Proslier - Localized magnetism on the Surface of Niobium: experiments and theory
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Localized magnetism exists on the surface of niobium, as evidenced by experiments and supported by theory. Point contact spectroscopy, SQUID, and EPR measurements show the presence of paramagnetic moments in niobium oxides that increase with mild baking and decrease with high temperature baking. Theoretical modeling of the residual resistance matches experimental data and attributes it to magnetic impurities in the niobium oxide layer, with a concentration of around 250 ppm or 6×1012/cm2. Surface treatments like baking modify the concentration of magnetic moments, providing an explanation for changes in cavity performance.
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![Surface magnetism: TheoryModel assumes: Homogeneous moment density on λ + London limit + dirty limit3 parameters: η, α: describe effect of magnetic impurities on the superconductor -> concentrationnormal conductivity σ0-> mean free path, ℓ.London & dirty limit:In the limit:Density of Normal electrons:Density of superconducting electrons:SRF film workshop-Oct 2010Solve self consistently g[T, e], f[T, e] and Δ[T]14](https://image.slidesharecdn.com/presentationproslierpadova-101005083420-phpapp01/85/Proslier-Localized-magnetism-on-the-Surface-of-Niobium-experiments-and-theory-19-320.jpg)
Proslier - Localized magnetism on the Surface of Niobium: experiments and theory
- 1. Localized magnetism on the Surface of Niobium: experiments and theoryTh. Proslier, M. Kharitonov, M.Pellin (ANL)J.F. Zasadzinski (IIT)G. Ciovati (JLAB)N. Groll, I. Chiorescu, A. Gurevich (NHMFL)C. Antoine (CEA)A. Romanenko, L.Cooley (FNAL)Funded by ARRA-DOE, Office of science, High Energy Physics.SRF film workshop-Oct 2010
- 2. 2Experimental evidence: PCT, SQUID, EPR, RAMAN
- 3. Theory: residual resistanceSRF film workshop-Oct 2010
- 4. 3Niobium surfaces are complex, important, and currently poorly controlled at the nm levelResidue from chemical processingInclusions,Hydride precipitatesSurface oxide Nb2O55-10 nm45 nm RF depthInterface: sub oxides NbO, NbO2often not crystalline (niobium-oxygen “slush”)Interstitials dissolved in niobium (mainly O, some C, N, H) e- flow only in the top 45 nmClean niobiumGrain boundariesProbe the surface superconductivitySRF film workshop-Oct 2010
- 5. 4Experimental evidences of Magnetism: Point Contact6 Tesla magnet1.6-300 K2Measure of the surface superconducting gap Δ
- 6. The ZBC value -> Number of normal electronNormal electrons in gap => dissipation and lower QIdeal BCS superconductorSRF film workshop-Oct 2010
- 7. Experimental evidences of Magnetism: Point Contact5Conc= 0.1-0.3%Th. Proslier, j. zasadzinski et al. APL 92, 212505 (2008)SRF film workshop-Oct 2010
- 8. 6Hot and cold spots in SRF cavity (from J-lab)“Normal” spectrumAnomalous spectrumOnly on hot spotsHot spots: show dissipative behavior Higher ZBC and anomalous spec. lower gap values (1.3<∆<1.55)Cold spots: “normal” dissipation Low ZBC values Normal gap values (1.5<∆<1.55)Correlates with cavities results! (once again)Origin of peculiar spectrum and dissipation?Th. Proslier, G. Ciovati to be submitted to PRST-AB (2010)SRF film workshop-Oct 2010
- 9. 7Hot and cold spots in SRF cavity, OriginTemp. dep: peak at 0 mV bias increasesKilling superconductivity by applying a mag. FieldNb/NbOx/AuTa/Ta2O5/Alfits with Appelbaum theory ->Magnetic impurities in the oxidesJ>0 -> antiferromagnetic coupling-First time measured on Nb oxides-Same behavior observed on unbaked Nb couponsTh. Proslier, J. zasadzinski et al. accepted to PRB (2010)SRF film workshop-Oct 2010
