Magneto-Resistance at the Pre-Martensite Transition in Ni$_{2}$MnGa

A magneto-resistance and angle resolved photoemission (ARPES) study of the pre-martensite phase of the ferromagnetic shape memory alloy single crystal Ni$_{2}$MnGa, reveals a temperature (235 \underline {$>$} T \underline {$>$} 190 K) and field dependent (0 -- 1 T) positive/negative magneto-resistance slope. Previous inelastic neutron scattering experiments (Zheludev et al., PRB \textbf{51}, 1995) on this Heusler alloy indicate a phonon branch [110]-TA$_{2}$ softening in the pre-martensite phase along \textbf{q} = (1/3, 1/3, 0).~ This phonon softening combine with our ARPES data show that significant depletion of states (pseudo gap) occur at the premartensitic transition temperature. Recent results (Shapiro et al., EPL \textbf{77}, 2007) reveal phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting.~ Our experimental results will be discussed in light of electron-phonon coupling.