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Scanning tunneling spectroscopy on Bi2Sr2CaCu2O6−8
Physiea C 185-189 (1991) 9 4 1 - 9 4 2 North-Holland
SCANNING TUNNELING SPECTROSCOPY ON Bi=Sr=CaCu=Os-~
K o i c h i ICHIMURA*, K a z u s h i g e NO~URA*, F u j i o MINAMI** and S h u n j i TAKEKAWA *°* *
~ H.
Department o f P h y s i c s , Hokkaldo U n i v e r s i t y , Sapporo 060, J a p a n Rc:zareh Institute of Applied Electricity, Sapporo 060, J a p a n National Institute f o r R e s e a r c h in I n o r g a n i c M a t e r i a l s , Tsukuba, I b a r a k i 305, J a p a n
T u n n e l i n g measurement was c a r r i e d o u t on s i n g l e c r y s t a l s o f s u p e r c o n d u e t i o n g o x i d e Bi=Sr=CaCu:Os_~ w i t h u s e o f t h e STM. T u n n e l i n g c o n d u c t a n c e v a r i e s w i t h t h e d i s t a n c e between t h e t i p and t h e sample s u r f a c e a t t h e o u t s i d e r e g i o n of s u p e r c o n d u c t i n g gap, w h i l e t h e midgap c o n d u c t a n c e i s i n d e p e n d e n t o f t h e t i p d i s t a n c e . The s u p e r c o n d u c t i n g gap p a r a m e t e r was o b t a i n e d as A=26±4 meV, and c o r r e s p o n d i n g l y 2~/kT¢=7±1. T e m p e r a t u r e d e p e n d e n c e of gap p a r a m e t e r i s c o n s i s t e n t w i t h t h e BCS. I n v a r i o u s h i g h T¢ s u p e r c o n d u c t i n g o x i d e s , larger
v a l u e s of e n e r g y gap A t h a n t h e BCS
prediction tunneling
have been r e p o r t e d studies.
wide s c a t t e r .
by s e v e r a l
These v a l u e s have shown a
For i n s t a n c e ,
v a l u e s of 2h/ksT=
c o n d u c t a n c e i s s t r o n g l y enhanced a t t h e o u t s i d e r e g i o n o f t h e gap with d e c r e a s i n g t h e t i p distance.
S i m i l a r b e h a v i o r was r e p o r t e d by
K i t a z a w a et a t .
[ 4 ] . They c l a i m e d t h a t the f i t
of t h e c o n d u c t a n c e curve t o Dynes' e q u a t i o n a s ,
from 6 t o 11 have been a s s i g n e d to Bi2SrzCaCu=08-8 [ 1 - 3 ] . tunneling
H e r e , we r e p o r t t h e
{1)
s p e c t r o s c o p y measurement p e r f o r m e d on
Bi2Sr2CaCu20o-~ by use o f a low t e m p e r a t u r e Single crystals
o f Bi2Sr2CaCuz08-~ were u s c d
s t u d y . The s u p e r c o n d u c t i n g t r a n s i t i o n
t e m p e r a t u r e was d e t e r m i n e d a s T~=87 K from t h e midpoint of the resistive
transition.
(dI/dV)-V characteristics
were measured in t h e
temperature at different
is not suitable. The origin of such a distancedependent tunneling conductance has not been
STM { S c a n n i n g T u n n e l i n g M i c r o s c o p e } . in t h i s
Ns(E)=Re(E-iF)/I(E-iF)z-AzI*/ZN~(E),
Tunneling
explained yet. The tunneling through Bi~O layer
may be Fartly r e s p o n s i b l e . In our present result, however, the inner gap conductance is almost ~ndependent of the tip
r a n g e from 4 . 2 K t o room t e m p e r a t u r e positions
>
s u r f a c e by s c a n n i n g t h e t i p . F i g u r e 1 shows d i f f e r e n t i a l c u r v e s o b t a i n e d by v a r y i n g
l:3r~=~
BiaSr~Cu2Oa. 6 6-2K
of the cleaved sample
6n
conductance
t h e d i s t a n c e betweel;
the tip
and t h e sample s u r f a c e .
We c o n t r o l l e d
the tip
d i s t a n c e by k e e p i n g t h e t u n n e l i n g
current
I constant for a constant bias voltage
of 150 mV, which i s much larger than the superconducting energy gap, before the voltage
uJ
.
