Hf-STJ development for COBAND― I-V characteristics studies ―2023年度
第２回TCHoU構成員会議・成果報告＆交流2023/12/18

Y. Takeuchi (TCHoU, Univ. of Tsukuba)
for the COBAND collaboration

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COBAND(Cosmic Background Neutrino Decay)
Search for Neutrino decay in Cosmic background neutrino➔To be observed as FIR photons around ~50m𝜸

𝝂𝟐

𝝂𝟑

Neutrino Decay

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Neutrino Decay signal and backgrounds
Surface Brightness I [MJy/sr]

CIB summary from Matsuura et al.(2011)ZE

AKARICMBCOBE

Expected Neutrino
Decay signalZL𝜏 = 1 × 1014 yr𝑠𝒎𝟑 = 𝟓𝟎 𝐦𝐞𝐕

ISD
DGL

ZodiacalEmission

SL

Wavelength [m]Current experimental
lower limit is 1012 yrs

E [meV]

We can identify  decay signal by highly precise measurement of photon energy
spectrum around ~50m
3➔ Require for the detector to detect and measure single photon energy at ~50m

Density of State

Density of State

超伝導エネルギーギャップ
𝑇 > 𝑇𝐶𝜖𝐹𝑇 < 𝑇𝐶𝜖𝐹

超伝導状態では，フェルミ準位𝜖𝐹 の付近の準位の電子二個が2の束縛エネル
ギーにより，Cooper 対を形成 ➔ エネルギー準位にギャップが生じる

SiNbTaTc[K]9.23 4.48
Δ[meV] 1100 1.550 0.7

AlHf
1.20 0.165
0.172 0.020

Δ~1.8𝑘𝐵 𝑇𝐶 (BCS theory)

Superconducting Tunnel Junction (STJ)
Superconductor / Insulator /Superconductor (SIS)
Josephson junction device
SuperconductorE2

Insulator
Ns(E)Superconductor
Superconductor
Insulator

Δ: Superconducting gap energy

A constant bias voltage (|V|<2Δ/e) is applied across the junction.

A photon absorbed in the superconductor breaks Cooper pairs and creates tunneling current ofquasi-particles proportional to the deposited photon energy.
• Much lower gap energy (Δ) than FIR photon ➔ Can detect FIR photon and measure its energy.
5STJ current-voltage characteristics
Current

EW/ light illumination
No light2Δ-2/eleak current

2/e

DC Josephson current

Suppressed by B
fieldVoltage
B field
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STJ candidates

SiNb

Al

Nb/Al-STJ
Tc[K]
9.23
1.20
• Well-established and commonly used.
• Δ~0.6meV by the proximity effect from Al Δ[meV] 1100 1.550 0.172• Operation temp. <400mK• Back-tunnelling gain G ~10• Nq.p.=25meV/1.7Δ10~ 250
E/E~10% for E=25meV (~50m)➔25meV single-photon detection is feasible ideally.
➔Candidate for the rocket experiments with diffraction grating.

Hf0.165
0.020

Hf-STJ• Not established as a practical photo-detector yet by any group• Nq.p.=25meV/1.7Δ~ 735• 2% energy resolution for a 25meV single-photon is achievable➔Spectrum measurement without a diffraction grating.
➔Developing for a future satellite experiment7

Superconducting device process equipment at KEK clean room
Hf-STJ samples are fabricated at KEK clean room byour group.
• Successful in etching Hf layer in 2008.
• Confirm SIS junction by Hf-HfOx-Hf in 2010.
• Confirm Hf-STJ response to visible light pulse in 2013.

ICP-RIE

Aligner in yellow hat

Metal film sputtering

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Slide from DPPD session, TCHoU workshop on 2021/3/29

X-ray illumination test at IBS CUP in Jun. 2019

Adiabatic Demagnetization Refrigeration(ADR) at temperature down to 30mK
◼ Hf-STJ I-V◼ Hf-STJ response to X-ray photon (55Fe)◼

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Slide from DPPD session, TCHoU workshop on 2021/3/29

X-ray illumination test at IBS CUP in Jun. 2019On ADR cold stage◼ STJ samples◼ Cu collimator◼ X-ray source (55Fe) sealedin polyester tape
◼ Solenoid coil with persistentcurrent switch
200m×200m

X-ray source(250nm)

STJ
Cu Collimator

cold stage(~2nm)

25mm(400nm)

chip carrier

10Slide from DPPD session, TCHoU workshop on 2021/3/29

200m sq. Hf-STJ sample under test
Anode oxidation

Al wireAl wireHf Upper layer (250nm)

300nm

HfOx (~2nm)

Hf Lower layer (400nm)
Si wafer (100)11

Slide from DPPD session, TCHoU workshop on 2021/3/29

Setup for I-V curve and X-ray response measurements at the IBS CUPCurrent
sourceIDifferential amplifierin room temp.

+−Hf-STJ

constant current

ADR

VSTJ I-V curve w/ light w/o light◼
◼

◼
◼

Sinusoidal current on STJ for I-V measurement
Constant current on STJ for X-ray response measurement
Voltage of STJ is read with differential amp. placed at roomtemp.
X-ray signal is shown up as a negative pulse in STJ voltage.
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Slide from DPPD session, TCHoU workshop on 2021/3/29

Signal fit to the template

Use the region from -50s to 200s in the template• Amplitude and baseline are fitting parameters.
• Also scan t from -50s to +50 s, and choose the t which gives the best 2

Slide from DPPD session, TCHoU workshop on 2021/3/29

Energy distribution

After selection on fit parameters for candidates, 1138 events left.
• K1+K2 X-rays with energy of 5.9 keV and a probability about 24.4%,• K X-rays with nominal energy of 6.5 keV and a probability about 2.85%
Assuming K:K ratio and peak energies
and same  for K and K,We fit the distribution and scaled.

