Tuesday, February 10, 2015

Rich Kondo physics in iron-based superconductors.

One of the most fascinating, challenging, and frustrating aspects of the iron-based superconductors is the presence of many competing energy scales, particularly associated with Hund's rule coupling.
There are debates about just how strong the correlations are, how large the spin moments are, and how relevant Mott physics is.

Furthermore, Hund's rule seems to lead to an unusual metallic state that is both difficult to characterise and describe theoretically. It has some signatures of a bad metal, as emphasised by (amongst others) Haule, Kotliar, Si, and Abrahams, but this characterisation is disputed by Johnston in his review [see Section 3.8.2].

Given the chemical and structural diversity of this large class of materials, we should be cautious about claiming the same physics is dominant in all the materials.

There is a nice paper that illustrates the richness of this system.
Local Quantum Criticality of an Iron-Pnictide Tetrahedron 
T. Tzen Ong and Piers Coleman

It shows how the Hund's rule coupling and interplay of orbital and spin degrees of freedom can suppress formation of the Fermi liquid state that would occur in a one band system.

One of our key observations, is that in addition to their spin physics, the iron-based tetrahedra develop an orbital degree of freedom associated with the degenerate eg orbitals. For conventional transition metal ions, the Hund’s coupling JH locks the unpaired electrons together into a high-spin configuration, exponentially suppressing the spin-Kondo temperature to low temperatures according to an effect discovered by Schrieffer [15] and recently noted by others [16]. Here we show that unlike their spin counterparts, orbital fluctuations are not subject to the “Schrieffer effect”, giving rise to a unique situation in which the orbital degrees of freedom behave as fluctuating quantum mechanical variables that result in an incoherent “non-Fermi liquid” ground state. While departures from perfect tetragonality will reestablish the Fermi liquid, a large temperature range of incoherent metal behavior is expected to remain.

One of the effects of the projection into the high-spin manifold is an S-fold reduction of the spin-Kondo coupling, JS=J/4S but strikingly, the strength of the orbital Kondo interaction is unaffected. This means that the Hund’s interaction will exponentially suppress the spin-Kondo effect down to a lower scale TspinK/D(TorbK/D)2S, as in conventional transition metal ions, while leaving the orbital Kondo effect unaffected. This schism between the orbital and spin-Kondo effect drastically affects the physics,

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