Effects of electron irradiation on YBa2CU307-δ superconductor

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Effects of electron irradiation on YBa2CU307-δ superconductor

A. Legris, F. Rullier-Albenque, E. Radeva, P. Lejay

To cite this version:

A. Legris, F. Rullier-Albenque, E. Radeva, P. Lejay. Effects of electron irradiation on YBa2CU307-δ superconductor. Journal de Physique I, EDP Sciences, 1993, 3 (7), pp.1605-1615.

�10.1051/jp1:1993203�. �jpa-00246819�

J. Phys. I France 3 (1993) 1605-1615 _JULY 1993, _PAGE 1605

Classification

Physic-s

Absfi.acts

74.70V 74.60M 61.80F

Effects of electron irradiation

_on

YBa~CU~O~_

superconductor

~

A.

Legris ('),

F.

Rullier-Albenque ('),

E. Radeva

(')

_{and P.}

Lejay (2)

(') Laboratoire des Sohdes Irradids (*), CEA-CEREM, Ecole

Polytechnique,

91128 Palaiseau Cedex, France

(~) CRTBT-CNRS, BP 166X, 38042 Grenoble Cedex, France

(Receit,ed /5 Janl~arv1993. revised I Marc-h /993, accepted J5 March J993)

Abstract. We have irradiated YBa~CU~O~_~ single crystals with electrons of energy E ranging from o-I _to 2.4MeV. From the measurements of the T~ reduction rate, AT~

d(AT~)/d(~Pt

) with ~Pi the irradiation fluence, _we have clearly evidenced for the first time the influence of hoth copper and oxygen defects on the critical temperature. The

analysis

of the data,

using

calculated

displacement

cross-sections, allows _{us to extract} the values of the

displacement

threshold energy for oxygen and copper atoms that _we have evaluated _as lo and 15eV respectively. We

clearly

show that irradiation effects have

nothing

to do with oxygen removal effects. However, the irradiation induced T~ decrease _{seems to} be due _{to a}

pair breaking

effect _as suggested for Zn doped

YBa~CU~O~_~

samples. On this subject, _we bring experimental evidence which shows that the

« classical

» Abrikosov-Gorkov depairing mechanism by magnetic

impurities

is _not

appropriate

to

explain

either the irradiation effects

or the Zn substitution

ones.

1. Introduction.

Copper

oxide

superconductors

have been found to be very sensitive _to

damage

caused

by

radiation with

highly energetic particles

^such as neutrons, ions _or electrons

[1-71.

In

particular,

the T~ reduction exceeds

significantly

that observed in conventional

superconductors

and _even in Al 5

alloys

under similar irradiation conditions. In the

YBa~CU~O~

5

(YBCO) compound,

^it

has been shown that the initial T~ reduction _can be

roughly

correlated _to the total number of atomic

displacements by

^{nuclear collisions}

irrespective

of the irradiation type

[8]

except ^for

heavy energetic

ions in which _case electronic collisions dominate

[9-lll.

Transmission

electron

microscopy (TEM)

has been u~ed to

study

the defect _structures induced

by

irradiation with electrons

[121,

_neutrons

[1,

131 and

heavy

ions

[141.

In

particular,

these studies have

allowed _{one to} establish _a clear correlation between the defects

morphology

and the

pinning

enhancement observed after _neutrons

II

or

high

_energy

heavy

ion irradiations

[15,

161 for _a

(n URA CNRS N° 1380.

review of defect structures see

[171

for instance.

Nevertheless,

_as far _as T~ is considered, the

precise

nature of the defects and the

underlying

mechanism

responsible

for the T~

degradation

has not been elucidated yet.

In

YBa~CU~O~_5,

it is well known that the oxygen content is the critical parameter

governing

_T~. As the oxygen atoms are the

lightest

of the structure and _are

loosely

bound at least

along

the chains

they

_can also be _more

easily displaced

under

irradiation,

and it is

tempting

to attribute irradiation effects

mainly

to oxygen sublattice

disordering.

In order to

investigate

the

validity

ol'this

assumption,

_we have irradiated _at low temperature

(20 K)

YBCO

single crystals by

electrons of different

energies,

E.

Indeed,

electron irradiation remains _one of the best controlled

techniques

to introduce

point

defects in _a

given

matrix.

