Nick Curro: Droplets of Magnetism in Cadmium Doped CeCoIn5

(Delayed Blog posting from Week 1, Thursday, 2nd Aug).



Nick Curro described the results of a new series of NMR measurements that his group (see Urbano et al.) has carried out on Cadmium doped CeCoIn5, Ce(Co_1-x Cd_x)In5 . CeCoIn5 is a heavy fermion superconductor. One of the recent excitements has been the discovery by Pham et al, that the addition of Cadmium induces antiferromagnetism within the superconductor. There is a lot of similarity between the phase diagram of the Cd doped CeCoIn5 and the phase diagram of CeRhIn5. (See Joe Thompson blog, below). Cd is like "negative pressure" in the CeRhIn5 phase diagram. Thus the Cadmium helps to experimentally unify the physics of the 115 materials.

Nick described CeCoIn5 as a Kondo lattice material, in which mobile electrons move through a lattice of localized moments, interacting with the moments via an antiferromagnetic super-exchange J. He showed the Doniach phase diagram, and compared it with the phase diagram of this Cadmium doped material. Cadmium doping is loosely equivalent to "hole doping", and by reducing the size of the conduction sea, the system is driven to the left on the Doniach diagram. (See figure).

NMR measurements are carried out on the Indium sites of this 115 material - there are two indium sites - one of high symmetry, lying in the Ce plane (I) and another of low symmetry, out-of-plane. When NMR is carried out, the NMR line at the I site splits into three peaks, corresponding to three different environments (A, B, C). The A environment is the predominant "bulk" environment, and the 1/T1 signal shows the higher Neel temperature and the lower superconducting temperature. Two interesting features here:

• The normal state above TN does not display a Korringa relaxation rate, but a slower T^1/4 temperature dependence.
• There is no observed effect on the T^1/4 region, due to Cadmium doping.

But the SC Tc is Cadmium dependent, and at high dopings, completely disappears.

The B and C sites are exposed to progressively higher local Weiss fields, and indicate a non-uniform antiferromagnetic environment. Curro says these results show that the Cd is inducing antiferromagnetic droplets, and the C and B sites may correspond to the nearest and next-nearest neighbors, respectively.

In conclusion, the effect of the Cadmium seems two fold:

1. It changes the uniform bulk environment by reducing the hole density and uniformlysuppressing the superconducting Tc
2. It induces droplets of antiferromagnetism which percolate to produce long-range order.