Electronic structure, phonons and electron–phonon interaction in MgXNi₃(X = B, C and N)

Abstract

Ab initio calculations have been performed to study the electronic structure, phonon dispersion relations, and the electron–phonon interaction for cubic superconductors MgXNi3 with X = B, C and N. The electronic calculations are based upon the application of a plane wave basis, ultrasoft pseudopotentials, and the local density approximation of the density functional scheme. The electronic structure results are used, within the implementation of a linear response technique, for calculations of phonon states. Our calculations predict anomalous dispersion of the transverse acoustic branch along [111] in MgCNi3, and of all the three acoustic branches in a larger region of the Brillouin zone for MgNNi3. An explanation for the differences in the phonon dispersion and electron–phonon coupling parameter between these three materials has been put forward.

Puzzling phonon dispersion curves and vibrational mode instability in superconducting MgCNi3

“…They have also generated the theoretical curves based on the fitted and adjusted inter atomic force constants in addition to comparing the experimental data with the results obtained from first principles calculation. 14 They obtained in general a reasonably well fit to few dips and anomalies with the adjusted parameters. In the case of first principles result the agreement is quite poor particularly in the -R line.…”

“…Further evidence for the phonon-mediated pairing mechanism is provided by theoretical estimates ranging between 0.8 to 1.5. 5,9,14 The electronic structure measurements and calculations 4,9,[18][19][20][21] indicate the large density of states (sharp peak) just below the Fermi level arising from an extended van Hove singularity which further supports the strong electron-phonon coupling (EPC). 4 A number of model and first principles investigations of lattice dynamics of MgCNi 3 have been performed.…”

“…The calculations of Heid et al 6 show that the instability appearing over a large area in reciprocal space which is most pronounced near the X-and M-zone edges. Ab-initio calculations of Tütünce and Srivastava 14 show that the phonon modes with imaginary frequency are present between X-and R-edge points along with a dip between R-and -points for TA mode. Imaginary frequencies which show instability of the dynamical structure and also unphysical particularly for a compound which 2158-3226/2012/2(2)/022120/9 C Author(s) 2012 2, 022120-1 022120-2 Jha, Gupta, and Gupta AIP Advances 2, 022120 (2012) is isostructure to well studied and dynamically stable pervoskite superconductor Ba 0.6 K 0.4 Bio 3 22, 23 and the large discrepancies amongst the theoretical calculations of phonon dispersion with regard to the area and position of unstable modes in the Brillouin zone are puzzling and still not understood.…”

“…Thus, in the recent past several attempts have been made to investigate the structural, electronic, vibrational and superconducting properties of MgCNi 3 . 1,[4][5][6][7][8][9][10][11][12][13][14] Experimental results 9,[15][16][17] have provided evidence for a medium to strong-coupling superconductivity driven by electron-phonon interactions. Further evidence for the phonon-mediated pairing mechanism is provided by theoretical estimates ranging between 0.8 to 1.5.…”

“…4 A number of model and first principles investigations of lattice dynamics of MgCNi 3 have been performed. [5][6][7]14 The Ni-derived acoustic phonon modes due to their likely link with the superconducting transition 21 and instability of the acoustic phonon modes at some points in Brillouin zone are of special interest. All previous theoretical calculations including our own model calculation 7 show soft modes, anomalies and regions of imaginary (negative) frequencies.…”