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Cyclotron resonance measurements of organic conductor α-(BEDT-TTF)2KHg(SeCN)4
ELSEVIER
Synthetic
Metals86
(1997) 201 I-2012
CYCLOTRON RESONANCE MEASUREMENTS OF ORGANIC CONDUCTOR a-(BEDT-TTF)zKHg(SeCN)d H. Ohta, Y. Yamamoto, K. Akioka, M. Motokawaa, T. Sasakia and T. Fukasea Department of Physics, Kobe University, Rokkodai, Nada, Kobe 657, Japan “MR, Tohoku University, Katahira, Aoba, Sendai 980-77, Japan
Abstract a-(BEDT-TTF)2KHg(SeCN)4 is the new organic conductor which has been synthesized by Sasaki recently. Although it has similar crystal structure with (BEDT-TTF)pKHg(SCN) 4, it does not show antifenomagnetic like transition down to 0.5 K. Therefore the comparison of the physical properties of these two substances is interesting. The cyclotron resonance of K-Se salt has been measured in the millimeter and submillimeter wave regions with the pulsed magnetic field up to 15 T at 1.8 K, and we observed only one cyclotron resonance with the effective mass of 0.96 w. It can be attributed to a-orbit , and this effective mass turned out to be smaller than 2.0 m, obtained by SdH oscillation. The circular polarization measurement has also been performed and this resonance turned out to be hole active which is consistent with the band calculation. Keywords:
magnetotransport, cyclotron resonance, organic conductors based on radical cation and/or anion salts
1.
Introduction Recently the temperature dependence cyclotron resonance (CR) measurement of a-(BEDT-TTF)zKHg(SCN)d has been performed from 1.8 K to the temperature above the density wave transition TD=~ K [I]. Four resonances, two of which are consistent with the previous result obtained by Singleton et a1.[2], were observed at 1.8 K, but only one resonance corresponding to the effective mass of 0.97 m, remained above TJJ and we attributed this resonance to the closed a-orbit On the other hand, Sasaki et al. synthesized new organic conductor a-(BEDT-TTF)2KHg(SeCN)4 recently [3]. As the difference is only between the S and Se atoms, the crystal structure is the same except for the very small differences in the lattice parameters, but the density wave transition is not present in K-Se salt down to 0.5 K. The band calculation suggests that there are two-dimensional hole closed orbit (aorbit) and one-dimensional electron open orbit (y-orbit) in KSe salt. Therefore only one Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillation or one CR are expected down to 0.5 K in K-Se salt. The magnetoresistance measurement of K-Se salt by applying the magnetic field up to 23 T perpendicular to the ac plane showed only one SdH oscillation and the effective mass was estimated to be 2.0 m, [3]. Therefore the observation of CR in K-Se salt will be good tests for our attribution of the CR to the closed a-orbit in the case of K-S salt, and for the suggestion by Singleton et al. [2] that the effective mass obtained by CR is smaller than that obtained by SdH or dHvA effects from the Kahn’s theorem [4].
2.
Experimental CR measurement of K-Se salt has been performed in the frequency region from 50 to 383 GHz in the temperature range 0379-6779/97/$17.00
0 1997 Else&r
PII SO379-6779(96)04699-1
science S.A All rights -,xt
from 1.8 to 50 K using the pulsed magnetic field up to 15 T which is applied perpendicular to the ac plane. The details of the experimental setup can be found in ref. [5]. As the sample
0
2
4
6
8 B
10
12
14
(T)
Fig. 1 Typical transmission spectra of a-(BEDTTTF)$.Hg(SCN)4 observed at 1.8 K. The magnetic field is applied perpendicular to the ac-plane. The CR’s are indicated by the arrows.
