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Soft phonon mode and superconducting properties of 2H-NbS 2

Soft phonon mode and superconducting properties of 2H-NbS 2. Maxime Leroux. maxime.leroux@grenoble.cnrs.fr. P. Rodière , K. Hasselbach , T . Klein, C . Marcenat (a), L. Cario (b), P. Diener (c), M-A. Measson (d), M . Le Tacon (e). Institut Néel CNRS/UJF – Grenoble, FRANCE

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Soft phonon mode and superconducting properties of 2H-NbS 2

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  1. Soft phonon mode and superconducting properties of 2H-NbS2 Maxime Leroux maxime.leroux@grenoble.cnrs.fr P. Rodière, K. Hasselbach, T. Klein, C. Marcenat (a), L. Cario (b), P. Diener(c), M-A. Measson(d), M. Le Tacon (e) • Institut Néel CNRS/UJF – Grenoble, FRANCE • (a) CEA-INAC-SPSMS Grenoble, FRANCE • (b) IMN Nantes, FRANCE • (c) SRON, Utrecht, NETHERLANDS • (d) LMPQ, CNRS/Univ Paris Diderot-Paris 7, FRANCE • (e) Max Planck Institut Stuttgart, GERMANY 1

  2. Framework: Superconductivity and electronicorderings Temperature ourwork 2H-NbS2 previously 2H-NbSe2 • High Tc: Pnictides (SDW), cuprates (AF, YBaCuO) • Heavy fermions: CeIn3 (AF), CeRh/CoIn5 (AF), UGe2 (F) • Organic: AF,SDW,CDW more or less strange Fermi liquid electronic ordering: AF, SDW, CDW... SC External parameter: pressure, doping concentration... Collaboration or competition? 2

  3. 2H-NbS2and 2H-NbSe2 hexagonal lattice isoelectronic compounds Nb mass decreases lower orbitals S or Se • 2H-NbSe2 : • Tc = 7 K • Unconventional superconductivity • TCDW = 33 K • 2H-NbS2 : • Tc = 6 K • No CDW 3

  4. STM: local electronicdensity of states 2H-NbSe2 2H-NbS2 T=100 mK E=EF - 4 meV superperiodicity= CDW atomiclattice No CDW Coll. Madrid : I. Guillamon & al, PRL 2008 4

  5. 2H-NbS2 • Unconventional superconducting properties ? • Any traces of a CDW ? 5

  6. Small superconducting gap in 2H-NbS2 and 2H-NbSe2 λ : Magnetic penetration depth previously 2H-NbSe2 2H-NbS2 BCS model samefastincrease small gap • For BCS isotropic gap Samesmallenergyscalecompared to BCS theory Fletcher, Carrington, Diener, Rodière et al. PRL 98, (057003) 2007 Diener, Leroux, Cario, Klein, and RodierePRB, 84 (054531) 2011 6

  7. Independent measurements Scanning Tunneling spectroscopy Coll. Madrid I. Guillamon, H.Suderow et al, PRL 2008 T=100 mK 1stcriticalfieldmeasurement 7

  8. Magneticfieldmeasurementwith« micro »-Hall probes network of 10 probes, sensitivity : 700 Ω/T = 70 mΩ/G 20 µm Probes: Martin Konczykowski and Vincent Mosser samplestuckwithgrease (1 G=10-4 T) Magneticfield B : profile outside sample 1 2 3 4 5 6 7 8 9 10 Probe number 8

  9. Temperaturedependence of Hc1 in NbS2by micro-Hall probe Samedeviationfrom BCS smallenergyscaleconfirmed by Hc1 Hc1(T) for H // c Diener, Leroux, Cario, Klein, and RodierePRB, 84 (054531) 2011 9

  10. 2H-NbS2 • Unconventional superconducting properties ? • Any traces of a CDW in 2H-NbS2 ? • 2H-NbSe2: CDW preceded by a soft phonon mode • Inelastic X-ray Scattering 10

  11. Ab initio phonon calculation versus experiment calculation : T= 0 K MatteoCalandra IXS phonon sample Incident beam 2H-NbS2 Scattered beam with e--phonon coupling center of Brillouin zone edge of Brillouin zone 11

  12. Ab initio phonon calculation versus experiment computation : T= 0 K experiment 12

  13. Soft phonon mode of 2H-NbSe2and 2H-NbS2 NbS2 : IXS NbSe2 : Inelastic X-ray Scattering , Weber, …, Reznik2011 NbSe2 : Inelastic Neutron Scattering, Ayache 1992 The soft mode does not become static in NbS2 C. Ayacheand R. Currat and P. Molinié, Physica B, 180 & 181(333-335), 1992 F. Weber , …, and Reznik., arXiv:1103.5755, 2011 (accepted for PRL) 13

  14. Conclusion Correlation : 2H-NbSe2 soft phonon mode 0 meV CDW 2H-NbS2 soft phonon mode 0 meV No CDW consequences on superconductivity ? anisotropic electron-phonon coupling Beneficial or detrimental ? Van Hove singularity ? Thanks for your attention! 14

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