Superconducting Materials

Z. Fisk, UC Irvine Gor’kov Memorial Symposium Tallahassee January 13, 2018

Where are the superconductors?

•  What was learned from heavy Fermion superconductivity?

•  What are similarities to other classes of superconductors?

•  Prevalence of metallic compounds satisfying valence rules

heavy Fermion superconductors

•  all appear near magnetic quantum critical point

•  antiferromagnetism can co-exist with superconductivity

•  no co-existence when superconductivity occurs at higher temperature than magnetic order

Phase diagram of CeRhIn5 (Park and Thompson)

Onuki et al. J. Alloys Cpds. 408-412, 27 (2006)

Stock et al. Phys. Rev. Lett. 100, 087001 (2008)

Conclusions

•  Heavy Fermion metals arise from a dense lattice of Kondo centers in which Kondo compensation competes with magnetic order

•  Coherence is synonymous with the formation of a Fermi surface incorporating the f-electron(s)

•  Superconductivity can gap out part of the magnetic fluctuation spectrum and provide a competitive route for loss of entropy at low temperature

pnictides

Wolfgang Jeitschko

2006 Hirano; 2008 Jorend

Superconductivity vs Bonding dP-P ~ 3.7Å BaRh2P2 Tc = 1.0K BaIr2P2 Tc = 2.1K dP-P ~ 2.25K CaRh2P2 no Tc

cuprates

La1.84Sr0.16CuO4 Tc = 40K

“valence” materials

•  LaFeOP, BaFe2As2

•  La2CuO4

•  MgB2

•  HfNCl:Li

BCS-type A15 superconductivity

•  Peierls-type lattice distortion above Tc •  strong coupling of soft lattice mode to

conduction electrons •  no lattice distortion when Tc occurs first

Batterman and Barrett Phys. Rev. 145, 296 (1966)

Schilling V3Si: TMartensitic / Tc

Unreasonable ineffectiveness of materials smarts for raising Tc

•  Cuprates •  Pnictides •  BCS

Where they are

•  Heavy Fermions: near magnetic order •  high Tc: near “charge” order •  BCS: near lattice instability

Highest Tc found in proximity to competing “localized” phase

•  holds for BCS, heavy Fermions, organics, cuprates and pnictides

•  the competing phase, afm in heavy Fermions, terminates where it intersects the Tcboundary and does not extend into the superconducting phase

•  “localized”: in BCS, lattice distorted; in heavy Fermions, local moment magnetism; in cuprates, the psuedo-gap phase.

•  electronics highly non-adiabatic •  superconductivity runs in structures •  superconductivity is where physics becomes chemistry

What is the chemistry?

•  Superconductivity solves a problem involving the electronics of a material

•  Superconductivity is diagnostic of this problem

•  Superconductivity is fundamentally a chemistry problem