= Solution
<Heavy fermion materials> are correlated metals whose low-energy <quasiparticles> have exceptionally large effective masses. Partly localized $f$-electron moments hybridize with broader conduction states. The <Kondo effect> and coherence below the <Kondo-lattice coherence temperature> produce a narrow low-energy band, so a small energy change corresponds to a large momentum-space state density.
For the <electronic heat capacity of a heavy Fermi liquid>, the resulting <density of states> gives a large low-<temperature> electronic specific-heat coefficient,
$$
\boxed{C_{\rm el}=\gamma T,\qquad\gamma=\frac{\pi^2k_B^2}{3}g(E_F),}
$$
and often an enhanced <magnetic susceptibility>. Here $g$ includes both spins and must be expressed per sample, mole or volume consistently with $\gamma$. A coherent Fermi-liquid regime often has $\rho=\rho_0+AT^2$ with an enhanced coefficient $A$. Above coherence, scattering by local moments can instead give a Kondo-like resistivity upturn.
The competition between moment screening and intersite magnetic interactions can yield a heavy paramagnetic metal, magnetic order, quantum-critical behaviour or unconventional <superconductivity>. Ce- and U-based compounds supply familiar examples. “Heavy” describes the emergent electronic <quasiparticles>, not the ionic mass; the simple heavy-band picture need not survive at a <quantum critical point>.<Heavy Fermions and the Kondo Lattice: A 21st Century Perspective by Piers Coleman>[Sections 1.1–1.3: heavy electrons, local moments and Kondo-lattice coherence]{ref}
\Image[/past-exam-of-the-quantum-condensed-matter-course-of-the-university-of-cambridge/2015/paper-1-correlated-metal-bands.png]
{title=Hybridization flattens a conduction band near a localized level, while an itinerant exchange field splits spin bands}
{source=paper-1-correlated-metal-bands.py}
Back to article page
