Experimental observation of phonons as spectators in FeSi electronic gap formation

The evolution of a phonon spectrum in a narrow-gap semiconductor FeSi was investigated in a wide range of temperatures (46 K $\le T\le 297$ K, $P=0.1$ MPa) and pressures (0.1 MPa $\le P\le 43$ GPa, $T=297$ K) using nuclear inelastic scattering of synchrotron radiation with the energy resolution 0.53 meV. Decreasing temperature as well as increasing pressure causes a strong reorganization of the phonon spectrum manifested in splitting and shifts of the phonon peaks. The phonon spectra measured under the temperature and pressure conditions corresponding to the same unit cell volume reveal nearly complete matching. On the contrary, the spectra measured under the conditions of the equal mean-square atomic displacements differ drastically. These observations suggest that the transformation of the electronic spectrum of FeSi is controlled predominantly by the change of the unit cell volume. The corresponding changes of the interatomic forces and the resulting modification of the phonon spectrum appear as the direct consequence of this transformation.

Figure 1

The evolution of the partial DOS of iron atoms in FeSi: (a) with temperature at ambient pressure; (b) with pressure at room temperature. For easier visibility, the curves are shifted vertically.

Figure 2

The temperature and pressure dependences of the splitting for the two lowest-energy peaks in the PDOS, displayed by the open and filled symbols respectively. The vertical arrows labeled by $T_c$ and $P_c$ indicate the critical temperature and pressure of the metal-semiconductor transition, respectively. Inset: The pressure dependence of the splitting for the two lowest-energy peaks in the PDOS for all range of investigated pressures.

Figure 3

The temperature and pressure dependences (a) of the mean-square amplitude $\langle u^2\rangle$ of atomic vibrations and (b) of the unit cell volume $V$. $T_c$ and $P_c$ indicate the temperature and pressure of the onset of the splitting of the first phonon peak, respectively. The dashed horizontal lines show the mean-square amplitude (a) and unit cell volume (b) at $T=T_c$.

Figure 4

The partial DOS for iron atoms in FeSi measured (a) for the same unit cell volume $V$ but different mean-square amplitudes $\langle u^2\rangle$ of atomic vibrations and (b) for the same mean-square amplitudes but different unit cell volumes.