K. Iyakutti

Effect of surface doping on the band structure of graphene: a DFT study

Abstract Various techniques, like doping, vacancy creation, strain engineering are tried to open a gap in the bandstructure of graphene and in some cases the gap has opened up. However, when the gap opens up the Dirac cones disappear. Without Dirac cones, graphene loses all its novelty. So opening a gap in graphene, retaining Dirac cones has become a challenging task. We, through first principles study using Density Functional theory, have done band gap tuning investigations. We have succeeded in opening the band gap, retaining the Dirac cones. Surface doping (adsorption) of various elements are tried and finally surface doping of sulfur is found to induce band gap opening in graphene. The Dirac cones are retained and the graphene is now a semiconductor with fast moving massless Dirac Fermions. We are reporting this type of calculations for the first time.

First principles study of structural, electronic, elastic and magnetic properties of cerium and praseodymium hydrogen system REHx (RE: Ce, Pr and x=2, 3)

Abstract The structural, electronic, elastic and magnetic properties of cerium, praseodymium and their hydrides REH x (RE=Ce, Pr and x =2, 3) were investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At zero pressure all the hydrides were stable in the ferromagnetic state. The calculated lattice parameters were in good agreement with the experimental results. The bulk modulus decreased with the increase in the hydrogen content for these hydrides. The electronic structure revealed that di-hydrides were metallic whereas trihydrides were half metallic at zero pressure. A pressure-induced structural phase transition from cubic to hexagonal phase was predicted in these hydrides. The computed elastic constants indicated that these hydrides were mechanically stable at zero pressure. The calculated Debye temperature values were in good agreement with experimental and other theoretical results. A half metallic to metallic transition was also observed in REH 3 under high pressure. Ferromagnetism was quenched in these hydrides at high pressures.

First principles study of structural, electronic and mechanical properties of transition metal hydrides (TMH, TM=Mo, Tc, Ru)

Abstract The structural, electronic and mechanical properties of transition metal hydrides (TMH, TM=Mo, Tc, Ru) are investigated by means of first principles calculation based on density functional theory with generalized gradient approximation. Among the five crystallographic structures that have been investigated, the cubic phase is found to be more stable than the hexagonal ones. A structural phase transition from ZB to WC in MoH, NaCl to NiAs in TcH and NaCl to ZB to NiAs in RuH is also predicted under high pressure. The calculated elastic constants indicate that all the three hydrides are mechanically stable at ambient pressure.

Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt): a DFT study

Abstract First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.

Theoretical insights into the minority carrier lifetime of doped Si—A computational study

Using density functional theory, we have analyzed the ways and means of improving the minority carrier lifetime (MCL) by calculating the band structure dependent quantities contributing to the MCL. We have computationally modeled silicon doped with different elements like B, C, N, O, P, Ti, Fe, Ga, Ge, As, In, Sn, Sb, and Pt and looked at the effect of doping on MCL. In co-doping, the systems Si-B-Ga, Si-B-Ge, Si-B-2Ge, Si-B-Pt, Si-Ga-Ge, Si-Ga-2Ge, and Si-Ga-Pt are investigated. From our calculation, it is found that by doping and co-doping of Si with suitable elements having “s” and “p” electrons, there is a decrease in the recombination activity. The predicted effective minority carrier lifetime indicates the possibility of significant improvements. Based on the above studies, it is now maybe possible, with suitable choice of dopant and co-dopant material, to arrive at part of a standard production process for solar grade Si material.Using density functional theory, we have analyzed the ways and means of improving the minority carrier lifetime (MCL) by calculating the band structure dependent quantities contributing to the MCL. We have computationally modeled silicon doped with different elements like B, C, N, O, P, Ti, Fe, Ga, Ge, As, In, Sn, Sb, and Pt and looked at the effect of doping on MCL. In co-doping, the systems Si-B-Ga, Si-B-Ge, Si-B-2Ge, Si-B-Pt, Si-Ga-Ge, Si-Ga-2Ge, and Si-Ga-Pt are investigated. From our calculation, it is found that by doping and co-doping of Si with suitable elements having “s” and “p” electrons, there is a decrease in the recombination activity. The predicted effective minority carrier lifetime indicates the possibility of significant improvements. Based on the above studies, it is now maybe possible, with suitable choice of dopant and co-dopant material, to arrive at part of a standard production process for solar grade Si material.