2012
H. Ohta et al. /SyntheticMetals
86 (1997) 201 I-2012
400 hole-active
electron-
350 2
300
5
250
xxx-h
T=86K
@TIzKH8GeW, 9OGI-h T=1.8K
electron-active ,, ”
n
A
‘
I
8
10
4B (T)’ Fig. 2
The frequency-field diagram of observed CR’s at 1.8 K.
is metalic, several single crystals are arranged on a polyethylene sheet with gaps of less than lmm among the single crystals, and the transmitted light through these gaps are detected. Circularly polarized light was obtained by attaching circular polarizer to the Gunn oscillator.
3.
Results and discussion Figure 1 shows the typical spectra of a- (B EDT TTF)zKHg(SeCN)q obtained at 1.8 K. Only one resonance is obtained and no CR corresponding to the effective mass of 2.0 m,, which is obtained by SdH, is obtained at low frequencies such as 120 or 140 GHz. The line width and the intensity of obtained resonance are comparable with those of CR obtained in the case of K-S salt. ESR was not observed clearly because we performed the measurements in low resolution. ESR is expected if we use the high resolusion system as in the case of a-(BEDT-TI’F)$H4Hg(SCN)4 [6]. The temperature dependence measurement from 1.8 to 50 K showed no change of resonance position within the experimental error. Obtained resonances are plotted in the frequency-field diagram shown in Fig. 2. The fitted line converges to the origin which suggests that obtained resonances are CR. The obtained effective mass is 0.96 h. As only one CR from the closed a-orbit is expected in K-Se salt from the band calculation, we can attribute this effective mass to that of closed a-orbit. This result is in good agreement with our previous attribution of the a-orbit above TD in case of K-S salt. Next we tried the measurement using circularly polarized light. This measurement will show clearly whether the CR is due to the electrons or holes. Figure 3 shows the results. The measurement of GaAs using our system is presented for comparison. The carrier concentration of this GaAs is 4x1015 /cc. The CR of GaAs is electron active as expected but the effective mass is rather large compared to the known value of O.O7m, which may be due to the large line width compared to the resonance field. At 4.2 K CR disappears
Fig. 3
CR measurements using circularly polarized light at 90 GHz.
because all the carriers fall into the levels below the conduction band. As the observing frequency is much lower than the plasma frequency of about 13 THz. the background increases as the field is applied is due to the increase of magnetoresistance. Compared to the result of GaAs, it is clear that the observed CR of K-Se salt is hole-active which is consistent with our attribution. In conclusion, we showed that only one CR, which can be attributed to the closed a-orbit, is observed in K-Se salt. This attribution is consistent with the measurement using circularly polarized light and with our result of K-S salt obtained previously. Obtained effectve mass of 0.96 m, is clearly smaller than that obtained by SdH oscillation. Acknowledgments The authors HO and YY are grateful to Dr. Uji of NRIM for the stimulating discussion and encouragement. The authors are also grateful to Professor N. Miura of ISSP, Tokyo University for providing high quality GaAs sample, and Professor H. Nojiri of IMR. Tohoku University and Dr. Y. Shimamoto of Central Research Laboratory, Hitachi Ltd. for valuable discussion on the cyclotron resonance of GaAs. References [l] H. Ohta, Y. Yamamoto, M. Motokawa and K. Kanoda, submitted to Phys. Rev. Lett. [2] J. Singleton, F.L. Pratt, M. Doporto, T.J.B.M. Janssen, M. Kurmoo, J.A.A.J. Perenboom. W. Hayes and P. Day, Phys. Rev. Lett. 68 (1992) 2500. [3] T. Sasaki, H. Ozawa, H. Mori, S. Tanaka, T. Fukase and N. Toyota, J. Phys. Sot. Jpn. 65 (1996) 213. [4] W. Kohn, Phys. Rev. 123 (1961) 1242. [5] M. Motokawa, H. Ohta and N.Makita, Int. I. Infrared MMW 12 (1991) 149. [6] K. Akioka. H. Ohta, Y. Yamamoto, M. Motokawa and K. Kanoda, ICSM96 (Snowbird, Utah, 1996) P3.336
