Mössbauer analysis of superconductivity in tin and tin-copper films vapour-deposited on cold substrates

Journal of Non-Crystalline Solids 117/118 (1990) 363-366 North-Holland

363

M~SSBAUER ANALYSIS OF SUPERCONDUCTIVITY IN TIN AND TIN-COPPER FILMS VAPOUR-DEPOSITED ON COLD SUBSTRATES

Masafumi TANIWAKI, Makoto UNETA (a), Kazuo KASAYA (b) and Masao MAEDA (c) Department of Electronic Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan

The electronic structure and lattice vibration in the defective tin and amorphous tin-copper films vapour-deposited on cold substrates below 10 K were investigated by 119 Sn M~ssbauer effect. The copper-concentration dependence of the isomer shift and the recoilless fraction in the films corresponded well to that of the superconducting transition temperature, Tc. The metallic bonding in quenched tin-copper film increased with copper-concentration at 5 - 10 at~ and decreased at 10 - 20 at%. The Debye parameter, eD(-1), decreased with increasing copper concentration at 5 - 10 at% and recovered at 10 - 20 atZ. The dependence of Tc on copper concentration was explained qualitatively by applying these changes in the electronic structure and the lattice vibration to the McMillan's equation.

tunneling measurement 5, specific heat measure-

i. INTRODUCTION In many metals, the superconducting tran-

ment 6 and

MNssbauer

spectroscopy 7 for

sition temperature, Tc, is enhanced by vapour-

films.

quenching on cold substrates.

effect of the electronic structure,

was

This phenomenon

first found by Shalmikov for tin in 1938.I

However,

important

tin

the efforts to examine the

factor determining

another

Tc, on the Tc

Studies on the Tc enhancement by vapour quench-

enhancement are very poor,

ing in various metals were performed by Buckel

phization should change the electronic struc-

and Hilsch. 2'3

ture as well as the lattice vibration.

found

that

In their study,

an addition

of

it was also

impurity

during

quenching strengthens the Tc enhancement.

although the amor-

The

authors have investigated the lattice vibration

In

and the electronic structure in pure Sn vapour-

the case of the addition of copper to tin, Tc

quenched on cold substrates below 10 K by 119Sn

is about 4.3 K at 0 - 5 at~ and it increased

MNssbauer effect spectroscopy and estimated

abruptly to 7.2 K around 8 atZ and decreased

their contribution to the Tc enhancement. 8

gradually with increasing Cu concentration. 4

this article, we report the lattice vibration

It was explained that the Tc enhancement

is

and the electronic structure in vapour-quenched

attributed to the structural defects introduced

Sn1_x-Cu x films(O ~ x ~ 20) and their effect on

by quenching and that the addition of impurity

the Tc enhancement.

stabilizes the disorder,

In

i.e., the amorphous

structure.

The BCS theory established in 1957

suggested

that the lattice softening induced

The films of Sn-Cu(O, 5, 10, 20 at~Cu) were

by the structural defects in the quenched films

produced by vapour-quenching on cold substrates

increases Tc.

of quartz

This was examined intensively by

2. E X P E R I M E N T A L P R O C E D U R E S

glass

in a cryostat.

Present address: a) NTT Applied Electronics Laboratories, Musashino-shi, Tokyo 180, Japan b) NTT Optelectronics Laboratory, Atsugi-shi, Kanagawa Prefecture 243-01, Japan c) Kanazawa Institute of Technology, Nonoichi-cho, Ishikawa-ken 921, Japan

0022 3093/90/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland)

During the

M. Taniwaki et al./MSssbauer analysis of superconductivity

364

Table I.

The parameters in Mossbauer spectra of vapeur-deposited tin-copper films.

sample

thickness (nm)

heat treatment

71

pure Sn

145

Sno.95Cuo.o 5

Sno.9oCuo.10

line position (mm/s)

as depodited

2.657(17)

annealed

2.612(17)

as deposited

2.660(5)

annealed

2.607(5)

152

8no.800u0.20

346

1.84(10) 1.75(8)

line area (~ mm/s)

1.084(64) 1.196(58)

3.14(28) 3.30(24)

3.57(4) 4.O9(4)

1.090(17)

6.11(13)

0.987(16)

6.34(17)

2.784(8)

2.52(3) 3.09(3)

1.240(26)

4.91 (17)

2.611(7)

1.119(21)

5.43(17)

as deposited

2.748(18)

3.28(9)

1.332(61)

6.86(50)

annealed

2.534(23)

3.83(14)

1.218(72)

7.33(60)

as deposited

2.692(8)

7.42(9)

1.379(28)

16.07(53)

annealed

2.531(6)

8.40(9)

1.271(18)

16.77(40)

of substrates was

After quenching,

the inner

part of the cryostat was rotated and MNssbauer s p e c t r u m of the d i s o r d e r e d or a m o r p h o u s film was m e a s u r e d at 4.2 K.

line w i d t h (mm/s)

as deposited

quenching, the t e m p e r a t u r e kept b e l o w 10 K.

