Soft phonon mode and superconducting properties of 2H-NbS2

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Maxime Lerouxmaxime.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

electronic ordering: AF, SDW, CDW...

Temperature

more or less strange Fermi liquid

SC

Framework: Superconductivity and electronic orderings

Collaboration or competition ?2

External parameter: pressure, doping concentration...

• High Tc: Pnictides (SDW), cuprates (AF, YBaCuO)

• Heavy fermions: CeIn3 (AF), CeRh/CoIn5 (AF), UGe2 (F)

• Organic: AF,SDW,CDW

previously

2H-NbSe2

our work2H-NbS2

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2H-NbS2 and 2H-NbSe2

hexagonal lattice

P S ClAs Se Br

Nb

S or Se

isoelectronic compounds

2H-NbS2 :• Tc = 6 K• No CDW

2H-NbSe2 :• Tc = 7 K• Unconventional

superconductivity• TCDW = 33 K

mass decreases

lower orbitals

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Coll. Madrid : I. Guillamon & al, PRL 2008

atomic latticeNo CDW

superperiodicity = CDW

STM: local electronic density of states

T=100 mKE=EF - 4 meV

2H-NbS22H-NbSe2

2H-NbS2

• Unconventional superconducting properties ?

• Any traces of a CDW ?

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Small superconducting gap in 2H-NbS2 and 2H-NbSe2

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BCS model

Fletcher, Carrington, Diener, Rodière et al. PRL 98, (057003) 2007

• For BCS isotropic gap

λ : Magnetic penetration depth

2H-NbS2

𝛿𝜆(𝑇 )∝√ 𝜋 Δ02𝑘𝐵𝑇

𝑒− Δ0𝑘𝐵𝑇

previously 2H-NbSe2

small gap

2H-NbS2 2H-NbSe2

Δ0 1.2(1) kBTc 1.1(1) kBTcsame fast increase

Same small energy scale compared to BCS theory

Δ0=1.2(1)𝑘𝐵𝑇𝑐

Diener, Leroux, Cario, Klein, and Rodiere PRB, 84 (054531) 2011

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Independent measurements

1) Scanning Tunneling spectroscopy

2) 1st critical field measurement

T=100 mK

Coll. Madrid I. Guillamon, H.Suderow et al, PRL 2008

network of 10 probes, sensitivity : 700 Ω/T = 70 mΩ/G

Magnetic field measurement with « micro »-Hall probes

Magnetic field B : profile

outside sample

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sample stuck with grease

Probes: Martin Konczykowski and Vincent Mosser

1 2 3 4 5 6 7 8 9 10 Probe number

20 µm

(1 G=10-4 T)

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Temperature dependence of Hc1 in NbS2 by micro-Hall probe

Hc1(T) for H // c

Same deviation from BCS

small energy scale confirmed by Hc1

Diener, Leroux, Cario, Klein, and Rodiere PRB, 84 (054531) 2011

2H-NbS2

• Unconventional superconducting properties ?

• Any traces of a CDW in 2H-NbS2 ? 2H-NbSe2 : CDW preceded by a soft phonon modeInelastic X-ray Scattering

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IXS

sample𝑘𝑖 ,ℏ𝜔 𝑖

𝑘 𝑓 ,ℏ𝜔𝑓Incident beam

Scattered beam

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Ab initio phonon calculation versus experimentcalculation : T= 0 KMatteo Calandra

center of Brillouin zone edge of Brillouin

zone

phonon

�⃗�

with e--phonon coupling

2H-NbS2

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Ab initio phonon calculation versus experimentcomputation : T= 0 K experiment

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Soft phonon mode of 2H-NbSe2 and 2H-NbS2

The soft mode does not become static in NbS2

NbSe2 : Inelastic Neutron Scattering, Ayache 1992

NbSe2 : Inelastic X-ray Scattering , Weber, …, Reznik 2011

NbS2 : IXS

C. Ayache and R. Currat and P. Molinié, Physica B, 180 & 181 (333-335), 1992F. Weber , …, and Reznik., arXiv:1103.5755, 2011 (accepted for PRL)

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anisotropic electron-phonon coupling

Conclusion

Thanks for your attention!

0 meV CDW2H-NbSe2 soft phonon mode

No CDW 0 meV2H-NbS2 soft phonon mode

Correlation :

consequences on superconductivity ?

Beneficial or detrimental ?Van Hove singularity ?