Structural, Electronic, and Mechanical Properties of CoN and NiN: An Ab Initio Study

Abstract The structural stabilities of cobalt mononitride (CoN) and nickel mono-nitride (NiN) were investigated among the crystal structures, namely, NaCl (B1), CsCl (B2), and zinc blende (B3). It was found that the zinc blende (B3) phase was the most stable phase for both nitrides. A pressure-induced structural phase transition from B3 to B1 phase was predicted in these nitrides. The computed lattice parameter values were in agreement with the experimental values and other theoretical values. The electronic structures reveal that these nitrides are metallic at zero pressure. The computed elastic constants indicate that CoN and NiN are mechanically stable in the B1 and B3 phases. The variations of the elastic constants, bulk modulus, shear modulus, Poisson’s ratio, and elastic anisotropy factor with pressure were investigated. The Debye temperature θD values are reported for both the nitrides in their B1 and B3 phases. The high-pressure NaCl phase of both CoN and NiN were found to be ferromagnetic.

First principles study of structural and magnetic properties of transition metal nitrides TMN (TM = Cr, Mn)

ABSTRACT The structural stability of chromium nitride (CrN) and manganese nitride (MnN) is investigated among four different structures, namely, NaCl (Fm3m), zinc blende (F4-3m), orthorhombic (Pnma) and tetragonal (I4/mmm). It is found that the most stable phase is the zinc blende phase for CrN and MnN. The structural phase transition from zinc blende to orthorhombic phase is predicted at high pressure. At normal pressure, CrN and MnN are found to be antiferromagnetic. As the pressure is increased, antiferromagnetic-to-nonmagnetic phase transition is observed at the pressures of 169.5 GPa in CrN and 206 GPa in MnN. The elastic constants obey the Born--Huang criteria, suggesting that they are mechanically stable. The calculated B/G values indicate that CrN and MnN are ductile in nature.

Quantum scar and breakdown of universality in graphene: A theoretical insight

Graphene has brought forward a lot of new physics. One of them is the emergence of massless Dirac fermions in addition to the electrons and these features are new to physics. In this theoretical study, the signatures for quantum scar and the breakdown of universality in graphene are investigated with reference to the presence of these two types of fermions. Taking the graphene quantum dot (QD) potential as the confining potential, the radial part of Dirac equations are solved numerically. Concentrations of the two component eigen-wavefunctions about classical periodic orbits emerge as the signatures for the quantum scar. The sudden variations, in the ratio of the radial wave-functions (large and small components), R(g/f), with mass ratio κ are the signatures for breakdown of universality in graphene. The breakdown of universality occurs for the states k = −1 and k = 1, and the state k = −1 is more susceptible to the breakdown of universality.

First Principles Study of Electronic Structure, Magnetic, and Mechanical Properties of Transition Metal Monoxides TMO(TM=Co and Ni)

Abstract The ground-state properties, electronic structure, magnetic and mechanical properties of cobalt oxide (CoO) and nickel oxide (NiO) are investigated using generalised gradient approximation parameterised by Perdew–Burke–Ernzerhof (GGA-PBE) and GGA-PBE+U formalisms. These oxides are found to be stable in the antiferromagnetic (AFM) state at normal pressure. The computed lattice parameters are in agreement with the experimental and other theoretical works. Pressure-induced magnetic transition from AFM to ferromagnetic (FM) state is predicted in NiO at a pressure of 84 GPa. Both these compounds are found to be mechanically stable in the AFM state at normal pressure.

Structural and magnetic phase transition in samarium hydrogen system SmHx (x=1, 2, 3)

We report ab-initio calculations for the structural and magnetic phase transition of SmHx (x= 1, 2, 3) using the Vienna ab-initio simulation package (VASP). The non-spin polarized (NSP) and spin polarized (SP) calculations are performed for these hydrides at normal and high pressure. It is found that these compounds are stable in ferromagnetic state at normal pressure. The calculated lattice parameters and bulk modulus of these hydrides are in good agreement with the available experimental results. A pressure-induced structural phase transition from cubic to hexagonal phase in SmH and SmH2 and hexagonal to cubic phase in SmH3 is predicted. A pressure-induced ferromagnetic to nonmagnetic phase transition is observed in SmH, SmH2 and SmH3 at the pressures of 104 GPa, 76 GPa and 81 GPa respectively. Ferromagnetism is quenched in mono, di and tri hydrides at high pressures.