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Register a new userOptical evidence for heavy charge carriers in FeGe
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The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 - 3.1 eV for different temperatures from 30 K to 296 K. The optical conductivity shows the evolution of a low energy (0.22 eV) 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 quasi-particles with a mass renormalization factor of 12. 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.
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The optical conductivity of charge carriers coupled to quantum phonons is studied in the framework of the one-dimensional spinless Holstein model. For one electron, variational diagonalisation yields exact results in the thermodynamic limit, whereas at finite carrier density analytical approximations based on previous work on single-particle spectral functions are obtained. Particular emphasis is put on deviations from weak-coupling, small-polaron or one-electron theories occurring at intermediate coupling and/or finite carrier density. The analytical results are in surprisingly good agreement with exact data, and exhibit the characteristic polaronic excitations observed in experiments on manganites.
We performed optical spectroscopy measurement on single crystal of CeTe$_3$, a rare-earth element tri-telluride charge density wave (CDW) compound. The optical spectra are found to display very strong temperature dependence. Besides a large and pronounced CDW energy gap being present already at room temperature as observed in earlier studies, the present measurement revealed the formation of another energy gap at smaller energy scale at low temperature. The second CDW gap removes the electrons near E$_F$ which undergo stronger scattering. The study yields evidence for the presence of multiple CDW orders or strong fluctuations in the light rare-earth element tri-telluride.
We report optical spectra of Lu$_5$Ir$_4$Si$_{10}$ and Er$_5$Ir$_4$Si$_{10}$, exhibiting the phenomenon of coexisting superconductivity or antiferromagnetism and charge density wave (CDW) order. We measure the maximum value of the charge density wave gap present on part of the Fermi surface of Lu5Ir4Si10, corresponding to a ratio 2Delta/k_B T_CDW approx 10, well above the value in the limit of weak electron-phonon coupling. Strong electron-phonon coupling was confirmed by analyzing the optical conductivity with the Holstein model describing the electron-phonon interactions, indicating the coupling to phonons centered at 30 meV, with a coupling constant lambda approx 2.6. Finally we provide evidence that approximately 16 % of the Fermi surface of Lu5Ir4Si10 becomes gapped in the CDW state.
We have grown single crystals of the type-VIII intermetallic clathrate Ba8Ga16Sn30 from both Sn and Ga flux, evaluated their compositions through electron microprobe analysis and studied their transport properties through measurements on temperature dependent resistivity, thermopower and Hall coefficient. Crystals grown in Sn flux show n-type carriers and those from Ga flux show p-type carriers, whereas all measured compositions remain very close to the stoichiometric 8:16:30 proportion of Ba:Ga:Sn, expected from charge-balance principles. Our results indicate a very high sensitivity of the charge carrier nature and density with respect to the growth conditions, leading to relevant differences in transport properties which point to the importance of tuning this material for optimal thermoelectric performance.
The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($sigma(omega)$) spectra. Remarkably, the temperature variation in the $sigma(omega)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $sigma(omega)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $sigma(omega)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.
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