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Heavy FermionHeavy fermion materials are a specific type of metallic compounds that have a lowtemperature specific heat whose linear term is up to 1000 times larger than the value expected from the freeelectron theory. The heavy fermion behaviour has been found in rare earth and actinide metal compounds at very low temperatures (<10 K) in a broad variety of states including metallic, superconducting, insulating and magnetic states. Additional recommended knowledge
The specific heat for normal metalsAt low temperature and for normal metals, the specific heat C_{P} consists of the specific heat of the electrons C_{P,el} depending linearly on temperature T and of the the specific heat of the crystal lattice vibrations or the phonons C_{P,ph} depending cubically on temperature C_{P} = C_{P,el} + C_{P,ph} = γT + βT^{3} with proportionality constants β and γ. In the temperature range mentioned above, the electronic contribution is the major part of the specific heat. For the freeelectron gas  a simple model system that neglects electron interaction  or metals that could be described by it, the electronic specific heat is given by with Boltzmann's factor k_{B}, the electron density n and the Fermi energy ε_{F} (the highest single particle energy of occupied electronic states). The proportionality constant γ is called the Sommerfeld parameter. Relation between heat capacity and "thermal effective mass"For electrons with a quadratic dispersion relation (as for the freeelectron gas), the Fermi energy ε_{F} is inversely proportional to the particle's mass m: where k_{F} stands for the Fermi wave number that depends on the electron density and is the absolute value of the wave number of the highest occupied electron state. Thus, because the Sommerfeld parameter γ is inversely proportional to ε_{F}, γ is proportional to the particle's mass and for high values of γ, the metal behaves as a free electron gas in which the conduction electrons have a high thermal effective mass. Example: heat capacity for UBe_{13} at low temperaturesExperimental results for the specific heat of the heavy fermion compound UBe_{13} show a peak at a temperature around 0.75 K that goes down to zero with a high slope if the temperature approaches 0 K. Due to this peak, the γfactor is much higher as for the freeelectron gas in this temperature range. In contrast, above 6 K, the specific heat for this heavy fermion compound approaches the value expected from freeelectron theory. Book ReferencesKittel, Charles (1996) Introduction to Solid State Physics, 7th Ed., John Wiley and Sons, Inc. 

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Heavy_Fermion". A list of authors is available in Wikipedia. 