proslier - localized magnetism on the surface of niobium: experiments and theory

Localized magnetism on the Surface of Niobium: experiments and theory

Th. 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

Experimental evidence: PCT, SQUID, EPR, RAMAN

Theory: residual resistance


Niobium surfaces are complex, important, and currently poorly controlled at the nm level

3

45

nm

RF

dep

thInclusions,

Hydride precipitates

Surface oxide

Nb2O5 5-10 nm

Interface: sub oxides

NbO, NbO2

often not crystalline

(niobium-oxygen

“slush”)

Interstitials

dissolved in

niobium (mainly O,

some C, N, H)

Grain boundaries

Residue from

chemical

processing

Clean niobiume- flow only in the top

45 nm

Probe the surface superconductivity


4

6 Tesla magnet 1.6-300 K

2

Ideal BCS superconductor

• Measure of the surface superconducting gap Δ

• The ZBC value -> Number of normal electron

Normal electrons in gap => dissipation and lower Q

Experimental evidences of Magnetism: Point Contact


Experimental evidences of Magnetism: Point Contact

5

Th. Proslier, j. zasadzinski et al. APL 92, 212505 (2008)

Conc= 0.1-0.3%


6

Hot and cold spots in SRF cavity (from J-lab)

Anomalous spectrum

Only on hot spots

“Normal” spectrum

Hot 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)

Origin of peculiar spectrum and dissipation?

Correlates with cavities results! (once again)

Th. Proslier, G. Ciovati to be submitted to PRST-AB (2010)


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Hot and cold spots in SRF cavity, Origin

Temp. dep: peak at 0 mV bias increases Killing superconductivity by applying a mag. Field

Th. Proslier, J. zasadzinski et al. accepted to PRB (2010)


Ta/Ta2O5/Al

Nb/NbOx/Au

fits with Appelbaum theory ->

Magnetic impurities in the oxides

J>0 -> antiferromagnetic coupling

-First time measured on Nb oxides

-Same behavior observed on unbaked Nb coupons

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Zero Bias Conductance (ZBC) peak: Spin Flip Tunneling

Ta-Ta2O5-TaNb-Nb2O5-Au (hot spots)

2 K

Kondo effect

Definite proof for localized paramagnetic moments

in the Niobium oxide

What is the origin ?


Δ=g.µ B .H -> g = 3.5

g=2

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Experimental evidences of Magnetism: Electron Paramagnetic resonance (EPR)

On niobium powders (high surface/volume ratio)

Localized paramagnetic moments

due to Oxygen vacancies.

2 values of g: g=1.2 and g=3.3

-> insolated spins

-> arrangement in 1D spin chains


Nb12O29

0.0 0.2 0.4 0.6 0.8

0.8

0.9

1.0

1.1

1.2

Sig

na

l (A

.U.)

Magnetic Field 0H (T)

5K

Cavity

15Kg~1.2

g~3.3

cavity background

A.Lappas, PRB 65, 134405 (2002)

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Experimental evidences of Magnetism: Superconducting Quantum Interference Device (SQUID)

χ= χ0 + C/(T+ θCW)=M/(B.Vol)

Background: Pauli term χ0[T] • Linear dependence-> surface magnetism


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Experimental evidences of Magnetism: Mild and HT baking in UHV of EP samples

Samples:

• 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 Rres


Vol =S.d

d=5 nm

-Factor of 10 in Curie cst with Cava ->

Real vol of oxides = 10 x the nominal vol

d:from 5 to 10 nm (neutron)

roughness -> factor of 5 easily (Raman)

Conc of magnetic moments:

~ 104 ppm = 1%, PCT ~ 0.1%

Raman spectroscopy

12

.

A

.

B

.

C

.

D

BCP Nb foil

Pits in cavity Nb

• Enhancement of the signal intensity

due to amorphous NbOx at

imperfection locations + roughness

-> Lead easily to factor of 5-10

in real volume.


Summary experimental results:

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Magnetic impurities present at the surface of Nb, in the oxides

Nb2O5-δ is magnetic [1]

Concentration is modified by surface treatment (mild, High T)

Hot spot show higher concentration of Mag. Moments (PCT)

Correlates with Cavity results

Theory: surface impedance [T]

1: Cava et al. Nature 350, 598 (1991) & PRB 41, 13 (1991) & PRB 72, 033413 (2005)


Surface magnetism: Theory

Density of Normal electrons:

Density of superconducting electrons:

London & dirty limit:

In the limit:

Model assumes: Homogeneous moment density on λ + London limit + dirty limit

3 parameters:

η, α : describe effect of magnetic impurities on the superconductor -> concentration

normal conductivity σ0 -> mean free path, ℓ.


Theory: The residual resistance

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α=0.02 meV, η =0.5



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α=0.033 meV, η =0.2


Surface magnetism: Theory

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η= 0.2 -> strong coupling


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Bake 120C Bake 180C

Th. Proslier, M. Kharitonov submitted to PRL (2010)


η =0.2

Bake 90C Bake 160C

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Δ and Tc Mean free path and λ

Mean Free path, ℓ, decreases after baking

consistent with BCS surface impedance fits

α~2.10-2 meV & η=0.2 -> 250 ppm in Nb -> 6.1012 /cm2 in Nb oxides


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Concentration of Mag. Impurities London penetration depth, λ

Concentration of Magnetic impurities increase after baking

consistent with SQUID data

Cornell 2010

conclusion


Non-linear Meissner effect: On-chip cavity

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60 nm Nb+5nm W

30 nm Nb+5nm W

60 nm Nb+ water

60 nm Nb+air

30 nm Nb+air

30 nm Nb+water

proslier - localized magnetism on the surface of niobium: experiments and theory

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