Optical evidence for heavy charge carriers in FeGe

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from $0.01\text{to}3.1\mathrm{eV}$ for different temperatures from $30\text{to}296\mathrm{K}$. The optical conductivity shows the evolution of a low-energy $\left(0.22\mathrm{eV}\right)$ interband transition and the development of a narrow free-carrier response with a strong energy and temperature dependence. The frequency-dependent effective mass and scattering rate derived from the optical data indicate the formation of dressed quasiparticles with a mass renormalization factor of 5. Similar to FeSi the spectral weight in FeGe is not recovered over a broad frequency range, an effect usually attributed to the influence of the on-site Coulomb interaction.

Figure 1

(Color) The normal-incidence reflectivity $R\left(\omega \right)$ (a), optical conductivity ${\sigma }_1\left(\omega \right)$ (b), and real part of the dielectric function ${ϵ}_1\left(\omega \right)$ (c) of FeGe derived from the ellipsometry and reflectivity measurements at several temperatures ranging from $30\text{to}296\mathrm{K}$ on a logarithmic scale.

Figure 2

(Color online) Spin-polarized density of states of FeGe. The energy is relative to $E_F$.

Figure 3

(Color online) The optical conductivity spectra of the helimagnetic state of FeGe. The solid and dashed lines denote the experimental and calculated optical spectra, respectively.

Figure 4

(Color) Extended Drude analysis of the optical conductivity of FeGe. The details of $1∕\tau \left(\omega \right)$ above $62\mathrm{meV}$ are due to the proximity of the onset of interband transitions at $0.1\mathrm{eV}$. (${\omega }_p=2.7\mathrm{eV}$ and ${ϵ}_{\infty }=264$ as described in the text).

Figure 5

(Color) The integrated spectral weight of FeGe for a number of representative temperatures.