Heavy fermions systems: The CeMIn₅ system (M=Co, Rh, Ir)

(F. P. Mena)

The physics of the single magnetic impurity in a non-magnetic metallic matrix is characterized by a single temperature T_K, known as Kondo temperature. Below T_K, the local moment of the impurity is screened by the conduction electrons and the system can be described as a Fermi liquid (that is, it has the same properties as a Fermi gas of electrons but with enhanced parameters). Moreover, all thermodynamic quantities scale with T/T_K. In the Kondo lattice (a lattice of magnetic centers) there are two characteristic temperatures: a temperature below which the moments are locally screened (T_K) and a temperature below which the Fermi liquid is formed (T*). This Fermi liquid state is characterized by a large enhancement from where the term “heavy fermion system” has been coined. At high temperatures, we can see these systems as the itinerant conduction electrons moving independently from the localized magnetic moments (fig.1). At temperatures T<<T*, these two bands hybridize [Millis and Lee, PRB 35 (1987)] and form the so-calleed hybridization gap (fig. 1). If the Fermi energy lies at the top of the lower band, we have a Fermi liquid. These picture is widely accepted but its existence has been just recently confirmed [Dordevic et al., PRL 86 (2001)]. However, the Fermi liquid state is not always formed and instead a magnetic ground state can be obtained. The CeMIn5 system allows us to study this interplay between these different states.

Figure 1: Schematic band structure of a heavy fermion system at large (gray dotted lines) and low temperartures (black solid lines).	Figure 2: Phase diagram of the CeMIn (M = Co, Rh, Ir) family (from Pagliuso et al., Physica B 312-313 (2002)].

Cerium based compounds are well known to exhibit strongly correlated electronic phenomena. Among them, the materials of the  CeMIn₅ group have recently attracted considerable attention. The reason is their unusual properties, specially their unconventional superconductivity, similar to the high T_C superconductors [Petrovic, et al. Europhys. Lett. 53 (2001). Sidorov et al., PRL 89(2002)] and, presumably, proximity to a quantum critical point (with pressure [Sidorov et al., PRL 89(2002). Mito et al., PRL 90 (2003)] and magnetic field [A. Bianchi et al., cond-mat/0302226] as tuning parameters). The analogy with the high T_C superconductors even starts with their crystal structure which can be seen as layers of CeIn₃  separated by MIn₂ layers. These heavy fermion compounds exhibit an ample gamut of properties that reveal the subtle interplay between magnetism and superconductivity (see fig. 2).

We have performed optical measuremnts in these compounds and confirmed the formation of the hybridization gap. Moreover, we have been able to see directly how the formation of antiferromagnetism destroys the Fermi liquid state (fig. 3). At low temperatures, just above the gap, we have also seen the formation of strong resonances, reminiscent of phonons but with a far larger spectral weight. The origin of these features is still not known but by performing Raman scattering measurements we hope to obtain insights to elucidate it. These measurements are in progress.

Figure 3: Optical conductivity of the CeMIn5 group obtained from the combination of reflectivity and ellipsometry.