..1
0
-200
normalized at 30 mV. We can see the sharp drop of the differential conductance corresponding to the superconducting gap. We also find that the
"
0
200
VOLTAGE ( mV )
sweep. As the current increases, the tip distance becomes narrowed. Each curve is
8aA
N
F i g . 1. d I / d Y vs V c h a r a c t e r i s t i c s at various t i p d i s t a n c e s . The t u n n e l i n g c u r r e n t I d e n o t e s t h e t i p d i s t a n c e as d e s c r i b e d i n t h e t e x t . The d o t t e d c u r v e r e p r e s e n t s t h e f i t o f t h e curve a t I=10 nA t o e q . ( 1 ) .
0921-4534/91/503.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
If. khimura et al, / Scanning tunneling spectroscopy on BizSrzCaCu20 ~
942
d i s t a n c e as seen in Fig. 1 and i s considered t o show an i n t r i n s i c p r o p e r t y of the superconduct-
........ ¢'q
determine the gap parameter as &=29 meV from the
<
s e p a r a t i o n between p o i n t s , where the conductance
I-.
the t i p d i s t a n c e . This value o f h corresponds to
.
..1
i v i t y of t h i s m a t e r i a l . We can, t h e r e f o r e ,
is 70~ of i t s maximum In v a l u e , i r r e s p e c t i v e of
... . . . .
<
BltSrmCoCu2Oa.8
=
•
|
m
]
O.S TITc
t h a t o b t a i n e d by a f i t of the (dI/dV)-V curve at I=10 nA to eq. (1). Although t h i s determination of ~ i s not based on any s p e c i f i e d theory, i t gives p r o b a b l y the reasonable value for the superconducting gap parameter.
Fig. 3. Normalized gap parameter &(T)/h(4.2 K) vs reduced temperature T/To, where &(4.2 K)=26 meV. The s o l i d curve r e p r e s e n t s the BCS prediction.
As we r e p o r t e d p r e v i o u s l y [5], the (dI/dV)-Y curve shows the s p a t i a l v a r i a t i o n . The gap
parameter A i s shown in Fig. 3. Because of
parameter & v a r i e s over a l e n g t h about 10 rim.
thermal expansions of STM u n i t m a t e r i a l s , we
This comes from both the s h o r t coherence l e n g t h
could h a r d l y keep the t i p a t the f i x e d p o s i t i o n
and the lnhomogeneity of the sample, and is one
of the sample s u r f a c e as the t e m p e r a t u r e
of the causes f o r s c a t t e r e d v a l u e s reported. As
changes. A c c o r d i n g l y , the s p a t i a l v a r i a t i o n of h
a result,
described above is also included in Fig. 3. At
the gap parameter was determined as
~=26±4 meV, and c o r r e s p o n d i n g l y 2a/kT¢=7±l. This
low temperature the gap parameter i s almost
i n d i c a t e s t h a t the s u p e r c o n d u c t i v i t y in
constant and decreases as the temperature
Bi2Sr=CaCu=08-~ is t h a t of s t r o n g - c o u p l i n g .
r a i s e s . The temperature dependence of a is
As shown in Fig. 2, (dI/dV)-Y curve v a r i e s with the temperature. As the temperature r a i s e s ,
q u a l i t a t i v e l y c o n s i s t e n t with the BCS theory. in summary, t u n n e l i n g conductance was
the s t r u c t u r e of the energy gap becomes smeared
i n v e s t i g a t e d on s i n g l e c r y s t a l s of
and d i s a p p e a r s at about To.
Bi=Sr=CaCu=Os-¢ with the STY. Although the
The temperature dependence of the gap
conductance curve v a r i e s with the t i p distance from the sample s u r f a c e at the o u t s i d e region of the s u p e r c o n d u c t i n g gap, the midgap conductance is independent of the t i p d i s t a n c e . The obtained gap parameter i n d i c a t e s the s t r o n g - c o u p l i n g s u p e r c o n d u c t i v i t y . The temperature dependence of the gap parameter is explained with the BCS.
:
:>
49K
'¢1 "o
References [1] T. Ekino and J. Akimitsu, Phys. Rev. B40, 6902 (1989) [2] H. I k u t a , A. Maeda, K. Uchinokura and lauaaa,
- 200
0
VOLTAGE (mY)
200
Fig. 2. dI/dY vs V c h a r a c t e r i s t i c s at v a r i o u s temperatures.
+~rl.
a~,
•
•
tl~O
.
~1
,
L X U O 0
(1988) [3] N. Miyakawa, V. Shimada, T. Kido and N. Tsuda, J. Phys. S~c. Jpn. 59, 2473 (1990} [4] K. Kitazawa, T. Hasegawa and M. Nantoh, Research Report on Mechanism o f S u p e c c o n d u c l i v i t y I, 287 (1991) [5] K. Ich~mura, K. Nomura, F. Minami and S. Takekawa, J. Phys,: Condens. Matter 2, 9961 (1990)
SCANNING TUNNELING SPECTROSCOPY ON Bi=Sr=CaCu=Os-~
K o i c h i ICHIMURA*, K a z u s h i g e NO~URA*, F u j i o MINAMI** and S h u n j i TAKEKAWA *°* *
~ H.
