Synthesis and characterization of...

Stefano TriviniFinal review27/09/2017

Synthesis and characterization of 𝜸/δ−NbNfor SRF cavity application and vortices study in superconductors

Supervisor: Mattia ChecchinCo-supervisor: MartinaMartinelloPPMS-MFMMentor: Zuhawn Sung

1. Synthesis and characterization of 𝜸/δ−NbN

2. Study of vortices in superconductors.

Overview

2 27/09/2017 Stefano Trivini | Final Presentation

1) Synthesis and characterization of 𝜸/δ−NbN


Task and purpose overview

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𝑄 ∝1𝑅+

QualityFactor𝑅+ ∝ 𝑒-.//.

Surfaceresistance

27/09/2017 Stefano Trivini | Final Presentation

Task and purpose overview

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𝑄 ∝1𝑅+

QualityFactor𝑅+ ∝ 𝑒-.//.

Surfaceresistance

Nb EP(Electropolished)𝑇1=9.25K

2-5𝜇𝑚 filmof:𝛿 − 𝑁𝑏𝑁𝑇1=15-17K𝛾 − 𝑁𝑏𝑁𝑇1=12-15K


Overview of the method

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Synthesisinfurnace:𝑝FG, 𝑇, 𝑡𝑖𝑚𝑒



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Characterization:SEM/EDS,AFM,MFM,PPMS,SIMS

“notgood”



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Characterization:SEM/EDS,AFM,MFM,PPMS,SIMS

Ifresultsare“good”theprocesswillbeappliedtoa1.3GHzSRFcavity.

“notgood”


Reference sample Electro Polished (EP) Nb


Morphology EP Niobium (SEM-AFM)

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SEM(ScanningElectronMicroscopy):

EDS(EnergyDispersiveX-raySpectrometry)

AFM(AtomicForceMicroscopy):


Reference Nb EP, AC Susceptibility (PPMS)

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Onetransition:𝑇1 =9.26K


Nb EP, Bulk Magnetization loops (PPMS)


Nb EP, H(T) phase diagram

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𝐻1L 𝑇 = 𝐻1L(0)1 − 𝑇/𝑇1 L

1 + 𝑇/𝑇1 L

𝐻1 𝑇 = 𝐻1 0 1 − 𝑇/𝑇1 L


• Within the Ginzburg-Landau (GL) theory is possible to determine: the GL parameter (k), the coherence length (𝜉) and the London penetration depth (𝜆S) (all at 0K).

Reference Nb EP, superconductor parameters

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𝐵1L(0)(Tesla) 0.394±0.005𝐵1(0)(Tesla) 0.148±0.005

𝑇1(K) 9.41±0.42k(0) 1.89±0.07

𝜆S(0)(nm) 54±2𝜉(0)(nm) 28.9±0.2

𝑘 0 =𝐵1L 02� 𝐵1(0)

𝜉 0 =ℏ

2𝑒𝐵1L(0)�

𝜆S(0) = 𝑘 0 𝜉(0)


N2 Treated samples


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N2 pressure dependence of Nb-N phase diagram

Phase of interest in this study:- 𝜹-NbN Tc=15-17,3K- 𝜸-NbN Tc=12-15K

M.Joguet,W.Lengauer,M.Bohn,J.Bauer,J.ofAlloys andCompounds 269(1998)233-237

1000°C


Samples

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-Nb ElectroPolishedà Startingmaterial

Pre-treatment:800°C3hnonitrogen

1)T=800°C pN2=25mTorr time=25min

2)T=1000°C pN2=10mTorr time=2h

3)T=1000°C pN2=50mTorr time=2h


Surface morphology in top view


Surface morphology (1)

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-T=800°C pN2=25mTorr time=25min

SEM trigonal nitrides AFM nitrides height ~30-70nm



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-T=1000°C pN2=10mTorr time=2h

SEM -Trigonal nitrides observed-Morphology dependent onthegrain

2𝜇𝑚



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-T=1000°C pN2=50mTorr time=2h


Surface morphology in cross section


SEM Cross section (1)

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800°C 25 mTorr 30min

Contrast in the electronic image suggests the presence of an over-layer different from the bulk.

No EDS Nitrogen signal



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1000°C 10 mTorr 2h

White layer of ~1𝜇𝑚 only on one face of the sample.



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1000°C 50 mTorr 2h

White layer of ~1𝜇𝑚 only on one face of the sample.




Stoichiometry


Stoichometry (2)

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LowsensitivityinallEDXmeasurementsfornitrogen(bothfortopandcross-sectionmeasurements).

Nitrogen signal covered fromcarbonandoxygen’s signals.


