Joe Thompson: "Proximity of Superconductivity and Magnetism in the 115s"

Joe described his work with T. Park and other collaborators on CeRhIn5,
a member of the 115-family of heavy fermion materials that are layered
derivatives of the cubic CeIn3. The attached figure shows the
magnetic/superconducting phase diagram as a function of pressure,
magnetic field, and temperature.

At a sufficiently high magnetic field (above 9T), increasing pressure
induces a quantum phase transition (at P1) from an antiferromagnetic metal
phase to a non-magnetic metal phase. There is some indication that
the transition is second order. Yet, dHvA measurements of Onuki's group
find a jump in the Fermi surface, with effective mass showing a tendency
of divergence at P1. In the low-pressure AF phase, the f-electrons are
localized since the measured Fermi surface is similar to that seen
in the f-less reference material LaRhIn5. In the higher-pressure non-magnetic
phase, on the other hand, the same f-electrons are itinerant because
the measured Fermi surface is similar to that of the bandstructure
calculations in which these f-electrons are assumed mobile.

Joe went on to describe the pressure-induced transitions inside the
superconducting part of the phase diagram. For finite fields (which
are smaller than 9T), there is evidence for the second-order nature
of the transition from a co-existing AF+SC phase (which appears to
be homogeneous) to a pure SC phase. At H=0, the residual specific
heat coefficient was found to undergo a rapid decrease as the pressure
is increased through a threshold value (P2, smaller than P1),
as did the Fermi velocity fitted from the finite-T Hc2.

Joe suggested that the f-electrons remain localized in the SC+AF
phase. He did so based on the aforementioned dHvA observation
of localized f-electrons in the high field AF state, along with
the observation that the ordered moment at temperatures just above
Tc of the AF+SC phase is large.

There was discussion about how strong an evidence the above entail
for the localized nature of the f-electrons inside the AF+SC phase.
There was also discussion on the extent to which the change of \gamma
and v_F across P2 should be associated with the transition in magnetism,
or is instead a reflection of a distinction in superconductivity between
the co-existing AF+SC phase and the pure SC one.

Joe went on to describe the effect of Cd-doping in Co-115 and Ir-115.
A co-existing AF+SC region occurs in the Cd-doped Co-115, but is absent
in the Cd-doped Ir-115.

The main questions that Joe raised are:

* What is the nature of the 4f electrons when they participate
simultaneously in magnetism and superconductivity?

* If we accept the notion that the f-electrons are localized while
participating in the superconductivity, what implications does it
have for the non-Fermi liquid behavior in the normal state?

* In particular, does it suggest some form of Kondo breakdown?