High Tc nanometric SC films

1. Accelerator R&D From Supratech Platform to theFuture of Accelerating RF Cavities Claire AntoineIrfuService des Accélérateurs, de Cryogénie et de MagnétismeLaboratoire d’Etude des Systèmes Accélérateurs Radiofréquence

2. High Tc nanometric SC films Champ magnétique B (mT) Coefficient de qualité Q0 Champ Accélérateur Eacc (MV/m) Barrière de surface et faible RBCS Happliqué HNb Surface interne de la cavité Vers extérieur cavité  • Composite nanometric SC : Multilayers Nb / insulator/ superconductor / insulator /superconductor… • Bulk Nb prevents perpendicular vortex penetration • Insulating layer ~ 15 nm : Josephson decoupling • High Tc SC ~ some 10 nm • High HS • Magnetic screening for parallel vortex • Lower RBCS Q0 ↑↑

3. 1rst experimental results : ~ 25 nm NbN ~ 15 nm insulator (MgO) 250 nm Nb “bulk” Reference sample R Test sample SL Monocrystalline sapphire ~ 12 nm NbN x 4 14 nm insulator (MgO) 250 nm Nb “bulk” Monocrystalline sapphire Test sample ML • Collaboration avec J.C. Villégier, CEA-Inac / Grenoble • High quality Model samples : • (Asserted techniques for superconducting electronics circuits preparation) • Magnetron sputtering • Flat monocrystalline substrates • Same as Josephson junction preparation

4. Characterization: • Standard characterisation : material quality (collabn CEA/INAC-Grenoble) • X Rays :low angle diffusion: • Peaks/phases Identification : Monocrystalline or highly (200) textured layers • Distortions :d(200) on NbN expanded: ~ 0,5 % • X Rays : reflectivity: • thickness and interface roughness • Quantum design PPMS: • Tc, conductivity * Sample R contains only Nb capped with a layer of NbO. It is obtained by RIE etching of sample SL. ** In ML the motive MgO/NbN is repeated 4 times. *** except the external, capping layer which is 5 nm

5. SQUID (1) : Oriented quartz holder Sample Quartz holder H Detection coils Quartz holder Detection coils H Sample Detectioncoils H Sample • H , longitudinal moment • H //, longitudinal moment • H //, transverse moment • Principle of measurement (5x5mm2 samples) : • Samples in uniform external field • Parallel and perpendicular field tested • Measurement difficulties for SC thin films in H //: • Very strong transverse signal vs longitudinal => the long. measurement appears non purely dipolar (in fact superposition of 2 signals) • Very strong sensitivity of M to angle => intensity comparison = difficult • Long. fits need to be reprocessed (Work in progress) Usual dipolar signal Signal as detected in the long. loop : strong even signal is due to “cross talk” with the transverse moment

6. SQUID (2) : Reference sample(250 nm Nb) • Comparison H// vs. H • H // : Comparison longitudinal/transverse moment Magnetic moment (e.m0u0) Magnetic moment (e.m0u0) Field (Oe) Field (Oe) • Transverse signal appears only in the SC state • Mtrans>> Mlong !? • Transverse and longitudinal behavior are similar at low field (same HC1) • The first transition appears at lower field when H is  (demagnetization factor ~20 000!)

7. SQUID (3) : Open questions… SL : Magnetization curve @ 4.5 K & 12 K : • NbN alone also exhibit a strong pinning behavior (SL @ 12 K) • Pinning feature disappear when Nb and NbN are SC (4.5K) SL Magnetization curve @ 4.5 K without / with normalization ??? • Why samples with ~ the same volume/mass have such different moments ?

8. SQUID (4) :Same samples, better centering • H // => main component of the moment is transverse ! • Samples with ~ the same volume/mass =>emu should be comparable without normalization • Flip of moment ? Moments in opposite direction in each layer ?

