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  • Molecular Diversity Preservation International  (18)
  • American Institute of Physics (AIP)  (13)
  • 2015-2019  (31)
  • 1
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    Effect of total and pair configurational entropy in determining dynamics of supercooled liquids over a range of densities (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-07-19
    Description: In this paper, we present a study of supercooled liquids interacting with the Lennard Jones potential and the corresponding purely repulsive (Weeks-Chandler-Andersen) potential, over a range of densities and temperatures, in order to understand the origin of their different dynamics in spite of their structures being similar. Using the configurational entropy as the thermodynamic marker via the Adam Gibbs relation, we show that the difference in the dynamics of these two systems at low temperatures can be explained from thermodynamics. At higher densities both the thermodynamical and dynamical difference between these model systems decrease, which is quantitatively demonstrated in this paper by calculating different parameters. The study also reveals the origin of the difference in pair entropy despite the similarity in the structure. Although the maximum difference in structure is obtained in the partial radial distribution function of the B type of particles, the rdf of AA pairs and AB pairs gives rise to the differences in the entropy and dynamics. This work supports the observation made in an earlier study [A. Banerjee et al. , Phys. Rev. Lett. 113 , 225701 (2014)] and shows that they are generic in nature, independent of density.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    Composition dependence of the glass forming ability in binary mixtures: The role of demixing entropy (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-07-19
    Description: We present a comparative study of the glass forming ability of binary systems with varying composition, where the systems have similar global crystalline structure (CsCl+fcc). Biased Monte Carlo simulations using umbrella sampling technique show that the free energy cost to create a CsCl nucleus increases as the composition of the smaller particles is decreased. We find that systems with comparatively lower free energy cost to form CsCl nucleus exhibit more pronounced pre-crystalline demixing near the liquid/crystal interface. The structural frustration between the CsCl and fcc crystal demands this demixing. We show that closer to the equimolar mixture, the entropic penalty for demixing is lower and a glass forming system may crystallize when seeded with a nucleus. This entropic penalty as a function of composition shows a non-monotonic behaviour with a maximum at a composition similar to the well known Kob-Anderson (KA) model. Although the KA model shows the maximum entropic penalty and thus maximum frustration against CsCl formation, it also shows a strong tendency towards crystallization into fcc lattice of the larger “A” particles which can be explained from the study of the energetics. Thus for systems closer to the equimolar mixture although it is the requirement of demixing which provides their stability against crystallization, for KA model it is not demixing but slow dynamics and the presence of the “B” particles make it a good glass former. The locally favoured structure around “B” particles is quite similar to the CsCl structure and the incompatibility of CsCl and fcc hinders the fcc structure growth in the KA model. Although the glass forming binary systems studied here are quite similar, differing only in composition, we find that their glass forming ability cannot be attributed to a single phenomenon.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 3
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    Experimental evidence of beam-foil plasma creation during ion-solid interaction (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-08-03
    Description: Charge state evolution of the energetic projectile ions during the passage through thin carbon foils has been revisited using the X-ray spectroscopy technique. Contributions from the bulk and the solid surface in the charge changing processes have been segregated by measuring the charge state distribution of the projectile ions in the bulk of the target during the ion–solid interaction. Interestingly, the charge state distribution measured in the bulk exhibits Lorentzian profile in contrast to the well-known Gaussian structure observed using the electromagnetic methods and the theoretical predictions. The occurrence of such behavior is a direct consequence of the imbalance between charge changing processes, which has been seen in various cases of the laboratory plasma. It suggests that the ion-solid collisions constitute high-density, localized plasma in the bulk of the solid target, called the beam-foil plasma. This condensed beam-foil plasma is similar to the high-density solar and stellar plasma which may have practical implementations in various fields, in particular, plasma physics and nuclear astrophysics. The present work suggests further modification in the theoretical charge state distribution calculations by incorporating the plasma coupling effects during the ion–solid interactions. Moreover, the multi-electron capture from the target exit surface has been confirmed through comparison between experimentally measured and theoretically predicted values of the mean charge state of the projectile ions.
