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A. Ghosh

Publications and source records attributed to A. Ghosh.

At least 163 records · Page 9Linked to original sources
arXiv2006

Quantitative Analysis of Hydrogenated DLC Films by Visible Raman Spectroscopy

The correlations between properties of hydrogenated diamond like carbon films and their Raman spectra have been investigated. The films are prepared by plasma deposition technique, keeping different hydrogen to methane ratio during the growth process. The hydrogen concentration, sp$^3$ content, hardness and optical Tauc gap of the materials have been estimated from a detail analysis of their Raman spectra. We have also measured the same parameters of the films by using other commonly used techniques, like sp$^3$ content in films by x-ray photoelectron spectroscopy, their Tauc gap by ellipsometric measurements and hardness by micro-hardness testing. The reasons for the mismatch between the characteristics of the films, as obtained by Raman measurements and by the above mentioned techniques, have been discussed. We emphasize on the importance of the visible Raman spectroscopy in reliably predicting the above key properties of DLC films.

cond-mat.mtrl-sci↗
arXiv2005

Local transport in a disorder-stabilized correlated insulating phase

We report the experimental realization of a correlated insulating phase in 2D GaAs/AlGaAs heterostructures at low electron densities in a limited window of background disorder. This has been achieved at mesoscopic length scales, where the insulating phase is characterized by a universal hopping transport mechanism. Transport in this regime is determined only by the average electron separation, independent of the topology of background disorder. We have discussed this observation in terms of a pinned electron solid ground state, stabilized by mutual interplay of disorder and Coulomb interaction.

cond-mat.mes-hall↗
arXiv2005

Euler buckling in red blood cells: An optically driven biological micromotor

We investigate the physics of an optically-driven micromotor of biological origin. A single, live red blood cell, when placed in an optical trap folds into a rod-like shape. If the trapping laser beam is circularly polarized, the folded RBC rotates. A model based on the concept of buckling instabilities captures the folding phenomenon; the rotation of the cell is simply understood using the Poincarè sphere. Our model predicts that (i) at a critical intensity of the trapping beam the RBC shape undergoes large fluctuations and (ii) the torque is proportional to the intensity of the laser beam. These predictions have been tested experimentally. We suggest a possible mechanism for emergence of birefringent properties in the RBC in the folded state.

physics.bio-ph↗
arXiv2005

An improved estimate of black hole entropy in the quantum geometry approach

A proper counting of states for black holes in the quantum geometry approach shows that the dominant configuration for spins are distributions that include spins exceeding one-half at the punctures. This raises the value of the Immirzi parameter and the black hole entropy. However, the coefficient of the logarithmic correction remains -1/2 as before.

gr-qc↗
arXiv2004

A dynamic localization of 2D electrons at mesoscopic length scales

We have investigated the local magneto-transport in high-quality 2D electron systems at low carrier densities. The positive magneto-resistance in perpendicular magnetic field in the strongly insulating regime has been measured to evaluate the spatial concentration of localized states within a mesoscopic region of the samples. An independent measurement of the electron density within the same region shows an unexpected correspondence between the density of electrons in the metallic regime and that of the localized states in the insulating phase. We have argued that this correspondence manifests a rigid distribution of electrons at low densities.

cond-mat.mes-hall↗
arXiv2003

Possible evidence of a spontaneous spin-polarization in mesoscopic 2D electron systems

We have experimentally studied the non-equilibrium transport in low-density clean 2D electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the non-linear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at non-zero B, we could associate them to the opposite spin states of the system, indicating a spontaneous spin polarization at B = 0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground state property of the low-density 2D systems.

cond-mat.str-el↗
arXiv2000

Pre-Big Bang Cosmology and Quantum Fluctuations

The quantum fluctuations of a homogeneous, isotropic, open pre-big bang model are discussed. By solving exactly the equations for tensor and scalar perturbations we find that particle production is negligible during the perturbative Pre-Big Bang phase.

hep-th↗
arXiv1999

Back-Reaction to Dilaton-Driven Inflation

We compute the leading-order back-reaction to dilaton-driven inflation, due to graviton, dilaton and gauge-boson production. The one-loop effect turns out to be non-vanishing (unlike the case for pure de-Sitter and for power-law inflation), to be of relative order $\ell_P^2H^2(t)$, and to have the correct sign for favouring the exit to a FRW phase.

hep-th↗
arXiv1999

Ward Identity for Membranes

Ward identities in the case of scattering of antisymmetric three form RR gauge fields off a D2-brane target has been studied in type-IIA theory.

hep-th↗
arXiv1999

On the Thermodynamics of a Gas of AdS Black Holes and the Quark-Hadron Phase Transition

We discuss the thermodynamics of a gas of black holes in five-dimensional anti-de-Sitter (AdS) space, showing that they are described by a van der Waals equation of state. Motivated by the Maldacena conjecture, we relate the energy density and pressure of this non-ideal AdS black-hole gas to those of four-dimensional gauge theory in the unconfined phase. We find that the energy density rises rapidly above the deconfinement transition temperature, whilst the pressure rises more slowly towards its asymptotic high-temperature value, in qualitative agreement with lattice simulations.

hep-th↗
arXiv1998

Quantum Fluctuations in Open Pre-Big Bang Cosmology

We solve exactly the (linear order) equations for tensor and scalar perturbations over the homogeneous, isotropic, open pre-big bang model recently discussed by several authors. We find that the parametric amplification of vacuum fluctuations (i.e. particle production) remains negligible throughout the perturbative pre-big bang phase.

hep-th↗
arXiv1998

Reply to comment by Zaslavskii on extremal black hole action

It is shown that Zaslavskii's misunderstanding of our published proof of the irrelevance of all extremal black hole configurations (whether with equal charge and mass or not) rests on his refusal to see the essential difference between the correct inequality governing extremal and non-extremal actions and his incorrect version.

hep-th↗
arXiv1998

Effectiveness of One-dimensional gas models for black holes

A one-dimensional gas model has been constructed and shown to provide correct expressions for entropies for extremal and near-extremal BTZ black holes. Recently suggested boosting of black strings is used to compute the entropy for the Schwarzschild black hole also from this gas model.

hep-th↗
arXiv1997

Quantum correction to thermodynamical entropy of black hole

The entropy of a black hole can differ from a quarter of the area of the horizon because of quantum corrections. The correction is related to the contribution to the Euclidean functional integral from quantum fluctuations but is not simply equal to the correction to the effective action. A (2+1) dimensional rotating black hole is explicitly considered.

gr-qc↗
arXiv1997

Understanding the area proposal for extremal black hole entropy

Whereas the usual understanding is that the entropy of only a non-extremal black hole is given by the area of the horizon, there are derivations of an area law for extremal black holes in some model calculations. It is explained here how such results can arise in an approach where one sums over topologies and imposes the extremality condition after quantization.

hep-th↗
arXiv1996

Entropy of Extremal Dyonic Black Holes

For extremal charged black holes, the thermodynamic entropy is proportional not to the area but to the mass or charges. This is demonstrated here for dyonic extremal black hole solutions of string theory. It is pointed out that these solutions have zero classical action although the area is nonzero. By combining the general form of the entropy allowed by thermodynamics with recent observations in the literature it is possible to fix the entropy almost completely.

hep-th↗
arXiv1995

Multidimensional Isotropic and Anisotropic Q-Oscillator Models

q-oscillator models are considered in two and higher dimensions and their symmetries are explored. New symmetries are found for both isotropic and anisotropic cases. Applications to the spectra of triatomic molecules and superdeformed nuclei are discussed.

hep-th↗