Snbose National Center For Basic Science

Kolkata, India

Snbose National Center For Basic Science

Kolkata, India
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Devi Y.C.,Snbose National Center For Basic Science | Ghosh K.J.B.,Snbose National Center For Basic Science | Chakraborty B.,Snbose National Center For Basic Science | Scholtz F.G.,Stellenbosch University | Scholtz F.G.,National Institute for Theoretical Physics NITheP
Journal of Physics A: Mathematical and Theoretical | Year: 2014

Beginning with a review of the completely operatorial formulation of noncommutative quantum mechanics in 2D and 3D spaces using Hilbert- Schmidt operators, we analyze the issue of maximal localization, both in position and phase space, of a single particle by using symplectic invariant formulation of uncertainty relation through the computation of the variance matrix. We then extend the analysis to multi-particles and carry out second quantization by introducing basis independent field operators. This then facilitates the computation of thermal correlation functions and the associated effective statistical potential in two- and three-dimensional non-commutative space using an operator formulation that makes no reference to a star product. The corresponding results for the Moyal and Voros star products are then easily obtained by taking the corresponding overlap with Moyal and Voros bases. The forms of the correlation function and the effective potential are found to be the same, except that in the Voros case the thermal wave length undergoes a non-commutative deformation, ensuring that it has a lower bound of the order of √θ. It is shown that in a suitable basis (called here quasi-commutative) in the multi-particle sector the thermal correlation function coincides with the commutative result both in the Moyal and Voros cases, with an appropriate non-commutative correction to the thermal length in the Voros case, and that the Pauli principle is restored. © 2014 IOP Publishing Ltd.

Garain S.K.,Snbose National Center For Basic Science | Ghosh H.,Indian Center for Space Physics | Chakrabarti S.K.,Snbose National Center For Basic Science | Chakrabarti S.K.,Indian Center for Space Physics
Monthly Notices of the Royal Astronomical Society | Year: 2013

Low-and intermediate-frequency quasi-periodic oscillations (QPOs) in black hole candidates are believed to be due to oscillations of the Comptonizing regions in an accretion flow. Assuming that the general structure of an accretion disc is a two-component advective flow (TCAF), we numerically simulate the light curves emitted from an accretion disc for different accretion rates and find how the QPO frequencies vary. We use a standard Keplerian disc residing at the equatorial plane as a source of soft photons. These soft photons, after suffering multiple scattering with the hot electrons of the low angular momentum, sub-Keplerian, flow emerge out as hard radiation. The hydrodynamic and thermal properties of the electron cloud is simulated using a total variation diminishing (TVD) code. The TVD code is then coupled with a radiative transfer code which simulates the energy exchange between the electron and radiation using Monte Carlo technique. The resulting localized heating and cooling are included also. We find that the QPO frequency increases and the spectrum becomes softer as we increase the Keplerian disc rate. However, the spectrum becomes harder if we increase the sub-Keplerian accretion rate. We find that an earlier prediction that QPOs occur when the infall time-scale roughly matches with the cooling time-scale, originally obtained using a power-law cooling, remains valid even for Compton cooling. Our findings agree with the general observations of low-frequency QPOs in black hole candidates.© 2013 The Authors.

Mondal S.,Snbose National Center For Basic Science | Ram Ghimire R.,Snbose National Center For Basic Science | Raychaudhuri A.K.,Snbose National Center For Basic Science
Applied Physics Letters | Year: 2013

We report that photoresponse of ZnO in ultraviolet (UV) can be enhanced substantially by simultaneously applying a gate bias in an Electric Double Layer Field Effect Transistor configuration fabricated on ZnO as a channel. The effect arises from synergy between UV illumination and applied gate bias, which leads to a substantial enhancement in the device current. We propose that large carrier density created by the illumination and the gate leads to neutralization of some of the oxygen charged vacancies which in turn reduce potential scattering leading to enhanced field effect mobility. This is verified by gate bias controlled Photo Luminescence experiment. © 2013 AIP Publishing LLC.

Ghosh B.,Snbose National Center For Basic Science | Das K.,Snbose National Center For Basic Science | Raychaudhuri A.K.,Snbose National Center For Basic Science
Journal of Applied Physics | Year: 2011

In this paper we report what happens to a virgin oxide junction Pr 0.5Ca 0.5MnO 3/SrTi 0.95Nb 0.05O 3 (PCMO/Nb:STO), when it is subjected to cycling of voltage bias of moderate value (±4 V). It is found that the initial cycling leads to formation of a permanent state of lower resistance where the lower resistance arises predominantly due to the development of a shunt across the device film (PCMO). On successive voltage cycling with increasing magnitude, this state transforms into states of successive lower resistance that can be transformed back to the initial stable state on cycling to below a certain bias. A simple model based on p-n junction with shunt has been used to obtain information on the change of the junction on voltage cycling. It has been shown that the observation can be explained if the voltage cycling leads to lowering of the barrier at the interface and also reduction in series resistance. It is suggested that this lowering can be related to the migration of oxygen ions, which causes the vacancies at the junction region. Cross-sectional imaging of the junction shows formation of permanent filamentary bridges across the thickness of the PCMO after the virgin p-n junction is first taken through a voltage cycle, which would explain the appearance of a finite shunt across the p-n junction. © 2011 American Institute of Physics.

