Dynamical instability and Fermi surface topology in ${\text{Ni}}_2$FeGa from first principles

The phonon spectrum of stoichiometric Heusler alloy ${\text{Ni}}_2$FeGa is calculated for the high-temperature cubic austenite phase by using first-principles density functional perturbation theory. We also compute the elastic constants of the alloy from the initial slopes of the acoustic phonon branch. The ${\text{TA}}_2$ phonon branch along [110] direction shows softening with a minimum dip at $\zeta =0.58$ which indicates the possibility of modulated phases prior to martensitic transformation. We also map the Fermi surface of this alloy both in 3D and 2D to check the presence of any nesting vectors. The observed nesting parameter is in good agreement with the above value of the wave vector in the [110] direction where phonon softening occurs.

Figure 1

L2${}_1$ structure of stoichiometric ${\text{Ni}}_2$FeGa alloy.

Figure 2

$k$-mesh convergence for different degauss values. The $k$ mesh of the larger degauss value converges faster.

Figure 3

${\text{TA}}_2$ mode of ${\text{Ni}}_2$FeGa alloy along [110] direction at different degauss values showing convergence at $\zeta =0.58\left(110\right)$.

Figure 4

Phonon spectra of ${\text{Ni}}_2$FeGa alloy showing softening along [110] with a minimum dip at $\zeta =0.58\left(110\right)$. The full (black) lines represent the spectrum for degauss 0.01 Ryd while the dashed (blue) lines represent the spectrum for degauss 0.005 Ryd.

Figure 5

Partial vibrational density of states (DOS) of each atomic species plotted together with total vibrational DOS of ${\text{Ni}}_2$FeGa.

Figure 6

3D Fermi surface topology of ${\text{Ni}}_2$FeGa alloy. The Fermi surfaces that occupy the center of each face and edge (red) are the 20th bands of minority spin bands while the other Fermi surfaces in between (green) represent the 19th bands of minority spin bands.

Figure 7

2D cross section of Fermi surface of ${\text{Ni}}_2$FeGa along $k_z=1.0$ (dotted lines) and $k_z=0.95$ (solid lines) planes showing Fermi nesting vector $\mathbf{q}=0.58\left(110\right)$ represented by the arrow (blue) extending from the 19th band to the 20th band.