Localized magnetism on the Surface of Niobium: experiments and theory<br />Th. Proslier, M. Kharitonov, M.Pellin  (ANL)<br...
2<br /><ul><li>Experimental evidence: PCT, SQUID, EPR, RAMAN
Theory: residual resistance</li></ul>SRF film workshop-Oct 2010<br />
3<br />Niobium surfaces are complex, important, and currently poorly controlled at the nm level<br />Residue from chemical...
4<br />Experimental evidences of Magnetism:  Point Contact<br />6 Tesla magnet<br />1.6-300 K<br />2<br /><ul><li>Measure...
The ZBC value -> Number of normal electron</li></ul>Normal electrons in gap => dissipation and lower Q<br />Ideal BCS supe...
Experimental evidences of Magnetism:  Point Contact<br />5<br />Conc= 0.1-0.3%<br />Th. Proslier, j. zasadzinski et al. AP...
6<br />Hot and cold spots in SRF cavity (from J-lab)<br />“Normal” spectrum<br />Anomalous spectrum<br />Only on hot spots...
7<br />Hot and cold spots in SRF cavity, Origin<br />Temp. dep: peak at 0 mV bias increases<br />Killing superconductivity...
8<br />Zero Bias Conductance (ZBC) peak: Spin Flip Tunneling<br />Nb-Nb2O5-Au (hot spots)<br />Ta-Ta2O5-Ta<br />2 K<br />g...
9<br />Experimental evidences of Magnetism:  <br />	Electron Paramagnetic resonance (EPR)<br />On niobiumpowders (high sur...
10<br />Experimental evidences of Magnetism:  <br />	Superconducting Quantum Interference Device (SQUID)<br />χ= χ0 + C/(T...
11<br />Experimental evidences of Magnetism:  <br />	Mild and HT baking in UHV of EP samples<br />Samples:<br /><ul><li> M...
Raman spectroscopy<br />12<br />BCP Nb foil<br />.D<br />.B<br />.C<br /><ul><li> Enhancement of the signal intensity</li>...
Summary experimental results:<br />13<br /><ul><li>Magnetic impurities present at the surface of Nb, in the oxides</li></u...
Hot spot show higher concentration of Mag. Moments (PCT)
Correlates with Cavity results
Theory: surface impedance [T] </li></ul>1: Cava et al. Nature 350, 598 (1991) & PRB 41, 13 (1991) & PRB 72, 033413 (2005) ...
Surface magnetism: Theory<br />Model assumes: Homogeneous moment density on λ + London limit + dirty limit<br />3 paramete...
Theory: The residual resistance<br />15<br />α=0.02 meV, η =0.5<br />SRF film workshop-Oct 2010<br />
Theory: The residual resistance<br />16<br />α=0.033 meV, η =0.2<br />SRF film workshop-Oct 2010<br />
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Proslier - Localized magnetism on the Surface of Niobium: experiments and theory

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Localized magnetism on the surface of Niobium: experiments and theory (Thomas Proslier - 40')
Speaker: Thomas Proslier - Argonne National Laboratory | Duration: 40 min.
Abstract
The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios (rods, foils, wires, powders) it was found that the impurity contribution is correlated with surface area. Tunneling measurements which utilize the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field > 4T, consistent with the Appelbaum-Anderson model for spin flip tunneling. Viewed together, the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments, likely caused by oxygen vacancies in Nb2O5. The computation of the surface impedance (RS) in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance.

