Showing papers by "Debnarayan Jana published in 2021"

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.

TPDH-graphene: A new two dimensional metallic carbon with NDR behaviour of its one dimensional derivatives

Abstract: A new two dimensional carbon allotrope TPDH-graphene (Tetra-Penta-Deca-Hexagonal-graphene) belonging to the tetragonal-pentagonal carbon ring family is proposed in this work using density-functional method. The allotrope satisfies all the conditions of structural stability. This allotrope has lower cohesive energy than many existing planar carbon allotropes. It can withstand temperature as high as 1000 K without loosing its structural integrity. However, its electronic structure reflects metallic character due to delocalised pz orbital near the Fermi level. Further, this mechanically stable elastically anisotropic structure shows directional variation of In-plane Young's modulus and Poisson's ratio. It is stronger than graphene in a particular direction. Moreover, The material can be identified by four characteristic peaks in the electron energy loss spectra within 10 eV energy. It has optical reflectance peaks in the visible range at ∼600 nm yellow colour. Interestingly, some nanoribbons of this material show semimetallic, semiconducting and metallic behaviour. Non-equilibrium Green's function method along with density-functional theory is employed to study the nanodevices made of these nanoribbons. Strong current regulation property and robust negative differential resistance effect with a peak-to-valley ratio 3.3 are observed in two nanodevices making TPDH-graphene an attractive material for use in nanoelectronics.

Impressive Thermoelectric Figure of Merit in Two-Dimensional Tetragonal Pnictogens: a Combined First-Principles and Machine-Learning Approach.

Abstract: Over the past decade, two-dimensional materials have gained a lot of interest due to their fascinating applications in the field of thermoelectricity. In this study, tetragonal monolayers of group-V elements (T-P, T-As, T-Sb, and T-Bi) are systematically analyzed in the framework of density functional theory in combination with the machine-learning approach. The phonon spectra, as well as the strain profile, dictate that these tetragonal structures are geometrically stable as well as they are potential candidates for experimental synthesis. Electronic analysis suggests that tetragonal pnictogens offer a band gap in the semiconducting regime. Thermal transport characteristics are investigated by solving the semiclassical Boltzmann transport equation. Exceptionally low lattice thermal conductivity has been observed as the atomic number increases in the group. The high Seebeck coefficient and electrical conductivity as well as the low thermal conductivity of T-As, T-Sb, and T-Bi lead to the generation of a very high thermoelectric figure of merit as compared to standard thermoelectric materials. Furthermore, the thermoelectric conversion efficiency of these materials has been observed to be much higher, which ensures their implications in thermoelectric device engineering.

Electronic and thermal transport in novel carbon-based bilayer with tetragonal rings: a combined study using first-principles and machine learning approach.

Abstract: In this article, the structural, electronic and thermal transport characteristics of bilayer tetragonal graphene (TG) are systematically explored with a combination of first-principles calculations and machine-learning interatomic potential approaches. Optimized ground state geometry of the bilayer TG structure is predicted and examined by employing various stability criteria. Electronic bandstructure analysis confirmed that bilayer TG exhibits a metallic band structure similar to the monolayer T-graphene structure. Thermal transport characteristics of the bilayer TG structure are explored by analysing thermal conductivity, the Seebeck coefficient, and electrical conductivity. The electronic part of the thermal conductivity shows linearly increasing behaviour with temperature, however the lattice part exhibits the opposite character. The lattice thermal conductivity part is investigated in terms of the three phonon scattering rates and weighted phase space. On the other hand, the Seebeck coefficient goes through a transition from negative to positive values with increasing temperature. The Wiedemann–Franz law regarding electrical transport of the bilayer TG is verified and confirms the universal Lorentz number. Specific heat of the bilayer TG structure follows the Debye model at low temperature and constant behaviour at high temperature. Moreover, the Debye temperature of the bilayer TG structure is verified by ab initio calculations as well as fitting the specific heat data using the Debye model.

