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Superconducting and normal states of Ba1−xKxBiO3 studied by high quality thin films
Physica C 185-189 (1991) 1343-1344 North-Holland
Supercond~ting
a n d Normal S t a t e s
of Ba,-.KsBiOa
Studied by High Quality Thin Films
H. SaiD*, T. Ido, S.Tajima*, M.Yosida +, K.Tanabe .+ and S. Uchida Dept. o f A p p l i e d P h y s i c s , U n i v . of Tokyo, Bunkyo-ku, Tokyo 113, Japan S u p e r c o n d u c t i v i t y R e s e a r c h L a b o r a t o r y , I n t e r n a t i o n a l S u p e r c o n d u c t i v i t y Technology C e n t e r , K o t o - k u , Tokyo 135, Japan* NTT Applied Electronics Laboratories, Tokai, Ibaraki 319-II, Japan,*
The physical properties of Ba~-,K, BiO~ including transport coefficients, optical spectra, Pa®~n spectra, the upper critical field and superconducting energy gap were measured on thin films. From the experimental results, we firmly conclude that the Bal-xK.BiO3 is close to a band metal in its metallic phase and the superconductivity is of 8CS-type.
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 temperature i n Bal-,KxBiOa i s now r e a c h i n g 40K1. This is t h e h i g h e s t Tc o b s e r v e d s o f a r e x c e p t f o r t h o s e i n the Cu-based oxides. Here we r e p o r t an e x tensive s t u d y of p h y s i c a l p r o p e r t i e s i n t h i s system u s i n g s p u t t e r e d t h i n f i l m s . The detc.?s of film synthesis have been described elsewhere 2,4. Although the resistivity of the Ba0.6K0.4BiO3 thin films was sensitive to slight difference of growth conditions as shown in figure i, all of the films exhibited superconducting transition between 20K and 23K with the transition width narrower than 0.5K. The current-voltage characteristics in the superconducting state had no feature due to grain boundary Josephson junctions, indicating the negligible effect of grain boundaries to the electrical conduction in these thin films. The sample dependence of resistivity probably due to difference in the density of scattering centers such as Ba-Bi anti-site defects. In our best sample, the resistivity was 2 4 0 ~ c m at room temperature and 140 ~ cm at 30K which are the lowest among the values so far reported. For physical measurement, we used films with the room temperature resistivity 500 ~ cm which were most reproducibly obtained. Figure 2 shows the reflectivity spectrum for the Ba0.6K0.4Bi03 thin film in the superconducting state at 5K divided by that in the normal state at 30K. The spectrum has a clear peak at 60cm -I. Following the result of Mattis-Bardeen calculation s, we obtained the
2.5
I
I
~"'-~-~-'-.
I
I
I
Bao.6Ko.,BiOsThin Film
2 E b .... ~....... •"
>-"
...
.....
l "
d
0.5 •,
1 50
~ [00
I I 150 200 Temperature(K)
I 250
300
Fig.1 The e l e c t r i c a l resistivity for the Bao.6Ko.4BiOa t h i n f i l m s , f a b r i c a t e d under various conditions.
superconducting gap d =3.7 ± 0.3meV from the peak position. The inset of figure 2 is the current-voltage characteristics for a SIN-type tunnel junction with Au counter electrode fabricated on a photolithography and ion milling. The zerobias conductanc~ of this junction in the superconducting state was as small as 2% of that in the normal state. The gap estimated from this I-V curve is 3.SmeV and coincides with the result of the infrared measurement. Using Tc=22.5K determined by resistivity measurement,
*Present address: NTT Basic Research Laboratories, Midori-cho,
Musashino-Shi, Tokyo 180, Japan
0921~534Dl/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights ~sen, ed.
H. Sato et aL / Superconducting and normal states of Baz.xKxBiO~
1344
t.l(
Bao.61
1.01 , ..,,..
0.98] T=5,0K 0.95 u--,.--,--,--,..-~ 30 80
130
180
Wave Number(/cm) F i g . 2 The r e f l e c t i v i t y a t 5K d i v i d e d by t h a t a t 30K f o r t h e Bao.aKo.4BiO3 t h i n f i l m . The inset is the I-V curve for a Bao.6Ko.4BiO3/Au tunnel junction on the thin film. The division of the horizontal axis is 2mY.
