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PRODID:UW-Madison-Physics-Events
BEGIN:VEVENT
SEQUENCE:0
UID:UW-Physics-Event-1878
DTSTART:20100809T150000Z
DURATION:PT1H0M0S
DTSTAMP:20260422T092323Z
LAST-MODIFIED:20100730T205705Z
LOCATION:5310 Chamberlin
SUMMARY:Niobium Oxidation\, Metal-oxide Interface Effects\, and rf Los
 ses\, R. G. Herb Condensed Matter Seminar\, Juergen Halbritter\, Resea
 rch Center Karlsruhe
DESCRIPTION:For superconducting Nb the electric surface impedance Z<su
 p>E</sup> is confined to a narrow (≲10nm) Nb<sub>2</sub>O<sub>5</sub
 >-Nb interfaces and the magnetic surface impedance Z<sup>H</sup> to λ
 <sup>B</sup>≲100 nm as magnetic penetration depth into Nb. The corre
 sponding surface resistances R<sup>E</sup> and R<sup>H</sup> are small
  below 2K but their increase with oxidation is for Nb cavities the lim
 iting obstacle at low E<sup>⊥</sup>≲10V/m and at high rf magnetic 
 fields B<sup>||</sup>≈0.1T and high rf electric fields E<sup>⊥</su
 p>≳5MV/m. Nb oxidation has been studied by angle resolved X-ray phot
 on electron spectroscopy (ARXPS)\, by surface resistance R<sup>B</sup>
 (T\,B\,ω)\, by flux pinning\, by penetration depth λ<sup>B</sup>(T\,
 B\,ω)\, and by voltage tunnel spectroscopy (VTS). Our studies showed 
 that oxidation is strongly inhomogeneous: by Nb<sub>2</sub>O<sub>5-y</
 sub> consisting of crystalline blocks (CB) built of {NbO<sub>6</sub>} 
 octahedra to sizes ~ 1nm with a barrier height Φ<sub>B</sub> = E<sub>
 C</sub> -E<sub>F</sub> ≃ 1 eV as difference between conduction band 
 E<sub>C</sub> and Fermi energy E<sub>F</sub>\, by CBs separated by cry
 stallographic shear planes (CS) with Φ<sub>S</sub> ≃ 0.1 eV housing
  localized states n<sub>L</sub>(z) ≃ 10<sup>19</sup>/cm<sup>3</sup>\
 , and by nuclei in Nb easing the strain relieve due to the CB volume i
 ncrease by more than 300% causing injection of oxidized weak links and
  NbO<sub>x</sub> precipitates up to depth between 0.1 - 50μm degradin
 g locally the superconducting energy gap δΔ/Δ≃100x/Δ by O<sub>x<
 /sub>. The latter deteriorations of superconductivity have been minimi
 zed successfully in recent years by Nb with minimal density of nuclei.
  Hence\, here we focus on the Nb-oxides and on Nb<sub>2</sub>O<sub>5-y
 </sub>/NbO<sub>z</sub>/Nb interface reaching up to 0.1μm deep for hig
 h purity Nb surfaces. Wet\, fast grown Nb<sub>2</sub>O<sub>5-y</sub> i
 s thicker then 4 nm and highly perturbed with a high density of CS and
  of n<sub>L</sub> enforcing weak link and NbO<sub>x</sub> injection in
 to Nb to relieve the strain. Slower oxidation is quantified by the Cab
 rera Mott process\, where nuclei poor Nb single crystals show the slow
 est Nb<sub>2</sub>O<sub>5-y</sub> growth rate with smaller CS and n<su
 b>L</sub> densities on one side and\, on the other side\, the smallest
  amount of O injection into Nb. Differently grown Nb<sub>2</sub>O<sub>
 5-y</sub> yield interfacial tunnel exchange (ITE) differing by oxide t
 hickness and CS and n<sub>L</sub> density\, yielding\, e.g.\, greatly 
 varying R<sup>E</sup> ∝ n<sub>CS</sub>n<sub>L</sub> dominated by n<s
 ub>L</sub> and CS densities.
\n
\nThe details of Nb<sub>2</sub>O<sub>5
 -y</sub> structure and growth will be elucidated explaining via ITE R<
 sub>res</sub> < 5nΩ(f/GHz)<sup>2</sup>\, noise\, electronic two level
  systems (ETLS) confined to the Nb surface and oxidation dependent Q-d
 rops quantitatively\, especially the much reduced Q-drop after UHV ann
 ealing. Differences between Nb\, Al\, and NbN oxidation and their cons
 equences for Z<sup>E</sup> and Z<sup>H</sup> will be elucidated.
URL:https://www.physics.wisc.edu/events/?id=1878
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