WAMSDO 14 November 2011 CERN

1. UNIVERSITÀ DEGLI STUDI DI GENOVA Irradiation of MgB2 Marina PuttiUniversity of Genova and CNR-SPIN V. Braccini, P.Brotto, C. Ferdeghini, V.Ferrando, E.Galleani, F. Gatti, P. Manfrinetti, A. Palenzona, I. Pallecchi, C. Tarantini, M. Tropeano WAMSDO 14 November 2011 CERN

2. H. U. Aebersold and E. Lehmann Paul Scherrer Institut, Villagen, Switzerland P. Postorino, D. Di Castro, A. Sacchetti, M. Lavagnini, M. Ortolani, P.Dore CNR-INFM-Coherenzia and Universita' di Roma "La Sapienza", Italy. R.Vaglio , R.Di Capua, M.Salluzzo, P.Orgiani CNR-INFM-Coherenzia and Universita' di Napoli, Italy. M. Affronte CNR-INFM-S3 and Universita` di Modena e Reggio Emilia, Italy R.Gonnelli and D.Daghero CNISM-INFM, Politecnico di Torino, Italy A.Orecchini, C.Petrillo and F.Sacchetti CNR-INFM CRS-SOFT and Università di Perugia, Italy S. Massidda, A. Sanna and F. Bernardini CNR-INFM-SLACS and Università di Cagliari, Italy X.X.Xi, Qi Li The Pennsylvania State University, University Park, PA 16802, USA I Sheikin GHMFL, CNRS, Grenoble, France E. G. Haanappel LNCMP, CNRS-UPS-INSA, Toulouse, France U. Gambardella INFN Frascati National Laboratory

3. Supercond. Sci. Technol. 21 (2008) 043001 Ruggero Vaglio CNR-SPIN and University of Naples, Italy John Rowell School of Materials, Arizona State University WAMSDO 14 November 2011 CERN

4. Outline • Brief remarks on MgB2 • Irradiation Experiments • Tc vs residual resistivity behaviour • Upper critical field • Critical Current • Density of states • Theoretical model for the degradation of Tc • Conclusions • All the results are systematically compared with similar experiments performed on A15 superconductors.

5. Magnesium diboride • Strong covalent bonds between the boron atoms. • Coupling of electron with the optical vibration mode of boron atoms (mode E2g). Tc = 39 K, close to the maximum value predicted by BCS theory

6. MgB2 first example of two-gap superconductor Dp(0)  2 meV ;Ds(0)  7 meV

7. MgB2 two-band superconductor Hole-p electron-p hole-s hole-s pbands 3D nearly electron-like weakly coupled with E2g mode sbands 2D hole-like strongly coupled with E2g mode Different parity Interband scattering is negligible • s and p Cooper pairs with pretty different character do not mix due to negligible interband impurity scattering • Tc is amplified (by a factor 2) by the occurnce of two decoupled bands

8. Role of disorder Ds Dp increasing disorder increasing disorder In a two-band s/c interband scattering mixes strong s-pairs with weak p-pairs andcauses pair breaking. A.A.Golubov and I.I.Mazin, PRB 55 (1977) • In the strong scattering limit • The critical temperature is expected to decrease down to a saturation value 19-25 K • In the strong scattering limit • The critical temperature is expected to decrease down to a saturation value 19-25 K • The energy gaps should merge to the BCS value 3.56 O.V. Dolgov et al. PRB 72, 024504 (2005)

9. Aims of the of irradiation experiment: • Investigating Tc suppression and multiband nature of MgB2 • Improve the superconducting properties (Hc2, Hirr, Jc) • Different kind of particles have been used for irradiating MgB2 : • neutrons • a-particles • protons • Ions

10. Neutron irradiation • Thermal neutrons produce defects by nuclear reactions • n + 10B  (1.7 MeV) 4He + (1 MeV) 7Li • Huge cross section at low neutron energies (3800 b at 25 meV) • With natural boron (~ 20% 10B), the penetration depth is ~200 mm • With enriched 11B (~ 0.5% 10B), the penetration depth is ~1 cm • Neutron irradiation facilities: • TRIGA MARK II nuclear reactor, Atomic Institute Vienna (fast/thermal neutron flux density: 7.6/6.1 × 1012 n/(cm2∙s) • Spallation neutron source SINQ Paul Sherrer Institut (PSI) of Zurich. thermal neutron flux density 1013 n/(cm2∙s∙) fast neutron flux density 1010 n/(cm2∙s∙)