we obtained 2 d/kaTe =3.8±0.3, which issmallet than those in AI5 superconductors 6. Similar values have been reported for Ba0.sK0.4BiO3 sintered samplesT,9. Ks shown in figure 3, the upper critical magnetic field was determined by resistivity measurement in magnetic fields. The GL coherence length ~cc=5.3±0.1nmwas obtained. The resistive transition remains sharp up to the field of 8T, in clear contrast to that observed for cuprate superconductors. In the normal state, the Ba0.6K0.4Bi03 thin film exhibited a negative Hall coefficient -3xl0"4cm3/C and a Drude-type plasma reflectivlty, confirming the previous result2,3. Moreover the Raman spectrum of the thin film indicates that no breathing-mode lattice distortion exists in the metallic phase even in microscopic scale. Based o n these experimental results for normal state properties, here we put an assumption that the electronic structure in Bao.6Ko.4Bi03 can be descril~ad by band theory. Using the Fermi velocity extracted from the result of band calculationS, we can estimate the meanfree-path L=0.69nm from the d and ~eL values obtained here. On the other hand L=0,64nm and L=0.74nm are estimated from the measured resistivity and the scattering rate in the plasma spectrum respectively. These values estimated from independent measurements coincide well each other. This consistency between normal
state and superconducting parameters indicates the validity of both the above assumption and the discription of the superconductivity in this system by the BCS theory. Only t h e f i l m s w i t h t h e room t e m p e r a t u r e r e s i s t i v i t y lower t h a n 700 ~ Q c m e x h i b i t m e t a l l i c b e h a v i o r of r e s i s t i v i t y i n f i g u r e I . Based on t h e above a s s u m p t i o n , t h i s r e s i s t i v i t y v a l u e 700 .~cm c o r r e s p o n d s t o the m e a n - f r e e - p a t h close to the lattice parameter. This is c o n sistent with the general trends that the lower bound of mean-free-path in the Boltzmann theory is set by the lattice parameter. In summary, we performed measurement of superconducting and normal state properties on Bal-xI~Bi03 thin films. From the experimental results, we conclude that Bai-xKxBi03 is close to a band metal in its metallic phase and the superconductivity is of BCS-type.
Bao,6Ko.4BiO3 Thin Film
5
"K 3 >
.~
2 1 0
~
10
15
20
25
30
Temperature(K)
Fig.3 The resistive superconducting transition in magnetic fields up to 8T for the Bao.6Ko.4Bi03 t h i n film.
REFERENCES I . D,Tseng and E.Ruckenstein, J.Mater.Res.5 (1990)742. 2. H.Sato et el., Nature 338(1989)241. 3. H.Sato eta!., Physics C !62-164(1989)1!2!. 4. H,Sato at al., Physics C 169(1990)391. 5. D,C.Mattis and J-Bardeen, Phys.Rev,lll(1958) 412. 6. F,Marsiglio and 3.P.Carbotte, Phys.Rev.B33 (1986)6141. 7. Z.Schlesinger et el., Phys.Rev.B40(1989) 6862. 8. L,F.Mattheiss and D.R.Hamman, Phys.Rev.Lett. 60(1988)2681. 9. Q,Huang et el., Nature 347(1990)369.
Supercond~ting
a n d Normal S t a t e s
of Ba,-.KsBiOa
Studied by High Quality Thin Films
H. SaiD*, T. Ido, S.Tajima*, M.Yosida +, K.Tanabe .+ and S. Uchida Dept. o f A p p l i e d P h y s i c s , U n i v . of Tokyo, Bunkyo-ku, Tokyo 113, Japan S u p e r c o n d u c t i v i t y R e s e a r c h L a b o r a t o r y , I n t e r n a t i o n a l S u p e r c o n d u c t i v i t y Technology C e n t e r , K o t o - k u , Tokyo 135, Japan* NTT Applied Electronics Laboratories, Tokai, Ibaraki 319-II, Japan,*
The physical properties of Ba~-,K, BiO~ including transport coefficients, optical spectra, Pa®~n spectra, the upper critical field and superconducting energy gap were measured on thin films. From the experimental results, we firmly conclude that the Bal-xK.BiO3 is close to a band metal in its metallic phase and the superconductivity is of 8CS-type.
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 temperature i n Bal-,KxBiOa i s now r e a c h i n g 40K1. This is t h e h i g h e s t Tc o b s e r v e d s o f a r e x c e p t f o r t h o s e i n the Cu-based oxides. Here we r e p o r t an e x tensive s t u d y of p h y s i c a l p r o p e r t i e s i n t h i s system u s i n g s p u t t e r e d t h i n f i l m s . The detc.?s of film synthesis have been described elsewhere 2,4. Although the resistivity of the Ba0.6K0.4BiO3 thin films was sensitive to slight difference of growth conditions as shown in figure i, all of the films exhibited superconducting transition between 20K and 23K with the transition width narrower than 0.5K. The current-voltage characteristics in the superconducting state had no feature due to grain boundary Josephson junctions, indicating the negligible effect of grain boundaries to the electrical conduction in these thin films. The sample dependence of resistivity probably due to difference in the density of scattering centers such as Ba-Bi anti-site defects. In our best sample, the resistivity was 2 4 0 ~ c m at room temperature and 140 ~ cm at 30K which are the lowest among the values so far reported. For physical measurement, we used films with the room temperature resistivity 500 ~ cm which were most reproducibly obtained. Figure 2 shows the reflectivity spectrum for the Ba0.6K0.4Bi03 thin film in the superconducting state at 5K divided by that in the normal state at 30K. The spectrum has a clear peak at 60cm -I. Following the result of Mattis-Bardeen calculation s, we obtained the
2.5
I
I
~"'-~-~-'-.
