Novel Aspects of Superconductivity, Andrey Chubukov, Piers ...

Novel Aspects of Superconductivity, Andrey Chubukov, Piers ...

Description and justification for the workshop

The novel and exciting aspects of superconductivity in strongly correlated

fermionic systems are associated with the new mechanisms that are responsible

for its emergence and the new and unconventional physical properties that are

exhibited by these systems, arising from the strong correlations between the

fermions that form Cooper pairs. While superconductivity is only one of many

ordered states that can occur in interacting fermionic systems, it is unique as it

best reflects the complex nature of the underlying normal state.

Many strongly correlated electron materials, such as the Ce-based heavy

fermion systems, cuprates, cobaltates, quasi one-dimensional and quasi twodimensional

organic charge transfer salts become unconventional superconductors

at low temperatures. The origin of superconductivity in most of these

systems cannot be explained using the conventional Bardeen-Fröhlich idea of

phonon-mediated pairing of fermions, but is rather (most likely) due to a strong

electron-electron interaction. While some researchers believe that the key physics

behind an electronic pairing mechanism is the proximity to a Mott insulator,

others propose that it is the proximity to a quantum critical point that gives

rise to unconventional superconductivity. There are other proposals as well.

Below are some examples of new directions in this area of research:

• In the high-T c superconductors, strong electronic correlations destroy or,

at least, strongly suppress the fermionic quasiparticle residue in the normal

state. The critical temperature, T c , varies non-monotonically with

doping, and the pseudogap phase occupies a substantial portion of the

phase diagram. Whether the pseudogap phase is uniquely related to superconductivity

is not clear.

• Although the normal state in the cuprates is a bad metal, the superconducting

state possesses well defined low energy quasiparticles. How the

fermionic coherence emerges and why it requires true superconductivity

(and not just pseudogap) is still a matter of debate.

• Why is unconventional superconducting state so ubiquitous – it is observed

in many different families of materials (the cuprates, heavy-fermion, organic

materials). Is unconventional superconductivity the fingerprint of a

highly unconventional normal state?

• Competition between superconductivity and other types of ordering (e.g.,

charge or spin ordering). This issue is related to ongoing discussion on

the nature of the pseudogap phase in the cuprates, but is also relevant for

other materials, such as magnetic superconductors, supersolids, etc.

• Numerous experiments in organic superconductors have demonstrated that

superconductivity is the strongest exactly at a first order phase boundary


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