1 RF Superconducting Materials Workshop at Fermilab A Novel Method to Measure the Absolute Value of the Magnetic Penetration Depth in Superconductors Vladimir V. Talanov*, Steven M. Anlage, Lucia Mercaldo # John H. Claassen (NRL) * Solid State Measurements, Inc., Pittsburgh, PA # ENEA- Portici Research Center - Italy

2 Motivation Measurement of gives insight into material properties and quality Most techniques measure  but not the absolute magnitude of varies with ℓ MFP, magnetic field (non-linear Meissner effect), direction, weak links, etc. superconductor vacuum y H(y) Typically  ~ 15 nm to 10’s of  m

3 4 Classes of Techniques to Measure 1)Absolute Length Scale Techniques Meissner state flux exclusion. One sample dimension (L) is known  f ~ (L –  ) x (area of the sample) (empty vs. pertubed) Problem: often / L ~ (0) found from fitting  (T) to theory 2) Reflection / Transmission Mutual inductance of two coils Microwave transmission “Missing area” sum-rule, KK analysis Problems: requires large (~cm 2 ) thin films 3) Probes of Internal Magnetic fields  SR Polarized Neutron Reflectometry Problems: very specialized techniques Requires model of mixed state or very flat large area surfaces 4) Josephson Tunneling Magnetic diffraction pattern Problem: requires creating a tunnel junction The Variable-Spacing Parallel Plate Resonator /L ~ and systematically larger Requires very accurate spectroscopy

4 The Variable-Spacing Parallel Plate Resonator Principle of Operation: Measure the resonant frequency, f 0, and the quality factor, Q, of the VSPPR versus the continuously variable thickness of the dielectric spacer (s), and to fit them to theoretical forms in order to extract the absolute values of and R s. Vary s s: contact – ~ 100  m in steps of 10 nm to 1  m The measurements are performed at a fixed temperature In our experiments L, w ~ 1 cm

5 The VSPPR Experiment Films held and aligned by two sets of perpendicular sapphire pins Dielectric spacer thickness (s) measured with capacitance meter

6 VSPPR: Theory of Operation V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000) US Patent # 6,366,096 Superconducting samples Quality Factor fringe effect SC Trans. line resonator Resonant Frequency Assumes: 2 identical and uniform films, local electrodynamics, R s (f) ~ f 2 f* is a reference frequency

7 High-T c Superconducting Thin Films at 77 K fit: 257 ± 25 nm R s fit: 200 ± 20 f* = 10 GHz L = 9.98 mm, w = 9.01 mm, film thickness d = 760 ± 30 nm, T c = 92.4 K Mutual Inductance Measurements ( )/2 = 300 ± 15 nm

8 VSPPR: Theory of Operation V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000) US Patent # 6,366,096 Normal Metal samples fringe effect NM Trans. line resonator Assumes: 2 identical and uniform films, local electrodynamics, R s (f) ~ f 1/2 Quality FactorResonant Frequency

9 Thick Copper Plates at Room Temperature Q(s) f 0 (s) Q(s)  skin fit (f* = 10 GHz): 0.79 ± 0.1  m (from f-fit) and 0.77 ± 0.1  m (from Q-fit) Theory:  Cu = 1.7  cm at 293 K, yielding  skin = 0.68  m (f* = 10 GHz) L = mm, w = 9.88 mm, plate thickness d = 0.7 mm

10 What Relevance for SRF? The VSPPR could act as a scanned probe for (x, y) on Nb sheets The “probe” film is a known reference standard Compare  skin (300 K) to (1.8 K) Correlate with surface analysis Nb Sheet probe film Employ different modes of the VSPPR to study k direction-dependence of n s /m tensor Vortex generation at defects at high powers Build conformal probe/reference films for investigation of  (r, , z) and (r, , z) of finished cavities Design new reference resonator structures (sphere, cylinder, ray-chaotic, …)

11 Conclusions The VSPPR offers the opportunity to measure the absolute screening length scale both in the normal and superconducting states The results have been validated with alternative data For more information and details, see: V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000) US Patent # 6,366,096 The VSPPR also provides the absolute surface resistance The VSPPR can be employed as a scanned probe of Nb surface properties

12 Details Requirement of an offset spacing s 0 : s = s c + s 0 Tilting of the plates: Measured to be less than 1 mrad Misalignment of the plates Measurement of low-Q resonant open-resonator modes Background subtraction Effect of non-flatness of the plates Secondary fitting parameters V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000) US Patent # 6,366,096

13 RF / Superconducting properties Coupling with surface analysis! Is the comparison Samples/Cavities relevant? What is the link between DC/RF properties, between low field/high field properties? Are there other parameters “easy” to measure that could give us better prediction of the cavity behavior? Thermal transfer: influence of annealing, grain boundaries…. Questions to be addressed

14 Capacitance Measurement of s In-situ Capacitance Measurements C-meter Cu (HTS) films In pads 50-  m-thick Au wires

15 R s --Standard for Characterization of Superconducting Materials for Microwave Applications Proposed definition for R s via the well- standardized quantities -- frequency and length: Effective Surface Resistance of 100  at 10 GHz is a FWHM = MHz of the resonance curve for the Ohmic Q-factor produced by the VSPPR with the effective dielectric spacer thickness s eff = s + 2 eff = 10  m

16 Variable PPR Experimental Setup: Variable PPR Differential Micrometer Head, 70 nm resolution Top flexure Be/Cu bearing Coaxial thin wall ss tubes Bottom flexure Be/Cu bearing Bottom film’s substrate Al pins Flexible clamps for top & bottom HTS films Top film’s substrate LN2 1 mm fine travel 12 “ PPR with  m Variable Dielectric Spacer filled by LN2 Films Aligner Slider Actuator Displacement Sensor, 25 nm resolution Sensor target: front mirror Antenna loops Coax cables OUTIN Coupling probes Top view

17 Thick Copper Plates at 77 K