- Email: [email protected]
Ultrafast and large reflectivity change by ultraviolet excitation of the metallic phase in the organic conductor (EDO-TTF)2PF6
ARTICLE IN PRESS Physica B 405 (2010) S350–S352
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Ultrafast and large reflectivity change by ultraviolet excitation of the metallic phase in the organic conductor (EDO-TTF)2PF6 K. Onda a,b,, M. Shimizu c, F. Sakaguchi c, S. Ogihara c, T. Ishikawa c, Y. Okimoto c, S. Koshihara b,c, X.F. Shao d,e, Y. Nakano d, H. Yamochi d, G. Saito f a
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Non-equilibrium Dynamics Project, ERATO/JST, Tsukuba, Ibaraki 305-0801, Japan c Department of Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan d Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan e Institute for Integrated Cell-Material Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan f Research Institute, Meijo University, Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan b
a r t i c l e in f o
Keywords: Organic conductor Charge transfer complex Photo-induced phase transition Ultrafast dynamics
a b s t r a c t We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF)2PF6. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1 eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7 ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band. & 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental detail
(EDO-TTF)2PF6 (EDO-TTF= ethylenedioxytetrathiafulvalene) is a quasi-one-dimensional, 3/4-filled conducting charge transfer complex at room temperature, which undergoes a metal-toinsulator phase transition at 280 K [1]. This complex also shows unique and diverse ultrafast photo-induced phase transitions (PIPTs) in the low-temperature phase and the characteristics of these PIPTs have been intensively investigated [2–7]. In contrast, the photo-induced dynamics in the high temperature phase have not been explored. Even in other conducting charge transfer complexes, the photo-induced response of the metallic phase has been much less studied than that of the insulating phase [8–10]. Considering that the metallic nature of this type of complex differs considerably from that of conventional inorganic metals, the photo-induced dynamics of the metallic phase are probably unique. Thus, we studied the photo-induced dynamics of the ‘‘metallic’’ phase in (EDO-TTF)2PF6.
Sub-picosecond reflectivity changes were measured by a pump-probe method using 120-fs Ti:sapphire laser. The second harmonics (photon energy= 3.1 eV) or fundamentals (1.55 eV) of a part of the amplified laser pulse (pulse energy: 1 mJ/pulse, repetition rate: 1 kHz) were used as the pump pulse. For the probe pulse, we generated the pulse light ranging from midinfrared (0.25 eV) to visible (2.07 eV) from the amplified pulse using optical-parametric amplification (OPA) and second-harmonic generation (SHG) or difference frequency generation (DFG). The static reflectivity spectra were measured using a Fourier transform infrared (FTIR) spectrometer and a monochromator equipped with a xenon- or tungsten-lamp. The (EDO-TTF)2PF6 crystal was made by the procedure given in a previous paper [1]. Unless otherwise indicated, all measurements were carried out in air at room temperature.
3. Results and discussion Corresponding author at: Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. Tel./fax: + 81 45 924 5891. E-mail address: [email protected] (K. Onda).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.11.054
Fig. 1 shows the reflectivity spectrum (solid line) and the optical conductivity spectrum calculated from the reflectivity spectrum (broken line) of the high temperature phase at 290 K. The polarization of the light was parallel to the EDO-TTF stacking
ARTICLE IN PRESS K. Onda et al. / Physica B 405 (2010) S350–S352
axis, and these spectra were in good agreement with the previous report [11]. The spectra have two characteristic bands: a Drudelike band below 1 eV, and an intra-molecular band above 1.6 eV. The Drude-like band implies that the electrons move relatively freely along the stacking axis. This is further supported by conductivity measurements and vibrational spectroscopy, in which the conductivity drastically increases above 280 K [1] and the Raman spectrum shows the charge of EDO-TTF molecules is uniformly 0.5 [11]. On the other hand, the intra-molecular band is an electronic transition, probably a p–p* transition in the EDOTTF molecule, as indicated by the absorption spectra of solutions of EDO-TTF salts [12]. For the pump and probe measurements, we excited the intra-molecular band at 3.1 eV or the gap between the Drude-like band and the intra-molecular band at 1.55 eV. The photon energy of the probe pulse was varied from 0.25 to 2.08 eV in order to monitor the change of both bands. When the intra-molecular band at 3.1 eV was excited, ultrafast ( o0.05 ps) and large reflectivity changes ( 10%) were observed with a low-intensity pulse of around 1010 W/cm2, whereas excitation out of the intra-molecular band at 1.55 eV induced no reflectivity change, even with a higher intensity pulse of
Fig. 1. Reflectivity spectrum and optical conductivity spectrum in (EDO-TTF)2PF6 in the metallic high-temperature phase at 290 K.