By varying

E, we can

produce

selective defects in the different sublattices,

displacing

first the

lightest

_one,

O,

at low energy E _w 400

kev,

and further the heaviest _ones, Cu and _even Y and Ba, as the E value

progressively

increases from 0.I to 2.4 MeV. We show

unambiguously

^that T~ is sensitive _to

displacements

of both O and Cu _atoms. Moreover, _{we are} able _to determine

with _a

good

_accuracy the

displacement

threshold

energies

for O and Cu atoms, I-e- the

minimum energy which has to be transferred to an atom to

produce

a

point

defect, which is the necessary

parameter

to

quantify

irradiation disorder.

Obviously,

to determine the mechanism which leads _to such _a drastic reduction of T~

by

irradiation one needs _a

comprehensive understanding

of the mechanism which induces

superconductivity.

Nevertheless different mechanisms _can be put ^forward to try to understand this reduction of T~. ^{In these}

compounds

it is well established that electronic

properties

and

superconductivity

are related _to the hole _content in the

CUO~ planes.

Then _one may

envisage

that disorder induces modifications of the

charge

^transfer

[181.

^On the other hand, it has been

suggested

that the

superconductivity damage

under irradiation may result from _a

pair-breaking

mechanism

[191.

It is worth

mentioning

here that _a connection between electron irradiation

induced

depression

of T~ ^{and formation of localized}

magnetic

moments has

already

been

evidenced

by magnetic susceptibility

measurements

[71.

We show that _our

experimental

results

namely

Hall effect measurements on irradiated

samples

_are also better

interpreted

in the

framework of _a

pair-breaking

mechanism. On this

subject

we

give

strong

experimental

evidence to show that the _« classical _» Abrikosov,Gorkov

depairing

mechanism

by magnetic impurities

is _not

appropriate

in this _case.

2.

Experimental,

2. I SAMPLES. All but the 100 kev irradiations were

performed

_on

single crystals

of YBCO

which _were grown

using

the standard flux method.

Samples

were

plates

of about

I _x I x 0.02

mm~.

_Electrical

resistivity

measurements were

performed using

the standard Van der Pauw method for all the

samples [201.

Good electrical _contacts

(contact

resistance of about 0.I

fl)

_were made

by sticking

Pt wires with silver paste ^{and then}

performing

_an

annealing

at

300 °C for _one hour. The critical temperature was determined at the middle

point

of the

resistivity

transition. Before irradiation, _{T~ for} all the

samples

was around 92 K, ^which shows the

good quality

^{of the}

crystals. Samples

SI, SII and SIII _were irradiated _at different

energies,

while

samples

HI and HII _were irradiated _at 0A and 2.4 MeV

respectively.

The irradiation at 100 kev _was carried _{out on two}

good quality

thin

films,

TI and

TII,

which

were laser ablated onto _a

(100) SrTiO~

substrate

[211. Typical

dimensions of the

samples

_were

2 _x 2 _x 0.001

mm~.

_{In this}

case,

good

contacts _were

performed by attaching

^{Au wires}

using

_a

In sonosolder. T~ before irradiation _was about 89 K for both

samples.

2.2 ELECTRON IRRADIATIONS. All but the 100keV irradiations were

performed

in the

VINKAC low temperature

facility

on the Van de Graaff electron accelerator at the LSI

N° 7 EFFECTS OF ELECTRON IRRADIATION ON

YBa~CU~O~_~

SUPERCONDUCTOR 1607

Palaiseau

[221. During irradiation,

the

samples

are immersed in

liquid H~ (20 K)

and the irradiation flux is limited _to 2 _x

10'~

_e~

/cm~

_s in order _to avoid

heating.

After _a

given

fluence, the

samples

_are maintained _at 95 K for 10 min in _an in situ fumace

[231

and transition _curves

are recorded.

During

this stage, ^the

temperature stability

is estimated to be 50 mK.

The 100 kev irradiations _were

performed

in _a modified transmission electron

microscope

column well described elsewhere

[241.

The

samples

_are irradiated under _vacuum and the

irradiation flux is limited _to 5 _x 10~~

e/cm~

_s in order _to maintain the temperature ^{below 100 K.}

After

given

fluences, the transition _{curves are} recorded

using

_a Lake Shore 330 temperature

controller,

which _{ensures a} temperature

stability

of about 20 mK.

It is worth

mentioning

here that for all the

energies,

the electron

path

_was

large compared

to the

sample

thickness thus

ensuring

_a

homogeneous damage throughout

the

sample. Moreover,

all irradiations _were

performed along

the _c axis.

