Optical evidence for heavy charge carriers in FeGe

Abstract

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 to 3.1 eV for different temperatures from 30 to 296 K. The optical conductivity shows the evolution of a low-energy ͑0.22 eV͒ 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. DOI: 10.1103/PhysRevB.75.155114 PACS number͑s͒: 78.20.Ϫe, 71.27.ϩa, 75.10.Lp, 75.50.Bb Cubic FeGe is a good metal at low temperature, which undergoes a transition to helimagnetic order 1 at T C = 280 K with the magnetic moment at the iron sites of 1 B . The helix changes its orientation in a temperature interval T 2 ±20 K and shows pronounced temperature hysteresis 2 between 211 and 245 K. This material crystallizes in the B20 structure and the cubic space group P2 1 3 lacking a center of symmetry which is responsible for this long-range order. The isoelectronic compound FeSi has the same crystal structure. It has a large magnetic susceptibility at room temperature, which vanishes as the temperature approaches zero due to a small ͑70 meV͒ semiconductor gap at E F . A continuous series FeSi 1−x Ge x can be formed, where the metal insulator transition 3 occurs for x Ϸ 0.25. Theoretical models, which have been proposed to explain this behavior, invoke disorder, 4 narrow bands, and different ways of incorporating electron correlations.5-8 The temperature-dependent disappearance of the gap has been explained as a result of a correlation gap using a two-band Hubbard model, 9,10 and excellent agreement was obtained with optical data, 10-12 but it has been shown that vibrational disorder, if sufficiently strong, also closes the gap. 13 Anisimov et al. 14 have predicted a magnetic-field-driven semiconductor to metal transition in FeSi 1−x Ge x , and argued that the difference in electronic structure between FeSi and FeGe in essence consists of a rigid relative shift of the majority and minority-spin bands for the latter material. According to this model the optical spectra at low energies are expected to be the superposition of a Drude peak and an interband transition across an energy range corresponding to the forementioned relative shift of the majority and minority bands. Experimentally relatively little is known about the electronic structure of FeGe, for example, no optical data have been published.Here, we report optical measurements on a cubic FeGe single crystal at different temperatures. The real and imaginary parts of the dielectric function were derived from the reflectivity and ellipsometry measurements. Optical spectra of FeGe reveal the presence of an important interband transition at 0.22 eV and unusu...

Importance of thermal disorder and electronic occupation for temperature dependence of optical conductivity in FeSi and MnSi

“…17͒ and in FeGe above ϳ0.1 eV. 20 The same reason of transition probabilities explains why the SW curve for a high-DOS material ͑MnSi, FeGe, etc.͒ is above that for a low-DOS material ͑FeSi, CoSi, etc.͒.…”

“…This method was applied recently to FM FeGe, and the sequence and the relative intensities of ͑E͒ agree favorably with experiment, 20 however, with two important differencies. First, the energy scale needs to be shifted by several tenths of an eV.…”

“…The recently measured SW in FeGe shows a decrease below ϳ0.1 eV and an increase above this energy for increasing T. 20 FeGe is isoelectronic with FeSi, but it is FM with a saturation moment of about 1 B / f.u. and with a high DOS of the minority states.…”

“…12 However, through high-resolution optical measurements, it has been possible to detect T-dependent details of the band structure in several B20 compounds. [15][16][17][18][19][20] The band filling of the small gap in FeSi as a function of raising T has been observed, but with an unexplained missing intensity of the total spectral weight ͑SW͒ ͑the integrated optical conductivity͒ at least for energies up to a few eV. This has been taken as an indication of exotic, Kondo-like, and band-unlike interactions in FeSi.…”

Optical spectra of the heavy fermion uniaxial ferromagnetUGe2

“…18͒ and helimagnetic FeGe. 19 The relatively large mass enhancement observed in UGe 2 suggests the evolution of renormalized itinerant charge carriers out of a Fermi gas coupled to a lattice of 5f local orbitals. It is generally accepted that the periodic Anderson model ͑PAM͒, which describes the hybridization of a localized level with a conduction band, captures the essential physics of such systems.…”