Superconductivity and Ferromagnetism of ZrZn₂

Zs. Major,^a S. B. Dugdale,^a R. Watts,^a G. Santi,^a M. A. Alam,^a S. M. Hayden,^a J. A. Duffy,^b J. W. Taylor,^b T. Jarlborg,^c E. Bruno,^d D. Benea^e and H. Ebert^e

^aH.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK; ^bDepartment of Physics, University of Warwick, Coventry CV4 7AL, UK; ^cDPMC, University of Geneva, 24 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland; ^dDeparment of Physics, Univeristy of Messina, Salita Sperone 31, 98166 Messina, Italy; ^eInstitute for Physical Chemistry, Ludwig-Maximilians University, Butenandtsrasse 5-13, 81377 Munich, Germany (zs.major@bristol.ac.uk)

The discovery of the ferromagnetism co-existing with superconductivity in the compound UGe₂ [1], followed by similar discoveries in ZrZn₂ [2] and URhGe [3], has re-opened the debate about the relationship between magnetism and superconductivity. At the centre of this debate is the question of the pairing mechanism for the superconductivity, with the speculation that it is magnetically rather than phonon-mediated.

Knowledge of the topology of the Fermi surface (FS) is vital to an understanding of the superconductivity. Furthermore, such measurements are a stringent test of electronic structure calculations, on which our models for the superconductivity rely. Our recent quantum oscillatory measurements of the FS of ferromagnetic ZrZn₂ [4] have revealed a FS topology that is in agreement with our LMTO band structure calculations; however, the quasiparticles are strongly renormalized, to such a degree that there is no clear experimental evidence of the heaviest sheet. This sheet is important because it contributes about half of the density-of-states at the Fermi level. Also, there is some indication that FS nesting could be important, both in terms of the possibility of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state [5], and also in terms of spin-fluctuations and magnetically mediated pairing [6,7]. Given its likely dominance in determining physical properties, information about this unobserved sheet is vital.

We present the results of our investigation into these specific aspects of the superconductivity and ferromagnetism in ZrZn₂ using two different momentum density measurements. First, our positron annihilation study of the (paramagnetic) FS, and our search for signatures of the heavy sheet, is discussed. Secondly, we have used magnetic Compton scattering to address the issue of the delocalisation of the magnetic moment as an additional insight into the electronic structure. Both these measurements are supported by first-principles electronic structure calculations, and their implications discussed in terms of possible mechanisms for the superconducting pairing.

References

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3 Aoki, D., et al. (2001) Nature (London) 413 613

4 Yates, S.J.C., et al. (2003) Phys. Rev. Lett. 90 057003

5 Singh, D.J. and Mazin, I.I. (2002) Phys. Rev. Lett. 88 187004

6 Santi, G., et al. (2001) Phys. Rev. Lett. 87 247004

7 Fay, D. and Appel , J. (1980) Phys. Rev. B 22 3173