2.3 ISOCHRONAL ANNEALING, HALL EFFECT MEASUREMENTS.

Annealing

measurements

were made _on

samples

HI and HII after low temperature irradiation _at 0.4 and 2.4 MeV

respectively, using

the in situ furnace mentioned above. The

annealing

steps _were ten minutes

long

and 10 K apart ^{between 100 and 360 K. The} fluences of each irradiation _were calculated in order _to

produce

the _same T~ reduction of about 15 K for each

sample.

The

precise

^{values of} T~, (T~ ^before

irradiation),

_T~~ (T~ ^after

irradiation),

_{T~~~ (T~} after

annealing

_at 360

K)

and the

fluences used are summarized in table I for both

samples.

After the

annealing

at 360

K,

the

samples

_were moved into _a cryostat

provided

with _a

superconducting

coil in order to measure the Hall effect in the normal _{state at} B

=

8 T. The Hall

resistivity

was determined

by measuring

the odd

voltage

contribution after

exchanging

the

voltage

and current

probes

_at the

diagonals

of the

samples.

The

longitudinal resistivity

_was

obtained

using

the standard Van der Pauw

technique.

From these measurements, we are able _to determine the

longitudinal

_~,~ and Hall ~,,

conductivity

variations between 100 and 300 K.

Table I. Values

of

_{T~,, T~~, T~~~,} E and at

for samples

HI and HII ir;adiated and annealed.

Electron Fluence T~,

(K)

_T~~

(K)

_T~~~

(K)

Sample

energy

(1018 e~/cm2)

_(T~ before (T~ after (T~ ^after

(MeV) irradiation) irradiation) annealing)

HI 0.42 148 92.8 76.3 86.7

HII 2.42 28 92.7 77.2 85.7

3. Results.

3.I CRITICAL TEMPERATURE DECREASE RATE.

Samples

SI, SIT and SIII _were irradiated at

different

energies

E

ranging

from 0.I _to 2.4 MeV. For each value of

E,

_we find _a linear decrease of the critical temperature as a function of the electron fluence

allowing

_us to

determine the T~ decrease _rate

unambiguously

as AT~ =

-AT~/A~Pt

where at is the irradiation fluence. In

figure

I _we have

plotted

the variations of T~ ^{with fluence for}

sample

SIII.

The

sample

_was

initially

irradiated with electrons of 2.42MeV and then

successively

irradiated with electrons of 0.55

(open circles),

2.42

(full circles),

0.55,

2.42,

1.2

(open

triangles)

^{and 2.42 MeV. It is}

important

to note that after _an irradiation at low

(high)

_energy,

ss

86

84

~

82 14

~~

~~

78 76

74 72

0 2 4 6 8 10

Fluence I io~~e",

an~

Fig, I. T~

dependence

_on electron fluence, ~Pt, for sample SIII

initially

irradiated with electrons of 2.42 MeV, and succes~ively irradiated with electrons of o.55 (open circles), 2.42 (full circles), 0.55, 2.42, 1.2 (open

triangles)

and 2.42 MeV.

the value of AT~ at

high (low)

_energy remains the _{same as} in as-grown

samples.

This

clearly

shows that the

physical

_parameter which determines the

damage

rate is not affected

by

irradiation within the energy and fluence range

investigated,

and allows _{us to} be _sure that AT~ ^values at a

given

_energy do _not

depend

on

previous

irradiation

damage.

Values of AT~ are

plotted

as a function of E in

figure

2. The linear

dependence

between AT~ ^{and at observed within the} whole energy range

investigated

_means that the created defects

are uncorrelated. One _can then write AT~ as a linear combination of the

displacement

_cross-

sections in the different sublattices

AT~ ₌

~j

a,

~[(E, E[) (1)

where

~j

and

E[

_are

respectively

the cross-section and the

displacement

threshold energy for

species

I.

0.6 0.6

$

_

2

"fi

^{°°~ °°~}

f

.£

0.4 ° 0.4

f

il

,/ «

~ 0.3 ^°

,~'~ 0.3~

#

,/

2$ 0.2

l'~

_,,-- 0.2

I

~"

_°

l' ___.,---""'~

~

_°"

/~"'~"'

°.~

0 0 1'

0 0.5 1.5 2 2.5

Energy(Mew

Fig.