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 4
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    Magnetic and magnetocaloric properties of quasi-one-dimensional Ising spin chain CoV2O6 (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-04-07
    Description: We have investigated the magnetic and magnetocaloric properties of antiferromagnetic Ising spin chain CoV 2 O 6 by magnetization and heat capacity measurements. Both monoclinic α -CoV 2 O 6 and triclinic γ -CoV 2 O 6 exhibit field-induced metamagnetic transitions from antiferromagnetic to ferromagnetic state via an intermediate ferrimagnetic state with 1/3 magnetization plateau. Due to the field-induced metamagnetic transitions, these systems show large conventional as well as inverse magnetocaloric effects. In α -CoV 2 O 6 , we observe field-induced complex magnetic phases and multiple magnetization plateaus below 6 K when the field is applied along c axis. Several critical temperatures and fields have been identified from the temperature and field dependence of magnetization, magnetic entropy change, and heat capacity to construct the H–T phase diagram. As compared to α -CoV 2 O 6 , γ -CoV 2 O 6 displays a relatively simple magnetic phase diagram. Due to the large magnetic entropy change and adiabatic temperature change at low or moderate applied magnetic field, γ -CoV 2 O 6 may be considered as a magnetic refrigerant in the low-temperature region below 20 K.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 5
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    Breaking the bottleneck: Use of molecular tailoring approach for the estimation of binding energies at MP2/CBS limit for large water clusters (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-03-10
    Description: A pragmatic method based on the molecular tailoring approach (MTA) for estimating the complete basis set (CBS) limit at Møller-Plesset second order perturbation (MP2) theory accurately for large molecular clusters with limited computational resources is developed. It is applied to water clusters, (H 2 O) n (n = 7, 8, 10, 16, 17, and 25) optimized employing aug-cc-pVDZ (aVDZ) basis-set. Binding energies (BEs) of these clusters are estimated at the MP2/aug-cc-pVNZ (aVNZ) [N = T, Q, and 5 (whenever possible)] levels of theory employing grafted MTA (GMTA) methodology and are found to lie within 0.2 kcal/mol of the corresponding full calculation MP2 BE, wherever available. The results are extrapolated to CBS limit using a three point formula. The GMTA-MP2 calculations are feasible on off-the-shelf hardware and show around 50%–65% saving of computational time. The methodology has a potential for application to molecular clusters containing ∼100 atoms.
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    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 6
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    Magnetic phase characterization of nanocrystalline La2NiMnO6 using alternating current conductance (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-07-17
    Description: The signature of various disordered phases is inferred from the measurement of the real part of alternating current conductance Σ( T , f ) of a nanocrystalline double perovskite La 2 NiMnO 6 . The system exhibits a paramagnetic insulating (PMI) phase at high temperatures, a ferromagnetic insulating (FMI) phase at low temperatures, and a Griffiths-like phase in the intermediate temperature range. In these three phases, Σ( T , f ) shows qualitatively similar variation with frequency f . At a fixed temperature T , Σ( T , f ) remains constant to its Ohmic value Σ 0 up to a certain frequency, known as the onset frequency f c and increases with increasing f beyond f c . Scaled appropriately, Σ( T , f ) versus f data corresponding to these three regimes fall on the same master curve indicating the universal nature of the scaling behaviour of alternating current conductance. This onset frequency f c scales with Σ 0 as f c ∼ Σ 0 x f with x f as the nonlinearity exponent. This exponent x f shows a gradual crossover from 1.025 ± 0.006 in FMI phase to 0.518 ± 0.07 in PMI phase in an intermediate temperature range signifying the presence of Griffiths-like phase. A simple phenomenological R–RC model consistent with the microstructural conduction mechanisms in PMI and FMI phases is developed to generate the qualitative non-Ohmic character of ac conductance, the onset frequency f c , and the nonlinearity exponent x f . Existing scaling theories with reliable models are used to analyze and compare the results of ac conductance in similar systems.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 7
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    Emergence of dynamic cooperativity in the stochastic kinetics of fluctuating enzymes (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-08-30
    Description: Dynamic co-operativity in monomeric enzymes is characterized in terms of a non-Michaelis-Menten kinetic behaviour. The latter is believed to be associated with mechanisms that include multiple reaction pathways due to enzymatic conformational fluctuations. Recent advances in single-molecule fluorescence spectroscopy have provided new fundamental insights on the possible mechanisms underlying reactions catalyzed by fluctuating enzymes. Here, we present a bottom-up approach to understand enzyme turnover kinetics at physiologically relevant mesoscopic concentrations informed by mechanisms extracted from single-molecule stochastic trajectories. The stochastic approach, presented here, shows the emergence of dynamic co-operativity in terms of a slowing down of the Michaelis-Menten (MM) kinetics resulting in negative co-operativity. For fewer enzymes, dynamic co-operativity emerges due to the combined effects of enzymatic conformational fluctuations and molecular discreteness. The increase in the number of enzymes, however, suppresses the effect of enzymatic conformational fluctuations such that dynamic co-operativity emerges solely due to the discrete changes in the number of reacting species. These results confirm that the turnover kinetics of fluctuating enzyme based on the parallel-pathway MM mechanism switches over to the single-pathway MM mechanism with the increase in the number of enzymes. For large enzyme numbers, convergence to the exact MM equation occurs in the limit of very high substrate concentration as the stochastic kinetics approaches the deterministic behaviour.