S351
1011 W/cm2. For conventional inorganic metals, comparable reflectivity changes are observed above 1012 W/cm2 [13], and such a large reflectivity change should be irreversible [14]. Thus, this is a new phenomenon, which takes place in this organic metal only upon excitation of the intra-molecular transition. Typical temporal profiles of reflectivity change (DR/R) at various probe photon energies are shown in Fig. 2. The excitation intensity was 0.9 1010 W/cm2 ( = 2 1015 photons/ cm2). The character of the temporal profiles exhibited a clear contrast between the Drude-like band ( o0.95 eV) and the intramolecular band ( 41.77 eV). At the Drude-like band, the reflectivity rapidly decreases followed by increases, but the reflectivity does not reach the original reflectivity, that is, a small long lifetime component remains. The lifetimes of the decrease and increase were estimated to be o0.05 and 0.2 ps, respectively, by simulation of a double-exponential function including the cross-correlation between the pump and probe pulses ( = 0.25 ps). In contrast, at the intra-molecular band, the reflectivity change increases, and then decreases, and the lifetime of the decrease was estimated to be 0.7 ps by the same simulation. The different lifetimes of the probed bands clearly indicate differences in their dynamics. Fig. 3 shows the photo-induced reflectivity spectrum (solid circles) for the 0.1 ps after photo-excitation derived from the temporal profiles of the reflectivity change. To clarify the spectrum change, the observed reflectivity changes were tripled in the derivation. The thick solid and dotted lines represent the reflectivity spectra of the high- and low-temperature phases, respectively. Comparison of the spectrum after the photo-induced change to that of the low-temperature phase shows no indication that a low-temperature phase is formed. Although the metallic phase spectrum of this complex is not in perfect agreement with that estimated using the Drude approximation (the phonon shielding is insufficient and the reflectivity increase at lower photon energies is too low), it is the best available first step of analysis. Based on the Drude model, the reflectivity spectrum (R(o)) is expressed mainly by the plasma frequency (op) and the relaxation time (t) as follows sffiffiffiffiffiffiffiffiffiffi nð ~ oÞ-1 2 o2P e~ ðoÞ ~ ; e ðoÞ ¼ e1 ; n~ ¼ RðoÞ ¼ ~ oÞ þ 1 e0 nð oðo þ it-1 Þ
Fig. 2. Temporal profiles of reflectivity changes at various probe photon energies in (EDO-TTF)2PF6 in the high-temperature phase by ultraviolet 3.1 eV pulse excitation.
ARTICLE IN PRESS S352
K. Onda et al. / Physica B 405 (2010) S350–S352
had different lifetimes and opposing reflectivity changes as mentioned above. Since the reflectivity increase is very fast (o0.05 ps) and the decrease has a rather long lifetime, the reflectivity change probably originates from transient absorption from the excited state of the EDO-TTF molecule.
4. Conclusion
Fig. 3. Photo-induced reflectivity spectrum (solid circles) in (EDO-TTF)2PF6 together with reflectivity spectra in the high- and low-temperature phases and a simulated reflectivity spectra based on the Drude model.
We observed large, ultrafast reflectivity changes in the metallic phase of the quasi-one-dimensional conducting charge transfer complex (EDO-TTF)2PF6. The reflectivity change took place only when the intra-molecular band was excited, and the lifetime of the photo-induced state was approximately 0.2 ps. Measurement of the transient reflectivity spectrum and simulation based on the Drude model revealed that the decrease of the relaxation time of conduction electrons by electronically excited molecules caused this reflectivity change. We believe this to be a new phenomenon, which would occur in general organic conductors but not in conventional inorganic metals.
