1 PROGRESS IN NEAR-FIELD MICROWAVE MICROSCOPY OF SUPERCONDUCTING MATERIALS Tamin Tai, Steven M. Anlage Department of Physics Center for Nanophysics and Advanced Materials University of Maryland 19 February, 2010 Funding from Department of Energy NSF Division of Materials Research Maryland Center for Nanophysics and Advanced Materials 6 th SRF Materials Workshop NHMFL, Tallahassee, Florida

2 APPROACH GOAL: To establish links between microscopic defects and the ultimate RF performance of Nb at cryogenic temperatures APPROACH: Work with the SRF community to develop well- characterized defects and subject them to 2 forms of microscopy: Near-Field Microwave Microscopy Laser Scanning Microscopy

3 NEAR-FIELD MICROWAVE MICROSCOPY The Idea 1) Stimulate Nb with a concentrated and intense RF magnetic field 2)Drive the material into nonlinearity (nonlinear Meissner effect) Why the NLME? It is very sensitive to defects… 3)Measure the characteristic field scale for nonlinearity: J NL 4) Map out J NL (x,y) 5) Relate J NL (x,y) maps to candidate defects (T, J RF ) Meissner-state properties now depend on current J → Nonlinearities in inductance and loss J RF Leads to Harmonic Generation f → 3f, etc.

4 Magnetic recording heads provide strong and localized B RF B RF ~ 1 Tesla Lateral size ~ 100 nm How to Generate Strong RF Magnetic Fields?

5 Read/Write Head Thickness (~1.2 mm) Suspension Au bond pads to reader and writer Air bearing surface Cross-section (next slide) Magnetic Recording Head Slider

6 Permalloy shields ~2  m Cu coils Read Sensor Write Pole Magnetic Recording Head Cross Section RF Magnetic Fields Air bearing surface 2  m

7 Measured Input Impedance of Write Head Seagate Longitudinal Head Pole width ~ 200 nm Pole gap ~ 100 nm Pole thickness ~ 1  m Coil turns 6 ~ 8 Impedance Matched to 50 

8 Next Generation Experiment D. Mircea, Phys. Rev. B 80, (2009) We need higher B RF and strongly localized field distributions Goals: B RF ~ 200 mT Lateral size ~ 100 nm RF Coil on slider Superconductor

9 Induce high  0 K ~ 200 mT K(x,y) sharply peaked in space ► Better spatial resolution Current distribution geometry factor What do We Learn About the Superconductor? D. Mircea, SMA, Phys. Rev. B 80, (2009) + references therein Simulated current distribution beneath probe

10 J NL (x) P 3f (x) Position x Defect 1Defect 2 What do We Learn About the Superconductor? P 3f

11 NEAR-FIELD MICROWAVE MICROSCOPY Expected Outcome Catalog of Defects and their Superconducting RF Properties DefectLocal RF Properties J NL (A/m 2 ) Etch Pit E-beam Weld Step Edge Grain Boundary … Locally (< 1  m) stimulate Nb surface with large (B RF ~ 200 mT) RF field and induce nonlinear response Map J NL (x,y) and relate to known defects

12 Measurements on Tl 2 Ba 2 CaCu 2 O 8 Film At a fixed location Noise floor Vortex Nonlinearity? TcTc ~40 dB higher response expected

13 Unresolved Issues Not achieving the P 3f response levels expected Accomplishments Suppressed background nonlinearity (3f) from write head Seeing clear, reproducible signal from superconducting sample Receiving technical assistance from the magnetic recording industry Proven that heads are impedance matched Dragos Mircea, Sining Mao (Western Digital) Tom Clinton (Seagate → N th Degree) Gap may be too far from sample: Dirt, roughness Gap may be too close to sample: Gap+SC creates large reluctance Stray fields from slider inducing vortices?

