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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.

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slide1

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

RF Superconducting Materials Workshop at Fermilab

slide2

Motivation

Measurement of l gives insight into material properties and quality

  • varies with ℓMFP, magnetic field (non-linear Meissner effect), direction,

weak links, etc.

Most techniques measure Dl, but not the absolute magnitude of l

H(y)

vacuum

superconductor

l

y

Typicallyl ~ 15 nm to 10’s of mm

slide3

4 Classes of Techniques to Measure l

  • Absolute Length Scale Techniques
    • Meissner state flux exclusion. One sample dimension (L) is known
    • df ~ (L – g dl) x (area of the sample)
    • (empty vs. pertubed)

The Variable-Spacing Parallel Plate Resonator

l/L ~ 10-1 and systematically larger

Problem: often l / L ~ 10-3 - 10-4

l(0) found from fitting dl(T) to theory

  • 2) Reflection / Transmission
    • Mutual inductance of two coils
    • Microwave transmission
    • “Missing area” sum-rule, KK analysis

Problems: requires large (~cm2) thin films

Requires very accurate spectroscopy

  • 3) Probes of Internal Magnetic fields
    • mSR
    • 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

slide4

The Variable-Spacing Parallel Plate Resonator

Vary s

s: contact –

~ 100 mm

in steps of

10 nm to 1 mm

Principle of Operation: Measure the resonant frequency, f0, 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 l and Rs.

The measurements are performed at a fixed temperature

In our experiments L, w ~ 1 cm

slide5

The VSPPR Experiment

Films held and aligned by two sets

of perpendicular sapphire pins

Dielectric spacer thickness (s)

measured with capacitance meter

slide6

VSPPR: Theory of Operation

Superconducting samples

Resonant Frequency

Quality Factor

fringe

effect

SC Trans.

line resonator

f* is a reference frequency

Assumes: 2 identical and uniform films, local electrodynamics, Rs(f) ~ f2

V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000)

US Patent # 6,366,096

slide7

High-Tc Superconducting Thin Films at 77 K

Mutual Inductance Measurements

l fit: 257 ± 25 nm

(l1+l2)/2 = 300 ± 15 nm

Rs fit: 200 ± 20mW @ f* = 10 GHz

L = 9.98 mm, w = 9.01 mm, film thickness d = 760 ± 30 nm, Tc = 92.4 K

slide8

VSPPR: Theory of Operation

Normal Metal samples

Resonant Frequency

Quality Factor

fringe

effect

NM Trans.

line resonator

Assumes: 2 identical and uniform films, local electrodynamics, Rs(f) ~ f1/2

V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000)

US Patent # 6,366,096

slide9

Thick Copper Plates at Room Temperature

f0(s)

f0(s)

Q(s)

Q(s)

dskin fit (f* = 10 GHz): 0.79 ± 0.1mm (from f-fit) and 0.77 ± 0.1 mm (from Q-fit)

Theory: rCu = 1.7 mWcm at 293 K, yielding dskin = 0.68 mm (f* = 10 GHz)

L = 11.97 mm, w = 9.88 mm, plate thickness d = 0.7 mm

slide10

What Relevance for SRF?

The VSPPR could act as a scanned probe for l(x, y) on Nb sheets

The “probe” film is a known reference standard

Compare dskin(300 K) to l(1.8 K)

Correlate with surface analysis

probe film

Nb Sheet

Employ different modes of the VSPPR to study

lk direction-dependence of ns/m tensor

Vortex generation at defects at high powers

Build conformal probe/reference films for investigation of d(r, f, z) and l(r, f, z)

of finished cavities

Design new reference resonator structures (sphere, cylinder, ray-chaotic, …)

slide11

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

The VSPPR also provides the absolute surface resistance

The VSPPR can be employed as a scanned probe of Nb surface properties

For more information and details, see:

V. V. Talanov, et al., Rev. Sci. Instrum. 71, 2136 (2000)

US Patent # 6,366,096

slide12

Details

Requirement of an offset spacing s0: s = sc + s0

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

slide13

Questions to be addressed

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….

slide14

Capacitance Measurement of s

In-situ Capacitance Measurements

C-meter

50-m-thick Au wires

In pads

Cu (HTS) films

slide15

Rs--Standard for Characterization of Superconducting Materials for Microwave Applications

Proposed definition for Rsvia the well-standardized quantities -- frequency and length:

Effective Surface Resistance of 100 at 10 GHz is a FWHM = 2.533 MHz of the resonance curve for the Ohmic Q-factor produced by the VSPPR with the effective dielectric spacer thickness

seff = s+ 2eff = 10 m

slide16

Experimental Setup:Variable PPR

Differential Micrometer Head,

70 nm resolution

Actuator

Displacement Sensor,

25 nm resolution

Sensor target:

front mirror

Top flexure

Be/Cu bearing

1 mm fine travel

Coaxial thin wall

ss tubes

Slider

12 “

Bottom flexure

Be/Cu bearing

Flexible clamps for

top & bottom HTS films

LN2

PPR with 0-200 m Variable

Dielectric Spacer filled by LN2

Films Aligner

Bottom film’s

substrate

Top film’s

substrate

Al pins

Antenna loops

Top view

Coupling

probes

Coax cables

IN

OUT