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Phonon structures in the tunneling conductance of Bi-cuprates
Physiea C 235-240 (1994) 1889-1890
PHYSICA
North-Holland
Phonon structures in the tunneling conductance of Bi-cuprates N. Tsuda, E. Arai, A. Mottate, T. Kogawa, N. Numasaki, and D. Shimada Department of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-kax, Tokyo 162, Japan
Reproducibility of phonon .,.truclures in the tunneling conductance of Bi-cuprates is demonstrated. Temperature dependence of the gap shows a downward deviation from the BCS relation.
1. INTRODUCTION Phonon structures have been observed in the t u n n e l i n g conductance of Bi 2 Sr 2-x La.x..CuO.~ . (Bi2201La),' BizSrzCaCuzOs(Bi2212)," ~ and BizSr2CazCu3Olo(Bi2223)a and we presented a phonon mechanism of h i g h - T superconductivity in which the most important concept is a number of effective phonon modes.4"s However, the identification as p h o n o n structures has not always been agreed. Therefore it will be necessary to present more tunneling data. In this letter, their reproduobility and a peculiar temperature dependence of the gap will be shown.
-~ ~ ~, -~ 0 0 0
2. EXPERIMENTAL Single crystals of BizSrzCaCu I 96COo0408 (Bi2212Co) and Bi2212, and smtered materials of Bi2201La and Bi2223 were prepared. Tunnehng junctions were: point contact GaAs:B~2201La, natural-barrier Au:Bi2201La, evaporated SnO:_~ :Bi2212 and B12212Co, point contact GaAs: Bi2223 "~and natural-barrier Au:Bi2223. The BTK parameter U' of the SnO2_,:Bi2212 junction was 0 . ~ . ' ' Preparatxon ot. .,.. .u.p., . aLc~, . xabHt.atlVnC . . . . , . ~ _ o~t:j .u. m. . . tions and the measuring method are essentially the same with those already reported, ae-s The critical temperature T was determined from disappearing temperature of the tunneling structures. The gap (2A) was determined from a phonon zero-frequency .-) posfllon (,.A h)'17 or a separatmn between conductance peaks a[ the gap edge(2Avp) ~
0
40 80 (v- A/e)/mV
0
40 80 (V- A / e ) / m V
"
FIGURE 1. Second denvatwe d://dV 7 or -d2V/dl z was plotted against V - Ate. Measurir, g temperature and A/e are re&cared beslde each curve. Left slcle from the top down: SnO:.-x:Bi2212Co, GaAs •B12212, I SnOz-~:Bi22i2, and GFDS of Bi22127 Right side from the top down" natural--barrier Bi2201La, natural-barrier Bi2201La, nalural-bamer Bi2223, natural-barrier Bi2223, and GaAs: Bi2223. 3 Only for Bi2223, -d2V/dI 2 was plotted. Phoaon structures should appear as dips in all the curves m th~s presentat~ov. The 4th uppermost curve(Bi2223) was obtameCJby numerically dtfferenhating dV/dI while the 3rd uppermost curve(Bi2223) is the observed second denvatwe curve for which the effectwe modulation amplitude of the current is larger than the former.
0921-4534D4/$07 00 © 1994 - Elsevier Sctence B V All n g h t s reserved SSDi 092 i -4534(94)01511-2
N Z~uda et al,/Physwa C 235-240 (1994) 1889-1890
1890
3. RESULTS AND DISCUSSIONS Figure 1 shows dZl/dV z of Bi2201La, Bi2212 l, Bi2212Co, and -d2V/d/2 of Bi22233 together with the generalized p h o n o n density of states(GPDS) of Bi2212. 9 The abscissa ts not voltage V but V - Ne. For Bi2212, we identified the structures in d"I/dV ~- as phonon structures in the order parameter. L~-5'7 They will not be due to inelastic tunnelingl° for the reasons explained in [5]. The structures were observed even tot a weakbarrier-strength contact and an apparently broad gap-edge structure does not hinder the observatmn of phonon structures. GPDS is not known for Bi2201La and Bi2223, but the reproducibility and the displacement of the structures in parallel with A suggest that they are also the phonon structures. A was determined for Bi2212 and Bi2212Co. F ~ Bi2201La and Bi2223, A was determined at the lowest temperature, and th~ gap at higher temperalure was estimated by using the amount of the displacement of the structures. Thus determined gap is indicated by a symbol A pph" Figure 2 shows temperature dependence of Aph, App, and A .. As seen there, each reduced gap deviates froP~"the BCS one H to the smaller side at higher temperatures. Similar deviatmn was also L
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T/T FIGURE 2. Temperature dependence of the gap BI2201La: +(7 8, 29), x (7.6, 29), B~2212. [] (26, 85), ~ A(!9, 831, B~2212Co" O(15, 75), B~2223 V (04, 98)," O (28, 108), e ( 2 8 , 1(}8). A0/meV and /LK were parenthes~ed, ~here Ao Is half the gap at 0 K For Bi2223, &~was also plotted( O ). For other gaps, see the text. Solid line represen|s the BCS relahon Error m A is typically lmeV
reported for YBazCu307.12 Since the ox,des are slrong-coupling superconductors, such a dewallon is anomalous. The deviation ~s larger for Bi2212Co. It suggests some magnetic pair-breaking effect for the cause. For instance, a gapped spin exmtalion !3 will effectwely break the pair at higher temperalures. Another cause might be a d - w a v e order parameter. Jiang et al. predicted that the energy of the mammum in the density of states shows such a temperature dependence when the order parameter has a node. a4 At that time, the o b s e r v a t i o n of narrow phonon structures wtil be compatible with an anisotropic gap when the density of states is anisotropic in the normal state.
