1. Ingot Niobium Summary Workshop – December 4, 2015

3. Field Cooled (FC): shows bulk property of SC Zero Field Cooled (ZFC): shows surface property of SC A. Polyanskii

4. Tri-crystal Bi-crystal GB (#1) Normal to Surface Bi-crystal GB (#1) Normal to Surface Tri-crystal Thickness of sheet is 1.88 mm GB #2 GB #1 RF field in-plane

5. Misorientation angle between grains ≈17.8 o Orientation Imaging Microscopy (OIM): from Abraimov After BCP MO H

6. M SC H

7. M H Zig-zag domain walls nucleate above the Meissner state due to in-plane components Hx, which are equal to zero above the zig-zag walls where only vertical components Hz exist.

8. MO H

9. GB trace on top face of sample No GB trace on front face GB trace on bottom face of sample This face has been imaged by MO, when sample was turned by 90 0 H MO indicator 2.78mm 2.17mm 1.89mm H = 80 mT ZFC

10. Isolated a small sample from the above bi-crystal then, rotated by ~90° to align the GB plane parallel to the surface normal And compared the BCP’ed with the smooth-polished. H = 80 mTH = 112 mT H = 0 mT ZFC T = 6.2 K FC T = 6.2 K Before polishing H = 80 mTH = 100 mT 1 mm H = 0 mT FC T = 6.4 K ZFC T = 6.4 K GB After polishing The bulk pinning of magnetic flux is symmetric and only the flux penetration is asymmetric. There is no topological effects on the preferential flux penetration H

11. H = 58 mT FC T = 6 K H = 0 mT ZFC T = 6 K H = 72 mTH = 0 mT BCP’ed EP’ed ZFC T = 6.5 K FC T = 6.5 K Reduced thickness, and then compared with the EP’ed bi- crystal cut from same GB 1 mm GB is a weak link only when H ext is aligned parallel with the GB plane The GB groove may not the cause of the preferential flux penetration H Both: GB ∥ H ext

12. ~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 (170 mT) and H c2 (200 mT) 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 ~150-250μm, 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

13. A deep (3-5 μm) 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 BCP'ed Single Crystal BCP'ed Bi-Crystal

14. A deep (3-5μm) and highly inclined groove Preferential flux flow H ext = 0.08 T to 0.28 T when the GB plane // H ext The # is the angle between the GB plane and H ext 0.08T 0.10T 0.05T H ext

15. Linear coordinates Very different responses. 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 BCP’ed EP’ed GB flux flow

16. Artificially grooved single crystal (using FIB) Very flat surface by ultra-fine polishing V-J response of 26° Bi-crystalV-J response of Single crystal 0.08T 0.10T 0.05T 0.13T 0.18T 0.20T Flux flow Preferential flux flow at the grain boundary may be not triggered by surface topological features when GB plane is parallel to H ext by FIB (Focused Ion beam)

17. BCP’ed – groove effectFlattened – No surface effect H = 0.08T Angular dependency of flux flow at GB becomes more pronounced in non-grooved sample compared to BCP’ed, GB-grooved one. When GB is angularly aligned to external filed, the GB may split vortices treading at the GB into two or more parts or enlarge the length of vortex channel. Thus GB enhances Jc The number of degree indicates the angle of between a plane of GB and external magnetic field (GB vs H ext )

18. 110 121 Darker contrast due to high misorientation angle across GB Grain BoundaryDislocations Possible dislocations Pile-up at G.B Prepared by 30min BCP after mechanical thinning ~30-50 μm Precipitation contrast at GB Several strain and dislocation contrast Uniform transmission contrast indicates no step at GB Prepared by 50min BCP after mechanical thinning ~80-100 μm

19. 19 GB Au-Pd Oxide GB Au-Pd Oxide Λ ~ 40nm GB Oxide Au-Pd Inclusions Native oxide : Nb 2 O 5 5-10 nm Interface : sub oxides + interstitial oxygen : some monolayers. interstitials : what concentration, what depth profile ? Grain boundaries Chemica l residue ~ 40nm Shallow oxide indentation at the GB Thickness of Nb oxide; ~ 5- 7nm Λ ~ 40nm A B Halbritter’s widely accepted model

20. Successful GB TEM foils allow us to perform µ­chemical investigation Thin Nb 2 O 5 film Reference – Gatan Atlas (HV = 200 kV BF) Energy loss spectrum position Spectrometer entrance aperture position (diaphragm : ~100nm) Nb – M 3 Nb – M 4,5 Nb – M 2 O – K Example with oxygen Example w/o oxygen Illustration of sampling area for EELS Location of peaks in example analyses with and without oxygen

21. Oxygen-K Possible O- K knee? Oxygen-K peak is detectable in about 80% of in grain regions (50­ 20 µm away from GB) Oxygen peak (K shell) not clearly visible in 100 nm diameter grain boundary analysis regions Fourier deconvolution & background subtraction Courtesy of R.F. Egerton Within the integration window (∆) ≈ 75eV D. Bach, et al. Micro. Micronal. 12, (2006)

22. Optical image after wire-EDM cutting of a tensile-tested single crystal screwdislocations out of surface screwandedge dislocations in slip plane b b and 1.5mm d

23. View || H ext After Mechanical Polishing + 30 min BCP Zero Field Cooled (ZFC) T = 7 K Rem, T = 7 K, H = 0 mT (after H= 60 mT) ZFC, T = 7 K, H = 68 mT Remn, T = 7 K, H = 0 mT (after H = 68 mT) Edge dislocation line direction and slip plane trace Slip plane H ext Flux penetration Low angle grain boundary (GB) trace revealed by BCP Surface optical image b Flux penetration b

24. After Mechanical Polishing + 30 m BCP Surface optical image View || H ext Screw dislocation Slip plane ZFC, T = 8.2 K Remn, T = 8.2 K, H = 0 mT (after H = 52 mT) ZFC, T = 8.2 K, H = 60 mT Remn, T = 8.2 K, H = 0 mT (after H = 60 mT) Flux penetration H ext d

25. ZFC, T = 7.2 K, H = 35.6 mT FC, T = 7.2 K, H = 0 mT (after FC in H = 68 mT) LAGBSs Surface optical image Flux penetration Roof pattern for strong bulk SC current ZFC, T = 7.2 K FC, T = 7.2 K H=0 mT (after FC in H = 120 mT) LAGBSs Flux penetration Roof pattern for strong bulk SC current b d

26. At room temp After cryogenic treatment in MO imaging Clean zones: no dislocations & no NbH x segregations Pits or craters of NbH x segregations FE-SEM image Surface optical image Trace of NbH x segregations along the LAGB ~0.5-0.6° misorientatio n Grain orientation across the LAGB IPFLocal misorientation mapMisorientation angle profile NbH x segregation (< 100 K) highly favorable during MO imaging below T c of Nb ~ 9.2 K