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.

The topology and robustness of two Dirac cones in S-graphene: A tight binding approach.

Abstract: Present work reports an elegant method to address the emergence of two Dirac cones in a non-hexagonal graphene allotrope S-graphene (SG). We have availed nearest neighbour tight binding (NNTB) model to validate the existence of two Dirac cones reported from density functional theory (DFT) computations. Besides, the real space renormalization group (RSRG) scheme clearly reveals the key reason behind the emergence of two Dirac cones associated with the given topology. Furthermore, the robustness of these Dirac cones has been explored in terms of hopping parameters. As an important note, the Fermi velocity of the SG system (vF $$\simeq $$ c/80) is almost 3.75 times that of the graphene. It has been observed that the Dirac cones can be easily shifted along the symmetry lines without breaking the degeneracy. We have attained two different conditions based on the sole relations of hopping parameters and on-site energies to break the degeneracy. Further, in order to perceive the topological aspect of the system we have obtained the phase diagram and Chern number of Haldane model. This exact analytical method along with the supported DFT computation will be very effective in studying the intrinsic behaviour of the Dirac materials other than graphene.

Twin T-graphene: a new semiconducting 2D carbon allotrope.

Abstract: Two dimensional carbon allotropes with multiple atomic layers have attracted significant interest recently. In this work a new three atomic layer thick 2D carbon allotrope, twin T-graphene, is proposed. The sp2 hybridized dynamically stable phase is a nonmagnetic semiconducting material with an indirect band gap of 1.79 eV. Thermal stability investigations indicate that the material undergoes no change in the bonding pattern at 2000 K. The study of its mechanical properties indicates that it is an elastically isotropic soft material with low values of elastic constants. The electron mobility of the semiconductor is found to be nearly 375 cm2 V-1 s-1. The material when doped with single nitrogen in the tetragonal site undergoes a drastic change in its electronic properties and manifests itself in the form of a bipolar magnetic semiconductor which is a potential spintronic material. The study of the optical properties clearly indicates an optical gap of 1.89 eV. The material shows optical response in the visible range. Two sharp characteristic EELS peaks indicate the presence of this material. All these characteristics indicate that this material can have potential photocatalytic activity and be an attractive material for the applications in field effect transistors.

Acetylenic linkage dependent electronic and optical behaviour of morphologically distinct '-ynes'.

Abstract: We have critically examined the key role of acetylenic linkages (–CC–) in determining the opto-electronic responses of dynamically stable tetragonal (T) ‘-ynes’ with the help of a density functional theory method. The presence of –CC– between two tetra-rings invariably flips the electronic bands about the Fermi level. The underlying physics has been critically addressed with the help of a real space renormalization group (RSRG) scheme under a tight binding (TB) approximation. Besides, we have proposed an elegant approach to introduce and tune a band gap in the customarily metallic T graphene allotrope. The quantum dots of these systems exhibit diode like current–voltage (I–V) characteristics and can be used in negative differential resistance devices. In addition, the anisotropic optical properties evidently support the electronic states of the systems. In particular, the static dielectric constants for some of these ‘-ynes’ are enhanced compared to graphene and T graphene. The effective number of electrons participating in an interband transition shows saturation over 30 eV. Furthermore, electron energy loss spectra (EELS) peaks are consistent with the plasma frequencies of the corresponding systems. The intrinsic responses of the –CC– in these systems are extremely important for basic science and nanodevice research.

Highly efficient photocatalytic activity of CuO quantum dot decorated rGO nanocomposites

Abstract: CuO quantum dots (QD) of size 4.5 nm decorated on a rGO sheet to form nanocomposites with different weight percentages via a simple soft chemical route was reported here. Tuning of CuO QD absorption towards the visible region from the UV region in the presence of rGO was also observed. The luminescence of rGO was found to be quenched in rGO–CuO nanocomposites due to charge transfer from the lowest unoccupied molecular orbital of the rGO layer to the conduction band of CuO. Systematic and concise studies of photocatalytic performance towards degradation of methylene blue (MB) dye by CuO QD along with rGO–CuO nanocomposites were presented in this work. A nanocomposite with an equal weight percentage of rGO and CuO degrades almost 99% of MB under irradiation of visible light for 50 min, showing maximum degradation efficiency.

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.