(~)

annealed

112

Sno.85Cuo.15

line intensity

After it, the f i l m was

annealed at room temperature

in the vacuum of

1.0 0

O


0.9

the cryostat, by which the amorphous Sn trans-

[

I

i

I

i

formed into ~-tin. Then the M~ssbauer spectrum

0.2

of the annealed film was measured at 4.2 K. I{/)

EOI 3. RESULTS

E

Observed spectra were analyzed by non-linear least square method using a single Lorentzian. The o b t a i n e d

M~ssbauer

line

parameters

are

!/

-0.1

o

O

O

I

l

I

I

I

t a b u l a t e d in table I with the film thickness 08 which was estimated from Massbauer absorption area of the a n n e a l e d film.

The line p o s i t i o n

is s h o w n relative to BaSnO 3 at room t e m p e r a ture.

Figure

dependences

I shows

the

of the i s o m e r

Cu-concentration shift

in the as-

quenched film, the difference between the line

E E 07 o

03 0.6

I

0

I

5 I0 15 Cu concentration (%)

I

2O

width in the as-quenched film(I.S.) and that in the annealed film(AW) and the ratio of the line area in a s - q u e n c h e d nealed film.

film

to that in the an-

The isomer shift, which shows the

effective s-electron density at Sn nucleus, was nearly

constant at 0 - 5 at ~ and increased at

Figure I Massbauer parameters of vapour-deposited Sn-Cu films are shown as a function of Cu concentration. I.S., A W and area ratio show the line position, the difference in the line width and the ratio of the line area, r e s p e c t i v e l y ( d e tails in the text).

M. Taniwaki et al. / MSssbauer analysis of superconductivity 5 - 10 at %, and then d e c r e a s e d

at 10 - 20 %.

The line width in each as-quenched

s -I than that in the annealed film.

This shows

that the asymmetry

introduced

by q u e n c h i n g excluding around

of the structure

is s i m i l a r

in all Sn-Cu films,

the effect of copper

tin.

The recoilless

configuration

fraction ratio was

n e a r l y c o n s t a n t at 0 - 5 at % and d e c r e a s e d

at

5 - 10 at %.

We

It r e c o v e r e d

at 10 - 20 at %.

can say that the C u - c o n c e n t r a t i o n of the i s o m e r tion,

which

correspond

dependences

shift and the r e c o i l l e s s

reflects

well

the

lattice

to that of Tc.

clearly that the electronic tice vibration

frac-

vibration,

This indicates

state and the lat-

govern the superconducting

sition temperature

4.2 Lattice Vibration

film except

the pure tin f i l m was g r e a t e r by about 0.1 mm

tran-

in the quenched Sn-Cu films.

At a low

temperature

3 ER

pure tin and tin c o m p o u n d s isomer

shift,

of tin

atom in

is i d e n t i f i e d

w h e r e ER is the r e c o i l e n e r g y of 119Sn and The Debye parameter so(-1) is

oo(_1)=3~ 2kB (f F(~)~ l d ~ )-1 where F(~) is

phonon density

Debye p a r a m e t e r , recoilless

(3)

Oo(-1)

fraction

of states.

is

obtained

I.S., by the equation

(I) derived

ratio of the quenched film

and the annealed film and by adopting 140 K( ~tin) as the Debye t e m p e r a t u r e film.

in the a n n e a l e d

A remarkable

lattice

The Debye

temperature

minimum

softening

was

ob-

in the quenched

at 10 - 20 at%.

at 0 The

value was 106 K at 10 at%Cu.

4.3 Tc Enhancement The McMillan's ting

0.38+3.10ns --0.20n~--O.17nsnp

The

by t h e

from

by Lees and Flinn. 9

AS.=

(2)

Sn-Cu film decreased with Cu concentration

configuration

f

fraction

f = e x p ( - - 2kBOo(_l))

- 10 at %, and i n c r e a s e d

The e l e c t r o n

recoilless

is

served.

4. ANALYSIS ANDDISCUSSION 4.1 Electron Configuration

365

(1)

equation for the superconduc-

transition

temperature

impropved

by

Dynes I0 is

ns +np--4 where

and

ns

electrons, shift

at

the n u m b e r s

npare

respectively 4.2

temperature.

K

to

BaSnO 3 at

of free e l e c t r o n s

is derived

on the assumption

has metallic bonding (5sSp3).

tion and Z o b t a i n e d

The

per one atom, that

the

increase

decrease

in

of metallic

electron

are t a b u l a t e d

Z,

bonding

configurain table 2.

where ~* is Coulomb

and

pseudopotential(

= 0.I<<~).

