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Author

Debnarayan Jana

Other affiliations: National Taiwan University
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.


Papers
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Journal ArticleDOI
TL;DR: Electric field induced band tuning, optical and thermoelectric responses in tetragonal germanene: a theoretical approach.
Abstract: In this article, we have systematically explored the electronic, optical and thermoelectric properties of tetragonal germanene (T-Ge) using first principles calculations. The ground state geometry of pristine T-Ge is buckled and exhibits nodal line semi-metallic behaviour. In addition, we have proposed a tight binding (TB) model Hamiltonian that efficiently explains the emergence of double Dirac points at the Fermi level of T-Ge. Furthermore, a hopping relation has been explored at which both Dirac points merge and then annihilate resulting in a direct band gap at the Γ point. To exploit the buckling of the system, we have employed a transverse electric field, which invariably breaks the sublattice symmetry and removes the degeneracies at the Fermi surface. Furthermore, the band gap at the Dirac points varies linearly with the external electric field strength. Our TB Hamiltonian adequately satisfies the first principles results even in the presence of an external electric field. Moreover, we have found that T-Ge offers efficient tuning of band gaps at the Dirac points compared to other buckled systems viz. hexagonal silicene and germanene. In addition, the optical behaviour of T-Ge has been explained in accordance with the electronic states of the system. The strong optical responses in a low energy region make the material efficient for optical nanodevice applications. Moreover, T-Ge shows relatively better thermoelectric behaviour than graphene. Therefore, the external electric field induced tunable band gap and intriguing low energy optical signals pave the way to choose T-Ge as a smart choice for optoelectronic device applications. Finally we have suggested probable routes for experimental realization of the T-Ge structure.

3 citations

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis involving optical properties of defected free standing (FS) germanene layer has been performed within density functional theory (DFT) framework, in which Arsenic (As), gallium (Ga), and beryllium (Be) are chosen as doping elements.
Abstract: Abstract Germanene, germanium version of graphene, is a novel member in the two-dimensional (2D) materials family. In this present study, a theoretical analysis involving optical properties of defected free standing (FS) germanene layer has been performed within density functional theory (DFT) framework. FS buckled germanene exhibits many fascinating and unconventional optical properties due to introductions of adatoms and voids. Arsenic (As), gallium (Ga) and beryllium (Be) are chosen as doping elements. Doping sites (same or different sub-lattice positions) play a crucial role to improve various optical properties. While Be doping, concentrations of Be are increased up to 18.75 % and void concentrations are increased up to 15.62 % (keeping fixed 3.12 % Be concentration). Emergence of several plasma frequencies occur in case of both parallel and perpendicular polarizations for defected germanene layers. Energy positions of peaks corresponding to maximum of imaginary parts of dielectric constants are red shifted for some As and Ga incorporated systems compared to pristine germanene. Absorption spectra peaks are more prominent for Be doped systems rather than void added systems. In addition, conductivity in infrared (IR) region is very high for the Be doped configurations in case of parallel polarization. Along with these, changes in other optical properties like refractive index, reflectivity, electron energy loss spectroscopy etc. are also analyzed briefly in this present study. We hope, this theoretical investigation may be regarded as an important tool to design novel opto-electronic tuning devices involving germanene in near future.

3 citations

Journal ArticleDOI
TL;DR: In this article, a scaling analysis of the variation of the real and the imaginary part of the dielectric permittivity ϵ1 and ϵ2 and the loss factor tan δ as a function of frequency and temperature is presented.
Abstract: The dielectric permittivity of yttrium-doped polycrystalline samples La 1 − x − y Y y Ca x MnO 3 with x = 0.05 , 0.33 and y=0.07 has been measured at frequencies f from 20 Hz to 2 MHz and at temperatures T from 80 K to 350 K . These samples were prepared in the form of bulk polycrystals(ceramics) by solid state reaction method and characterized by X-ray diffraction technique. With the decrease in temperature, this system exhibits a phase transition from paramagnetic insulating to ferromagnetic metallic at a temperature T MI = 225 K for x=0.05 and at 170 K for x=0.33. In this report, it is shown that such phase transition can be characterized by the exponents obtained from the scaling analysis of the variation of the real and the imaginary part of the dielectric permittivity ϵ1 and ϵ2 and the loss factor tan δ as a function of frequency and temperature. Results are analyzed systematically from the existing theoretical models and the scaling formalism.

2 citations

Posted Content
TL;DR: In this article, it was shown that charged scalar fields are always diamagnetic, even in the presence of interactions and at finite temperatures, and this generalises earlier work on the diamagnetism of charged spinless bosons to the case of infinite degrees of freedom.
Abstract: We show that charged scalar fields are always diamagnetic, even in the presence of interactions and at finite temperatures. This generalises earlier work on the diamagnetism of charged spinless bosons to the case of infinite degrees of freedom.

2 citations

Journal ArticleDOI
TL;DR: In this paper , the electronic and optical responses of multiferroic Bismuth ferrite (BiFeO3; BFO) have been critically explored based on the DFT + U approach, which is essential for its multidirectional application perspectives.
Abstract: Based upon Hubbard U corrected density functional theory (DFT), the electronic and optical responses of multiferroic Bismuth ferrite (BiFeO3; BFO) have been critically explored. Treating BFO as a strongly correlated insulating system, a Hubbard U parameter is considered for accurate description of localised Fe-3d states. However, the U parameter for the Fe-3d state is not sufficient as 2p states of O atoms are greatly influenced by Fe-3d states. Similar U parameters for Fe-3d as well as O-2p states are considered. An effective U value around 4.3 eV not only solves the band gap mismatch of BFO from both a theoretical and experimental aspect, but also predicts the exact nature of the band gap. Moreover, within this framework, the optical responses of BFO are critically investigated. Multiple transitions appearing in both theoretical and experimental absorption spectra are well explained by crystal field transitions of Fe3+ ions in a d5 high spin state, supported by the density of states curve of BFO. Therefore, based upon the DFT + U approach, accurate electronic and optical characteristics of BFO are critically explored, which is essential for its multidirectional application perspectives.

2 citations


Cited by
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Proceedings Article
14 Jul 1996
TL;DR: The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of ~2µK.
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.

3,530 citations

01 Sep 1955
TL;DR: In this paper, the authors restrict their attention to the ferrites and a few other closely related materials, which 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.
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.

2,659 citations

Journal ArticleDOI
TL;DR: In this paper, the authors 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 and 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 flexible membranes.
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.

1,367 citations