Anomalous magnetic properties of weak-links-bearing superconductors

Anomalous magnetic properties of weak-links-bearing superconductors

. . . . . . . J Physics C 185-189 (1991) 1527-1:528 North-Holland ANOMALOUS MAGNETIC PROPERTIES OF WEAK-LINKS-BEARING S~ERCONDUCTORS Sergei SERG...

191KB Sizes 1 Downloads 74 Views

. . . . . . . .

J

Physics C 185-189 (1991) 1527-1:528 North-Holland

ANOMALOUS MAGNETIC PROPERTIES OF WEAK-LINKS-BEARING S~ERCONDUCTORS Sergei

SERGEENKOV

Joint I n s t i t u t e

for Nuclear

Research,

Dubna 1 4 1 9 8 0 ,

USSR

A possible scenario for dislocation-induced intragrain weak l i n k s g e n e r a t i o n i s p r o p o s e d . The t h e o r y p r e d i c t s a reentrant-like behavior of the critical current density versus magnetic field due to the sample deoxygenation. A qualitative agreement with high-T c single crystals anomalies is discussed. P r o b a b l y , t h e r e a r e a l r e a d y no d o u b t s that, due t o small coherence length, practically any defects (imperfections)

We a r e i n t e r e s t e d in a concord between a finite dislocation density p(x) inside the crystal and an appearance of the

in high-T c single crystals may contrib-

Josephson supercurrent J s [ P ( x ) ] . The k e y p o i n t of t h e p r e s e n t t h e o r y i s t h e a s s o ciation of the (SIS-type) weak l i n k s j u n c t i o n s w i t h t w o phases r e s u l t i n g from the dislocations paths through the crystal. Supposing, for simplicity, the twinning boundaries a s t h e weak l i n k s sources, we are able to use for their treatment the dislocation theory of elastic twinning 5. If, for instance, a

to both the weak-links properties and the flux pinning 1-4. In a recent pa-

ute

per of Daeumling et

a l . 3,

for

instance,

a rather intriguing correlation between defect pinning, intragrain weak links, and oxygen deficiency i n YBa2Cu307_ ~ single crystals is discussed. In this paper a possible scenario of the intragrain weak links generation via dislocation mechanism is proposed, and their influence on the Josephson critical

current

density

vs.

magnetic

field

us

consider

Josephson contacts

the

model

of

magnetic

small

with length L~Aj,

is the Josephson penetration depth, strong

field

such

that

Aj

in a

(second)

thickness. critical

In

junction, this

current

pedbythe

t is insulator

case

density

the

maximum

through

the

field WO ) is stop-

etc,

then

density distribution law 5:

the

dislocation

is governed by the

p(x) = PO[ L~x } I/2

(3)

Here L = 2b/~ 0 is the twinning length, b is the total Burgers vector.

Integrating

Eq.(2) with the dislocation density (3) over o(x), and averaging over random junction size d, one obtains for the maximum critical current density

contact is : Js(X) = Jcsin#[p(x)], (I) where the variation of the superconduc--

Js(H ) = jc/(I+H2/H~)

(2)

,

:

(4)

where

ring phase obeys the law : 2~d dp -I d# dp - ~0-''H(~-~)

external

crystalline grain boundaries,

H

#O/2ffAjd, where d=ALI+AL2+t, ALl(2 ) is the London penetration depth of the first

homogeneous interfaces

is discussed as well. Let

freely growing twinning boundary (in the

Hp

~0 2ndoAp

0921-4534D1/$(73.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.

4b 2 [02+ 462 ] - 1

Ap

S. $ergeenkov / Weak4inks-bearingsuperconductors

1528 As

is seen,

describes

Eq.(4),

at

an ordinary

least

1.00

formally,

decreasing

of the

critical current density with H, but ac-

zO.80

tually this

a

the

is not the case

implicit dependence

of

of a character-

istic "penetration

depth"

ternal

sources

affecting

cation

(via the

~0

in v i e w

Ap on the ex-

twinning

the

dislo-

length

L,

see

Eq.(3)). To prove this statement,

let us

define the form of these sources.

First

of a11, the

if the dislocation

boundary

rent

of

magnetic

line

two phases

lies on

with

susceptibilities

Eo.6o o0.40

~

diffe(~I

0.20

0.00

0.00

and

1.00

2.00

MAGNETIC

FIELD

3.00

X2), then such a dislocation will be influenced,

in nonzero magnetic

field,

by

a force : fm = ~ ( ~ 1 - ~ 2 )H2 " Here a is an interplane

(5) distance. The

FIGURE I N o r m a l i z e d critical current density (4) versus reduced m a g n e t i c field h=H/Ho :

I)

6=0, 2) 6=0.01, 3) 6=0.05.

force (5) will be equalized by the force of a dry friction

(which is always

sent inside crystal),

pre-

and by the osmotic

overcomes this barrier

(existing only

for

becomes

6#0),

the

pinning

force fo due to an excess nonequilibrium

fective,

concentration of vacancies cac 0 (or oxy-

the critical current

gen deficiency 6=c/c0-I ) :

a=10~, (6)

I n accordance w i t h E q s . ( 4 ) - ( 6 ) , inal

oxygen d e f i c i e n c y

t h e nom-

~aO l e a d s t o

appearance of an effective field H*(6) = ~(T)[log(l+6)] 1/2,

the

b=10nm,

:

field

H*(0.05)~4T,

the

fields

Taking

and supposing that ~ 2 ~ I ,

for T=70K and 6=0.05,

a rough estimate

nomena in (7)

is observed.

tains to

ef-

and a rather smooth fall-off of

X2=-I/4~,

kBT f0 = .........b 2 1 o g ( t + 6 ) .

less

where

one

of the effective

which

is very

the anomalous

YBa2Cu307_6

ob-

single

close phe-

crystals

have been observed 3.

where

f 2 BT I/2 z(T) = taa---~-~-~ } When t h e

applied

REFERENCES •

magnetic

field

reaches

I. G. Deutscher and K.A. MUller, Rev. Lett. 59 (i987) 1745.

Phys.

the effective one H*(6), the moving dislocation is blocked, enhancement as

a

of

result,

the to

thus leading to the pinning

the

forces

recovery

2. J.R. Clem, Physica 162-164C (1989) 1137.

and,

of

the

3. M. Daeumling, Larbalestier,

J.M. Seuntjens and D.C. Nature 346 (1990) 332.

critical current density (see Fig, l). It is

seen

H=H*(6),

that

at

the

the additional

points, peaks

where

4. R.L. Peterson and J.W. Ekin, Phys. Rev. 37B (1988) 9848.

are gene-

rated in agreement with the observations of Daeumling et al. 3 When magnetic field

5. L.D. Landau and E.M. Lifshitz, Theory of Elasticity (Pergamon, New York, 1960).