Synthetic
Metals86
(1997) 201 I-2012
CYCLOTRON RESONANCE MEASUREMENTS OF ORGANIC CONDUCTOR a-(BEDT-TTF)zKHg(SeCN)d H. Ohta, Y. Yamamoto, K. Akioka, M. Motokawaa, T. Sasakia and T. Fukasea Department of Physics, Kobe University, Rokkodai, Nada, Kobe 657, Japan “MR, Tohoku University, Katahira, Aoba, Sendai 980-77, Japan
Abstract a-(BEDT-TTF)2KHg(SeCN)4 is the new organic conductor which has been synthesized by Sasaki recently. Although it has similar crystal structure with (BEDT-TTF)pKHg(SCN) 4, it does not show antifenomagnetic like transition down to 0.5 K. Therefore the comparison of the physical properties of these two substances is interesting. The cyclotron resonance of K-Se salt has been measured in the millimeter and submillimeter wave regions with the pulsed magnetic field up to 15 T at 1.8 K, and we observed only one cyclotron resonance with the effective mass of 0.96 w. It can be attributed to a-orbit , and this effective mass turned out to be smaller than 2.0 m, obtained by SdH oscillation. The circular polarization measurement has also been performed and this resonance turned out to be hole active which is consistent with the band calculation. Keywords:
magnetotransport, cyclotron resonance, organic conductors based on radical cation and/or anion salts
1.
Introduction Recently the temperature dependence cyclotron resonance (CR) measurement of a-(BEDT-TTF)zKHg(SCN)d has been performed from 1.8 K to the temperature above the density wave transition TD=~ K [I]. Four resonances, two of which are consistent with the previous result obtained by Singleton et a1.[2], were observed at 1.8 K, but only one resonance corresponding to the effective mass of 0.97 m, remained above TJJ and we attributed this resonance to the closed a-orbit On the other hand, Sasaki et al. synthesized new organic conductor a-(BEDT-TTF)2KHg(SeCN)4 recently [3]. As the difference is only between the S and Se atoms, the crystal structure is the same except for the very small differences in the lattice parameters, but the density wave transition is not present in K-Se salt down to 0.5 K. The band calculation suggests that there are two-dimensional hole closed orbit (aorbit) and one-dimensional electron open orbit (y-orbit) in KSe salt. Therefore only one Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillation or one CR are expected down to 0.5 K in K-Se salt. The magnetoresistance measurement of K-Se salt by applying the magnetic field up to 23 T perpendicular to the ac plane showed only one SdH oscillation and the effective mass was estimated to be 2.0 m, [3]. Therefore the observation of CR in K-Se salt will be good tests for our attribution of the CR to the closed a-orbit in the case of K-S salt, and for the suggestion by Singleton et al. [2] that the effective mass obtained by CR is smaller than that obtained by SdH or dHvA effects from the Kahn’s theorem [4].
2.
Experimental CR measurement of K-Se salt has been performed in the frequency region from 50 to 383 GHz in the temperature range 0379-6779/97/$17.00
0 1997 Else&r
PII SO379-6779(96)04699-1
science S.A All rights -,xt
from 1.8 to 50 K using the pulsed magnetic field up to 15 T which is applied perpendicular to the ac plane. The details of the experimental setup can be found in ref. [5]. As the sample
0
2
4
6
8 B
10
12
14
(T)
Fig. 1 Typical transmission spectra of a-(BEDTTTF)$.Hg(SCN)4 observed at 1.8 K. The magnetic field is applied perpendicular to the ac-plane. The CR’s are indicated by the arrows.
