# Presentation: 937850

## A Novel Method to Measure the Absolute Value of the Magnetic Penetration Depth in Superconductors

**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 **

## 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

## 4 Classes of Techniques to Measure 

**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 ~ 10****-3**** - 10****-4**
**	****   ********(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 ~ 10****-1**** and systematically  larger**

**Requires very accurate spectroscopy**

## 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**

## The VSPPR Experiment

**The VSPPR Experiment**

**Films held and aligned by two sets**
**of perpendicular sapphire pins**

**Dielectric spacer thickness (*****s*****)**
**measured with capacitance meter**

## 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

## High-Tc 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**

**(********1****+********2****)/2 = 300 ****± 15**** nm**

## 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 Factor**

**Resonant Frequency**

## Thick Copper Plates at Room Temperature

**Thick Copper Plates at Room Temperature**

**Q(s)**

**f****0****(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***** = 11.97 mm, *****w***** = 9.88 mm, plate thickness *****d***** = 0.7 mm**

## What Relevance for SRF?

**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, ...)**

## 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**

## 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 **

## Questions to be addressed

**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**

## Capacitance Measurement of s

***In-situ***** Capacitance Measurements**

- C-meter

- Cu (HTS) films

- In pads

- 50-m-thick Au wires

## Rs--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 = 2.533 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

## Experimental Setup: 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 0-200 m Variable
- Dielectric Spacer filled by *LN2*

**Films Aligner**

**Slider**

**Actuator**

- Displacement Sensor,
- 25 *nm* resolution

- Sensor target:
- front mirror

-  Antenna loops

- Coax cables

- OUT

- IN

**Coupling**
**probes**

*Top view*

## Thick Copper Plates at 77 K

**Thick Copper Plates at 77 K**