The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Suppressed superconductivity on the surface of SRF quality niobium for particle accelerating cavities Z.H. Sung, A.A. Polyanskii, P.J. Lee, A. Gurevich, and D.C. Larbalestier Applied Superconductivity Center National High Magnetic Field Laboratory Florida State University

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Outline Issues – GBs are defect responsible for Q 0 drop at high field regime? Previous work - Premature flux penetration by MO imaging technique DC transport measurements - Preferential flux flow at the GB Field dependent flux flow resistivity of the GB and its angular dependence, and GB flux flow as f(θ) Local magnetic characteristics at the GBs Summary Current search - local magnetic characteristics on the PITs

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN GBs are defect responsible for Q 0 drop at high field regime? G. Ciovati et al. Proc. of the 2006 LINAC, paper TUP033 No major difference of Q 0 between grain sizes G. Eremeev et al. Proc. of EPAC’06, Edinburgh, MOPCHI176  40% of all hot-spots associated with GB G. Ciovati et al. Phys. Rev. ST Accel Beam 13, 2010 So far… Not clear about the GBs effects on SRF Nb cavity

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Quench locations: Z111 TESLA 6 cell cavity G. Ciovati et al. SRF Material Workshop at MSU, 2009 Courtesy of W. Singer. ILC Cavity Group 9th Meeting. January 27 th. 2009

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Very weak dissipation at H < H c1 (Q 0 = ) Q drop due to vortex dissipation at H > H c1 Nb has the highest lower critical field H c1 Gurevich et al. PRL 88, (2002) GBs can locally reduce superconducting gap (Δ) and the depairing current density, J b.gb → suppress the onset of vortex penetration → increase R s → increase power dissipation GB is a hot spot site Thermodynamic critical field H c (surface barrier for vortices disappears) Gurevich et al. Physica C, 441 (2006)

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Description of specific grains and grain boundaries Large grain Nb sheet from JLAB (P. Kneisel and Co-workers) “Thinner” GBs have planes which are closer to the surface perpendicular. “Thicker” GBs are inclined ~20-30° from the perpendicular. This as-received slice (RRR ~280, 3.1mm thick) has a very large grain size (~50-100mm), which allowed us to isolate multiple bi-crystals. OIM: GBs are ~ 25-36° misoriented Overview of the as-received niobium slice

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Premature flux penetration at the grain boundary when the GB plane // H ext (by MO imaging) P. Lee, et al., Physica C, 441 (1), (2006) H = 58 mT FC T = 6 K H = 0 mT ZFC T = 6 K H = 72 mT H = 0 mT BCP’ed EP’ed ZFC T = 6.5 K FC T = 6.5 K 1 mm H Both: GB//H ext Top & Bottom 100min BCP H = 80 mT ZFC Lack of flux penetration at the tilted GB 3D image: 23.6° tilted GB

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN ~20-30 mT 200mT As-receivedMechanically ground Surface image of I-shape sample after BCP treatment 1 mm 100μm Expand the gap between H c1 (170mT) and H c2 (200mT) at 4.2 K → Make vortex penetration at lower H ext 1.Cut samples into I-shape with wire-EDM 2.Mechanically grind down the bottom of the sample surface to ~ μm, so the top surface remain as-received condition 3.Ultra fine polish with vibratory polisher (Vibromet ® Buehler) 4.Finalize all surfaces with either BCP or EP - Make surfaces representative of real cavity surface 5.Further reduce the bridges of some I-shape single- & bi- crystals with extra BCP 6.Artificially groove with FIB and mechanically smear away the grooved produced by the chemical treatments The procedures DC transport V-I characterization with 1T Electromagnet Higher-H c SC Nb

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Preferential flux flow on the deep GB groove A deep (3-5μm) and highly inclined groove No groove (~0.5–2.0μm roughness) The V-J characteristics show that the grain boundary is a channel of preferential flux flow (FF) by weakly pinned vortices. Flux flow evidence from H = 0.08 T to 0.28 T However, the slightly non-ohmic V-I response suggests that flux flow is not just confined to a single vortex row flowing along the grain boundary 0.05T 0.08T 0.10T

