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Heavy electrons and the symplectic symmetry of spin

Nature Physics volume 4, pages 643–648 (2008)Cite this article

Abstract

The recent discovery of two heavy-fermion materials PuCoGa5 and NpPd5Al2, which transform directly from Curie paramagnets into superconductors, reveals a new class of superconductors where local moments quench directly into the superconducting condensate. Unlike other heavy-electron superconductors, where Cooper pairing is thought to be driven by spin fluctuations, these higher-transition-temperature materials do not seem to be close to a magnetic instability. Large-N expansions have been invaluable in describing heavy-fermion metals, but so far cannot treat superconductivity. Here, we introduce a new class of large-N expansion that uses symplectic symmetry to protect the odd time-reversal parity of spin and sustain Cooper pairs as well-defined singlets. We show that when a lattice of magnetic ions exchange spin with their metallic environment in two distinct symmetry channels, they can simultaneously satisfy both channels by forming a condensate of composite pairs between local moments and electrons. In the tetragonal crystalline environment relevant to PuCoGa5 and NpPd5Al2, the lattice structure selects a natural pair of spin exchange channels, predicting a unique anisotropic paired state with either d- or g-wave symmetry. This pairing mechanism also predicts a large upturn in the NMR relaxation rate above Tc and strong enhancement of Andreev reflection in tunnelling measurements.

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Figure 1: Composite pairing in two crystal field screening channels.
Figure 2: Phase diagram for a two-channel Kondo lattice, computed in the symplectic large-N limit for tetragonal symmetry, where spin is exchanged via channels Γ1≡Γ7+ and Γ2≡Γ7−.
Figure 3: Upturn in the NMR relaxation rate created by the cooperative interference of the Kondo effect in two channels at different sites, for the extreme case of maximum Tc, where J1=J2 (blue line), compared with measured NMR relaxation rate in PuCoGa5 (ref. 31) (yellow circles).

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Acknowledgements

This research was supported by the National Science Foundation grant DMR-0605935 and US Department of Energy grant DE-FE02 00ER45790 (M.D.). The authors would like to thank N. Andrei, K. Basu, E. Miranda, R. Moessner, P. Pagliuso, S. Sachdev, N. Read, G. Zarand and particularly S. Thomas, for discussions related to this work.

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  1. Center for Materials Theory, Rutgers University, Piscataway, New Jersey 08854, USA

    Rebecca Flint, M. Dzero & P. Coleman

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  1. Rebecca Flint
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Correspondence to P. Coleman.

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Flint, R., Dzero, M. & Coleman, P. Heavy electrons and the symplectic symmetry of spin. Nature Phys 4, 643–648 (2008). https://doi.org/10.1038/nphys1024

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