11. 2.1 Irradiation experiments in MgB2 2 MeV 4He irradiation experiments: Gandikota R et al 2005 Appl. Phys. Lett.86 012508 Gandikota R et al 2005 Appl. Phys. Lett. 87 072507 Neutron irradiation experiments: Kar’kin et al 2001 JETP Lett. 73 570 Eisterer et al 2002 Supercond. Sci. Technol. 15 L9 Wang et al 2003 J. Phys. Condens. Matter 15 883 Zehetmayer et al 2004 Phys. Rev. B 69 054510 Putti et al 2005 Appl. Phys. Lett. 86 112503 Tarantini et al 2006 Phys. Rev. B 73 134518 Wilke et al 2006 Phys. Rev. B 73 134512 Ferrando et al 2007 J. Appl. Phys. 101, 043903

12. 2.2 Irradiation experiments in MgB2 300 keV O2+ irradiation experiments:

13. 2.3 Irradiation experiments in A15 Sweedler et al 1974 Phys. Rev. Lett. 33 168 Sweedler et al 1978 J. Nucl. Mater. 72 50 Poate et al 1976 Phys. Rev. Lett. 37 168 Ghosh A K and Myron Strongin 1980 Superconductivity in d- and f-Band Metals 305 Wiesman et al 1978 J. Low Temp. Phys. 30 513 Rowell J M and Dynes R C “Bad metals, good Superconductors” Alterovitz et al 1981 Phys. Rev. B 24 90 Noolandi J and Testardi L R 1977 Phys. Rev. B 15 5462 Sweedler A R and Cox D E 1975 Phys. Rev. B 12 147 Cox D E and Tarvin J A 1978 Phys. Rev. B 18 22 Burbank R D, Dynes R C and Poate J M 1979 J. Low Temp. Phys. 36 573 Testardi et al 1977 Phys. Rev. Lett. 39 716 Flukiger R 17th International Conference on Low Temperature Physics-LT-17, Ref. 4, 609 Nolscher C and Saemann-Ischenko G 1985 Phys. Rev. B 32 1519 Bett R 1974 Cryogenics 14 361 Vonzovski S V, Izyumov Yu A and Kurmaev E K 1982 Superconductivity of Transition Metals (Berlin: Springer) Alterovitz S A, et al., 1981 Phys. Rev. B 24 90 Karkin A E, Mirmelshtein A V, Arkhipov V E and Goshchitskii B N 1980 Phys. Status Solidi a 61 K117 Cort B, Stewart G R, Snead C L Jr, Sweedler A R and Moechlecke S 1981 Phys. Rev. B 24 3794 Phys. Rev. Lett. 37 168 Ghosh A K, Gurvitch M, Wiesmann H and Strongin M 1978 Phys. Rev. B 18 6116 …………

14. Tc (K) MgB2 A15s 3. Tc vs r0 behaviour

15. Comparison with A15s Tc/Tc0 vs G Tc/Tc0 vs r0

16. 4.1 Upper critical field of MgB2 Neutron irradiated Mg11B2 polycrystals Upper critical field nearly doubles in the sample with Tc = 33 K

17. 4.2 Upper critical field of MgB2 : Hc2(0) vs Tc irradiated

18. 4.2 Comparison with A15 Hc2(0) vs Tc/Tc0

19. 5.1 Critical current of MgB2 Neutron irradiated Mg11B2 polycrystals

20. Nb3Sn wires NbTi, Nb3Sn, Nb3Al wires 5.2 Critical current of A15

21. Neutron fluence 1.4 1020cm-2 Pinning Force Mg11B2 Amorphous regions Typical diameter 2÷4 nm F/FPmax Nb3Sn wires Comparable with the coherence length [010] [100] 5.3 Pinning mechanisms The shift of the FP peak means that a new pinning mechanisms is working Similar behaviour was observed in Nb3Sn wires

22. 6. DOS dependence on disorder Slope of Hc2 Tc within the BCS model increasing irradiation

23. 30 15 0 EF N(E) (States/eV cell) V3Si TOTAL DOS MgB2 21 14 7. 0 Nb3Sn TOTAL DOS G 7.1 Theoretical model for the degradation of Tc. Testardi-Matteis model: smearing of the DOS

24. DOS as a function of r0

25. V3Si Nb V Tc vs r in A15

26. 8.2 A model for suppression of Tc in MgB2 M.Putti et al EPL(2007) • Interband scattering and two band isotropization • Interband scattering and two band isotropization • Intraband scattering and smearing of the DOS Interband scattering Interband + Intraband scattering

27. Conclusion • MgB2 is a two gap superconductor: this feature amplifies its Tc but makes it sensitive to the disorder • Tc is halved by a fluence of 1019 neutron /cm2 and of 61016 a-particles /cm2 • Upper critical field and critical current are improved for neutron fluence around 1018 /cm2 • Similar results have been obtained in A15 superconductors despite of the different ingredients that make Tc high in MgB2 and A15 WAMSDO 14 November 2011 CERN