4. CONCLUSIONS Reproducibility of fine structures m the tunneling conductance was repeatedly demonstrated for Bi2201La, Bi2212, Bi2212Co and Bi2223. The structures will be the phonon structures. The gap showed a downward deviation from the BCS relation at higher temperatures.
REFERENCES 1. N. Miyakawa et aL: J. Phys. Soc. Jpn. 62(1993) 2445 and refs thereto. 2 S. l. Vedeneev e t a l : Phvslca C198(1992)47 ,rod refs thereto 3 M. Ohuchl et al.: Jpn. J. Appt. Phys. 32(1993) L251 and refs. thereto. a Y Shiina et aL : to be published in M a m f e s t a ttons o f the E l e c t r o n - P h o n o n btteractton (Proc. Second CINVESTAV Supercond. Syrup, ) ed. R. Baquero (World Scientific, Singapore, 1994), 5 D Shimada et at.: Z. Phys. B85(1991)7. 6 G E. Blonder et aL: Phys. Rev. B25(1982)4515. 7 D J. Scalapmo et al.: Phys. Rev. 148(1966)263, 8 Y Shnna ct al. : Physica C212(1993)173. 9 B. Renker et al • 7_. Phys B77(!989)65. 10 R S, Gonnetli et aL Phys. Rev,B49(1994)1480 ii J B,urdeen e t a l : Phvs Rev 108(1957)1175. 12 E PolluraketaL, Phvs Rex' B47(1993)5270. 13 J P,ossat-MJgnod e t a l Physica C185/189 (1991)86. t4 C Jlanget al Phvs Rex B47(1993)5325
PHYSICA
North-Holland
Phonon structures in the tunneling conductance of Bi-cuprates N. Tsuda, E. Arai, A. Mottate, T. Kogawa, N. Numasaki, and D. Shimada Department of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-kax, Tokyo 162, Japan
Reproducibility of phonon .,.truclures in the tunneling conductance of Bi-cuprates is demonstrated. Temperature dependence of the gap shows a downward deviation from the BCS relation.
1. INTRODUCTION Phonon structures have been observed in the t u n n e l i n g conductance of Bi 2 Sr 2-x La.x..CuO.~ . (Bi2201La),' BizSrzCaCuzOs(Bi2212)," ~ and BizSr2CazCu3Olo(Bi2223)a and we presented a phonon mechanism of h i g h - T superconductivity in which the most important concept is a number of effective phonon modes.4"s However, the identification as p h o n o n structures has not always been agreed. Therefore it will be necessary to present more tunneling data. In this letter, their reproduobility and a peculiar temperature dependence of the gap will be shown.
-~ ~ ~, -~ 0 0 0
2. EXPERIMENTAL Single crystals of BizSrzCaCu I 96COo0408 (Bi2212Co) and Bi2212, and smtered materials of Bi2201La and Bi2223 were prepared. Tunnehng junctions were: point contact GaAs:B~2201La, natural-barrier Au:Bi2201La, evaporated SnO:_~ :Bi2212 and B12212Co, point contact GaAs: Bi2223 "~and natural-barrier Au:Bi2223. The BTK parameter U' of the SnO2_,:Bi2212 junction was 0 . ~ . ' ' Preparatxon ot. .,.. .u.p., . aLc~, . xabHt.atlVnC . . . . , . ~ _ o~t:j .u. m. . . tions and the measuring method are essentially the same with those already reported, ae-s The critical temperature T was determined from disappearing temperature of the tunneling structures. The gap (2A) was determined from a phonon zero-frequency .-) posfllon (,.A h)'17 or a separatmn between conductance peaks a[ the gap edge(2Avp) ~
0
40 80 (v- A/e)/mV
0
40 80 (V- A / e ) / m V
"
FIGURE 1. Second denvatwe d://dV 7 or -d2V/dl z was plotted against V - Ate. Measurir, g temperature and A/e are re&cared beslde each curve. Left slcle from the top down: SnO:.-x:Bi2212Co, GaAs •B12212, I SnOz-~:Bi22i2, and GFDS of Bi22127 Right side from the top down" natural--barrier Bi2201La, natural-barrier Bi2201La, nalural-bamer Bi2223, natural-barrier Bi2223, and GaAs: Bi2223. 3 Only for Bi2223, -d2V/dI 2 was plotted. Phoaon structures should appear as dips in all the curves m th~s presentat~ov. The 4th uppermost curve(Bi2223) was obtameCJby numerically dtfferenhating dV/dI while the 3rd uppermost curve(Bi2223) is the observed second denvatwe curve for which the effectwe modulation amplitude of the current is larger than the former.