14 Greg Ruchti, Senior Thesis, UMD Next Steps Micro-Loops  = 1.3 x 10 6 A 3 /m 2 Original loop probes had  ~ 10 3 A 3 /m 2 Slider gap - sample distance control

15 LASER SCANNING MICROSCOPY New round of experiments in March+April on bulk Nb + films Experiments in collaboration with Alexey Ustinov and Alexander Zhuravel at Uni. Karlsruhe, Germany

16 Near-field microwave microscopy and Laser Scanning Microscopy are quantitative measurement techniques capable of addressing important materials issues in Nb The microscopes are flexible, simple, broadband, and have frequency- independent spatial resolution Looking for materials collaborators with well-characterized local defects Objectives: Develop B surf ~ 200 mT,  m-scale probes for RF Critical Field imaging Develop compact resonators with Nb coupon samples Relate local cryogenic RF properties to microstructure Conclusions

17 LASER SCANNING MICROSCOPY The Idea 1) Create a compact resonant structure involving a Nb coupon 2) While exciting on resonance, scan a focused laser spot on sample 3)Image the Photo-Thermal effects: 4) Relate these images to candidate defects J RF (x, y) - RF current density in A/cm 2 Thermo-Electric imaging J NL (x,y) – Nonlinearity current scale in A/cm 2 RF vortex breakdown, flow, critical state

18 LASER SCANNING MICROSCOPY Preliminary Results

19 LASER SCANNING MICROSCOPY Preliminary Results on YBa 2 Cu 3 O 7 Patterned Film Reflectivity Image Thermo-Electric Image (room temperature) DC Critical State Image (T I c ) J RF (x,y) Image (5.5 GHz, T < T c )

20 LASER SCANNING MICROSCOPY Preliminary Results RF Defect Imaging in bulk Nb

21 LASER SCANNING MICROSCOPY Expected Outcome Candidate Resonant Structures Bulk Nb “Short Sample” structures to measure RF breakdown fields and relate to defects and surface processing Measure RF breakdown fields in SC / Ins / SC / Ins … multilayer samples Gurevich, Appl. Phys. Lett. (2006) Image variations in the thermal healing length to better understand the role of heat dissipation in RF performance

22 PLANS FOR THE FUTURE Laser Scanning Microscope Collaborators: Dr. Alexander Zhuravel, B. I. Verkin Inst. of Low Temperature Physics Kharkiv, Ukraine Prof. Dr. Alexey Ustinov, University of Karlsruhe, Germany We seek representative samples containing well-characterized defects and/or surface treatments

23 Collaborators Johan Feenstra Andrew Schwartz C. Canedy A. Stanishevsky Alexander Tselev F. C. Wellstood R. Ramesh J. Melngailis David Steinhauer Atif Imtiaz Sheng-Chiang Lee Wensheng Hu Ashfaq Thanawalla Sangjin Hyun (Seoul Nat. Univ.) Dragos Mircea Hans Christen (Neocera) Vladimir Talanov (Neocera  SSM) Robert Hammond (STI) Andrew Schwartz (Neocera) Steve Remillard (Agile Devices) C. P. Vlahacos Sudeep Dutta Jonah Kanner Post-Docs Graduate Students Undergraduate Students NSF REU Students Nadia Fomin (Georgetown) Eric Wang (UC Berkeley) Tom Hartman (Princeton) Industrial Collaborators Acknowledgements Greg Ruchti Akshat Prasad Young-noh Yoon Yi Qi

24 Single-tone sinusoidal microwave signal with single frequency input, f Superconductor 3f : NLME Nonlinearities Microwave current induced on the sample 2f : Vortex Nonlinearities K max ~ 50 A/m NEAR-FIELD MICROWAVE MICROSCOPY Preliminary Results Create a sample with well-characterized defects and probe them with localized RF currents in the superconducting state B max ~ 60  T

25 Nonlinear Response Image of Known Defect Bi-Crystal Grain Boundary in YBa 2 Cu 3 O 7-  T = 60 K f = 6.5 GHz S.-C. Lee, et al., Phys. Rev. B 72, (2005) x (  m) y (  m)