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Isotope effect in the organic superconductor κ-ET2Cu[N(CN)2]Br
PHIIIIIil
Physica C 185-189 (1991) 2663-2664 North-Holland
ISOTOPE EFFECT IN THE ORGANIC SUPERCONDUCTOR ~-ET2Cu[N(CN)2]Br Yasumoto TANAKA, Nobumori.KINOSHITA,Madoka TOKUMOTO and Hiroyuki *ANZAI Electrotechnical Laboratory, Umezono 1-1-4, Tsukuba, Ibaraki 305, Japan. *Himeji Institute of Technology, Harima Science Garden City 1479-1 Kanaji, Kamigori-cho, Ako-gun, Hyogo 678-12, Japan
In order to study the difference in the isotope shi;t of intramolecu!ar vibration between two organic superconductors which are ~:-ET2Cu[N(CN)2]Br and K-ET2Cu(NCS)2, infrared reftectivity spectra for both deuterated and undeuterated single crystals of the former were measured and compared with those for the latter. A difference was found in the frequency of C=C stretching mode coupled to the charge carriers, suggesting the difference in the electron-molecular vibration or the bare frequency of C=C stretching mode between the two superconductors. 1. INTRODUCTION The isotope effect plays an essential role in the study of the mechanism of superconductivity. We had reported a "normal" isotope effect in ~ET2Cu[N(CN)2]Br that is the critical temperature
ET2Cu(NCS)2 as shown in Fig. 1.5,6 The a axis and ¢ axis in the former conducting plane correspond to the ¢ axis and b axis of the latter respectively. Figure 2 shows the polarized reflectivity spectra. Comparing some reported spectra of ~ET2Cu(NCS)2, 7 we could not find any difference between the spectra of these two compounds except an intramolecu~ar v,br,~, , - ' : ~,,,. ~ T,. ,,,e most prominent intramolecular vibration is the totally
T¢ of the deuterated compound is lower than that of the undeuterated one 1 in contrast to the "inverse" isotope effect in ~-ET2Cu(NCS)22. Yamaji proposed that in organic superconducting charge transfer salts composed of "Fl'F-ana!og molecules, the attractive interaction between charge carries can be mediated by totally symmetric (ag mode) intramolecular vibrations as well as by the acoustic phonon. 3 We calculated the frequency shift of the intramolecular vibration and the electronmolecular vibration (e-my) coupling constant for both deuterated and undeuterated "FFF molecules and demonstrated that the "inverse" isotope effect could be explained in the framework of this model. 4 In this paper we report on the infrared s#ectroscopy of ~-ET2Cu[N(CN)2]Br and discuss the difference on the isotope effect on the intramolecular vibration between ~:ET2Cu[N(CN)2]Br and K-ET2Cu(NCS)2. 2. RESULTS AND DISCUSSION The packing pattern of the ET molecules of ~E T 2 C u [ N ( C N ) 2 ] B r resembles that of ~:-
-
I
" :::
a
:
C
,..'--. , - ~
"
C,~.~< ~
I
~ f /C ~~
b) I~
;7 . . . . .
, ~
7c
,
%~
~ ' ~~'S~
~ II
-~ ~/:'-~
C--<'--- ,_9 i
FIGURE 1 The packing pattern of the ET moRecuDes. (a) ~ET2Cu[N(CN)2]Br. (b) ~:-ET2Cu(NCS)2. In the right figures the molecules are projected to the conducting plane normal to the incident light.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rescrved.