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Proslier - Localized magnetism on the Surface of Niobium: experiments and theory

  1. 1. Localized magnetism on the Surface of Niobium: experiments and theory<br />Th. Proslier, M. Kharitonov, M.Pellin (ANL)<br />J.F. Zasadzinski (IIT)<br />G. Ciovati (JLAB)<br />N. Groll, I. Chiorescu, A. Gurevich (NHMFL)<br />C. Antoine (CEA)<br />A. Romanenko, L.Cooley (FNAL)<br />Funded by ARRA-DOE, Office of science, High Energy Physics.<br />SRF film workshop-Oct 2010<br />
  2. 2. 2<br /><ul><li>Experimental evidence: PCT, SQUID, EPR, RAMAN
  3. 3. Theory: residual resistance</li></ul>SRF film workshop-Oct 2010<br />
  4. 4. 3<br />Niobium surfaces are complex, important, and currently poorly controlled at the nm level<br />Residue from chemical processing<br />Inclusions,<br />Hydride precipitates<br />Surface oxide <br />Nb2O55-10 nm<br />45 nm <br />RF depth<br />Interface: sub oxides NbO, NbO2<br />often not crystalline <br />(niobium-oxygen “slush”)<br />Interstitials dissolved in niobium (mainly O, some C, N, H) <br />e- flow only in the top 45 nm<br />Clean niobium<br />Grain boundaries<br />Probe the surface superconductivity<br />SRF film workshop-Oct 2010<br />
  5. 5. 4<br />Experimental evidences of Magnetism: Point Contact<br />6 Tesla magnet<br />1.6-300 K<br />2<br /><ul><li>Measure of the surface superconducting gap Δ
  6. 6. The ZBC value -> Number of normal electron</li></ul>Normal electrons in gap => dissipation and lower Q<br />Ideal BCS superconductor<br />SRF film workshop-Oct 2010<br />
  7. 7. Experimental evidences of Magnetism: Point Contact<br />5<br />Conc= 0.1-0.3%<br />Th. Proslier, j. zasadzinski et al. APL 92, 212505 (2008)<br />SRF film workshop-Oct 2010<br />
  8. 8. 6<br />Hot and cold spots in SRF cavity (from J-lab)<br />“Normal” spectrum<br />Anomalous spectrum<br />Only on hot spots<br />Hot spots: show dissipative behavior<br /> Higher ZBC and anomalous spec.<br /> lower gap values (1.3<∆<1.55)<br />Cold spots: “normal” dissipation<br /> Low ZBC values <br /> Normal gap values (1.5<∆<1.55)<br />Correlates with cavities results! (once again)<br />Origin of peculiar spectrum and dissipation?<br />Th. Proslier, G. Ciovati to be submitted to PRST-AB (2010)<br />SRF film workshop-Oct 2010<br />
  9. 9. 7<br />Hot and cold spots in SRF cavity, Origin<br />Temp. dep: peak at 0 mV bias increases<br />Killing superconductivity by applying a mag. Field<br />Nb/NbOx/Au<br />Ta/Ta2O5/Al<br />fits with Appelbaum theory -><br />Magnetic impurities in the oxides<br />J>0 -> antiferromagnetic coupling<br />-First time measured on Nb oxides<br />-Same behavior observed on unbaked Nb coupons<br />Th. Proslier, J. zasadzinski et al. accepted to PRB (2010)<br />SRF film workshop-Oct 2010<br />
  10. 10. 8<br />Zero Bias Conductance (ZBC) peak: Spin Flip Tunneling<br />Nb-Nb2O5-Au (hot spots)<br />Ta-Ta2O5-Ta<br />2 K<br />g=2<br />Kondo effect<br />Δ=g.µ B .H -> g = 3.5<br />Definite proof for localized paramagnetic moments<br /> in the Niobium oxide<br />What is the origin ?<br />SRF film workshop-Oct 2010<br />
  11. 11. 9<br />Experimental evidences of Magnetism: <br /> Electron Paramagnetic resonance (EPR)<br />On niobiumpowders (high surface/volume ratio)<br />Nb12O29<br />Localized paramagnetic moments<br />due to Oxygen vacancies.<br />2 values of g: g=1.2 and g=3.3<br />-> insolated spins<br />-> arrangement in 1D spin chains<br />A.Lappas, PRB 65, 134405 (2002)<br />SRF film workshop-Oct 2010<br />