2. T~ decrease rate,

Ai~,

_{as a} function of the electrons energy E _: open squares, experimental values solid line,

fitting

_curve

using

calculated

displacement

cross-sections- The dotted line represents the contribution of the oxygen atoms

(Et

lo eV and the dashed line the contribution of the copper

ones

(Ej~

= 15 eV ).

N° 7 EFFECTS OF ELECTRON IRRADIATION ON YBa~CU~O~_

~ SUPERCONDUCTOR 1609

The best fit for the

experimental

results

using equation (I)

is also

reported

in

figure

2. The cross-sections have been calculated

using

_an

analytical

method

developed by

D. Lesueur

[25].

It is worth

mentioning

that in this calculation the number of

displaced j

atoms fi,~

by

a

primary

k _are taken into account. Here, the

fitting

parameters _are

only

the two

displacement

threshold

energies, Et

and

E[~.

Indeed~ the low energy

(E

< 0.4 MeV

experimental values,

_can be fitted

by considering

the sole oxygen

displacements

_: for

El

=

lo _± I eV, _we found _a linear

dependence

between AT~ ^and

~)(E, Ei)

_with

a

slope

A

=

(3.2

_± 0.4 ) x

10~

_{K. For}

higher

electron

energies, displacements

in the copper ^sublattice must be considered

resulting

in the

following expression

AT~ A

«j(E, El

_{) +}

B~j~(E, Ej~) ₍₂₎

with A

=

(3.2

_± 0.4 _x 10~ K _as aforementioned while B

=

(8 _± 0.7 _x 10~ K and

Ej~

=15±1eV _were determined in _a similar way as A and

E).

In this

expression

~)

and

~j~

are

displacement

cross-sections normalized _to the total number of atoms, which

means that

~j

_x at represents ^the

displacement

_{per atom}

d.p.a.

for the I

species,

I-e- the

ratio between the total number of _atoms

displaced

and the total number of _atoms. Then, we see for instance that I §l of

d.p.a.

^for O

produces

_{a T~} reduction of 32 _± 4 K and 80 _± 7 K if _we

consider Cu _atoms.

3.2 CRITICAL TEMPERATURE RECOVERY AFTER ANNEALING. In order _to

Study

the differen-

ces between low and

high

_energy electron irradiation induced defects, _we have irradiated

samples

HI and HII

respectively

_at 0.4 and 2.4 MeV and then studied the T~ recovery

by

isochronal

annealing

between 100 and 360K. In

figure

3a _we have

represented

the

T~ recovery ^rate (T~ _T~~II_{(T~~ T~~} as a function of the

annealing

temperature. ^{For HI} we can

clearly distinguish

two recovery stages at 180 and 350 K. These _two

stages

are also present ⁱⁿ the

high

_energy irradiated

sample although they

are less

sharp

than in the

previous

case. At 0.4 MeV the

analysis presented

above shows that almost

only

_oxygen atoms are

displaced~

and therefore the _two recovery stages at 180 and 350 K could be associated with two different

recombination processes of oxygen defects. We _can make _a

rough

estimation of the amount of

copper defect recombination in the 2.4MeV irradiated

sample by simply subtracting

the

contribution of oxygen defects

assuming

that formula

(2)

remains valid

during annealing.

^Such recombination processes seem to occur within _a broad temperature range between 200 and 300 K

(cf. Fig. 3b).

3.3 HALL _EFFECT MEASUREMENTS lN THE NORMAL STATE. After irradiation and

annealing~

the two

samples

HI and HII present a T~

depression

^of about 6 K. The Hall constant, measured before and after irradiation and

annealing,

is _not affected

by

this T~

depression.

In

fact, previous

^{results obtained} after ion or neutron irradiation

[5, 261

show that the Hall coefficient is

relatively

insensitive _to irradiation disorder _{even at} much

higher

fluences. The _same result is

also observed when disorder is introduced

by

Zn substitution _on copper sites

[271.

