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Superconducting Transition Edge Sensors
& Topological Defects Formation
Dept. of PhysicsSlovak Technical UniversityIlkovičova 3812 19 BratislavaSlovak Republic
Valko Pavol
[email protected]://www.valko.net
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What are TES
- sensor- low TC superconductor
- native (tungsten)
- proximity structure (Al-Ag)
- special phase (-Ta)
- deposited as thin film- sputtering, evaporation
- strip form- lithography, shadow mask
- absorber- with low heat capacity
- dielectric crystals (Al2O3)
- semiconductors (Si, Ge)
- superconductors (Nb, etc.)
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Motivation & Method• already extensively studied as radiation detectors with “fast”
phase transition guarantied (to achieve high count rate)• possibility to cause local or global heating of tested samples
by choosing source of energy (laser, radiation, particles...)
• spontaneous magnetic flux could modify the state of sample,i.e. different resistance at fixed temperature might be a signal
• “missing energy” like signals should be observable for high resolution detectors
• broad range of superconductors (native, composite, anisotropic, heavy-fermions, ...) available for tests at various temperatures, geometries ....
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-Tantalum with particles -Tantalum sputtered (200 nm thick) film on
silicon (500 m Si) absorber 5.4 MeV particles (241Am source) used as localized
“heater” releasing energy over less than 4 m path (most of it near the end point)
affected “heated” region of superconductor is of similar size right above track end-point
superconductor is directly heated by phonons propagating spherically from track end-point (ignoring focusing properties of crystal)
only small part of superconductor heated above critical temperature, followed by fast cooling (quasiparticle - phonon system might follow various energy spread processes )
pulse amplitudes recorded only
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-Tantalum experimental results
0 1000 2000 3000 4000 5000
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
SQ
UID
sig
nal [
V]
Time [s]
rise
= 20 s
decay1
= 3 ms
decay2
= 5 ms
• “hotspot” cooling rate > 50 K/s (from pulse rise time)
• energy spectra with large tails (possible signal)
• no clear “missing” energy signal seen
typical pulse shapetypical spectrum
0 10 20 30 40 50 60 700
100
200
300
400
500
Temperature 582 mKBias current 280 nA
Num
ber
of c
ount
s
Channel number
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-Tungsten tests with X rays• experiment with “global” sample heating• tungsten film (200 nm) sputtered on heated sapphire substrate (20 x
10 x 0.5 mm3) and lithographically structured to 1 x 0.5 mm2 area• 55Fe source X-rays used as “heat source” (5.8 and 6.4 keV ) with
sample hit rate of 0.7 Hz• X-ray induced events originated
directly from in metal film (17%) and dielectric crystal (83%)
• whole traces recorded for eachevent
• search of “satellite” peaks at fixed bias point
• temperature scan over sensor R-T transition
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-Tungsten experimental results
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 24000
1
2
3
4
5
6
7
8
9
10
11
Res
ista
nce
[m]
Trigger number
• observed “satellite” peak for events originating from tungsten film
• observed “multiple” transitions during R-T scan
• both effects could be associated with defect creation
temperature scan over transitiontypical X-ray spectrum
0 200 400 600 800 1000 1200 1400 16000
10
20
30
40
50
60
70
N
umbe
r of
cou
nts
Channel number
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Possible ways to improve
• to perform 3He + n like defect creation experiments in bulk superconductors searching for “missing energy” using TES or STJ as energy deficit sensors – native short coherence length (or artificially reduced)
superconductors are preferred
• nuclear fission would be ideal local energy source – large localized energy depositions are required to achieve
comparable density of deposited energy vs. condensation energy in “realistic” superconductors
• dedicated TES experiments with fast SQUID read-out testing localised energy release deep in bulk– similar experiments with particles, extremely pure Nb and
STJ’s arrays were already performed (Gaitskell et. al., 1991)
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THE END• Major concerns
– effect of residual and bias current induced magnetic fields– large and shallow pinning centres– properties of thin superconductor films (d~ξ) near
(at) critical temperature– intermediate (mixed) state resistance dynamics– interactions of quasiparticles and phonons in
superconductors• Links to previous observations
– possible source of observed “extra” noise in TES detectors (deKorte et al.)
– multi decay time constants of observed pulses
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20 40 60 80 100 120 140 1600
20
40
60
80
100
120
140
N
umbe
r of
cou
nts
Channel number
Temperature 582.25 mKBias current 280 nA
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20 40 60 80 100 120 140 1600
50
100
150
200
250
300
350
400
Temperature 583 mKBias current 280 nA
Channel number
Nu
mb
er
of c
ou
nts
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0 500 1000 1500 2000
1860
1870
1880
1890
Bas
e lin
e ch
anne
l
Trigger number
Base line channel spread
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0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 24000
1
2
3
4
5
6
7
8
9
10
11
Res
ista
nce
[m]
Trigger number
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0 200 400 600 800 1000 1200 1400 1600 1800 2000
2050
2100
2150
2200
2250
2300
2350
Tra
ce r
ecor
der
chan
nel
Time [s]