0921-4534D4/$07 00 © 1994 - Elsevier Sctence B V All n g h t s reserved SSDi 092 i -4534(94)01511-2
N Z~uda et al,/Physwa C 235-240 (1994) 1889-1890
1890
3. RESULTS AND DISCUSSIONS Figure 1 shows dZl/dV z of Bi2201La, Bi2212 l, Bi2212Co, and -d2V/d/2 of Bi22233 together with the generalized p h o n o n density of states(GPDS) of Bi2212. 9 The abscissa ts not voltage V but V - Ne. For Bi2212, we identified the structures in d"I/dV ~- as phonon structures in the order parameter. L~-5'7 They will not be due to inelastic tunnelingl° for the reasons explained in [5]. The structures were observed even tot a weakbarrier-strength contact and an apparently broad gap-edge structure does not hinder the observatmn of phonon structures. GPDS is not known for Bi2201La and Bi2223, but the reproducibility and the displacement of the structures in parallel with A suggest that they are also the phonon structures. A was determined for Bi2212 and Bi2212Co. F ~ Bi2201La and Bi2223, A was determined at the lowest temperature, and th~ gap at higher temperalure was estimated by using the amount of the displacement of the structures. Thus determined gap is indicated by a symbol A pph" Figure 2 shows temperature dependence of Aph, App, and A .. As seen there, each reduced gap deviates froP~"the BCS one H to the smaller side at higher temperatures. Similar deviatmn was also L
I
l
[
1
l
I
I
i
l
"2
1.0 ~
®
~
JI
"--.
v
<10"5 r-
o+ o
0 0, [.
00
I
I
,
ta \
t
0.5
I
q
I
1.0
T/T FIGURE 2. Temperature dependence of the gap BI2201La: +(7 8, 29), x (7.6, 29), B~2212. [] (26, 85), ~ A(!9, 831, B~2212Co" O(15, 75), B~2223 V (04, 98)," O (28, 108), e ( 2 8 , 1(}8). A0/meV and /LK were parenthes~ed, ~here Ao Is half the gap at 0 K For Bi2223, &~was also plotted( O ). For other gaps, see the text. Solid line represen|s the BCS relahon Error m A is typically lmeV
reported for YBazCu307.12 Since the ox,des are slrong-coupling superconductors, such a dewallon is anomalous. The deviation ~s larger for Bi2212Co. It suggests some magnetic pair-breaking effect for the cause. For instance, a gapped spin exmtalion !3 will effectwely break the pair at higher temperalures. Another cause might be a d - w a v e order parameter. Jiang et al. predicted that the energy of the mammum in the density of states shows such a temperature dependence when the order parameter has a node. a4 At that time, the o b s e r v a t i o n of narrow phonon structures wtil be compatible with an anisotropic gap when the density of states is anisotropic in the normal state.
4. CONCLUSIONS Reproducibility of fine structures m the tunneling conductance was repeatedly demonstrated for Bi2201La, Bi2212, Bi2212Co and Bi2223. The structures will be the phonon structures. The gap showed a downward deviation from the BCS relation at higher temperatures.
REFERENCES 1. N. Miyakawa et aL: J. Phys. Soc. Jpn. 62(1993) 2445 and refs thereto. 2 S. l. Vedeneev e t a l : Phvslca C198(1992)47 ,rod refs thereto 3 M. Ohuchl et al.: Jpn. J. Appt. Phys. 32(1993) L251 and refs. thereto. a Y Shiina et aL : to be published in M a m f e s t a ttons o f the E l e c t r o n - P h o n o n btteractton (Proc. Second CINVESTAV Supercond. Syrup, ) ed. R. Baquero (World Scientific, Singapore, 1994), 5 D Shimada et at.: Z. Phys. B85(1991)7. 6 G E. Blonder et aL: Phys. Rev. B25(1982)4515. 7 D J. Scalapmo et al.: Phys. Rev. 148(1966)263, 8 Y Shnna ct al. : Physica C212(1993)173. 9 B. Renker et al • 7_. Phys B77(!989)65. 10 R S, Gonnetli et aL Phys. Rev,B49(1994)1480 ii J B,urdeen e t a l : Phvs Rev 108(1957)1175. 12 E PolluraketaL, Phvs Rex' B47(1993)5270. 13 J P,ossat-MJgnod e t a l Physica C185/189 (1991)86. t4 C Jlanget al Phvs Rex B47(1993)5325