and an applied temperature gradient. It is now well known that, in the absence of phonon contribution, a weakly coupled non-interacting quantum dot can operate at thermoelectric efficiencies approaching the Carnot limit. However, it has also been recently pointed out that such peak efficiencies can only be achieved when operated in the reversible limit, with a vanishing current and hence a vanishing power output. In this paper, we point out three fundamental results affecting the thermoelectric performance due to the inclusion of Coulomb interactions: a) The reversible operating point carries zero efficiency, b) operation at finite power output is possible even at peak efficiencies approaching the Carnot value, and c) the evaluated trends of the the maximum efficiency deviate considerably from the conventional {\it{figure of merit}} $zT$ based result. Finally, we also analyze our system for thermoelectric operation at maximum power output.Comment: 10 pages, 6 figures, Resubmission- to be published in Phys. Rev.

Perfect Teleportation, Quantum state sharing and Superdense Coding through a Genuinely Entangled Five-qubit State

We investigate the usefulness of a recently introduced five qubit state by Brown \it et al. \normalfont \cite{Brown} for quantum teleportation, quantum state sharing and superdense coding. It is shown that this five-qubit state can be utilized for perfect teleportation of arbitrary single and two qubit systems. We devise various schemes for quantum state sharing of an arbitrary single and two particle state via cooperative teleportation. We later show that this state can be used for superdense coding as well. It is found that five classical bits can be sent by sending only three quantum bits.Comment: 8 Pages, added sections on state sharin

Biomedical engineering: current status and issues for developments in India

Biomedical engineering is an important field of engineering science that plays pivotal role in the modern health care systems. Rapid developments in medical technology has boosted the medicare system to a large extent. These developments greatly improved the quality, availability and efficiency of the health care delivery system. Technological advancement in the developed countries contributed their advancement in developing countries to certain extend. In countries like Indian these technological advancements can be beneficial only in the shorter period. It is rather impossible to visualize these benefits in the long term unless otherwise plan are devised to sustain these developments. Therefore, it is high time to start more biomedical engineering programs in the country. In this regard various technical and medical bodies must unit and cooperate in the proper molding of programs. Different biomedical societies and related organizations can play a greater role in the curriculum development. These organizations must contribute their efforts in designing course, projects, set short and long term goals. Also create an environment to establish active interactions with various institutions of interest. Initially plans should be drawn to modify the existing curricula by introducing new and need based programs. Also provide expertise to the institutes starting new biomedical engineering under graduate, post graduate and research programs. This paper outlines some of these issues related to the development and modernization of biomedical engineering programs in India and other developing countries

An analogue of Raynaud’s theorem :weak formal schemes and dagger spaces

We study the relationship between the categories of weak formal schemes and dagger spaces. We introduce the notion of weak formal blowups of weak formal schemes and show that they correspond to rational subdomains of the associated dagger spaces via the generic fiber functor. In analogy with Raynaud’s theorem in formal and rigid geometry, we establish an equivalence of categories between the localized category of quasiparacompact admissible weak formal schemes by weak formal blowups, and the category of quasi-paracompact quasi-separated dagger spaces

Probing electronic excitations in molecular conduction

We identify experimental signatures in the current-voltage (I-V) characteristics of weakly contacted molecules directly arising from excitations in their many electron spectrum. The current is calculated using a multielectron master equation in the Fock space of an exact diagonalized model many-body Hamiltonian for a prototypical molecule. Using this approach, we explain several nontrivial features in frequently observed I-Vs in terms of a rich spectrum of excitations that may be hard to describe adequately with standard one-electron self-consistent field (SCF) theories.Comment: Significantly different content -- inadequacy of SCF approach described with simple model, and a whole new class of experiments showing gate modulated current steps discussed in terms of excitations in the molecular many-body spac

Rectification by charging -- the physics of contact-induced current asymmetry in molecular conductors

We outline the qualitatively different physics behind charging-induced current asymmetries in molecular conductors operating in the weakly interacting self-consistent field (SCF) and the strongly interacting Coulomb Blockade (CB) regimes. A conductance asymmetry arises in SCF because of the unequal mean-field potentials that shift a closed-shell conducting level differently for positive and negative bias. A very different current asymmetry arises for CB due to the unequal number of open-shell excitation channels at opposite bias voltages. The CB regime, dominated by single charge effects, typically requires a computationally demanding many-electron or Fock space description. However, our analysis of molecular Coulomb Blockade measurements reveals that many novel signatures can be explained using a {{simpler}} orthodox model that involves an incoherent sum of Fock space excitations and {\it{hence treats the molecule as a metallic dot or an island}}. This also reduces the complexity of the Fock space description by just including various charge configurations only, thus partially underscoring the importance of electronic structure, while retaining the essence of the single charge nature of the transport process. We finally point out, however, that the inclusion of electronic structure and hence well-resolved Fock space excitations is crucial in some notable examples.Comment: 12 pages, 10 figure

