Abstract:

A Conventional superconductor results from a pairing between electrons based on lattice vibrations or phonons and is extremely well understood
in terms of the BCS theory (by Bardeen, Cooper, and Schreiffer).  Unfortunately, these conventional superconductors have relatively low
transition temperatures making them difficult for applications and therefore there is a strong interest in studying unconventional
superconductors where the pairing mechanism may not rely strictly upon phonons.

The apparent competition between superconductivity and antiferromagnetism has been a topic of much interest recently mainly as
the result of the discovery of high-temperature superconductivity in the two dimensional cuprates and unusually high transition temperatures in
the heavy fermion CeCoIn5 system.  In the cuprates, superconductivity results by doping an insulating two-dimensional antiferromagnet with
either holes or electrons.  In contrast, in the two dimensional CeTIn5 system, superconductivity emerges from a metallic spin-density wave
ground state.  I will present an experimental overview of the magnetic properties in the CeCoIn5 and YBa2Cu3O6+x superconductors. I will show
that the low-energy response of CeCoIn5 is dominated by a S=1 triplet excitation similar to magnetic dimer systems.  I will compare these
results to those of the cuprates and discuss how the magnetism in both compounds might be consistently understood.   Comparison to theories
involving stripes and resonating valence bonds will be made.