Ranber Singh

Magnetism in graphene due to single-atom defects: dependence on the concentration and packing geometry of defects

The magnetism in graphene due to single-atom defects is examined by using spin-polarized density functional theory. The magnetic moment per defect due to substitutional atoms and vacancy defects is dependent on the density of defects, while that due to adatom defects is independent of the density of defects. It reduces to zero with decrease in the density of substitutional atoms. However, it increases with decrease in density of vacancies. The graphene sheet with B adatoms is nonmagnetic, but with C and N adatoms it is magnetic. The adatom defects distort the graphene sheet near the defect perpendicular to the sheet. The distortion in graphene due to C and N adatoms is significant, while the distortion due to B adatoms is very small. The vacancy and substitutional atom (B, N) defects in graphene are planar in the sense that there is in-plane displacement of C atoms near the vacancy and substitutional defects. Upon relaxation the displacement of C atoms and the formation of pentagons near the vacancy site due to Jahn-Teller distortion depends upon the density and packing geometry of vacancies.

Temperature dependence of the thermodynamic functions of strongly interacting liquid alloys

A model is proposed to obtain expressions for thermodynamic functions like the heat of mixing (HM), excess specific heat ( Delta CP), volume of mixing (VM) and isothermal compressibility ( chi T) of liquid binary alloys with a strong compound-forming tendency in order to cover a large range of temperature. This has been used to analyse the temperature dependence of the free energy of mixing (GM), HM and Delta CP for liquid Li-Sn alloys. The specific heat of the liquid Li7Sn2 alloy has been determined experimentally by drop calorimetric heat content measurements and is found to support the theoretical value, which is based on EMF analysis. The results indicate self-coordination towards the Sn-rich end and strong hetero-coordination in the region 0.33<or=cLi<or=1.0, being maximum around Li3Sn. The influence of the chemical short-range order (CSRO) on the concentration fluctuations, Scc(0), chemical diffusion, D, and order parameter, alpha 1, has been considered. Scc(0) registers a dominant temperature effect in the self-coordinated region, whereas the effect of temperature on the diffusion coefficient is most visible in the hetero-coordinated region.

Sample dependence of the structural, vibrational, and electronic properties of a − Si : H : A density-functional-based tight-binding study

In order to investigate the sample dependence of various properties of hydrogenated amorphous silicon (a-Si:H), we have generated four samples with 216 silicon atoms and 24 hydrogen atoms using the density functional based tight binding molecular dynamics simulations. The overall structural properties of these model samples are in agreement with the previous theoretical and experimental results. While the Si-Si and Si-H pair correlation functions are independent of preparation procedure as well as initial conditions, the H-H pair correlation functions are sample dependent. The distribution of hydrogen atoms in all the samples is nonuniform and depends upon the preparation procedure as well as the initial structure from which the hydrogenated amorphous silicon sample is generated. The Si-Si bond length and Si-Si-Si bond angle distributions are nearly independent of sample preparation procedure, but Si-H bond length distributions are sample dependent. The peaks in the vibrational density of states (VDOS) at high frequencies, which are due to the Si-H bond vibrations, are in reasonable accord with the experimental results. The positions of the high frequency peaks are found to be dependent on the local environment which changes from one sample to another. While the high frequency vibrational modes related to Si-Si bond vibrations are moderately localized, the vibrational modes related to Si-H bond vibrations in a-Si:H samples are highly localized. In samples generated

Spin-polarized density functional investigation into ferromagnetism in C-doped (ZnO) n clusters; n = 1-12, 16

We investigate the ferromagnetism in ZnO clusters due to vacancy defects and C impurities doped at substitutional O or Zn sites, and interstitial sites. The total energy calculations suggest C at the O site is more stable than that at the Zn site in ZnO clusters. The total magnetic moments of Zn(n)O(n-m)C(m) clusters are 2.0 μ(B)/C. However, when two C atoms are bonded to the same Zn atom they interact antiferromagnetically and the total magnetic moment becomes less than 2.0 μ(B)/C. The interstitial C defects in ZnO clusters induce small magnetic moments. The combination of substitutional and interstitial C defects in ZnO clusters leads to magnetic moments of 0.0-2.0 μ(B)/C. The presence of vacancy defects in addition to substitutional C defects gives magnetic moments of greater than 2.0 μ(B)/C. These results suggest that the experimentally observed sample dependence of magnetic moments in ZnO systems is largely due to the different concentrations of substitutional and interstitial C impurities and the presence of vacancy defects in ZnO samples prepared under different growth conditions.

Phonons in nanocrystalline fcc nickel

The enhancement in the phonon DOS of nanocrystalline fcc Ni at low and high energies is due to the anisotropic changes in the interatomic force constants as compared to the bulk phase fcc Ni. The specific heat and vibrational entropy of nanophase is found to be greater than that of the bulk phase. There occurred a peak in the excessive specific heat of nanocrystalline fcc Ni at low temperatures. The excessive entropy of nanocrystalline Ni also has a peak at low temperatures but it becomes constant at high temperatures. The free energy of nanophase is lower than that of the bulk phase. The difference in the free energies of bulk phase and nanophase Ni increases linearly with temperature.

Model potential theory of the dynamic susceptibility of metals: application to vanadium, copper and aluminium

Expressions for both the orbital and spin contributions to the dynamic susceptibility are derived using the Shaw-Harrison model wavefunction transformation. For the Heine-Abarenkov-type transition-metal model potential, they are separated into two parts; one caused by the non-nodal character of the model wavefunction and other by the depletion hole around the ion core. The formalism is applicable to d and non-d band metals depending upon the choice of the model potential. Numerical results are presented for simple, noble and transition metals at various values of momentum and energy transfer. Al, Cu and V are chosen to be the metals used in the present investigations. The depletion-hole contribution is found to be of the same order of magnitude as the contribution of the smooth part. It is noticed that the structural features of the magnetic susceptibility are dominated by the spin response rather than by the orbital response. The orbital susceptibility as a whole is found to be about 10-80% that of the spin susceptibility except at very small momentum transfer where it dominates over the spin susceptibility. Therefore it is emphasised that the orbital contribution cannot justifiably be neglected when comparing the results with experimental measurements. The diamagnetic behaviour dominates in Cu and Al while the paramagnetic behaviour dominates in V.

Effects of charged defects on the electronic and optical properties of self-assembled quantum dots

We investigate the effects of point charge defects on the single particle electronic structure, emission energies, fine structure splitting and oscillator strengths of excitonic transitions in strained In

Phonon density of states in nanocrystalline57Fe

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Manipulating fine structure splitting in semiconductor quantum dots

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Dynamics of hydrogen in hydrogenated amorphous silicon

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