page_title: Heavy fermion material
section_title: Optical properties
hierarchical_section_title: Heavy fermion material / Properties / Optical properties
caption_attribution_description: English: Typical frequency-dependet optical conductivity of a heavy fermion compound. Blue line: T &gt; Tcoh. Red line: T &lt; Tcoh
context_page_description: In solid-state physics, heavy fermion materials are a specific type of intermetallic compound, containing elements with 4f or 5f electrons in unfilled electron bands. Electrons are one type of fermion, and when they are found in such materials, they are sometimes referred to as heavy electrons. Heavy fermion materials have a low-temperature specific heat whose linear term is up to 1000 times larger than the value expected from the free electron model. The properties of the heavy fermion compounds often derive from the partly filled f-orbitals of rare-earth or actinide ions, which behave like localized magnetic moments. The name "heavy fermion" comes from the fact that the fermion behaves as if it has an effective mass greater than its rest mass. In the case of electrons, below a characteristic temperature, the conduction electrons in these metallic compounds behave as if they had an effective mass up to 1000 times the free particle mass. This large effective mass is also reflected in a large contribution to the resistivity from electron-electron scattering via the Kadowaki–Woods ratio.
context_section_description: In order to obtain the optical properties of heavy fermion systems, these materials have been investigated by optical spectroscopy measurements. In these experiments the sample is irradiated by electromagnetic waves with tunable wavelength. Measuring the reflected or transmitted light 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. Compared to a good metal however, 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 that 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 time is also enhanced, leading to a narrow Drude roll-off at very low frequencies compared to normal metals.
  The lowest such Drude relaxation rate observed in heavy fermions so far, in the low GHz range, was found in UPd₂Al₃.
  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.
caption: Typical frequency-dependent optical conductivity of a heavy fermion compound. Blue line: T > Tcoh. Red line: T < Tcoh.
exif: {}