Saha-Dasgupta T.,Snbose National Center For Basic Science
Journal of Superconductivity and Novel Magnetism | Year: 2012

Double-perovskite compounds with general formula ABB′O6, have attracted a lot of attention in recent years due to a variety of properties exhibited by them. In this paper, we will review our recent study on a number of double-perovskite compounds, namely La-doped Sr2FeMoO6, Cr-based family of compounds, Sr2CrXO6 (X=W, Re, Os), characterized with spectacularly high ferromagnetic transition temperatures and the magneto-capacitive compound, La2NiMnO6, We will discuss the signature of hybridization-driven antiferromagnetism in La-doped Sr2FeMoO6, while the parent compound, Sr2FeMoO6, is a half-metallic ferromagnet. The magnetism in the 3d-5d double perovskite Sr2CrXO6 (X=W, Re, Os) is found to be driven by the interplay of the hybridization-driven mechanism and the superexchange, which resulted into progressive increase of Tc as one moves from W to Re to Os at the B′ site. Our work, in the context of La2NiMnO6, identifies its superexchange-driven microscopic origin being responsible for the near room-temperature insulating ferromagnetic behavior. © 2012 Springer Science+Business Media New York.

Ghosh N.,Vellore Institute of Technology | Samanta S.,Snbose National Center For Basic Science | Ghosh B.,Snbose National Center For Basic Science
Journal of Superconductivity and Novel Magnetism | Year: 2014

Fabrication of a nanojunction by making a nano- hole on PMMA layer deposited in between Au and Y1Ba2Cu3O 7-δ (YBCO) films is reported. We have measured the current-voltage characteristics across Au/YBCO nanojunction at different temperatures and studied Andreev Reflection (AR) phenomena. The presence of Zero Bias Conductance Peak (ZBCP) due to AR, which is considered to be the well-known feature of d-wave superconductors, has been noticed at temperatures below and above the transition temperature (T C <87 K) up to 130 K. The result obtained indicates the possible- existence of the phase-incoherent Cooper pairs above T C for Au/YBCO. © 2013 Springer Science+Business Media New York.

Rajini Kanth B.,Snbose National Center For Basic Science | Bhattacharjya D.,M. S. University of Baroda | Mukhopadhyay P.K.,Snbose National Center For Basic Science
Materials Science Forum | Year: 2010

Study o Ferromagnetic shape memory alloys (FSMAs) is an interesting topic of present day research because of their large magnetic field induced shape recovery. They are important materials for the development of sensors and actuator based applications. Attempt for miniaturization of these actuators and sensors have led to the study of thin films. Bulk CoNiAl alloys are promising FSMAs because of their higher ductility and large range of control over the magnetic and structural transformation temperatures. To investigate the physical properties in thin film form we fabricated CoNiAl alloy films by D.C. sputtering method on glass substrates (kept at room temperature) under various conditions. They were annealed in vacuum at 500°C for 1h. The samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and ellipsometry. Thickness of the films was found to be lying between 70 and 272 nm. Fine grained microstructure was found for all the deposited films. The transformation temperatures of the samples were taken from the resistivity measurement done between 80 and 350 K. Magnetization measurements were also done between 80 and 400 K by using a vibrating sample magnetometer, but the Curie temperatures of the present films were not found to be below 400 K and magnetoelastic couplings were found to be rather weak. © (2010) Trans Tech Publications.

Deb S.,University of Calcutta | Patra H.K.,University of Calcutta | Lahiri P.,University of Calcutta | Dasgupta A.K.,University of Calcutta | And 2 more authors.
Nanomedicine: Nanotechnology, Biology, and Medicine | Year: 2011

The nanoparticle (NP) response of platelets is shown to be critically dependent on extent of preactivation of platelets by an agonist like ADP. A transition from de-aggregatory to aggregatory state is triggered in the presence of gold NPs (AuNP) only in such critical conditions. Adhered and suspended platelets respond differentially to NPs. Preactivation in the adhered state induced by shear force explains such observation. The NP effect is associated with enhanced release reaction, tyrosine phosphorylation and CD62P expression level. Unlike cancer cells, whose response is maximal when NP size is optimal (within the range 50 - 70 nm), the platelet response monotonically increases with reduction of the AuNP size. The uptake study, using quenching of quinacrine hydrochloride fluorescence by AuNP, indicates that accumulation 18 nm AuNP is several-fold higher than the 68 nm AuNP. It is further shown that AuNP response can provide a simple measure for thrombotic risk associated with nano-drugs. © 2011 Elsevier Inc.

Toschi A.,Vienna University of Technology | Hansmann P.,Vienna University of Technology | Hansmann P.,Max Planck Institute for Solid State Research | Sangiovanni G.,Vienna University of Technology | And 3 more authors.
Journal of Physics: Conference Series | Year: 2010

Significant progress in the theoretical description of Mott-Hubbard metal-to-insulator transitions has been made in the last years, especially thanks to the LDA+DMFT approach (local density approximation + dynamical mean field theory). Obviously the main attention has been focused on the transition itself, as, for example, in the textbook case of the Cr-doped V 2O3. As we discuss here, however, also the study of the insulating phase, characterized by the opening of a visible Mott-Hubbard gap in the spectral functions is far from being trivial: Strong-correlation effects make this phase strongly sensitive to small changes of external parameters, much more than one would expect for an insulator. In this situation, requiring a full consistency of the theoretical calculations with data from different spectroscopies may provide the most precise estimate for the local Coulomb interaction U in the LDA+DMFT approach. © 2010 IOP Publishing Ltd.

Chatterjee S.,Chennai Mathematical Institute | Lahiri A.,Snbose National Center For Basic Science | Sengupta A.N.,Louisiana State University
Journal of Geometry and Physics | Year: 2014

We develop a new differential geometric structure using category theoretic tools that provides a powerful framework for studying bundles over path spaces. We study a type of connection forms, given by Chen integrals, over path spaces by placing such forms within a category-theoretic framework of principal bundles and connections. A new notion of 'decorated' principal bundles is introduced, along with parallel transport for such bundles, and specific examples in the context of path spaces are developed. © 2013.

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