Superconducting energy gap in copper oxide SN system

ICA

Physiea C 185-189 (1991) 2573-2574 North-Holland

Superconducting E n e r g y M.Suga,

T.Nishino,

Central

Research

Gap in Copper Oxide S-N System

Y.Tarutani, Laboratory,

and K.Takagi

Hitachi

Ltd.,

Kokubunji,

Tokyo

185, Japan

A tunneling s t u d y i s p e r f o r m e d on t h e s u r f a c e of a normal-conducting copper oxide Lal.sBal.sCu3OT-y(LBCO) in S(HoBazCu3Ov-y(HBCO))-N(LBCO) b i - l a y e r s y s t e m . Measured t u n n e l i n g dI/dV c u r v e h a s a g a p - l i k e s t r u c t u r e . The p e a k - t o - p e a k v o l t a g e 2A in t h e t u n n e l i n g dI/dV curve e x p o n e n t i a l l y d e c r e a s e s w i t h i n c r e a s i n g LBCO l a y e r t h i c k n e s s . T h e s e f a c t s s u g g e s t t h a t a s u p e r c o n d u c t i v i t y i s i n d u c e d in t h e LBCO l a y e r . The c h a r a c t e r i s t i c decay length ~ of A is 260±50nm, which i s two o r d e r s l a r g e r than t h e v a l u e e x p e c t e d from t h e c o n v e n t i o n a l t h e o r y of the proximity effect.

I. INTRODUCTION

specimens are 50, 200, and 400nm, respectively.

Recently, a supercurrent was observed flowing through

S-N-S

junctions,

superconductors o x i d e s 1-3

In

In Fig.l, one pair of peaks is identified

in

which

both

each dl/dV curve, which are indicated by arrows.

and n o r m a l - c o n d u c t o r s

were

Absolute voltages at which the peaks exist in

these

in

junctions,

N

layer

t h e d I / d V c u r v e (we c a l l

these voltages

peak

thicknesses were two orders larger than expected decay lengths of superconducting order parameter in N layers,

which were calculated

from the

proximity effect theory 4. To clarify the origin of these supercurrents,

it is necessary

to know

the density of states in N layer in S-N bi-Jayer systems. For t h i s purpose, tunneling study was performed on the surface of the N(Lal.sBal.sCu3OT-y(LBCO)) S(HoBalCu3Oz-y(HBCO))-N(LBCO)

layer

in the

-30

with S-N bi-layer

0

15

30

v {my)

bi-layer system.

2. EXPERIMENTS

The specimens

-15

\

structure

j"

(shown in Fig. l) were fabricated on SrTi03 (110) substrates 3. Tunnel junctions were fabricated by evaporation of a metal (Au) directly on the LBCO

[N LSCO

s(H~co~

surface ~,e. Tunneling spectra were obtained by measuring I-V characteristics beLween Au and the

LBCO l a y e r

at

-10

4.2K.

3. RESULTS AND DISCUSSION Tunneling dl/dV curves for bi-layer specimens are shown in Fig. l . LBCO layer thicknesses, d of

-5

f

0 v (my)

5

lO

FIGURE 1 T u n n e l i n g c h a r a c t e r i s t i c s of Lhe S(HBCO)-N(LBCO) b i - l a y e r specimen measured on the s u r f a c e of L~e LBC0 l a y e r .

0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All righLs reserved.

M. 5uga et aL / Superconducting energy gap in copper oxide S-N system

2574

voltages hereafter) are 24± 2mY for d=5Onm, 1 4 . 3 ± 0.3mV for d=2OOnm,

and 5 . 7 ± 0.8mY

found

for

that

A

increasing

decreases

LBCO

layer

exponentially thickness.

with

This

is

d=400nm. At just lower (inside) voltage region

consistent with the proximity effect theory 4 if

than the peak voltage,

the origin

gradient of dI/dV

is

of the peaks

in the dI/dV curves

larger than those in other regions in each dl/dV

(Fig.l)

curve

From the above discussion,

it is considered that

the origin

in the dI/dV curves

(these

voltage

regions

are

called

divergence regions hereafter). The origin of the peaks in the dl/dV curves (Fig.l)

is considered

to be a superconducting

energy gap (SC-gap) induced in the N(LBCO) layer

is a SC-gap induced in the LBCO layer.

(Fig. l) is a SC-gap induced in the LBCO layer by contact

with the HBCO

LBCO layer,

following

fitting,

First,

each

dl/dV

curve

(Fig.l)

has

layer.

A characteristic decay length ~ of A

by contact with the S(HBCO) layer because of the two reasons.

of the peaks

which was obtained

is 260+_50nm.

in the

from Fig.2 by

This value is two orders

a

larger than an expected value of 4.2nm, from the

gap-like structure consisting of following three

proximity effect theory 4 at 4.2K with measured

characteristics.

hall concentration

I One pair of peaks exists in

each dl/dV curve.

II The absolute values of the

and resistivity

and free

electron mass. An origin of this discrepancy is

peak voltages are the same for the two peaks. III

an open question. The superconductivity

There exist the dlvergence regions in each dl/dV

might

be

induced

by

the

new

in LBCO

mechanism.

curve. These gap-like structures are similar to those

found in tunneling

superconductors s,e.

This

studies of high Tc suggests

that

origin of the peaks in the dl/dV curves

the is a

SC-gap.

ACKNOWLEDGEMENTS The authors would like to thank Dr.Hiratani for his valuable discussions,

and Mr. Fukazawa,

Mr.Akamatsu and Mr.Tsukamoto for providing high

Second, a dependence of the peak voltage exponential

with

respect

to

thickness, d. The peak voltage A

LBCO

is

layer

as a function

of LBCO layer thickness is shown in Fig.2. It is

quality thin films. The present achievement performed Asseciat~en

under

the m a n a g e m e n t

of the

is R&D

for FED as a part of the R&D of

Basic Technology for Future Industries supported by NEDO.

100 REFERENCES

E 10~°~°~o~.

>

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0

200 400 i00 Thickness of LBCO (nm)

FIGURE 2 Peak voltage A in t u n n e l i n g dl/dV curves as a f u n c t i o n of LBCO l a y e r t h i c k n e s s .

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