Debnarayan Jana

Bio: Debnarayan Jana is an academic researcher from University of Calcutta. The author has contributed to research in topics: Graphene & Density functional theory. The author has an hindex of 23, co-authored 127 publications receiving 2328 citations. Previous affiliations of Debnarayan Jana include National Taiwan University.

Magnetic phase characterization of nanocrystalline La2NiMnO6 using alternating current conductance

Abstract: 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 La2NiMnO6. 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 Σ0 up to a certain frequency, known as the onset frequency fc and increases with increasing f beyond fc. 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 fc scales with Σ0 as fc∼Σ0xf with xf as the nonlinearity exponent. This exponent xf shows a gradual crossover from 1.025 ± 0.006 in FMI phase...

Beyond T-graphene: Two-dimensional tetragonal allotropes and their potential applications

Abstract: Breakthrough of graphene dictates that decreasing dimensionality of the semiconducting materials can generate unusual electronic structures, excellent mechanical, and thermal characteristics with remarkable stability. Silicene, germanene, and stanene are the next 2D stable counterparts of other elements belonging to the same group. Since these monolayers possess hexagonal symmetry, scientists had already explored the possibility in the post graphene era of whether hexagonal symmetry was the main and utmost criterion for achieving Dirac cone. This motivation gave birth to T-graphene, a tetragonal network comprised of carbon atoms. However, T-graphene is not the only candidate for exhibiting Dirac fermion. In recent days, tetragonal monolayers of Si and Ge, i.e., T-Si and T-Ge, have been predicted to be stable. These 2D tetragonal allotropes remarkably possess double Dirac cones in their electronic band structure. As these monolayers possess buckling similar to silicene and germanene, the electronic bandgap can be easily introduced in the presence of an external electric field. Another technique to open bandgap is to apply strain in hydrogenated tetragonal networks. Tunable electronic properties in these tetragonal systems make them efficient for optoelectronics as well as thermoelectric applications. Moreover, due to delocalized π electrons, quantum dot systems comprised of tetragonal Si and Ge network show remarkable characteristics in the field of nonlinear optics. Recently, based on theoretical calculations, a bilayer T-graphene system is predicted with excellent mechanical strength relative to its monolayer variant. Not only group-IVA, group-VA elements also exhibit stable monolayer structures. Rather than T-graphene, T-Si, and T-Ge, these monolayers, however, possess intrinsic semiconducting properties, which enable them as a potential candidate for optoelectronic applications. Furthermore, several possible routes have been introduced to realize these systems experimentally. In this topical Review, we would critically explore the recent advancements of 2D tetragonal networks containing group-IVA and VA elements and their possible application perspectives in the field of thermoelectrics and nano-photonics.

Effect and Characterization of Stone–Wales Defects on Graphene Quantum Dot: A First-Principles Study

Abstract: A first principles based density functional theory (DFT) has been employed to identify the signature of Stone–Wales (SW) defects in semiconducting graphene quantum dot (GQD). Results show that the G mode in the Raman spectra of GQD has been red shifted to 1544.21 cm − 1 in the presence of 2.08% SW defect concentration. In addition, the intensity ratio between a robust low intense contraction–elongation mode and G mode is found to be reduced for the defected structure. We have also observed a Raman mode at 1674.04 cm − 1 due to the solo contribution of the defected bond. The increase in defect concentration, however, reduces the stability of the structures. As a consequence, the systems undergo structural buckling due to the presence of SW defect generated additional stresses. We have further explored that the 1615.45 cm − 1 Raman mode and 1619.29 cm − 1 infra-red mode are due to the collective stretching of two distinct SW defects separated at a distance 7.98 A. Therefore, this is the smallest separation between the SW defects for their distinct existence. The pristine structure possesses maximum electrical conductivity and the same reduces to 0.37 times for 2.08% SW defect. On the other hand, the work function is reduced in the presence of defects except for the structure with SW defects separated at 7.98 A. All these results will serve as an important reference to facilitate the potential applications of GQD based nano-devices with inherent topological SW defects.

Bose-Einstein condensation in a gas of sodium atoms

Abstract: Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Intrinsic ripples in graphene

Abstract: The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.