9. SQUID (5) : H // sample plane; longitudinal moment 1rst penetration field Bp : (longitudinal) ML : 4 layers NbN 12 nm SL : 1 layer NbN 25 nm X 4.5 X 1.8 • SQUID : magnetization curve @ 4.5 K • normalization : H = -M (probably not legitimate ? Clarity purpose) • estimation of edge/geometrical effects: difficult • Meanwhile, with only one monolayer 25 nm of NbN deposited on Nb : • Behavior of SL/ML is totally different as compared to each individual layer. • First penetration (longitudinal) field Bp is higher with NbN top layer(s). • Bplong >> Bptrans in SL and ML (2D signature ?). • Lower hysteresis surface, reversible behavior (less pinning of vortices ?)

10. Local magnetometry Differential Locking Amplifier Excitation/Detection coil (small/sample) = T/Tc • 3rd harmonic measurement, coll. INFM Napoli • local measurement : sample size >> excitation/measuring coil • perpendicular field  @ Napoli (// field underdevelopment @ Saclay) • b0cos (wt) applied in the coil • temperature ramp • third harmonic signal appears @ BC1 Sample SL : third harmonic signal for various b0

11. Perspectives : depositing and testing RF cavities • depositing and testing RF Cavities: • IPN (Orsay) : 3GHz, • LKB (Paris) : 50GHz • Cavités 1.3 GHz @ Saclay. TE011, ~3 GHz IPNO 1.3 GHz Irfu 50 GHz LKB

12. Conclusions et perspectives: • Positive points: • Nb/NbN Single layer (and in some extent ML) exhibit a very different behavior than individual layers. • First penetration field Bp is higher with NbN top layer (longitudinal) • Similar behavior observed with local measurement of BC1. • Bplong >> Bptrans in SL • Lower hysteresis surface, reversible behavior (less pinning of vortices ?) • Limitations of these results: • Reference is only 250 nm thick, and exhibit a very low HC1 compared to bulk Nb => Results obtained at ~low field • What will be the behavior of ML films on bulk Nb ? • What will be the behavior of ML films in RF ? • What is the optimum thickness, # of layers ? • Still a lot of work needed !

13. Compléments

14. Characterization 1: • Quantum design physical properties measurement system (PPMS):

15. Xray Characterization : • 1) Large angle diffusion: • Peak/phase identification • Distortions

16. Xray Characterization : • 2) Reflectivity: • Thickness and roughness of layers

17. Multilayers optimization Nb NbN Al2O3 MgO Cu Metallic substrates more realistic): • SC structure optimization • Deposition techniques optimization • Magnetron sputtering Inac (Grenoble), • Atomic Layer Deposition INP (Grenoble) • From samples to cavities : • ALD involves the use of a pair of reagents • Application of this AB Scheme • Reforms a new surface • Adds precisely 1 monolayer • Viscous flow (~1 Torr) allows rapid growth • No line of site requirements • => uniform layers, larges surfaces, well adapted to complex shapes : cavities! • up grade of existing cavities ?

18. HC1, HS enhancement • Interactions w. image vortices =>  surface barrier ! • Thin films w. d< l=> multiple interactions • Vortex field in a film decays over the length d/x instead ofl (interaction with many images) • => enhancement of HC1 et Hs : • → • Thermodynamic critical field Hc (surface barrier for vortices disappears): • →

19. Bulk Nb ultimate limits : not far from here ! • Cavité 1DE3 : • EP @ Saclay • T- map @ DESY • Film : courtoisie • Gössel + • D. Reschke • (DESY, • Début 2008) The hot spot is not localized : the material is ~ equivalent at each location => cavity not limited /local defect, but by material properties ?

20. SIMS : ppb/ppm element analysis with nm depth resolution C 400 µm O Cycle 76 Cycle 11 Cycle 6 Cycle 1 : Surface Cycle 16 (fin de la couche d’oxyde) Table (mini) SIMS : simplified robust analyzing tool Courtesy of Millbrook

21. Rappels sur les principaux supras