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    Topics: Chemistry and Pharmacology , Physics
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  • 8
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    High-endurance megahertz electrical self-oscillation in Ti/NbOx bilayer structures (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-05-27
    Description: Electrical self-oscillation is reported for a Ti/NbO x negative differential resistance device incorporated in a simple electric circuit configuration. Measurements confirm stable operation of the oscillator at source voltages as low as 1.06 V, and demonstrate frequency control in the range from 2.5 to 20.5 MHz for voltage changes as small as ∼1 V. Device operation is reported for 〉6.5 × 10 10 cycles, during which the operating frequency and peak-to-peak device current decreased by ∼25%. The low operating voltage, large frequency range, and high endurance of these devices makes them particularly interesting for applications such as neuromorphic computing.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 9
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    Self-assembly of an NbO2 interlayer and configurable resistive switching in Pt/Nb/HfO2/Pt structures (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-10-01
    Description: A configurable resistive switching response is reported for Pt/Nb/HfO 2 /Pt devices subjected to different set compliance currents. When operated at a low compliance-current (∼100  μ A), devices show uniform bipolar resistive switching behavior. As the compliance current is increased (∼500  μ A), the switching mode changes to integrated threshold-resistive (1S1M) switching, and at still higher currents (∼1 mA), it changes to symmetric threshold switching (1S) characteristic of threshold switching in NbO 2 − δ . These switching transitions are shown to be consistent with the development of an NbO 2−δ interlayer at the Nb/HfO 2 interface that is limited by the set compliance current due to its effect on oxygen transport and local Joule heating. The proposed mechanism is supported by finite element modeling of the 1S1M response assuming the presence of such an interlayer. These findings help to understand role of interface reactions in controlling device performance and provide a means for the self-assembly of integrated 1S1M resistive random access memory structures.
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    Topics: Physics
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  • 10
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    Kinetics of protein adsorption on gold nanoparticle with variable protein structure and nanoparticle size (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-10-29
    Description: The spontaneous protein adsorption on nanomaterial surfaces and the formation of a protein corona around nanoparticles are poorly understood physical phenomena, with high biological relevance. The complexity arises mainly due to the poor knowledge of the structural orientation of the adsorbed proteins onto the nanoparticle surface and difficulties in correlating the protein nanoparticle interaction to the protein corona in real time scale. Here, we provide quantitative insights into the kinetics, number, and binding orientation of a few common blood proteins when they interact with citrate and cetyltriethylammoniumbromide stabilized spherical gold nanoparticles with variable sizes. The kinetics of the protein adsorption was studied experimentally by monitoring the change in hydrodynamic diameter and zeta potential of the nanoparticle-protein complex. To understand the competitive binding of human serum albumin and hemoglobin, time dependent fluorescence quenching was studied using dual fluorophore tags. We have performed molecular docking of three different proteins—human serum albumin, bovine serum albumin, and hemoglobin—on different nanoparticle surfaces to elucidate the possible structural orientation of the adsorbed protein. Our data show that the growth kinetics of a protein corona is exclusively dependent on both protein structure and surface chemistry of the nanoparticles. The study quantitatively suggests that a general physical law of protein adsorption is unlikely to exist as the interaction is unique and specific for a given pair.
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    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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