Order Parameter Symmetries of Organic Superconductors

1
Order Parameter Symmetries of Organic
Superconductors

• REU Symposium 2002
• Robert Cherng (MIT)
• Carlos Sá De Melo (GaTech)

2
Superconductivity

• Zero Resistance, Meissner effect
• Observed by Onnes in 1911
• Microscopic theory developed by Bardeen, Cooper,
  Schrieffer in 1957

http//www.aber.ac.uk/publications/brochures/physi
cs/phys6.html
3
Outline

• Microscopic BCS Theory
• Quasi 1-D Organic Superconductors
• Possible Order Parameter Symmetries
• Specific Heat
• Other Quantities
• ?(T), µ(T), N(?), Fcond(T), ?ij(T), ?(q,?)

4
BCS Theory

• Fermi Sea Instability
• Bound States Cooper Pairs
• Large Pair Overlap
• Quantum Many-Body problem
• Order Parameter Description of the
  superconducting state

5
Quasi 1-D Organic Superconductors

• Salts of Planar Organic Molecules
• Highly Anisotropic Quasi 1-D
• Rich phase diagram
• Unconventional Superconductivity Evidence for
  Triplet Pairing

http//www.lps.u-psud.fr/Activites/ThemeB.asp
6
Triplet Weak Spin-Orbit Coupling Order Parameters
py-Wave d(k)c?0Y
px-Wave d(k)c?0X
pz-Wave d(k)c?0Z
Fxyz-Wave d(k)c?0XYZ
Xsin(kxa), Ysin(kyb), Zsin(kzc)
Figures from R. Duncan, R. W. Cherng, C. A. R Sá
de Melo cond-matt/0203570
7
Specific Heat

• CvdQ/dT
• Probes excitation spectrum Ek
• Discontinuity at Tc
• Sensitive to node structure of order parameter ?k
  at low temperatures

8
Specific Heat (cont.)
9
Specific Heat px-Wave
10
Specific Heat py-Wave
11
Specific Heat pz-Wave
12
Specific Heat fxyz-Wave
13
Px, Py, Pz, or Fxyz?

• Upper critical field, NMR suggest triplet pairing
  (Lee et. al.)
• Weak spin-orbit coupling since heaviest element
  is Se
• Thermal conductivity inconsistent with gap nodes
  (Belin and Behnia)
• But thermodynamics alone cannot completely
  distinguish the symmetries

14
Order Parameter and Chemical Potential
µ(T) Tc1.5 N0.5
?0(T) Tc1.5 N0.5
15
Density of States
N(?) T0
16
Spin Susceptibility
?(q0,?) T1
?(q0,?) T0
17
Superfluid Density
?xx(T)
?yy(T)
?zz(T)
18
Condensation Energy
FSC(T)-FN(T)
19
Conclusions

• The analysis of underlying symmetries is a deep
  and powerful tool
• Superconductivity is a quantum many-body
  phenomena
• Quasi 1-D organic superconductors may have a
  fully-gapped triplet order parameter

20
Acknowledgements

• Dr. Carlos Sá de Melo
• Dr. James Gole
• NSF REU

21
Order Parameters

• Phase Transitions and Symmetries
• Symmetry Breaking and Order Parameters
• Paramagnetism vs. Ferromagnetism M
• Normal State vs. Superconductor?