<~> is defined by

< ~> =fa'(~)F(~)d~ /fa'(~)F(~)~ ~d~ where

i.e., the

is an average

az(~)

phonon interaction. constant

in 5s electrons

5p electrons,

(5)

--

k

is

N(O)<

gZ >

of the

electron-

Electron-phonon

coupling

(6)

M< wz>

the decrease

of c o v a l e n t b o n d i n g were observed. tion of Cu had the maximum

(4)

pure Sn or

(5s 2 5p 2) and cova-

By addition of Cu, the increase and

room

From the electron configuration,

lent bonding

Tc=exp(_ 1.04(1+X) ) 1.20 X-- ~*(1+ 0.62~)

and I.S. is the line

relative

the n u m b e r

Sn-Cu

of 5s and 5p

The addi-

effect at 10 at% Cu.

where

M, N(O)

electronic

and is the a t o m i c mass, the

density

and the average

of states

surface

over the F e r m i surface of the

square of the e l e c t r o n i c spectively.

at Fermi

<~2> is

m a t r i x element,

re-

366

M. Taniwaki et al./ MiSssbauer analysis of superconductivity Table 2.

The list of the electron configuration, the Debye parameter, the parameters in the McMillan's equation and the calculated Tc in fl -tin and as deposited Sn-Cu films. electronconfiguration

co(-1) Z N(O) [ K electrons states per K per atom eV atom

-tin

5s1"2565p2"744

140

1.024

0.186

93

9790

8.24

0.74

3.72

pure Sn

5s1"2755p 2"725

122

1.100

0.190

81

7440

8.24

0.99

5.52

Sno.95Cuo.05

5s1"2765p2"724

125

1.104

0.191

83

7813

8.24

0.95

5.38

Sn0.9oCu0.10

5s1"3295p 2"671

I06

1.316

0.202

71

5660

8.24

1.39

7.52

Sno.85Cu0.15

5s1"31LSp 2"686

116

1.256

0.199

77

6692

8.24

1.16

6.62

5s1-2905p2.710

123

1.160

0.194

82

7618

8.24

0.99

5.66

sample

Sno.8oCuo.20

<~2>



K2

e V A -2

X

Tc(cal) K

tained Tc 4 by Fortmann and Buckel is shown as a function of Cu concentration in Fig. 2. see a good agreement experimental

We can

between calculated Tc and

Tc.

In this M~ssbauer effect study, it was shown that the superconducting o6 I--

Tc in the vapour-quenched

Q

Sn-Cu films is attri-

buted to the lattice vibration

Ol

/ I

tronic

Tc(calc) Tc(exp) ........

I

..........

transition temperature

1

structure.

electronic

The

structure

and the elec-

contribution

of the

to the Tc enhancement

is

not so large comparing with that of the lattice 4

I 0

I I I 5 I0 15 Cu concentration (%)

I 20

softening, quenched lattice

Figure 2 The calculated superconducting transition temperature f r o m the M ~ s s b a u e r parameters, Tc(calc), and the e x p e i m e n t a l l y obtained Tc(exp) of v a p o u r - d e p o s i t e d Sn-Cu films are shown as a function of Cu concentration.

however,

Sn-Cu

films.

softening

and

significant

in

the

It is noted that the the

increase

in free

electrons by quenching are not independent each other 9 but occur simultaneously.

REFERENCES 1. A. Shalmikov, Nature 142(1938)74 2. W. Buckel and R. Hilsch, Z. Physik 132(1952)420 3. W. BuckelandR. Hilsch, Z. Physik

By assuming that ~2(~) is independent of ~,

<~

138(1954)109

4. J. Fortmann and W. Buckel, Z. Physik 162(1961)93

>, and N(O) are obtained from Debye param-

5. K. Knorr and N. Barth, Solid State Commun. 8 (1975)1085

eter

6. S. Ewert and W. Sander, Z. Physik 2~7(1971)21

OD(-1) and Z under Debye model and free

electron

model.

obtained

in ref. 8 was adopted.

parameters

As



in equation

the

value

of 8.24

The various

(4) and the calculated

To in the quenched film were listed in table 2. The calculated

Tc and the e x p e r i m e n t a l l y

ob-

7. J. Boltz and F. Pobell, Z. Physik R20(1975)95 8. M. Taniwaki, M. Uneta and M. Maeda, Japan. J. Appl. Phys. 26 Suppl. 26-3(1987)1321 9. J.K. Lees and P.A. Flinn, J. Chem. Phys. 48 (1967)882 10. R.C. Dynes, Solid State Commun. 10(1972)615