Department o f P h y s i c s , Hokkaldo U n i v e r s i t y , Sapporo 060, J a p a n Rc:zareh Institute of Applied Electricity, Sapporo 060, J a p a n National Institute f o r R e s e a r c h in I n o r g a n i c M a t e r i a l s , Tsukuba, I b a r a k i 305, J a p a n
T u n n e l i n g measurement was c a r r i e d o u t on s i n g l e c r y s t a l s o f s u p e r c o n d u e t i o n g o x i d e Bi=Sr=CaCu:Os_~ w i t h u s e o f t h e STM. T u n n e l i n g c o n d u c t a n c e v a r i e s w i t h t h e d i s t a n c e between t h e t i p and t h e sample s u r f a c e a t t h e o u t s i d e r e g i o n of s u p e r c o n d u c t i n g gap, w h i l e t h e midgap c o n d u c t a n c e i s i n d e p e n d e n t o f t h e t i p d i s t a n c e . The s u p e r c o n d u c t i n g gap p a r a m e t e r was o b t a i n e d as A=26±4 meV, and c o r r e s p o n d i n g l y 2~/kT¢=7±1. T e m p e r a t u r e d e p e n d e n c e of gap p a r a m e t e r i s c o n s i s t e n t w i t h t h e BCS. I n v a r i o u s h i g h T¢ s u p e r c o n d u c t i n g o x i d e s , larger
v a l u e s of e n e r g y gap A t h a n t h e BCS
prediction tunneling
have been r e p o r t e d studies.
wide s c a t t e r .
by s e v e r a l
These v a l u e s have shown a
For i n s t a n c e ,
v a l u e s of 2h/ksT=
c o n d u c t a n c e i s s t r o n g l y enhanced a t t h e o u t s i d e r e g i o n o f t h e gap with d e c r e a s i n g t h e t i p distance.
S i m i l a r b e h a v i o r was r e p o r t e d by
K i t a z a w a et a t .
[ 4 ] . They c l a i m e d t h a t the f i t
of t h e c o n d u c t a n c e curve t o Dynes' e q u a t i o n a s ,
from 6 t o 11 have been a s s i g n e d to Bi2SrzCaCu=08-8 [ 1 - 3 ] . tunneling
H e r e , we r e p o r t t h e
{1)
s p e c t r o s c o p y measurement p e r f o r m e d on
Bi2Sr2CaCu20o-~ by use o f a low t e m p e r a t u r e Single crystals
o f Bi2Sr2CaCuz08-~ were u s c d
s t u d y . The s u p e r c o n d u c t i n g t r a n s i t i o n
t e m p e r a t u r e was d e t e r m i n e d a s T~=87 K from t h e midpoint of the resistive
transition.
(dI/dV)-V characteristics
were measured in t h e
temperature at different
is not suitable. The origin of such a distancedependent tunneling conductance has not been
STM { S c a n n i n g T u n n e l i n g M i c r o s c o p e } . in t h i s
Ns(E)=Re(E-iF)/I(E-iF)z-AzI*/ZN~(E),
Tunneling
explained yet. The tunneling through Bi~O layer
may be Fartly r e s p o n s i b l e . In our present result, however, the inner gap conductance is almost ~ndependent of the tip
r a n g e from 4 . 2 K t o room t e m p e r a t u r e positions
>
s u r f a c e by s c a n n i n g t h e t i p . F i g u r e 1 shows d i f f e r e n t i a l c u r v e s o b t a i n e d by v a r y i n g
l:3r~=~
BiaSr~Cu2Oa. 6 6-2K
of the cleaved sample
6n
conductance
t h e d i s t a n c e betweel;
the tip
and t h e sample s u r f a c e .
We c o n t r o l l e d
the tip
d i s t a n c e by k e e p i n g t h e t u n n e l i n g
current
I constant for a constant bias voltage
of 150 mV, which i s much larger than the superconducting energy gap, before the voltage
uJ
.