Y. Tanaka et aL
2664
/ Isotope effect in the organic superconductor r-ET2Cu[N(CN)2]Br
symmetric C=C stretching mode activated by the e-mv coupling observed around 1200~1300 cm-1. This peak is obscured by antiresonance dips due to C-H bendings for the spectrum of the undeuterated sample and its frequency could not be determined. In the case of deuterated sample the frequency of C-D bendings are much lower than that of C=C stretching mode. Then the frequency of the C=C stretching mode can be determined. We obtained that its frequency is 1220_+7 cm -1 which seemed to be at least about 10 cm -I higher than that of Ic-ET2Cu(NCS)2 for 0.8
3. SUMMARY We found that the frequency of C--C stretching mode for deuterated =¢-ET2Cu[N(CN)2]Br is higher than that for deuterated ~:-ET2Cu(NCS)2. A possibility that this discrepancy has some influence on the isotope effect of these two compounds is discussed.
0.8
0.6
>I--
>
E//a
I--
i0.4
0.4
O
LL LU
0.2
rr"
la) 0.(
,
,
, , ~ r
2500 5000 FREQUENCY(1/cm)
o.o
2500 500, FREQUENCY(I/cm)
FIGU~.E 2 The polarized reflectivity spectra of EET2Cu[N(CN)2]Br between 350 "~nd 5500 cm-l. (a) deuterated sample. (b) undeuterated sample.
~" 800!
•
•
.
.
.
.
.
,
.
.
.
,
.
600 ! >~>_ 400 t-o
c~ Z O o
200 0 I300
.;! /
"
\
(b) i000 1200 I400 FREQUENCY(llcm)
E//a as shown in Fig. 3. This frequency depends on the e-mv coupling constant, the position of the mid-infrared band and the bare frequency which is the frequency in the absence of the e-my coupling. 8 For E//a the position of the midinfrared band is about 2400 cm-1 which is same as that of ~-ET2Cu(NCS)2. Then it is possible that the difference is either in the bare frequency or in the e-mv coupling of C=C stretching mode. We would like to point out that the difference in the isotope effect on T¢ may originate the difference in the intramolecular vibration.
i~00
FIGURE 3 The conductivity spectra between 800 and 1600 cm-1 with about 1 cm -1 resolution. (a)E//a for ~ - E T 2 C u [ N ( C N ) 2 ] B r (b) E//c for ~ET2Cu(NCS)2. Absolute value of the latter spectrum was changed so as to be easy to compare two spectra.
REFERENCES 1. M. Tokumoto, N. Kinoshita, Y. Tanaka & H. Anzai, J. Phys. Soc. Jpn. 60(1991)1424. 2. K. Oshima, H. Urayama, H. Yamochi & G. Saito, Synthetic Metals 27(1988)A473. 3. K. Yamaji, Soild State Commun. 61 (1987)413. K. Yamaji, Synthetic Metals 27(1988)A115. 4.M. Tokumoto, K. Murata, N. Kinoshita, K. Yamaji, H. Anzai, Y. Tanaka, Y. Hayakawa, K. Nagasaka & Y. Sugawara, Mol. Cryst Liq. Cryst. 181(1990)295, Y. Tanaka, N. Kinoshita, Y. Asai, M. Tokumoto, Y. Hayakawa, K. Nagasaka & Y. Sugawara, Synthetic Metals 42, 2231 (1991 ). 5. H. Urayama, H. Yamochi, G. Saito, K. Nozawa, T. Sugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto & J. Tanaka, Chem. Lett. (1988)55. 6.A.M. Kini, U. Geiser, H. H. Wang, K. D. Carlson, J. M. Williams, W. K. Kwok, K. G. Vandervoort, J. E. Thompson, D. L. St,Jpka, D. Jung & M.-H. Whangbo, /norg. Chem. 29(1990)2555. U. Geiser, A. J. Schultz, H. H. Wang, D. M. Watkins, D. L. Stupka, J. M. Williams, J. E. Schirber, D. L Overmyer, D. Jung, J. J. Novoa & M.-H. Whangbo, Physica O174, 475-486(1991 ). 7. K. Kornelsen, J. Eldddge, H. H. Wang & J. M. Williams, Solid State Commun. 74(1990)501 ,and references there~in. 8 M. J. Rice, Soiid State Commun. 31 (1979)93. M. J. Rice, Phys. Roy. Lett. 37(1976)36.