It has been

suggested by

Anderson

[281

that _a better

analysis

of Hall data _can be

performed by studying

the

cotangent ^{of the Hall}

angle &~, given by

cotg

(&~)

=

~,,/~,,

^{which is}

expected

to increase

linearly

with the square of temperature

cotg

(&~

)

=

E _x T~ ₊ F

These _{curves are}

plotted

in

figure

4, for

sample

HI before and after irradiation. We observe that this relation holds _even after

irradiation,

the

slope

E

being independent

of the

damage

introduced and the coefficient F

increasing

_{as T~} decreases. Our

goal

is _not to

give

1.i

1 ++++++++

'~~ ~+~

+

~ 0.9

, C* ^~

~ 0.8 C*

~i

~+

0.7

~'

Qu

^~

~ ~n ⁺⁺

~_~ ^0.6

~

, +

'u _0.5 . ~~

~$ O 0.4 MeV irradiated sample ^{~* ~}

+

~ +

~

°°~

. 2.4 MeV irradiated sample _O ^~+

0.3

50 loo 150 200 250 300 350 400 loo 150 200 250 300 350 400

T

~~lK)

_T

_~lK)

a) hi

Fig. ^3. alT~ recovery rate, (T~ T~,II (T~r T~, (where _{T~, and T~r} are

respectively

_{the T~} values before and after irradiation) _{as a} function of the annealing temperature ^for

samples

HI (open circles) and HII (full circles) b) Contribution of copper defects recombination processes to the T~ recovery for

sample

HII

irradiated by 2.4 MeV electrons.

300

250

~~200

~ 150 lF

/~

loo

50

0

0 2 3 4 5 6

T~l10'K~)

Fig.

4. Cotg

(&~

₌

«,,/«,,

(with _&~ the Hall

angle)

_versus T~ for sample ^{HI before} (full circles) and after irradiation (open circles) by 0.4 MeV electrons.

experimental

_support to Anderson's

theory,

but in this

form,

to compare our results with those of Chien _et al.

[271

^{obtained from} a sequence of Zn

doped samples.

Indeed, when _we

plot

the AT~/T~o

dependence

_on F

(see Fig. 5)

_we observe _a very

good

_agreement between their results

and _ours, which indicates that irradiation effects _on T~ seem to be of the _same kind _as those

produced by

Zn

doping.

4. Discussion.

We have

clearly

shown that T~ is sensitive to

displacements

of oxygen and copper atoms with

respective displacement

threshold

energies E~

of lo and 15 eV. Determination of the oxygen

threshold energy

by

electron

microscopy

leads _to

higher

values of

E~=15

eV with the

conclusion that YBCO is insensitive to 100 kev electron irradiation

[12, 291.

^These results are

in contradiction with _ours. It is worth

mentioning

here that electron fluences and rates used in

N° 7 EFFECTS OF ELECTRON IRRADIATION ON

YBa~CU~O~_~

SUPERCONDUCTOR 161

0.5

O,

0.4

fi'

I'

J

_0.3

,'

, /

$~

_0.2

~'

l'

~°~

O~$"

/

0

0 20 40 60 80 loo120140

F

Fig. 5. T~ reductions normalized to its initial value _{as a} function of F (see text) for irradiated samples HI and HII (full circles) and Zn doped samples ^from [271 (open circles).

electron

microscopy experiments

are much

higher

than ours and _so defects visible

by

this

technique

_may be different from those

responsible

for the initial T~ decrease. In

particular,

Basu et al.

[29]

determined the oxygen

displacement

^threshold energy from the

changes

in the

splitting

of the

(I,

1,

0)

diffraction spots ^{in the}

[0,

0,

II

diffraction pattern ^induced

by

the redistribution of oxygen ^atoms in the CUO chain

planes.

However, we have verified that _a

sample

of YBCO irradiated

by

2.4 MeV electrons up ^to T~ =

0 is still orthorhombic

[30],

which indicates that the

damage

_{on T~} should

precede

_any visible modification of orthorhombi-

city.

The

key questions

are

II

from which sites the oxygen and copper ^atoms

responsible

for the T~ decrease _are

ejected 2) by

^which mechanism do the

ejections

induce the T~ decrease. Since

we have to deal with _a very

complicated

structure, it is very difficult _to

bring

_a clear-cut _answer to the first

question.

Nevertheless, it is

interesting

to consider two

particular

_cases first, _we

may assume that

only

the

displacements

of the chain _atoms induce _a T~ reduction _;

secondly,

that

only

the

displacements

of the _atoms

belonging

to the

Cu02 Planes play

a role in the T~ decrease.