Zebrafish retinal stem cell differentiation mechanisms are disrupted by embryonic ethanol exposure

poster abstractPrenatal alcohol exposure can lead to a wide range of developmental abnormalities, which are included under the umbrella term fetal alcohol spectrum disorder (FASD). To understand the genesis of FASD defects, the zebrafish is important mechanistic animal model, particularly for retinal development. Previous work from our laboratory showed that ethanol treatment during gastrulation through somitogenesis in zebrafish embryos could recapitulate human ocular defects including microphthalmia, optic nerve hypoplasia, and photoreceptor defects. Ethanol-treated embryos showed increased retinal proliferation in the outer nuclear layer (ONL), inner nuclear layer (INL), and ciliary marginal zone (CMZ). Retinoic acid (RA) and folic acid (FA) co-supplementation rescued most ethanol-induced retinal defects, suggesting that nutrient deficiencies contribute to FASD. To better understand the genesis of ethanol-induced retinal cell differentiation defects, effects of ethanol exposure on retinal stem cell populations in the CMZ and Müller glial cell populations were examined. Ethanol treated retinas had an expanded CMZ, and a reduced expression domain for the cell cycle exit marker, cdkn1c. Ethanol treated retinas also showed reduced GFAP-positive Müller glial cells, which are a stem cell population in the central retina. At 72 hpf, the ONL of ethanol exposed fish showed few photoreceptors expressing terminal differentiation markers. Importantly, these poorly differentiated photoreceptors co-expressed the bHLH differentiation factor, neuroD, indicating that ethanol exposure produced immature and undifferentiated photoreceptors. Reduced differentiation along with increased progenitor marker expression and proliferation suggest cell cycle exit disruption due to ethanol exposure. Ethanol exposure severely disrupted Wnt and Notch signaling, which are critical for stem cell behavior and differentiation. These defects were rescued by Wnt signaling agonist, RA, and FA treatments. These results suggest ethanol disrupted retinal cell differentiation mechanisms. Further analysis of underlying molecular mechanisms will provide insight into the ethanol-induced retinal defects and potential therapeutic targets

Ethanol-induced Retinal Defects are Rescued by Retinoic Acid Supplement in Developing Zebrafish Embryos

poster abstractFetal Alcohol Spectrum Disorder (FASD) is caused by prenatal alcohol exposure, producing a spectrum of defects including facial abnormalities, sensory (visual and auditory) deficits, impaired fine motor skills and learning deficits including mental retardation. Our laboratory has used a zebrafish model for FASD that exposes embryos to ethanol during early development (midblastula transition through somitogenesis). Children diagnosed with FASD frequently show severe eye defects ranging from small eyes, underdeveloped optic nerve, and cataract. Zebrafish embryos exposed to ethanol showed defects similar to human eye birth defects. Presence of ethanol affected the differentiation of many retinal cell types including, retinal ganglion cells and photoreceptors. We hypothesize that ethanol may affect retinal patterning by competing with Retinaldehyde dehydrogenase (Raldh), reducing retinoic acid (RA) synthesis and signaling. Co-treatment of embryos with ethanol and 10-9 M RA could rescue the photoreceptor and retinal ganglion cell differentiation defects in the retina. RA plays a crucial role in the dorso-ventral patterning of the retina, and the enzymes involved in RA biosynthesis are expressed in the ventral retina during mid-somitogenesis stage. Our experiments showed that ethanol exposure during that critical time window when Raldh is expressed in the ventral retina causes severe defects in retinal cell specification. No defects were induced by ethanol exposure at the earlier stages. Presence of RA during photoreceptor differentiation could rescue ethanol-induced photoreceptor differentiation defects. Future work will dissect molecular mechanisms underlying ethanol defects, including retinoic acid-mediated eye development mechanisms. Determining the effects of ethanol exposure on retinal morphogenesis and differentiation will help identify potential therapeutic targets for ocular defects in this regrettably frequent birth defect syndrome