Physica C 185-189 (1991) 2663-2664 North-Holland
ISOTOPE EFFECT IN THE ORGANIC SUPERCONDUCTOR ~-ET2Cu[N(CN)2]Br Yasumoto TANAKA, Nobumori.KINOSHITA,Madoka TOKUMOTO and Hiroyuki *ANZAI Electrotechnical Laboratory, Umezono 1-1-4, Tsukuba, Ibaraki 305, Japan. *Himeji Institute of Technology, Harima Science Garden City 1479-1 Kanaji, Kamigori-cho, Ako-gun, Hyogo 678-12, Japan
In order to study the difference in the isotope shi;t of intramolecu!ar vibration between two organic superconductors which are ~:-ET2Cu[N(CN)2]Br and K-ET2Cu(NCS)2, infrared reftectivity spectra for both deuterated and undeuterated single crystals of the former were measured and compared with those for the latter. A difference was found in the frequency of C=C stretching mode coupled to the charge carriers, suggesting the difference in the electron-molecular vibration or the bare frequency of C=C stretching mode between the two superconductors. 1. INTRODUCTION The isotope effect plays an essential role in the study of the mechanism of superconductivity. We had reported a "normal" isotope effect in ~ET2Cu[N(CN)2]Br that is the critical temperature
ET2Cu(NCS)2 as shown in Fig. 1.5,6 The a axis and ¢ axis in the former conducting plane correspond to the ¢ axis and b axis of the latter respectively. Figure 2 shows the polarized reflectivity spectra. Comparing some reported spectra of ~ET2Cu(NCS)2, 7 we could not find any difference between the spectra of these two compounds except an intramolecu~ar v,br,~, , - ' : ~,,,. ~ T,. ,,,e most prominent intramolecular vibration is the totally
T¢ of the deuterated compound is lower than that of the undeuterated one 1 in contrast to the "inverse" isotope effect in ~-ET2Cu(NCS)22. Yamaji proposed that in organic superconducting charge transfer salts composed of "Fl'F-ana!og molecules, the attractive interaction between charge carries can be mediated by totally symmetric (ag mode) intramolecular vibrations as well as by the acoustic phonon. 3 We calculated the frequency shift of the intramolecular vibration and the electronmolecular vibration (e-my) coupling constant for both deuterated and undeuterated "FFF molecules and demonstrated that the "inverse" isotope effect could be explained in the framework of this model. 4 In this paper we report on the infrared s#ectroscopy of ~-ET2Cu[N(CN)2]Br and discuss the difference on the isotope effect on the intramolecular vibration between ~:ET2Cu[N(CN)2]Br and K-ET2Cu(NCS)2. 2. RESULTS AND DISCUSSION The packing pattern of the ET molecules of ~E T 2 C u [ N ( C N ) 2 ] B r resembles that of ~:-
-
I
" :::
a
:
C
,..'--. , - ~
"
C,~.~< ~
I
~ f /C ~~
b) I~
;7 . . . . .
, ~
7c
,
%~
~ ' ~~'S~
~ II
-~ ~/:'-~
C--<'--- ,_9 i
FIGURE 1 The packing pattern of the ET moRecuDes. (a) ~ET2Cu[N(CN)2]Br. (b) ~:-ET2Cu(NCS)2. In the right figures the molecules are projected to the conducting plane normal to the incident light.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights rescrved.
Y. Tanaka et aL
2664
/ Isotope effect in the organic superconductor r-ET2Cu[N(CN)2]Br
symmetric C=C stretching mode activated by the e-mv coupling observed around 1200~1300 cm-1. This peak is obscured by antiresonance dips due to C-H bendings for the spectrum of the undeuterated sample and its frequency could not be determined. In the case of deuterated sample the frequency of C-D bendings are much lower than that of C=C stretching mode. Then the frequency of the C=C stretching mode can be determined. We obtained that its frequency is 1220_+7 cm -1 which seemed to be at least about 10 cm -I higher than that of Ic-ET2Cu(NCS)2 for 0.8
3. SUMMARY We found that the frequency of C--C stretching mode for deuterated =¢-ET2Cu[N(CN)2]Br is higher than that for deuterated ~:-ET2Cu(NCS)2. A possibility that this discrepancy has some influence on the isotope effect of these two compounds is discussed.