The first

assumption

comes

directly

^from the fact that chain atoms are

weakly

bound and then _can be

easily displaced. By

_now

rewriting expression (2)

_{as a} function of the _cross

sections normalized to the chain atoms

~)~~'~

=

(l/7 ~)

_{+ l/3}

~)~) (3)

and

noting

that the

experimental

B/A involved in

(2)

is

equal

to

7/3,

_we obtain the relation

AT~ ₌ ^C x

~)~~'~ (4)

with C

=

(23

±

3)10~K.

This _assumes that the

probability

for oxygen and copper ^atom

displacements

does _not

depend

on their sites, ^and that

only displacements

in the chain affect T~.

Expression (4)

leads to the conclusion that

displacements

of copper or oxygen chain _atoms

have the _same effect _on T~. ^{In other} words~ it _seems that the

important

fact is to break the CUO chains

disregarding

the site where the break takes

place.

For electron irradiation at low

energies (E<

0.35

MeV),

_we found that

only

_oxygen

displacements

^affect T~ as observed in oxygen

depletion experiments.

But this

analogy

no

longer

holds in _a

quantitative comparison.

Indeed from formula

(4)

0.I i&

d.p.a.

leads to a

T~ reduction of about 23 K. The _{same amount} of oxygen removal from the chains leads to

0.4 K of T~ reduction for _an initial

fully oxygenated sample [311.

This indicates that the

damaging

mechanism induced

by

irradiation is

totally

different from the _one caused

by

_oxygen

depletion,

even if _we suppose that irradiation

only

affects the chain _structure. In the latter _case, it is _now well

accepted

that the T~

depression

^is due _{to a} reduction of the hole concentration in

the

CUO~ planes [3 II

This effect is related _to modifications in the band _structure

produced by short/long

_range

ordering

^of the chain structure

depending

on the oxygen content. Such

arrangements ^{have been observed}

by

^{TEM and, in}

particular,

when the oxygen content

equals

6.5 _{a new} orthorhombic structure

consisting

of _{one over two} chains

fully oxygenated

in the ah

planes

_was observed

[311.

It is worth

mentioning

here that calculations of the electronic

structure of _a

fully oxygenated sample

have shown that redistribution of oxygen atoms in the

chains results in _a drastic reduction Of

charge

transfer and hole

density

in the

CUO~ plane

even

if the oxygen content remains constant. Such _a

mechanism,

which has been

proposed

to

explain

effects of irradiation

damage [181, Predicts

that the loss of

superconductivity

should be associated with _a transition from the orthorhombic to a

tetragonal

structure. This situation _was

clearly

_not observed in electron irradiated

samples

_as mentioned above

[301.

From Hall data, it is

tempting

_{to compare} irradiation effects

with,

Zn substitution _on the Cu sites.

Concerning

these

experiments,

the

precise

site where the substitution _occurs is still _a controversial issue. Nevertheless, it _seems that the main effect of Zn

doping

is the appearance of local

magnetic

moments in the

CUO~ planes

associated with the disorder induced

by

the Zn

atoms substituted _on the

CUO~ planes.

^{Wherever the} substitution takes

place,

_a nominal

doping

of 0.I i& will induce _a T~ reduction of about 4 K

[27, 321.

In order to compare irradiation and Zn

doping

effects, let _us consider the situation in which T~ is

mainly

affected

by displacements

within the

CUO~ planes. Then, expressing

the variations

of

Ai~

_{as a} function of

~)P

=

4/7 _x

~)

and

~)~P

= 2/3 _x

~)~,

the

displacement

_cross sections for oxygen and copper ^atoms in the

CUO~ planes respectively,

we obtain

Ai~

m D _x

(~jP

₊ 2 _x

~)~P) (5)

where D m6

x10~K

_{and the}

constant 2 wich

multiplies ~)~P

_were deduced from the

experimental

values A and B in (2).

According

to formula (5), the

displacement

of _a copper atom is twice _more efficient than that of _an oxygen ^atom. This _can be understood since

ejection

of a copper atom will break the array of Cu-O-Cu _atoms in both

[l~

0, 01 and [0,1,

0]

directions~ while

ejection

of _an oxygen atom will break this arrangement

only along

one of these directions. Within this

interpretation~

0.I i&

d-p-

a. Cu will induce _a T~ reduction of about 12 K, which is

comparable

with the 4 K obtained for _an

equivalent

amount of Zn substitution.

