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.

Electronic and optical properties of non-hexagonal Dirac material S-graphene sheet and nanoribbons

Abstract: In this paper, we have critically explored the electronic, magnetic and optical properties of stable non-hexagonal carbon allotrope S-graphene (SG). The band structure of SG sheet exhibits two Dirac cones at distinct k → points. The ab-initio molecular dynamics study reveals the dynamical stability of the system up to 600 K. Besides, the sheet possesses appreciable mechanical stability. Furthermore, the electronic properties of the ribbons strongly depend on the edge geometry and width. The A-type S-graphene nanoribbons (SGNRs) with even N exhibit width-dependent direct bandgap semiconducting nature. Whereas, A-SGNRs with odd N are semi-metallic. In contrary, B-SGNRs are uniformly semi-metallic except for extremely narrow N = 1 and 2 case. Besides, the unexpected occurrence of Dirac point for the N = 4.5 C-SGNR has been explored. Moreover, the optical response of the sheet and nanoribbons are highly anisotropic. We have critically obtained the plasma frequency for the collective electron oscillation of the sheet at 5.37 eV under parallel incidence of electromagnetic waves. Besides, the effective number of electrons participating in direct interband transitions in sheet show a quasi-saturation between 3-10 eV. We thus expect these theoretical predictions will provide a basic understanding of the opto-electronic responses towards possible application of this non-hexagonal graphene network with multiple Dirac cones.

8-16-4 graphyne: Square-lattice two-dimensional nodal line semimetal with a nontrivial topological Zak index

Abstract: An unprecedented graphyne allotrope with square symmetry and nodal line semimetallic behavior has been proposed in the two-dimensional (2D) realm. The emergence of the Dirac loop around the high-symmetry points in the presence of both the inversion and time-reversal symmetries is a predominant feature of the electronic band structure of this system. Besides, the structural stability in terms of the dynamic, thermal, and mechanical properties has been critically established for the system. Following the exact analytical model based on the real-space renormalization group scheme and tight-binding approach, we have inferred that the family of 2D nodal line semimetals with square symmetry can be reduced to a universal four-level system in the low-energy limit. This renormalized lattice indeed explains the underlying mechanism responsible for the fascinating emergence of 2D square nodal line semimetals. Besides, the analytical form of the generic dispersion relation of these systems is well supported by our density-functional theory results. Finally, the nontrivial topological properties have been explored for the predicted system without breaking the inversion and time-reversal symmetry of the lattice. We have obtained that the edge states are protected by the nonvanishing topological index, i.e., Zak phase.

First principles Raman study of boron and nitrogen doped planar T-graphene clusters

Abstract: Tetragonal graphene (TG) is one of the theoretically proposed dynamically stable graphene allotropes. In this study, the Raman spectra, IR spectra and some electronic properties of pristine and doped (single boron (B) and nitrogen (N)) TG have been investigated by first-principles based density functional theory (DFT) at the B3LYP/6-31G(d) level. Formation energy computation indicates that for the pristine structures, stability increases with increasing cluster size. In addition, for a particular cluster size, single B doping introduces some distortion in the system while single N doping increases the stability of it. The Raman spectrum of the N doped system is dominated by a single peak but for the B doped system several intense lines are found. For all the structures low intensity similar breathing-like modes have been observed. Besides, relatively low (high) intensity Raman lines are found for single B (N) doping compared to those of the pristine one. The vibrational study also reveals the existence of a prominent phonon Raman line for pristine clusters which hardly changes its position and nature of vibration with varying cluster size. So this mode can be used for identification of pristine TG structures. Unlike pristine TG, the doped structures possess non-zero finite dipole moments due to asymmetry in charge distribution. Large values of the HOMO–LUMO gap as well as the absence of DOS at the Fermi level lead to the semiconducting nature of all the structures. All these theoretical predictions from DFT calculations may shed light on experimental observations involving TG systems.

Defects in 700 keV oxygen ion irradiated ZnO

Abstract: It is well known that energetic oxygen ions induce heavy crystalline disorder in ZnO, however, systematic study on this regard is very much limited Here, we present photoluminescence (PL), optical absorption and sheet resistance measurements on poly and single crystalline ZnO samples irradiated with 700 keV O ions Results have been compared with the effects of 12 MeV Ar irradiation on similar ZnO target Colour change of the samples with increasing O irradiation fluence has also been noted Non-monotonic variation of room temperature sheet resistance with the increase of fluence has been observed for polycrystalline ZnO Such an outcome has been understood as point defects transforming to bigger size clusters Near band edge (NBE) PL emission is largely reduced due to O ion irradiation However, at 10 K NBE emission can be observed for irradiated polycrystalline samples Irradiated ZnO single crystal does not show any band to band transition even at 10 K It is evident that dynamic recovery of defects is more effective in polycrystalline samples Ultraviolet–visible absorption spectrum of the irradiated ZnO crystal show pronounced sub-band gap absorption Oxygen irradiation generated new absorption band in ZnO is at 305 eV In the light of earlier reports, this particular band can be ascribed to absorption by neutral oxygen vacancy 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.