Acknowledgements ~ oÞ is the complex refractive index, e~ ðoÞ is the complex where nð dielectric constant, e0 is the dielectric constant in vacuum, and e1 is a constant representing transitions not involving conduction electrons. Using these equations, we simulated the reflectivity spectra of the high temperature phase (thin dashed line) and photo-induced state (thin solid line, also DR/R is exaggerated), as shown in Fig. 3. The plasma frequencies for both spectra were the same: op = 1.8 eV, whereas their relaxation times were different: tHT = 2.8(1) 10 15 s for the high temperature phase and tPI =2.5(1) 10 15 s for the photo-induced phase. Since the relaxation time for conventional metals is in the order of 10 14 s, for example: t = 2.7 10 14 s for Cu and t =2.7 10 14 s for Al at 273 K [15], the relaxation time of (EDO-TTF)2PF6 is approximately one tenth that of conventional metals. The 10% decrease in relaxation time by photo-excitation ((tHT tPI)/tHT) indicates that the electronically excited state of EDO-TTF molecules perturbs the conduction electrons more than those in the ground state. Considering the short lifetime of 0.2 ps for the photo-induced state, only the electrically excited molecules perturb the conduction electrons, while the excited phonons created by thermalization do not significantly affect the conduction electrons. Instead, the long lifetime component of more than a picosecond in the reflectivity change is probably due to the phonon excitation. To confirm this, we measured the reflectivity spectra at higher temperatures in thermal equilibrium. However, we found that the spectrum became irreproducible above 350 K, and did not change within experimental error (a few percents) below 340 K; thus, we could not conclusively determine the origin of the long lifetime component. Finally, different dynamics must be considered as the cause for the reflectivity changes probed at 1.77 and 2.07 eV, because they
This work was in part supported by a Grant-in-Aid for Scientific Research (B) no. 20340074 and a Grant-in-Aid for Scientific Research on Innovative Areas no. 21110512 from MEXT Japan, and by the G-COE program of JSPS Japan.
References [1] A. Ota, H. Yamochi, G. Saito, J. Mater. Chem. 12 (2002) 2600. [2] M. Chollet, L. Guerin, N. Uchida, S. Fukaya, H. Shimoda, T. Ishikawa, H. Yamochi, G. Saito, R. Tazaki, S. Adachi, S. Koshihara, Science 307 (2005) 86. [3] K. Onda, T. Ishikawa, M. Chollet, X. Shao, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 21 (2005) 216. [4] K. Onda, S. Ogihara, T. Ishikawa, Y. Okimoto, X.F. Shao, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Cond. Mat. 20 (2008) 224018. [5] K. Onda, S. Ogihara, K. Yonemitsu, N. Maeshima, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, Phys. Rev. Lett. 101 (2008) 067403. [6] K. Onda, S. Ogihara, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 148 (2009) 012002. [7] S. Ogihara, K. Onda, M. Shimizu, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 148 (2009) 012008. [8] S. Iwai, F. Hiramatsu, H. Nakaya, Y. Kawakami, K. Yakushi, Phys. Rev. B 77 (2008) 125131. [9] S. Iwai, K. Yamamoto, A. Kashiwazaki, F. Hiramatsu, H. Nakaya, Y. Kawakami, K. Yakushi, H. Okamoto, H. Mori, Y. Nishio, Phys. Rev. Lett. 98 (2007) 097402. [10] H. Okamoto, K. Ikegami, T. Wakabayashi, Y. Ishige, J. Togo, H. Kishida, H. Matsuzaki, Phys. Rev. Lett. 96 (2006) 37405. [11] O. Drozdova, K. Yakushi, A. Ota, H. Yamochi, G. Saito, Syn. Met. 133–134 (2003) 227. [12] A. Ota, Structures and properties of charge transfer complexes based on TTF derivatives, Ph.D. Thesis, Kyoto University, 2004. [13] A. Ng, P. Celliers, A. Forsman, R.M. More, Y.T. Lee, M.W.C. Dharma-wardana, G.A. Rinker, Phys. Rev. Lett. 72 (1994) 3351. [14] H.M. Milchberg, R.R. Freeman, S.C. Devey, Phys. Rev. Lett. 61 (1988) 2364. [15] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Thomson Learning, 1976.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Ultrafast and large reflectivity change by ultraviolet excitation of the metallic phase in the organic conductor (EDO-TTF)2PF6 K. Onda a,b,, M. Shimizu c, F. Sakaguchi c, S. Ogihara c, T. Ishikawa c, Y. Okimoto c, S. Koshihara b,c, X.F. Shao d,e, Y. Nakano d, H. Yamochi d, G. Saito f a
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Non-equilibrium Dynamics Project, ERATO/JST, Tsukuba, Ibaraki 305-0801, Japan c Department of Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan d Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan e Institute for Integrated Cell-Material Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan f Research Institute, Meijo University, Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan b
a r t i c l e in f o
Keywords: Organic conductor Charge transfer complex Photo-induced phase transition Ultrafast dynamics
a b s t r a c t We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF)2PF6. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1 eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7 ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band. & 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental detail
(EDO-TTF)2PF6 (EDO-TTF= ethylenedioxytetrathiafulvalene) is a quasi-one-dimensional, 3/4-filled conducting charge transfer complex at room temperature, which undergoes a metal-toinsulator phase transition at 280 K [1]. This complex also shows unique and diverse ultrafast photo-induced phase transitions (PIPTs) in the low-temperature phase and the characteristics of these PIPTs have been intensively investigated [2–7]. In contrast, the photo-induced dynamics in the high temperature phase have not been explored. Even in other conducting charge transfer complexes, the photo-induced response of the metallic phase has been much less studied than that of the insulating phase [8–10]. Considering that the metallic nature of this type of complex differs considerably from that of conventional inorganic metals, the photo-induced dynamics of the metallic phase are probably unique. Thus, we studied the photo-induced dynamics of the ‘‘metallic’’ phase in (EDO-TTF)2PF6.