..1
0
-200
normalized at 30 mV. We can see the sharp drop of the differential conductance corresponding to the superconducting gap. We also find that the
"
0
200
VOLTAGE ( mV )
sweep. As the current increases, the tip distance becomes narrowed. Each curve is
8aA
N
F i g . 1. d I / d Y vs V c h a r a c t e r i s t i c s at various t i p d i s t a n c e s . The t u n n e l i n g c u r r e n t I d e n o t e s t h e t i p d i s t a n c e as d e s c r i b e d i n t h e t e x t . The d o t t e d c u r v e r e p r e s e n t s t h e f i t o f t h e curve a t I=10 nA t o e q . ( 1 ) .
0921-4534/91/503.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
If. khimura et al, / Scanning tunneling spectroscopy on BizSrzCaCu20 ~
942
d i s t a n c e as seen in Fig. 1 and i s considered t o show an i n t r i n s i c p r o p e r t y of the superconduct-
........ ¢'q
determine the gap parameter as &=29 meV from the
<
s e p a r a t i o n between p o i n t s , where the conductance
I-.
the t i p d i s t a n c e . This value o f h corresponds to
.
..1
i v i t y of t h i s m a t e r i a l . We can, t h e r e f o r e ,
is 70~ of i t s maximum In v a l u e , i r r e s p e c t i v e of
... . . . .
<
BltSrmCoCu2Oa.8
=
•
|
m
]
O.S TITc
t h a t o b t a i n e d by a f i t of the (dI/dV)-V curve at I=10 nA to eq. (1). Although t h i s determination of ~ i s not based on any s p e c i f i e d theory, i t gives p r o b a b l y the reasonable value for the superconducting gap parameter.
Fig. 3. Normalized gap parameter &(T)/h(4.2 K) vs reduced temperature T/To, where &(4.2 K)=26 meV. The s o l i d curve r e p r e s e n t s the BCS prediction.
As we r e p o r t e d p r e v i o u s l y [5], the (dI/dV)-Y curve shows the s p a t i a l v a r i a t i o n . The gap
parameter A i s shown in Fig. 3. Because of
parameter & v a r i e s over a l e n g t h about 10 rim.
thermal expansions of STM u n i t m a t e r i a l s , we
This comes from both the s h o r t coherence l e n g t h
could h a r d l y keep the t i p a t the f i x e d p o s i t i o n
and the lnhomogeneity of the sample, and is one
of the sample s u r f a c e as the t e m p e r a t u r e
of the causes f o r s c a t t e r e d v a l u e s reported. As
changes. A c c o r d i n g l y , the s p a t i a l v a r i a t i o n of h
a result,
described above is also included in Fig. 3. At
the gap parameter was determined as
~=26±4 meV, and c o r r e s p o n d i n g l y 2a/kT¢=7±l. This
low temperature the gap parameter i s almost
i n d i c a t e s t h a t the s u p e r c o n d u c t i v i t y in
constant and decreases as the temperature
Bi2Sr=CaCu=08-~ is t h a t of s t r o n g - c o u p l i n g .
r a i s e s . The temperature dependence of a is
As shown in Fig. 2, (dI/dV)-Y curve v a r i e s with the temperature. As the temperature r a i s e s ,
q u a l i t a t i v e l y c o n s i s t e n t with the BCS theory. in summary, t u n n e l i n g conductance was
the s t r u c t u r e of the energy gap becomes smeared
i n v e s t i g a t e d on s i n g l e c r y s t a l s of
and d i s a p p e a r s at about To.
Bi=Sr=CaCu=Os-¢ with the STY. Although the
The temperature dependence of the gap
conductance curve v a r i e s with the t i p distance from the sample s u r f a c e at the o u t s i d e region of the s u p e r c o n d u c t i n g gap, the midgap conductance is independent of the t i p d i s t a n c e . The obtained gap parameter i n d i c a t e s the s t r o n g - c o u p l i n g s u p e r c o n d u c t i v i t y . The temperature dependence of the gap parameter is explained with the BCS.
:
:>
49K
'¢1 "o
References [1] T. Ekino and J. Akimitsu, Phys. Rev. B40, 6902 (1989) [2] H. I k u t a , A. Maeda, K. Uchinokura and lauaaa,
- 200
0
VOLTAGE (mY)
200
Fig. 2. dI/dY vs V c h a r a c t e r i s t i c s at v a r i o u s temperatures.
+~rl.
a~,
•
•
tl~O
.
~1
,
L X U O 0
(1988) [3] N. Miyakawa, V. Shimada, T. Kido and N. Tsuda, J. Phys. S~c. Jpn. 59, 2473 (1990} [4] K. Kitazawa, T. Hasegawa and M. Nantoh, Research Report on Mechanism o f S u p e c c o n d u c l i v i t y I, 287 (1991) [5] K. Ich~mura, K. Nomura, F. Minami and S. Takekawa, J. Phys,: Condens. Matter 2, 9961 (1990)