0.8
0.6
>I--
>
E//a
I--
i0.4
0.4
O
LL LU
0.2
rr"
la) 0.(
,
,
, , ~ r
2500 5000 FREQUENCY(1/cm)
o.o
2500 500, FREQUENCY(I/cm)
FIGU~.E 2 The polarized reflectivity spectra of EET2Cu[N(CN)2]Br between 350 "~nd 5500 cm-l. (a) deuterated sample. (b) undeuterated sample.
~" 800!
•
•
.
.
.
.
.
,
.
.
.
,
.
600 ! >~>_ 400 t-o
c~ Z O o
200 0 I300
.;! /
"
\
(b) i000 1200 I400 FREQUENCY(llcm)
E//a as shown in Fig. 3. This frequency depends on the e-mv coupling constant, the position of the mid-infrared band and the bare frequency which is the frequency in the absence of the e-my coupling. 8 For E//a the position of the midinfrared band is about 2400 cm-1 which is same as that of ~-ET2Cu(NCS)2. Then it is possible that the difference is either in the bare frequency or in the e-mv coupling of C=C stretching mode. We would like to point out that the difference in the isotope effect on T¢ may originate the difference in the intramolecular vibration.
i~00
FIGURE 3 The conductivity spectra between 800 and 1600 cm-1 with about 1 cm -1 resolution. (a)E//a for ~ - E T 2 C u [ N ( C N ) 2 ] B r (b) E//c for ~ET2Cu(NCS)2. Absolute value of the latter spectrum was changed so as to be easy to compare two spectra.
REFERENCES 1. M. Tokumoto, N. Kinoshita, Y. Tanaka & H. Anzai, J. Phys. Soc. Jpn. 60(1991)1424. 2. K. Oshima, H. Urayama, H. Yamochi & G. Saito, Synthetic Metals 27(1988)A473. 3. K. Yamaji, Soild State Commun. 61 (1987)413. K. Yamaji, Synthetic Metals 27(1988)A115. 4.M. Tokumoto, K. Murata, N. Kinoshita, K. Yamaji, H. Anzai, Y. Tanaka, Y. Hayakawa, K. Nagasaka & Y. Sugawara, Mol. Cryst Liq. Cryst. 181(1990)295, Y. Tanaka, N. Kinoshita, Y. Asai, M. Tokumoto, Y. Hayakawa, K. Nagasaka & Y. Sugawara, Synthetic Metals 42, 2231 (1991 ). 5. H. Urayama, H. Yamochi, G. Saito, K. Nozawa, T. Sugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto & J. Tanaka, Chem. Lett. (1988)55. 6.A.M. Kini, U. Geiser, H. H. Wang, K. D. Carlson, J. M. Williams, W. K. Kwok, K. G. Vandervoort, J. E. Thompson, D. L. St,Jpka, D. Jung & M.-H. Whangbo, /norg. Chem. 29(1990)2555. U. Geiser, A. J. Schultz, H. H. Wang, D. M. Watkins, D. L. Stupka, J. M. Williams, J. E. Schirber, D. L Overmyer, D. Jung, J. J. Novoa & M.-H. Whangbo, Physica O174, 475-486(1991 ). 7. K. Kornelsen, J. Eldddge, H. H. Wang & J. M. Williams, Solid State Commun. 74(1990)501 ,and references there~in. 8 M. J. Rice, Soiid State Commun. 31 (1979)93. M. J. Rice, Phys. Roy. Lett. 37(1976)36.