Mechanical characteristics and electric field-influenced thermoelectric and optical responses of tetragonal germanene

Abstract: In this report, we systematically explore the mechanical characteristics and electric field-induced thermoelectric and optical responses of tetragonal germanene (T-Ge). Elastic investigations confirm the mechanical stability of the T-Ge structure and its softness relative to graphene. Pristine T-Ge exhibits node line semi-metallic behaviour with double Dirac cones. In the presence of an electric field, its semi-metallic band structure gains a semiconducting nature. These electronic modifications also bring significant enhancement in its thermoelectric and optical spectra. The thermoelectric figure of merit for the T-Ge structure reaches a maximum in the presence of a 0.625 V Å−1 electric field. The thermoelectric power also varies with the different field strengths. In the optical spectra, the low-energy dielectric behaviour is significantly enhanced with the external field. The static dielectric constant exhibits a typical variation with the external field. The strength of the EELS response peaks is also modified with the electric field strengths. These optical and thermoelectric modifications in the presence of an electric field increase the feasibility of the T-Ge structure as a smart choice for nano-device applications.

First-principles study of the optical and thermoelectric properties of tetragonal-silicene.

Abstract: We report the optical and thermoelectric properties of the two-dimensional Dirac material T-silicene (TS) sheet and nanoribbons (NRs) by first-principles calculations. Both the optical and thermoelectric properties of TS can be modified by tailoring the sheet into nanoribbons of different widths and edge geometries. The optical response of the structures is highly anisotropic. A π interband transition occurs in the visible range of incident light with parallel polarization. The optical response for asymmetric arm-chair TS nanoribbons (ATSNRs) is larger than for symmetric ATSNRs. The absorptions of asymmetric ATSNR are redshifted due to a decrease in the bandgap with the width of the NRs. Plasma frequencies of the sheet and the NRs are identified from the imaginary part of the dielectric function and electron energy loss spectra curves. Thermoelectric properties like electrical conductivity, Seebeck coefficient, power factor, and electronic figure of merit are also studied. Compared with graphene, the TS sheet possesses a higher electrical conductivity and a better figure of merit. Among the NRs, asymmetric ATSNRs exhibit a better thermoelectric performance. All these intriguing features of TS may shed light on fabricating smart opto-electronic and thermoelectric devices.

Emergence of magnetic anisotropy by surface adsorption of transition metal dimers on γ-graphyne framework.

Abstract: In this paper a systematic study is carried out to demonstrate the structural stability and magnetic novelty of adsorbing transition metal (TM) dimers (A-B) on graphyne (GY) surface, GY@A-B. Our research points out that the dimers are strongly adsorbed onto GY due to their large natural pores and the electron affinity of the sp-hybridized carbon atoms. Electronic properties of these dimer-graphyne composite systems are of particular importance as they behave as degenerate semiconductors with partial occupation of states atEF. Furthermore, their remarkable spin polarization (>80%) at Fermi energy (EF) can be of paramount importance in spintronics applications. Most of the GY@A-B structures exhibit large magnetic anisotropies as well as magnetic moments along the out-of-plane direction with respect to the GY surface. Particularly, GY@Co-Ir, GY@Ir-Ir and GY@Ir-Os structures possess positive magnetic anisotropic energies (MAE) of 121 meV, 81 meV and 137 meV, respectively, which are comparable to other well-known TM dimer doped systems. The emergence of high MAE can be understood using the second-order perturbation theory on the basis of the strong spin-orbit coupling (SOC) between the two TMs and the degeneracy of their d-orbitals nearEF. A close correspondence between the simulated and the analytical results has been established through our work. Further, a simple estimation shows that, GY@A-B structures have the potential to store data up to 64 PB m-2. These intriguing electronic characteristics along with magnetism suggest GY@A-B to be a promising material for future magnetic storage devices.