Optical Properties
In order to obtain the optical properties of heavy fermion systems, these materials have been investigated by optical spectroscopy measurements. Therefore the sample is irradiated by electromagnetic waves with tuneable wavelength. Measuring the reflected or transmitted light then reveals the characteristic energies of the sample.
Above the characteristic coherence temperature, heavy fermion materials behave like normal metals; i.e. their optical response is described by the Drude model. But compared to a good metal, heavy fermion compounds at high temperatures have a high scattering rate because of the large density of local magnetic moments (at least one f-electron per unit cell), which cause (incoherent) Kondo scattering. Due to the high scattering rate, the conductivity for dc and at low frequencies is rather low. A conductivity roll-off (Drude roll-off) occurs at the frequency which corresponds to the relaxation rate.
Below, the localized f-electrons hybridize with the conduction electrons. This leads to the enhanced effective mass and a hybridization gap develops. In contrast to Kondo insulators, the chemical potential of heavy fermion compounds lies within the conduction band. These changes lead to two important features in the optical response of heavy fermions.
The frequency-dependent conductivity of heavy-fermion materials can be expressed by, containing the effective mass and the renormalized relaxation rate . Due to the large effective mass, the renormalized relaxation rate is also enhanced, leading to a narrow Drude roll-off at very low frequencies compared to normal metals.
The gap like feature in the optical conductivity represents directly the hybridization gap, which opens due to the interaction of localized f-electrons and conduction electrons. Since the conductivity does not vanish completely, the observed gap is actually a pseudogap. At even higher frequencies we can observe a local maximum in the optical conductivity due to normal interband excitations.
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