Nobuhiro Fukumoto, Yukinori Onoa, Masahiro Horia, Ryo Chikaokaa, Yosuke Hayakawaa, Shigenori Moriwaki and Norikatsu Mio

Photon Science Center, Univ. of Tokyoa Graduate School of Science and Engineering Univ. of Toyama, Toyama

Electron spin resonance measurement of sapphire for KAGRA mirrors

Japanese gravitational wave detectionproject KAGRA

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Gravitational wave Propagating as space strain Strain is very small No evidence of direct observationGravitational wave interferometer Huge Michelson interferometer Efforts for decreasing noises ・ mono-crystal sapphire mirror

space strain(plus mode)

Sapphire mirrorKAGRA interferometer

Mono-crystal sapphire mirror

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We need information about impurities in Mono-crystal sapphire

Merit・ low mechanical loss（ lower at cryogenic measurement ）・ high thermal conductivity

Demerit・ IR (infrared) absorption　 By impurities （ Fe3+ , Cr3+ ,Ti3+ … ） Sample IR absorption　　　　→ 30~680ppm /cm KAGRA requirement →50ppm /cm 　　　　　　　　　　　　　　　　　　

𝑯𝟎

Theory・ Solve degeneracy by external field　 absorption condition 　　

Electron Spin Resonance （ ESR ）

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Energy level transition(Free Electron)

𝑔=h𝜐𝛽𝐻 0

： Energy of microwave： Lande - factor： magnetic moment

Electron Spin Resonance （ ESR ）

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Setup・ Cavity　 Excite standing microwave・ Electromagnet　 Sweep external field

Inside of cavityStanding microwave fields

High sensitive measurement observing each spin

Instruments Setup

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ESR measurement instruments（ Graduate School of Science and Engineering Univ. of Toyama ,Ono.lab ）　 microwave frequency ： 9.38 GHz （ X-band ）　 Sweeping magnetic Field ： 0 ～ 10 kG

Electromagnet and cavityESR instrument

Sample preparation

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Crystals for IR absorption measurement High IR absorption crystal(680 ppm/cm): AC150 Low IR absorption crystal (30 ppm/cm): P401Cut samples for ESR measurement　 Cut into 27 blocks(from A1 to C9)

C-axis

Sample cuttingIR absorption crystal　 diameter: 10mm　 length: 40mm

Measurement Summary

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ESR peak summary

ESR peaks at 3 regions around 2 kG around 3 kG around 5 kG

Comparison of peaks between samples among blocks

Measurement conditionTemperature: room temperatureMicrowave power: 1mWMicrowave frequency: 9.38GHz

We have measured 27 x 2=54 samples.

Around 2kG

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Inte

nsity

(a.u

.)

Field(G)Each peak position is different→ different origin

P401_A8AC150_A3

Peak1930G

Peak 1705G

Around 3kG

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Inte

nsity

(a.u

.)

P401_A8

AC150_A3

Peak: 3345G

Peak: 3345G

Peak at the same positionField(G)

Around 3kG, another block

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Inte

nsity

(a.u

.)

No peak→ depend on position

P401_C8

AC150_C3

Field(G)

Around 5kG

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Inte

nsity

(a.u

.)

Field(G)

Peak: 5233G

Only high absorption sample has peak→possibility of contribution to IR absorption

P401_A1

AC150_A3

Peak intensity distribution around 3 kG

Ａ ＣＢ

AC150

Ａ ＣＢ

P401

Intensity is largest at red part and decreases with distance→possibility of local impurity/affix in cutting

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Peak intensity distribution diagram Red: HighYellow: MiddleBlue: Low

Identification of impurity

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Around C1 and C2, high IR absorption sample has peak, but around C2 it has no peak.

Around F3, both samples has peak , but around F1 and F2, they has no peak.

𝑔=h𝜐𝛽𝐻 0(b):RT

Around Ti3+ peak, samples have peak, but, these peaks are same as Fe3+ case.

Cr3+

Fe3+

R. S. de BIAS1 and D. C. S. RODRIGUES J. Am. Cerum. Soc., 68 [7] 409 (1985)

HighLowBoth

9.50GHzRT

Conversion peak position(calculated from g-value) Ti3+

Radiation Measurements 43 (2008) 295 – 299

Summary

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We observed ESR signals of the samples fabricated from the mono-crystals of different IR absorptions.

We observed peaks with intensity dependence on the block position around 3kG. It may come from local impurity or affix in cutting.

We observed peaks from only high IR absorption crystal sample around 5kG. It can be related to IR absorption.

Future work

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We have not identified the impurity yet.

In order to do so, we will prepare mono-crystal samples in which impurities (Fe3+, Ti3+ Cr3+…) are intentionally doped and measure them for impurity identification.

Thank you for your attention

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