M.C. Valsakumar

Temperature dependent structural properties and bending rigidity of pristine and defective hexagonal boron nitride

Structural and thermodynamical properties of monolayer pristine and defective boron nitride sheets (h-BN) have been investigated in a wide temperature range by carrying out atomistic simulations using a tuned Tersoff-type inter-atomic empirical potential. The temperature dependence of lattice parameter, radial distribution function, specific heat at constant volume, linear thermal expansion coefficient and the height correlation function of the thermally excited ripples on pristine as well as defective h-BN sheet have been investigated. Specific heat shows considerable increase beyond the Dulong-Petit limit at high temperatures, which is interpreted as a signature of strong anharmonicity present in h-BN. Analysis of the height fluctuations, ⟨h2⟩, shows that the bending rigidity and variance of height fluctuations are strongly temperature dependent and this is explained using the continuum theory of membranes. A detailed study of the height-height correlation function shows deviation from the prediction of harmonic theory of membranes as a consequence of the strong anharmonicity in h-BN. It is also seen that the variance of the height fluctuations increases with defect concentration.

Evolutionary algorithm based structure search for hard ruthenium carbides

An exhaustive structure search employing evolutionary algorithm and density functional theory has been carried out for ruthenium carbides, for the three stoichiometries Ru1C1, Ru2C1 and Ru3C1, yielding five lowest energy structures. These include the structures from the two reported syntheses of ruthenium carbides. Their emergence in the present structure search in stoichiometries, unlike the previously reported ones, is plausible in the light of the high temperature required for their synthesis. The mechanical stability and ductile character of all these systems are established by their elastic constants, and the dynamical stability of three of them by the phonon data. Rhombohedral structure is found to be energetically the most stable one in Ru1C1 stoichiometry and hexagonal structure , the most stable in Ru3C1 stoichiometry. RuC–Zinc blende system is a semiconductor with a band gap of 0.618 eV while the other two stable systems are metallic. Employing a semi-empirical model based on the bond strength, the hardness of RuC–Zinc blende is found to be a significantly large value of ~37 GPa while a fairly large value of ~21GPa is obtained for the RuC–Rhombohedral system. The positive formation energies of these systems show that high temperature and possibly high pressure are necessary for their synthesis.

Ab initio study on stacking sequences, free energy, dynamical stability and potential energy surfaces of graphite structures

Ab initio simulations have been performed to study the structure, energetics and stability of several plausible stacking sequences in graphite. These calculations suggest that in addition to the standard structures, graphite can also exist in AA-simple hexagonal, AB-orthorhombic and ABC-hexagonal type stacking. The free energy difference between these structures is very small (∼1xa0meV/atom), and hence all the structures can coexist from purely energetic considerations. Calculated x-ray diffraction patterns are similar to those of the standard structures for 2θxa0⩽xa070°. Shear elastic constant C44 is negative in AA-simple hexagonal, AB-orthorhombic and ABC-hexagonal structures, suggesting that these structures are mechanically unstable. Phonon dispersions show that the frequencies of some modes along the Γ–A direction in the Brillouin zone are imaginary in all of the new structures, implying that these structures are dynamically unstable. Incorporation of zero point vibrational energy via the quasi-harmonic approximation does not result in the restoration of dynamical stability. Potential energy surfaces for the unstable normal modes are seen to have the topography of a potential hill for all the new structures, confirming that all of the new structures are inherently unstable. The fact that the potential energy surface is not in the form of a double well implies that the structures are linearly as well as globally unstable.

Resonant tunnel magnetoresistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

The non-equilibrium Greens function technique is used to study the transport characteristics of double-barrier magnetic tunnel junctions. The exchange coupling strength of the electrodes is found to be crucial in deciding the magnetoresistance characteristics of these devices. At sufficiently large values of the magnetic coupling strength, the device is found to exhibit resonant tunnel magnetoresistance and its magnitude is found to be large. The existence of pure spin currents in these devices when there is antiferromagnetic coupling between the end electrodes is found to be the primary cause of resonant tunnel magnetoresistance. The influence of the band occupation of the electrodes and the many-body interaction present in the electrode regions on the spin current and magnetoresistance are also studied.

Middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

Using the non-equilibrium Greens function modeling of the transport characteristics of tunnel devices, we have found the middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions. It is observed from our study that even a weak ferromagnetism of the middle metallic layers is capable of promoting resonant tunnel magneto-resistance in these devices and the strength of the ferromagnetism is found to have strong influence on the bias dependence of the resonant tunnel magneto-resistance. The spin-up and spin-down currents flow in opposite directions for certain band occupancies and at certain bias voltage ranges when there is antiferromagnetic coupling between the electrodes of the tunnel junction. Resonant tunnel magneto-resistance occurs when the net current (sum of spin-up and spin-down currents) becomes very small at situations mentioned above. We have further studied the influence of band occupation of the electrode layers and the many-body interactions present in the electrode region on the spin current and magneto-resistance of these devices.

Structural analysis of graphene and h-BN: A molecular dynamics approach

Classical molecular dynamics simulation is employed to analyze pair correlations in graphene and h-BN at various temperatures to explore the integrity of their respective structures. As the temperature increases, the height fluctuations in the out-of-plane direction of both graphene and h-BN are found to increase. The positional spread of atoms also increases with temperature. Thus the amplitude of the peak positions in the radial distribution function (RDF) decreases with temperature. It is found that FWHM of peaks in the RDF of h-BN is smaller as compared to those of graphene which implies that the structure of h-BN is more robust as compared to that of graphene with respect to their respective empirical potential.