Heavy fermion material
[2] 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.
Ott in 1975, who observed enormous magnitudes of the linear specific heat capacity in CeAl3.
[3] While investigations on doped superconductors led to the conclusion that the existence of localized magnetic moments and superconductivity in one material was incompatible, the opposite was shown, when in 1979 Frank Steglich et al. discovered heavy fermion superconductivity in the material CeCu2Si2.
[4] In 1994, the discovery of a quantum critical point and non-Fermi liquid behavior in the phase diagram of heavy fermion compounds by H. von Löhneysen et al. led to a new rise of interest in the research of these compounds.
[5] Another experimental breakthrough was the demonstration in 1998 (by the group of Gil Lonzarich) that quantum criticality in heavy fermions can be the reason for unconventional superconductivity.
[6] Heavy fermion materials play an important role in current scientific research, acting as prototypical materials for unconventional superconductivity, non-Fermi liquid behavior and quantum criticality.
The actual interaction between localized magnetic moments and conduction electrons in heavy fermion compounds is still not completely understood and a topic of ongoing investigation.
[citation needed] Heavy fermion materials belong to the group of strongly correlated electron systems.
Several members of the group of heavy fermion materials become superconducting below a critical temperature.
The Fermi liquid theory of Lev Landau provides a good model to describe the properties of most heavy fermion materials at low temperatures.
Measuring the reflected or transmitted light reveals the characteristic energies of the sample.
, 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.
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.
[8] 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.
[8][9] The lowest such Drude relaxation rate observed in heavy fermions so far, in the low GHz range, was found in UPd2Al3.
[11] At even higher frequencies we can observe a local maximum in the optical conductivity due to normal interband excitations.
In the temperature range mentioned above, the electronic contribution is the major part of the specific heat.
For electrons with a quadratic dispersion relation (as for the free-electron gas), the Fermi energy εF is inversely proportional to the particle's mass m: where
, the metal behaves as a Fermi gas in which the conduction electrons have a high thermal effective mass.
In contrast, above 6 K, the specific heat for this heavy fermion compound approaches the value expected from free-electron theory.
The presence of local moment and delocalized conduction electrons leads to a competition of the Kondo interaction (which favors a non-magnetic ground state) and the RKKY interaction (which generates magnetically ordered states, typically antiferromagnetic for heavy fermions).
By suppressing the Néel temperature of a heavy-fermion antiferromagnet down to zero (e.g. by applying pressure or magnetic field or by changing the material composition), a quantum phase transition can be induced.
[12] For several heavy-fermion materials it was shown that such a quantum phase transition can generate very pronounced non-Fermi liquid properties at finite temperatures.
Such quantum-critical behavior is also studied in great detail in the context of unconventional superconductivity.