Superconducting properties


Superconductive properties (1)

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Comparisonbetween:-DCMagnetometry (bulk):

only one transition𝑇1 ≈ 9.2 K

-ACSusceptibility (surface):three transitions𝑇1 ≈ 9.0 K𝑇1 ≈ 9.1 K𝑇1 ≈ 9.2 K

Poor SCsurface phases.

1000°C10mTorr 2h



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1000°C50mTorr 2h

Newsignalat~17K

Couldbesystematic(1pt)

Moreprecisemeasurements:8.5-10K 15- 18.5K



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1000°C50mTorr 2h

8.0-10K 15-18.5K

Newtransition:Tc=8.33KNotobservedbefore

NoNbxNy SCphasesareknownwiththis𝑇1.

NewSCphaseornonstoichiometricNbN?


Summary NbN synthesis

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1)Evidenceofnitridelayerformation,compositioncannotconfirmedwithEDS(Nsignaltoolow)

2)EvidentsuperconductingtransitionwithTcdifferentfromallthecommonnitridephases.

3)Depositiondependentongrainorientation.

Future:- SIMSanalysistoinvestigatestoichiometry.- StudyofthelowTzoneoftheNbN phase

diagram.


2) Direct observation of vortices in superconductors


Magnetic Force Microscopy (MFM) imaging

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Figureofproperty ofattocube systems

Purpose:measurethemagneticfieldnearthesurface.

It’sfundamentaltoseparatetopologicalandmagneticeffects.


Clem’s model in point dipole approximation

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Whatwemeasureisthephaseshiftbetweenthedrivingforceandtheoscillationofthetipinpointdipoleapproximation:

𝚫𝝓 = −𝑸𝒌𝝏𝑭𝒛𝝏𝒛

Than:

𝑭𝒛 =𝝏𝑬𝒕𝒊𝒑-𝒔𝒂𝒎𝒑𝒍𝒆

𝝏𝒛InMFM𝐸nop-+qrpst canbeexpressedas:

𝑬𝒕𝒊𝒑-𝒔𝒂𝒎𝒑𝒍𝒆 = 𝒎𝒕𝒊𝒑𝑩𝒛Andso:

𝜟𝝓 = −𝑸𝒎𝒕𝒊𝒑

𝒌𝝏𝟐𝑩𝒛𝝏𝒛𝟐

.

Europhys.Lett.58,582(2002)


Clem’s model in point dipole approximation

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AnexpressionfortheBfieldisobtainedfromClemModel:𝝏𝟐𝑩𝒛(𝒛, 𝒓)

𝝏𝒛𝟐 =𝚽𝟎𝒌𝟐

𝟐𝝅𝝀𝟐 } 𝒅𝒌𝒌𝑱𝟎(𝒌𝒓)𝒌𝟐 + 𝝀-𝟐

𝒌𝟐 + 𝝀-𝟐�

𝒌 + 𝒌𝟐 + 𝝀-𝟐� 𝒆-𝒌𝒛�

𝟎

Welookatrelativevaluesof𝝏𝟐𝑩𝒛(𝒛,𝒓)𝝏𝒛𝟐

sowecannormalizethedata.

Infiguredifferentprofilescalculatedfordifferentvaluesofz.

Carneiro, G., & Brandt, E. H. (2000). Vortex lines in films: Fields and interactions. Physical Review B, 61, 6370-6376.


Vortices size at different MFM’s scan height

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vortexprofile width∝ Scan height

Figuretaken byT.G.Rappoport,L.Ghivelder,J.C.Fernandes,R.B.Guimaraes,M.A.Continentino, Phys.Rev.B75,054422(2007)


Experimental data


Sample: Nb Electro polished

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SurfaceNbEPAFM



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50 nmMFMScan Height 70 nmMFMScan Height

T=4K zerofieldcoolmode B=30mT (fixed)



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90 nmMFMScan Height 120 nmMFMScan Height

T=4K zerofieldcoolmode B=30mT (fixed)


Vortex profile fitting

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FWHMvaluesextractedwithmulti-gaussian fit.


Experimental data and simulated data

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Similartrends

HighdisagreementofFWMvalues.

Pointdipoletipisaroughmodel.


Dependence of vortex profile on surface

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AverageFWHM=1,2𝜇𝑚

DoublethanEPNb!Surfaceeffectsmatters.

70 nmMFMScan Height


Summary of vortices study

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1)MFMisagoodmethodtomapthevortices.

2)FWHMdependsstronglyonsurfaceLondonpenetrationdepth.It’spossibletomeasuresurfacepropertiesseparatelyfrombulkproperties.

Future:thepointdipoletip-sampleinteractionmodelistoorough.Amoresophisticatedmodelhastobeused:

MFMimageà ConvolutionofTip+sample


Thanks for the attention


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