Sub-picosecond reflectivity changes were measured by a pump-probe method using 120-fs Ti:sapphire laser. The second harmonics (photon energy= 3.1 eV) or fundamentals (1.55 eV) of a part of the amplified laser pulse (pulse energy: 1 mJ/pulse, repetition rate: 1 kHz) were used as the pump pulse. For the probe pulse, we generated the pulse light ranging from midinfrared (0.25 eV) to visible (2.07 eV) from the amplified pulse using optical-parametric amplification (OPA) and second-harmonic generation (SHG) or difference frequency generation (DFG). The static reflectivity spectra were measured using a Fourier transform infrared (FTIR) spectrometer and a monochromator equipped with a xenon- or tungsten-lamp. The (EDO-TTF)2PF6 crystal was made by the procedure given in a previous paper [1]. Unless otherwise indicated, all measurements were carried out in air at room temperature.
3. Results and discussion Corresponding author at: Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. Tel./fax: + 81 45 924 5891. E-mail address: [email protected] (K. Onda).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.11.054
Fig. 1 shows the reflectivity spectrum (solid line) and the optical conductivity spectrum calculated from the reflectivity spectrum (broken line) of the high temperature phase at 290 K. The polarization of the light was parallel to the EDO-TTF stacking
ARTICLE IN PRESS K. Onda et al. / Physica B 405 (2010) S350–S352
axis, and these spectra were in good agreement with the previous report [11]. The spectra have two characteristic bands: a Drudelike band below 1 eV, and an intra-molecular band above 1.6 eV. The Drude-like band implies that the electrons move relatively freely along the stacking axis. This is further supported by conductivity measurements and vibrational spectroscopy, in which the conductivity drastically increases above 280 K [1] and the Raman spectrum shows the charge of EDO-TTF molecules is uniformly 0.5 [11]. On the other hand, the intra-molecular band is an electronic transition, probably a p–p* transition in the EDOTTF molecule, as indicated by the absorption spectra of solutions of EDO-TTF salts [12]. For the pump and probe measurements, we excited the intra-molecular band at 3.1 eV or the gap between the Drude-like band and the intra-molecular band at 1.55 eV. The photon energy of the probe pulse was varied from 0.25 to 2.08 eV in order to monitor the change of both bands. When the intra-molecular band at 3.1 eV was excited, ultrafast ( o0.05 ps) and large reflectivity changes ( 10%) were observed with a low-intensity pulse of around 1010 W/cm2, whereas excitation out of the intra-molecular band at 1.55 eV induced no reflectivity change, even with a higher intensity pulse of
Fig. 1. Reflectivity spectrum and optical conductivity spectrum in (EDO-TTF)2PF6 in the metallic high-temperature phase at 290 K.