～6.7%(39622eV)

We found the energy resolution is6.7% (39622eV)

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I-V curve about 34mK w/ B field
200m square Hf-STJ @ T～34mK• Applied magnetic field ～10G on STJ• Leak current of ～3A(@20V)

Normal resistance: ～2.15Ω
Dynamic resistance: ～4.2Ω

This leakage is much larger than our expectation. (Our goal is ～pA)
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I-V curve about 30mK w/o and w/ B field
200m square Hf-STJ @ T～30mK• DC Josephson current is shown up without magnetic field, and it issuppressed with magnetic field (～10G).
I-V curve at 30mK in no magnetic field

I-V curve at 31mK in magnetic field

Magnet Off

Magnet On

zoom

This confirms the SIS junction on the STJ.

zoom16

Examples of Hf-STJ smpl.D I-V for temp. dep. after ACC, offset correction
214.3mK#1

150.2mK#1000

197.5mK#260

64.7mK#2596

Temperature dependence of Gap energy

Density of StateΔBCS,0𝑘𝑇𝑐

BCS 理論ΔBCS,0𝑘𝑇𝑐~1.7639 (BCS)

𝑇 < 𝑇𝐶𝜖𝐹𝑇/𝑇𝑐

Define V from I-V curve

4Vmes (μeV)

Temperature dependence of Gap energy
Δmes,0 = 25.988 ± 0.001𝜇𝑒𝑉

Dot: DataCurve: 𝜅BCS
ΔBCS (𝑇/𝑇𝑐 ) from gap eqn.

• Temperature dependence of V matched the BCS theory very well.
ΔBCS,0 ~37.7𝜇𝑒𝑉, 𝜅~0.69• Found to be 𝑇 = 247.742 ± 0.004 ± 4.6𝑚𝐾
𝑐19

Temperature dependence of STJ Leakage
BCS 理論Δ𝐼𝑡ℎ ∝ 2𝑁0 2𝜋Δ𝑘𝑇exp −𝑘𝑇

BCS theory (P.W.Epperlein 1978)

Δ 𝑇
Δ 𝑇𝑐
= 2𝑁0 𝑘𝑇𝑐 2𝜋exp −𝑘𝑇𝑐 𝑇𝑐
𝑘𝑇𝑐 𝑇
BCS 理論で 𝑇𝑐 に対応するΔより小さくなっていると仮定

ΔBCSΔ′𝜈ΔBCS
→=𝑘𝑇𝑐
𝑘𝑇𝑐
𝑘𝑇𝑐

ΔBCS,0𝑘𝑇𝑐~1.7639 (BCS)Temperature dependence of Leakage
𝑇𝑐 = 247.7𝑚𝐾 ，ΔBCS,0~1.7639を仮定して
𝑘𝑇𝑐

𝑉 = 10, 15, 20, 25, 30𝜇𝑉
Dot：Data
Curve：BCS theory

𝐼𝑡ℎ = 𝑃0ΔBCS 𝑇𝜈ΔBCS 𝑇𝑐
exp −+ 𝑃2
𝑘𝑇𝑐 𝑇𝑐𝑘𝑇𝑐 𝑇

Extracted 𝜈ΔBCS,010V: 27.1 eV
15V: 27.7 eV
20V: 29.1 eV
25V: 30.4 eV30V: 31.5 eV• Temperature dependence of leakage matched the BCS theory.
• Found to be 𝜈ΔBCS,0 ~29𝜇𝑉

21Constant terms in Leakage
𝐼𝑡ℎ = 𝑃0ΔBCS 𝑇𝜈ΔBCS 𝑇𝑐
exp −+ 𝑃2
𝑘𝑇𝑐 𝑇𝑐𝑘𝑇𝑐 𝑇• V dependence of constantterm• Ｖ＝10, 15, 20, 25μVの4点を使い直線fit• Constant term は，STJ に並列に存在する抵抗(4.8Ω)成分？
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Constant terms in Leakage
パラレル抵抗成分の大きさは，冷凍機のサーマルサイクルごとに異なることが確認されている。
➔絶縁膜層の欠陥ではない？

• 地磁気，ADR漏れ磁場などのSTJに対して垂直成分の磁束トラップによるnormal current path の形成？
• 磁気シールドによる改善の可能性。

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Constant terms in Leakage
Ｉ-Ｖカーブからパラレル抵抗成分の除去
Magnet On IV Data @31mK24• Because of quasi-particle lifetime, gap energy Δ has
imaginary part. (Dynes 1978).

𝑇 < 𝑇𝐶𝜖𝐹DoS (a.u.)• In this case, density of state distribution changes

Density of State

Imaginary component in gap energy?

22Re()Im()=0.05Re()25

I (A)

I (A)

DoS(a.u.)

Imaginary component in gap energy?

V (mV.)

Measured I-VV (mV.)

測定されたI-V から gap energy の複素成分(準粒子寿命の情報)が引き出せる可能性

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Summary• Hf-STJ is under development for application to COBAND project, aiming at farinfrared single photon detector and spectrometer.
• We successfully fabricated Hf-STJs with confirmed SIS junction and tested themwith X-ray source.
• We confirmed the clear signals for X-ray single photon from 55Fe with a Hf-STJsample and found that energy resolution is about 6.7% for 5.9keV.
These are the world first results for Hf-STJ.
• We measured and compare temperature dependence of gap energy  and I-V
curve of Hf-STJ with BCS theory. These strongly support that our Hf-STJ sample
is fabricated as the SIS junction successfully.
• Unexpectedly large leakage is likely due to normal current path other than SIS
junction. This indicates the possibility of reducing the leakage.
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