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Field dependent GB resistivity : R f (H) The width of the FF channel (W) & The number of vortices flowing on the channel (dV row ) at H = 0.08T are μm and μm, representing 1.15 and 3.04 rows, respectively from the low and high V portions of the V-I curve Single vortex row Collective depinning of multiple vortex rows along GB Multi vortex rows Gurevich’s model 7° [001] tilted YBa 2 Cu 3 O 7-δ GB 1 st linear fitting

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN High GB flux flow tendency when H ext lies on the GB plane A deep (3-5μm) and highly inclined groove Preferential flux flow H ext = 0.08 T to 0.28 T At the GB plane // H ext The # is the angle between the GB plane and H ext 0.08T 0.05T 0.10T

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Mechanisms of suppression of flux flow on the GB a A-J core size: The A-J cores overlap if > a, or Viscous flux motion R(B) is independent of B, if a single vortex chain moves along GB, while > a Gurevich et al. PRL 88, (2002) V = (I - I b )R H > (J b /J d ) 2 H c2 = J 2 / =  J d /J b 1. Split A vortex 2. Increase of GB area J θ GB α A > α B Transformation of Abrikosov vortex to mixed Abrikosov Josepshon vortex at the grain boundary of SRF-quality niobium is still unclear but GB weakness is shown at V-I responses

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Do BCP and EP have different effects on flux penetration? Linear coordinates Flux flow? No distinct flux flow evidence at the electropolished GB, similar to BCP’ed Single crystal However, traces of flux flow along the electropolished GB are visible Linear coordinates H = 58 mT ZFC T = 6 K BCP’ed EP’ed H = 72 mT ZFC T = 6.5 K GB flux flow

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN GB flux flow as f(θ) : crystallographic misorientation angle More misoriented GB retards the progress of preferential flux flow The aspect ratio is defined as the ratio of the width to length of the bridge of I-shape, so it means the higher the ratio is, the more demagnetization is 36° bi-crystal (the aspect ratio ~ 24.3)22° bi-crystal (the aspect ratio ~ 12.5)

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Field enhancement by severe surface H ext //surface As-received Magnetic field enhancement model at the slope of GB E-beam WELDED AREA Machine marks, large grains and height steps at GBs J. Knobloch, 8 th SRF workshop Β m  height, angle, aspect ration and radius of curvature at the corner of the grain boundary

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Local field enhancement characterization using micro Hall sensor 15 Micro Hall probe Hall probe sensor Fabricated by Dr. Milan Polak (2009) Al 2 O 3 BCP’ed Nb bi-crystal GB H Glass (t~126μm) Spec: ~ 2mm by 2mm Total thickness: ~0.4mm 0.4mm thick GaAs substrate 5μm thin InSb layer Activation area: 50μm by 50 μm Minimum distance between the sample surface and the Hall probe is ~0.2mm

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Field vs response voltage on Hall probe Field enhancements at highly inclined GB Roughness profile at the GB Slope change from 10.5° → 25.7° The field enhancement at the grain boundary region is almost 5 times higher than in-grain For example, at H ext ≈ 50mT, the induced H gb, ≈ mT, and at H ext ≈ 100 mT, H gb ≈ mT, accounting that local areas of the grain boundary region is normal state.

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN GB // H RF GBs Cavity wall μ structure and chemical properties ? GB Oxide Au-Pd Preferential GB flux penetration Preferential GB flux flowsGB Field enhancement

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Summary GBs can preferentially admit magnetic flux before it is admitted to grains when the GB plane // H ext Topological features introduced by BCP or EP were not the cause of the preferential nucleation of magnetic flux when H ext // the GB plane Transport measurements on BCP’ed samples showed clear evidences of preferential flux flow on the GB. Consistent with the results of MO imaging, the V-I characterizations showed that the grain boundary weakness is greatly enhanced when the plane of GB is parallel to the H ext vector. The highly inclined slope of grain boundary produced by BCP causes localized field enhancement when H ext // the sample surface. High resolution analytical microscopy is highly recommended to further understanding of intrinsic grain boundary properties