I
I
I
Bao.6Ko.,BiOsThin Film
2 E b .... ~....... •"
>-"
...
.....
l "
d
0.5 •,
1 50
~ [00
I I 150 200 Temperature(K)
I 250
300
Fig.1 The e l e c t r i c a l resistivity for the Bao.6Ko.4BiOa t h i n f i l m s , f a b r i c a t e d under various conditions.
superconducting gap d =3.7 ± 0.3meV from the peak position. The inset of figure 2 is the current-voltage characteristics for a SIN-type tunnel junction with Au counter electrode fabricated on a photolithography and ion milling. The zerobias conductanc~ of this junction in the superconducting state was as small as 2% of that in the normal state. The gap estimated from this I-V curve is 3.SmeV and coincides with the result of the infrared measurement. Using Tc=22.5K determined by resistivity measurement,
*Present address: NTT Basic Research Laboratories, Midori-cho,
Musashino-Shi, Tokyo 180, Japan
0921~534Dl/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights ~sen, ed.
H. Sato et aL / Superconducting and normal states of Baz.xKxBiO~
1344
t.l(
Bao.61
1.01 , ..,,..
0.98] T=5,0K 0.95 u--,.--,--,--,..-~ 30 80
130
180
Wave Number(/cm) F i g . 2 The r e f l e c t i v i t y a t 5K d i v i d e d by t h a t a t 30K f o r t h e Bao.aKo.4BiO3 t h i n f i l m . The inset is the I-V curve for a Bao.6Ko.4BiO3/Au tunnel junction on the thin film. The division of the horizontal axis is 2mY.
we obtained 2 d/kaTe =3.8±0.3, which issmallet than those in AI5 superconductors 6. Similar values have been reported for Ba0.sK0.4BiO3 sintered samplesT,9. Ks shown in figure 3, the upper critical magnetic field was determined by resistivity measurement in magnetic fields. The GL coherence length ~cc=5.3±0.1nmwas obtained. The resistive transition remains sharp up to the field of 8T, in clear contrast to that observed for cuprate superconductors. In the normal state, the Ba0.6K0.4Bi03 thin film exhibited a negative Hall coefficient -3xl0"4cm3/C and a Drude-type plasma reflectivlty, confirming the previous result2,3. Moreover the Raman spectrum of the thin film indicates that no breathing-mode lattice distortion exists in the metallic phase even in microscopic scale. Based o n these experimental results for normal state properties, here we put an assumption that the electronic structure in Bao.6Ko.4Bi03 can be descril~ad by band theory. Using the Fermi velocity extracted from the result of band calculationS, we can estimate the meanfree-path L=0.69nm from the d and ~eL values obtained here. On the other hand L=0,64nm and L=0.74nm are estimated from the measured resistivity and the scattering rate in the plasma spectrum respectively. These values estimated from independent measurements coincide well each other. This consistency between normal
state and superconducting parameters indicates the validity of both the above assumption and the discription of the superconductivity in this system by the BCS theory. Only t h e f i l m s w i t h t h e room t e m p e r a t u r e r e s i s t i v i t y lower t h a n 700 ~ Q c m e x h i b i t m e t a l l i c b e h a v i o r of r e s i s t i v i t y i n f i g u r e I . Based on t h e above a s s u m p t i o n , t h i s r e s i s t i v i t y v a l u e 700 .~cm c o r r e s p o n d s t o the m e a n - f r e e - p a t h close to the lattice parameter. This is c o n sistent with the general trends that the lower bound of mean-free-path in the Boltzmann theory is set by the lattice parameter. In summary, we performed measurement of superconducting and normal state properties on Bal-xI~Bi03 thin films. From the experimental results, we conclude that Bai-xKxBi03 is close to a band metal in its metallic phase and the superconductivity is of BCS-type.
Bao,6Ko.4BiO3 Thin Film
5
"K 3 >
.~
2 1 0
~
10
15
20
25
30
Temperature(K)
Fig.3 The resistive superconducting transition in magnetic fields up to 8T for the Bao.6Ko.4Bi03 t h i n film.
REFERENCES I . D,Tseng and E.Ruckenstein, J.Mater.Res.5 (1990)742. 2. H.Sato et el., Nature 338(1989)241. 3. H.Sato eta!., Physics C !62-164(1989)1!2!. 4. H,Sato at al., Physics C 169(1990)391. 5. D,C.Mattis and J-Bardeen, Phys.Rev,lll(1958) 412. 6. F,Marsiglio and 3.P.Carbotte, Phys.Rev.B33 (1986)6141. 7. Z.Schlesinger et el., Phys.Rev.B40(1989) 6862. 8. L,F.Mattheiss and D.R.Hamman, Phys.Rev.Lett. 60(1988)2681. 9. Q,Huang et el., Nature 347(1990)369.