- 10. 8Zero Bias Conductance (ZBC) peak: Spin Flip TunnelingNb-Nb2O5-Au (hot spots)Ta-Ta2O5-Ta2 Kg=2Kondo effectΔ=g.µ B .H -> g = 3.5Definite proof for localized paramagnetic moments in the Niobium oxideWhat is the origin ?SRF film workshop-Oct 2010
- 11. 9Experimental evidences of Magnetism: Electron Paramagnetic resonance (EPR)On niobiumpowders (high surface/volume ratio)Nb12O29Localized paramagnetic momentsdue to Oxygen vacancies.2 values of g: g=1.2 and g=3.3-> insolated spins-> arrangement in 1D spin chainsA.Lappas, PRB 65, 134405 (2002)SRF film workshop-Oct 2010
- 12. 10Experimental evidences of Magnetism: Superconducting Quantum Interference Device (SQUID)χ= χ0 + C/(T+ θCW)=M/(B.Vol) Background: Pauli term χ0[T] Linear dependence-> surface magnetismSRF film workshop-Oct 2010
- 13. 11Experimental evidences of Magnetism: Mild and HT baking in UHV of EP samplesSamples: M.B. Increase conc. of magnetic impurities(similar to Casalbuoni) High Temp. baking decrease it.Cavities: -at 1.8 K higher Rres after mild baking-High Temp. baking decrease RresVol =S.dd=5 nmFactor of 10 in Curie cst with Cava -> Real vol of oxides = 10 x the nominal vold:from 5 to 10 nm (neutron)roughness -> factor of 5 easily (Raman)Conc of magnetic moments:~ 104ppm = 1%, PCT ~ 0.1%SRF film workshop-Oct 2010
- 14. Raman spectroscopy12BCP Nb foil.D.B.C Enhancement of the signal intensitydue to amorphous NbOx at imperfection locations + roughness -> Lead easily to factor of 5-10 in real volume. Pits in cavity Nb .ASRF film workshop-Oct 2010
- 15. Summary experimental results:13Magnetic impurities present at the surface of Nb, in the oxides Nb2O5-δ is magnetic [1]Concentration is modified by surface treatment (mild, High T)
- 16. Hot spot show higher concentration of Mag. Moments (PCT)
- 17. Correlates with Cavity results
- 18. Theory: surface impedance [T] 1: Cava et al. Nature 350, 598 (1991) & PRB 41, 13 (1991) & PRB 72, 033413 (2005) SRF film workshop-Oct 2010
- 19. Surface magnetism: TheoryModel assumes: Homogeneous moment density on λ + London limit + dirty limit3 parameters: η, α: describe effect of magnetic impurities on the superconductor -> concentrationnormal conductivity σ0-> mean free path, ℓ.London & dirty limit:In the limit:Density of Normal electrons:Density of superconducting electrons:SRF film workshop-Oct 2010Solve self consistently g[T, e], f[T, e] and Δ[T]14
- 20. Theory: The residual resistance15α=0.02 meV, η =0.5SRF film workshop-Oct 2010
- 21. Theory: The residual resistance16α=0.033 meV, η =0.2SRF film workshop-Oct 2010
- 22. Surface magnetism: Theory17η= 0.2 -> strong coupling
- 23. Theory: The residual resistance18η =0.2Bake 180CBake 120CBake 160CBake 90CTh. Proslier, M. Kharitonov submitted to PRL (2010)SRF film workshop-Oct 2010
- 24. 19Theory: The residual resistanceΔ and TcMean free path and λMean Free path, ℓ, decreases after baking consistent with BCS surface impedance fitsα~2.10-2meV & η=0.2 -> 250 ppm in Nb -> 6.1012 /cm2 in Nb oxidesSRF film workshop-Oct 2010
- 25. 20Theory: The residual resistanceconclusionLondon penetration depth, λConcentration of Mag. ImpuritiesConcentration of Magnetic impurities increase after baking consistent with SQUID dataCornell 2010SRF film workshop-Oct 2010
- 26. Non-linear Meissner effect: On-chip cavity2160 nm Nb+5nm W30 nm Nb+5nm W60 nm Nb+ water60 nm Nb+air30 nm Nb+water30 nm Nb+air