  12. 12. 10<br />Experimental evidences of Magnetism: <br /> Superconducting Quantum Interference Device (SQUID)<br />χ= χ0 + C/(T+ θCW)=M/(B.Vol) <br />Background: Pauli term χ0[T]<br /><ul><li> Linear dependence-> surface magnetism</li></ul>SRF film workshop-Oct 2010<br />
  13. 13. 11<br />Experimental evidences of Magnetism: <br /> Mild and HT baking in UHV of EP samples<br />Samples:<br /><ul><li> M.B. Increase conc. of </li></ul>magnetic impurities<br />(similar to Casalbuoni)<br /><ul><li> High Temp. baking decrease it.</li></ul>Cavities: <br />-at 1.8 K higher Rres after mild baking<br />-High Temp. baking decrease Rres<br />Vol =S.d<br />d=5 nm<br /><ul><li>Factor of 10 in Curie cst with Cava -> </li></ul>Real vol of oxides = 10 x the nominal vol<br />d:from 5 to 10 nm (neutron)<br />roughness -> factor of 5 easily (Raman)<br />Conc of magnetic moments:<br />~ 104ppm = 1%, PCT ~ 0.1%<br />SRF film workshop-Oct 2010<br />
  14. 14. Raman spectroscopy<br />12<br />BCP Nb foil<br />.D<br />.B<br />.C<br /><ul><li> Enhancement of the signal intensity</li></ul>due to amorphous NbOx at <br />imperfection locations + roughness <br />-> Lead easily to factor of 5-10 <br />in real volume. <br />Pits in cavity Nb <br />.A<br />SRF film workshop-Oct 2010<br />
  15. 15. Summary experimental results:<br />13<br /><ul><li>Magnetic impurities present at the surface of Nb, in the oxides</li></ul> Nb2O5-δ is magnetic [1]<br /><ul><li>Concentration is modified by surface treatment (mild, High T)
  16. 16. Hot spot show higher concentration of Mag. Moments (PCT)
  17. 17. Correlates with Cavity results
  18. 18. Theory: surface impedance [T] </li></ul>1: Cava et al. Nature 350, 598 (1991) & PRB 41, 13 (1991) & PRB 72, 033413 (2005) <br />SRF film workshop-Oct 2010<br />
  19. 19. Surface magnetism: Theory<br />Model assumes: Homogeneous moment density on λ + London limit + dirty limit<br />3 parameters: <br />η, α: describe effect of magnetic impurities on the superconductor -> concentration<br />normal conductivity σ0-> mean free path, ℓ.<br />London & dirty limit:<br />In the limit:<br />Density of Normal electrons:<br />Density of superconducting electrons:<br />SRF film workshop-Oct 2010<br />Solve self consistently g[T, e], f[T, e] and Δ[T]<br />14<br />
  20. 20. Theory: The residual resistance<br />15<br />α=0.02 meV, η =0.5<br />SRF film workshop-Oct 2010<br />
  21. 21. Theory: The residual resistance<br />16<br />α=0.033 meV, η =0.2<br />SRF film workshop-Oct 2010<br />
  22. 22. Surface magnetism: Theory<br />17<br />η= 0.2 -> strong coupling<br />
  23. 23. Theory: The residual resistance<br />18<br />η =0.2<br />Bake 180C<br />Bake 120C<br />Bake 160C<br />Bake 90C<br />Th. Proslier, M. Kharitonov submitted to PRL (2010)<br />SRF film workshop-Oct 2010<br />
  24. 24. 19<br />Theory: The residual resistance<br />Δ and Tc<br />Mean free path and λ<br />Mean Free path, ℓ, decreases after baking <br /> consistent with BCS surface impedance fits<br />α~2.10-2meV & η=0.2 -> 250 ppm in Nb -> 6.1012 /cm2 in Nb oxides<br />SRF film workshop-Oct 2010<br />
  25. 25. 20<br />Theory: The residual resistance<br />conclusion<br />London penetration depth, λ<br />Concentration of Mag. Impurities<br />Concentration of Magnetic impurities increase after baking<br /> consistent with SQUID data<br />Cornell 2010<br />SRF film workshop-Oct 2010<br />
  26. 26. Non-linear Meissner effect: On-chip cavity<br />21<br />60 nm Nb+5nm W<br />30 nm Nb+5nm W<br />60 nm Nb+ water<br />60 nm Nb+air<br />30 nm Nb+water<br />30 nm Nb+air<br />

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