Superconducting Transition Edge Sensors

& Topological Defects Formation

Dept. of PhysicsSlovak Technical UniversityIlkovičova 3812 19 BratislavaSlovak Republic

Valko Pavol

valko@elf.stuba.skhttp://www.valko.net

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.)

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 ....

-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

-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

-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

-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

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)

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

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

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

0 500 1000 1500 2000

1860

1870

1880

1890

Bas

e lin

e ch

anne

l

Trigger number

Base line channel spread

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

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]