2012
H. Ohta et al. /SyntheticMetals
86 (1997) 201 I-2012
400 hole-active
electron-
350 2
300
5
250
xxx-h
T=86K
@TIzKH8GeW, 9OGI-h T=1.8K
electron-active ,, ”
n
A
‘
I
8
10
4B (T)’ Fig. 2
The frequency-field diagram of observed CR’s at 1.8 K.
is metalic, several single crystals are arranged on a polyethylene sheet with gaps of less than lmm among the single crystals, and the transmitted light through these gaps are detected. Circularly polarized light was obtained by attaching circular polarizer to the Gunn oscillator.
3.
Results and discussion Figure 1 shows the typical spectra of a- (B EDT TTF)zKHg(SeCN)q obtained at 1.8 K. Only one resonance is obtained and no CR corresponding to the effective mass of 2.0 m,, which is obtained by SdH, is obtained at low frequencies such as 120 or 140 GHz. The line width and the intensity of obtained resonance are comparable with those of CR obtained in the case of K-S salt. ESR was not observed clearly because we performed the measurements in low resolution. ESR is expected if we use the high resolusion system as in the case of a-(BEDT-TI’F)$H4Hg(SCN)4 [6]. The temperature dependence measurement from 1.8 to 50 K showed no change of resonance position within the experimental error. Obtained resonances are plotted in the frequency-field diagram shown in Fig. 2. The fitted line converges to the origin which suggests that obtained resonances are CR. The obtained effective mass is 0.96 h. As only one CR from the closed a-orbit is expected in K-Se salt from the band calculation, we can attribute this effective mass to that of closed a-orbit. This result is in good agreement with our previous attribution of the a-orbit above TD in case of K-S salt. Next we tried the measurement using circularly polarized light. This measurement will show clearly whether the CR is due to the electrons or holes. Figure 3 shows the results. The measurement of GaAs using our system is presented for comparison. The carrier concentration of this GaAs is 4x1015 /cc. The CR of GaAs is electron active as expected but the effective mass is rather large compared to the known value of O.O7m, which may be due to the large line width compared to the resonance field. At 4.2 K CR disappears
Fig. 3
CR measurements using circularly polarized light at 90 GHz.
because all the carriers fall into the levels below the conduction band. As the observing frequency is much lower than the plasma frequency of about 13 THz. the background increases as the field is applied is due to the increase of magnetoresistance. Compared to the result of GaAs, it is clear that the observed CR of K-Se salt is hole-active which is consistent with our attribution. In conclusion, we showed that only one CR, which can be attributed to the closed a-orbit, is observed in K-Se salt. This attribution is consistent with the measurement using circularly polarized light and with our result of K-S salt obtained previously. Obtained effectve mass of 0.96 m, is clearly smaller than that obtained by SdH oscillation. Acknowledgments The authors HO and YY are grateful to Dr. Uji of NRIM for the stimulating discussion and encouragement. The authors are also grateful to Professor N. Miura of ISSP, Tokyo University for providing high quality GaAs sample, and Professor H. Nojiri of IMR. Tohoku University and Dr. Y. Shimamoto of Central Research Laboratory, Hitachi Ltd. for valuable discussion on the cyclotron resonance of GaAs. References [l] H. Ohta, Y. Yamamoto, M. Motokawa and K. Kanoda, submitted to Phys. Rev. Lett. [2] J. Singleton, F.L. Pratt, M. Doporto, T.J.B.M. Janssen, M. Kurmoo, J.A.A.J. Perenboom. W. Hayes and P. Day, Phys. Rev. Lett. 68 (1992) 2500. [3] T. Sasaki, H. Ozawa, H. Mori, S. Tanaka, T. Fukase and N. Toyota, J. Phys. Sot. Jpn. 65 (1996) 213. [4] W. Kohn, Phys. Rev. 123 (1961) 1242. [5] M. Motokawa, H. Ohta and N.Makita, Int. I. Infrared MMW 12 (1991) 149. [6] K. Akioka. H. Ohta, Y. Yamamoto, M. Motokawa and K. Kanoda, ICSM96 (Snowbird, Utah, 1996) P3.336