SUPERCONDUCTING MICROBRIDGES UNDER IRRADIATION WITH AN ELECTRON BEAM

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SUPERCONDUCTING MICROBRIDGES UNDER

IRRADIATION WITH AN ELECTRON BEAM

P. Stöhr, K. Noto, R. Huebener

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque

C6, supplhment au no 8, Tome 39, aolit 1978, page C6-527

SUPERCONDUCTING M I C R O B R I D G E S UNDER I R R A D I A T I O N W I T H A N ELECTRON B E A M

P.L. Stijhr, K. Noto and R.P. ~uebener*

PhysikaZisches Institut, Universitat Tiibingen, Germany

RCsum6.- La destruction de la supraconductivit6 dans des microponts en couches minces de plomb pen- dant l'irradiation par un faisceau 6lectronique a BtB examinee

P

llaide d'un cryostat installs dans un microscope Blectronique

P

balayage. Un tel appareil fournit des mdthodes nouvelles intsressantes pour Qtudier le flux de chaleur et les propri6tBs de non Bquilibre des supraconducteurs en couche mince.

Abstract.- The breakdown of superconductivity in Pb-microbridges during irradiation with the electron beam of a scanning electron microscope has been investigated by incorporating a low-temperature stage into the instrument. Such an apparatus provides interesting new ways for studying heat-transfer mechanisms and nonequilibrium properties of thin-film superconductors.

By incorporating a low-temperature stage into a scanning electron microscope (Cambridge Model S 4-10) the breakdown of superconductivity under irradiation with the electron beam can be studied. Here the large variety of possible manipulations of the beam (energy, diameter, temporal and spatial struc- ture) represents an interesting feature for investi- gating new aspects of superconductivity under nonequilibrium conditions. In the following we present results obtained by irradiating microbridges of Pb at 4.2 K.

The samples of 1.1 Um thickness were prepared by vapour deposition on sapphire substrates (1 mm

thick). The microbridges were 3 mm long and 100 um wide. At both ends they were attached to wide sec- tions of Pb providing contact areas for current and voltage leads (see inset of figure 1). Within the vacuum chamber of the SEM, the Pb film was located

Fig. 1 : Current-voltage curves of the Pb microbridge for a beam voltage of 20 kV and different beam currents ; T = 4.2 K. Scanning over an area of 120 vm X 40 vm symmetrically across the entire width of the Pb bridge (frame frequency = 30 Hz). The dif- ferential resistance at the upper end of the curve for.Ibeam = 100 nA corresponds to 0.7% of the normal

resistance of the bridge.

L

sing power of the electron beam as a heating effect, on the front side of the substrate, the superconduc-

we conclude from figure 2 that the Pb film reaches T

tor being exposed directly to the electron beam. The 1 backside of the sample substrate was in direct con-

tact with liquid helium. Figure 1 shows a series of A

V(1) characteristics obtained for different beam m

-

N

Fig. 2 :

I:'~

versus beam power ; T = 4.2 K. Sean- nlng mode identical to that for figure I.

*~ehrstuhl ~x~erimentalphysik 11, Universitzt ~iibingen, Morgens telle 14, 7400 Tiibingen 1

at the beam power of 2.6 X 1 0 - ~ W. From this and the

i ,

, , , ,

, , , -

_*

Pb 2 W

*.

T . L 2 \ X&KV -

':

0.20Kv

\*

S b -IOW -

*

\.

0 - 5Kv

currents and constant beam voltage. In figure 2 the

-

< \ critical current I is plotted versus the beam power, "Y

O S - h _{\ -}

-

_,

resulting in a universal curve for the different - .'v

values of voltage and current of the beam.

* \

\

* S .

Interpreting the reduction of In with increa- _F

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irradiated area of 120 um X 40 pm we find for the heat transfer coefficient

a

= 18 W/K cm2

.

The thermal healing length r)

-

(K

.

d/a)1/2 within the Pb film is about 69 um aasming the more reasonable value

a

= 2 w/cm2 K and obtaining the heat conductivity

K from the normal electric resistivity of the Pb film (d = film thickness). Taking this length of 17

into account we see that the heated sample area must be increased by about a factor of 3 beyond the area irradiated directly by the beam. The data of figure 2 then yield a 2 6 W/K cm2 which appears reasonable.

Figure3 shows the thermal relaxation measured in a pulse experiment, indicating a thermal relaxation time of about 2 us. An estimate of the relaxation time from the relation

-rth

= C/& yields the

0 5 0 K 20

tome I p I

Fig. 3 : Voltage versus time in a pulse experiment. Fhe beam pulse (30 kV, 10-2 W) is indicated on the top. Sample current = 0.1 A, T = 4.2 K. The beam is focussed on a single spot near the center of the Pb bridge.

value T~~ = 0.02 vs. Here C is the heat capacity and A the surface area of the F'b film. We note that this value is by two orders of magnitucle smaller than the experimentally observed relaxation time, Such a discrepancy may indicate appreciable heating of the

substrate during each pulse. The voltage responds to the beam pulse with a delay of about 1 ps. A similar delay has been reported for irradiation with light pulses /l/.

By focussing the electron beam (minimum dia-

0

meter about 100 A) on different locations of the microbridge, the spatial dependence of the sample response can easily be investigated. Such dependence

may arise from the metallurgical microstructure or from an inhomogeneous distribution of the transport

current density. As an example we show in figure 4 the voltage versus the coordinate of the beam focus across the microbridge. By scanning the beam slowly

Fig. 4 : Voltage versus the location of the beam focus observed by scanning along a straight line across the Pb bridge (scanning time = 20 S). Beam power = 6 X 10-' W, sample current = 0.7 A, T=4.2

K.

The location of the superconductor is indicated at the bottom.

across the sample a voltage appears as soon as the beam crosses the sample edge. The voltage disappears as soon as the beam leaves the sample at the oppo- site edge. The two sharp voltage maxima near both sample edges seen in figure 4 are likely caused by the fact that the transport-current density is sharp ly maximized near the two edges of the superconducting strip.

Reference