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Investigation of SC breakdown on the PITs Cold spot Courtesy of Dr. Romanenko from BCP’ed cavity #3 – SE2 Image PIT-BSD image After Diamond cutting EBSD scanned area PIT on this triple point did not cause excessive heating

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN PIT-BSD image #3-PIT : 3D topology 20.8 μm Surface topology Surface topological features of the PIT Surface profiles By scanning laser confocal microscope ( ~ few tens of nm z-direction resolution)

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Local magnetic characterizations using micro Hall Array Sensor 1. Micro-Hall Sensor Using 7-8 Channels ~ 420  m 2. Micro-Hall Array Sensor Enough resolution to detect a single vortex quantum (Φ ≈ × T/m 2 ) 7-8 channels of 21 channels the entire length of the array (~420 μm) Provided by Dr. Milan Polak (2009) Provided by Dr. Eli Zeldov (2009)

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Local vortex avalanches in superconducting Nb E. Altshuler, Phys Rev B, 70 R, & Physica C, , (2004) MO imaging on Nb thin film HAS

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN PIT from cold spot sample on HAS PIT on the HAS PITS Hall array sensors Decreased the dimension of #3 part (see slide #4) with a precision diamond, then carefully mechanically reduced its thickness up to half of the original

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Voltage responses of PIT on the single Hall sensor Kink by field enhancements Kink by field enhancements: due to the topological effect of a pit

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Voltage responses of the PIT on the HAS Kink by field enhancements due to the topological effect of a pit The tendency of voltage responses are very similar to the plots by the single Hall sensor, but the onsets of Kinks by field enhancements are delayed up to ~ 20 – 50 mT

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Acknowledgement We would like to thank Peter Kneisel and Ganapati Rao Myneni and their colleagues at TJNL for providing the Nb slice. Lance Cooley, SRF Materials Group Leader, at FNAL. Special thanks to Ian Winger (Physic department, FSU) for wire- EDM Dr. Milan Polak and Dr. Eli Zeldov for supplying a micro-Hall sensors This work was supported by the US-DOE under grants DE-FG02- 05ER41392 and DE-FG02-07ER41451 and by the State of Florida support for the National High Magnetic Field Laboratory

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN What is the real microstructure at the GB after BCP? HRTEM used to observe the vicinity of the GB (with FIB technique) GB Au-Pd Oxide GB Au-Pd Oxide Λ ~ 40nm GB Oxide Au-Pd Inclusions Native oxide : Nb 2 O nm Interface : sub oxides + interstitial oxygen : some monolayers. interstitials : what concentration, what depth profile ? Grain boundaries Chemical residue ~ 40nm No oxide indentation at the GB Thickness of Nb oxide; ~ 5-7nm Λ ~ 40nm A B Halbritter’s widely accepted model

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN J c enhancements on meandered GBs of YBCO Planar GB (PLD) Feldmann et al., JAP 102 (2007): J.Am.Ceram.Soc (2008) Meandered GB (PLD & MOD) Planar GB B.C. Cai et al., Phil. Mag. B. (1987): A. Dasgupta et al, Phil. Mag. B. (1978) I c enhancement when H ext // the GB plane of Nb

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Further efforts for the GB depairing critical current BCP’ed Nb Bi-crystal (26° misoriented GB) B perp = H - η·M η = demagnetization factor M = magnetization Multi vortices rows Single vortex row 7° [001] tilted YBa 2 Cu 3 O 7-δ GB Micro-Hall Sensor Arrays GaAs/AlGaAs heterostructure Courtesy of Dr. Eli Zeldov ~ 420  m

The Applied Superconductivity Center The National High Magnetic Field Laboratory Florida State University Z.H. Sung 1 st SSTIN Field dependent GB resistivity : R f (H) FieldsCh1Ch2Ch3Ch4Ch5Ch6Ch7Ch8 10mT mT mT mT mT mT mT mT mT Local magnetization: B prep on the BCP’ed bi-crystal Multi vortices rows Single vortex row