In any case, we have to remember that the two

pictures proposed

above _are

limiting

_cases

that we have considered in _an attempt to establish _a

quantitative comparison

between

irradiation effects _on the _one hand~ and oxygen or Zn

doping

effects _on the other hand. The real situation is

certainly

more

complicated

and it is

possible

that

displacements

of

apical

_oxygen

and even of

yttrium

_or barium _atoms could

play

a role

concerning

_{the T~}

damage. Nevertheless~

the contribution of the heaviest _atoms should be of the second order within the energy range studied here. Moreover, it is worth

mentioning

that the

apical

_oxygen is

expected

to

participate

tO the

charge

^transfer

by providing

_a fourfold square

planar

coordination to the Cu chain _atoms when the chains _are present

[181.

In this context, the

major

contribution of _an

apical

_oxygen

displacement

should be related _{to a}

degradation

of the

charge

transfer which _we have shown to be _a minor effect in the _case of electron irradiations.

Since the presence of

magnetic

moments has been observed in irradiated

samples

[71 as well

as in the Zn substituted _ones

[32

for

instancel,

it _seems natural _to consider _a

pair-breaking

mechanism due to the

breaking

of the time reversal symmetry. ^{For this} purpose, the classical

N° 7 EFFECTS OF ELECTRON IRRADIATION ON

YBa~CU~O~_

~ SUPERCONDUCTOR 1613

Abrikosov-Gorkov mechanism has been evoked

[191.

Within this

theory,

a BCS

point-like

interaction between carriers is considered.

Furthermore,

it is assumed that the time reversal correlation

function, (K(t), K(0)), decays exponentially

in time

leading

to an

ergodic

situation.

Namely

_we have _:

(K(t), K(0))

cc

exp(- t/r,) (6)

Then,

the T~ reduction _rate is found _to be

proportional

to the inverse relaxation time r,, which in the presence of uncorrelated

magnetic impurities

leads _to

[3311

AT~ cc

J~

x N

(0)

_{x n~}

(7)

where J is the

exchange integral

between carriers and

magnetic impurities,

_n~ is the

magnetic impurities

concentration, and

N(0)

is the

density

of _{states at} the Fermi level.

From formula

(7),

the T~ ^decrease must be linear _on fluence and the T~

damage

rate

proportional

to N

(0). Thus,

_one should expect a

poorly oxygenated sample

to be less sensitive than _a rich

oxygenated

one, that is _to say that if _we consider _a sequence of oxygen

depleted samples,

_{the T~} reduction _must be faster for

higher

values of T~o. ^{We have observed}

exactly

the

opposite

when _we have irradiated sintered

samples

with different oxygen ^contents

by

electrons of 2.4 MeV

[341.

The _same results have been obtained

by Vichery

et al. [61 on

single crystals

irradiated

by

^?.5 MeV electrons. All this

experimental

evidence weakens the

predictions

of the Abrikosov-Gorkov model and further

clearly

establishes _a difference between oxygen

depleted

and irradiated

samples.

Indeed, if the irradiation effects _on T~ were the _{same as} those due _to oxygen removal, then the irradiation induced T~ decrease _rate should

progressively

increase _as T~ reduces (as observed in oxygen

depleted samples)

_: this is in total

disagreement

with _our results. On the other

hand,

irradiation results are consistent with results obtained from Zn substituted

samples

for which the T~ reduction _rate induced

by

Zn

doping

decreases with the oxygen ^content

[321.

5~ Conclusion~

We have

brought

the first

experimental

^{evidence of the influence of}

^displacing

copper ^atoms

by

electron irradiation _on the T~ reduction of YBCO

single crystals.

Furthermore _we have determined threshold

energies

for oxygen and copper ^{atoms to} be 10 and 15 eV

respectively.

Our results

clearly

show that irradiation effects have

nothing

to do with oxygen removal effects, while

they

show that the former _are similar to Zn

doping

effects.

Finally,

_we have

shown that the conventional Abrikosov-Gorkov

depairing

model induced

by

^localized

magnetic

moments is _not

appropriate

to

explain

_{the T~}

depression

either _on irradiated

samples

or in Zn

doped

ones.

Acknowledgments~

It is _a

pleasure

to thank Dr. G. Koren who gave us YBCO

films,

Dr. D. Lesueur for his _cross- section calculations and

helpful

discussions. We also thank P.

Laplace

^{for his technical}

assistance

during

irradiation

experiments.

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YBa~CU~O~_~

SUPERCONDUCTOR 1615

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