S351
1011 W/cm2. For conventional inorganic metals, comparable reflectivity changes are observed above 1012 W/cm2 [13], and such a large reflectivity change should be irreversible [14]. Thus, this is a new phenomenon, which takes place in this organic metal only upon excitation of the intra-molecular transition. Typical temporal profiles of reflectivity change (DR/R) at various probe photon energies are shown in Fig. 2. The excitation intensity was 0.9 1010 W/cm2 ( = 2 1015 photons/ cm2). The character of the temporal profiles exhibited a clear contrast between the Drude-like band ( o0.95 eV) and the intramolecular band ( 41.77 eV). At the Drude-like band, the reflectivity rapidly decreases followed by increases, but the reflectivity does not reach the original reflectivity, that is, a small long lifetime component remains. The lifetimes of the decrease and increase were estimated to be o0.05 and 0.2 ps, respectively, by simulation of a double-exponential function including the cross-correlation between the pump and probe pulses ( = 0.25 ps). In contrast, at the intra-molecular band, the reflectivity change increases, and then decreases, and the lifetime of the decrease was estimated to be 0.7 ps by the same simulation. The different lifetimes of the probed bands clearly indicate differences in their dynamics. Fig. 3 shows the photo-induced reflectivity spectrum (solid circles) for the 0.1 ps after photo-excitation derived from the temporal profiles of the reflectivity change. To clarify the spectrum change, the observed reflectivity changes were tripled in the derivation. The thick solid and dotted lines represent the reflectivity spectra of the high- and low-temperature phases, respectively. Comparison of the spectrum after the photo-induced change to that of the low-temperature phase shows no indication that a low-temperature phase is formed. Although the metallic phase spectrum of this complex is not in perfect agreement with that estimated using the Drude approximation (the phonon shielding is insufficient and the reflectivity increase at lower photon energies is too low), it is the best available first step of analysis. Based on the Drude model, the reflectivity spectrum (R(o)) is expressed mainly by the plasma frequency (op) and the relaxation time (t) as follows sffiffiffiffiffiffiffiffiffiffi nð ~ oÞ-1 2 o2P e~ ðoÞ ~ ; e ðoÞ ¼ e1 ; n~ ¼ RðoÞ ¼ ~ oÞ þ 1 e0 nð oðo þ it-1 Þ
Fig. 2. Temporal profiles of reflectivity changes at various probe photon energies in (EDO-TTF)2PF6 in the high-temperature phase by ultraviolet 3.1 eV pulse excitation.
ARTICLE IN PRESS S352
K. Onda et al. / Physica B 405 (2010) S350–S352
had different lifetimes and opposing reflectivity changes as mentioned above. Since the reflectivity increase is very fast (o0.05 ps) and the decrease has a rather long lifetime, the reflectivity change probably originates from transient absorption from the excited state of the EDO-TTF molecule.
4. Conclusion
Fig. 3. Photo-induced reflectivity spectrum (solid circles) in (EDO-TTF)2PF6 together with reflectivity spectra in the high- and low-temperature phases and a simulated reflectivity spectra based on the Drude model.
We observed large, ultrafast reflectivity changes in the metallic phase of the quasi-one-dimensional conducting charge transfer complex (EDO-TTF)2PF6. The reflectivity change took place only when the intra-molecular band was excited, and the lifetime of the photo-induced state was approximately 0.2 ps. Measurement of the transient reflectivity spectrum and simulation based on the Drude model revealed that the decrease of the relaxation time of conduction electrons by electronically excited molecules caused this reflectivity change. We believe this to be a new phenomenon, which would occur in general organic conductors but not in conventional inorganic metals.
Acknowledgements ~ oÞ is the complex refractive index, e~ ðoÞ is the complex where nð dielectric constant, e0 is the dielectric constant in vacuum, and e1 is a constant representing transitions not involving conduction electrons. Using these equations, we simulated the reflectivity spectra of the high temperature phase (thin dashed line) and photo-induced state (thin solid line, also DR/R is exaggerated), as shown in Fig. 3. The plasma frequencies for both spectra were the same: op = 1.8 eV, whereas their relaxation times were different: tHT = 2.8(1) 10 15 s for the high temperature phase and tPI =2.5(1) 10 15 s for the photo-induced phase. Since the relaxation time for conventional metals is in the order of 10 14 s, for example: t = 2.7 10 14 s for Cu and t =2.7 10 14 s for Al at 273 K [15], the relaxation time of (EDO-TTF)2PF6 is approximately one tenth that of conventional metals. The 10% decrease in relaxation time by photo-excitation ((tHT tPI)/tHT) indicates that the electronically excited state of EDO-TTF molecules perturbs the conduction electrons more than those in the ground state. Considering the short lifetime of 0.2 ps for the photo-induced state, only the electrically excited molecules perturb the conduction electrons, while the excited phonons created by thermalization do not significantly affect the conduction electrons. Instead, the long lifetime component of more than a picosecond in the reflectivity change is probably due to the phonon excitation. To confirm this, we measured the reflectivity spectra at higher temperatures in thermal equilibrium. However, we found that the spectrum became irreproducible above 350 K, and did not change within experimental error (a few percents) below 340 K; thus, we could not conclusively determine the origin of the long lifetime component. Finally, different dynamics must be considered as the cause for the reflectivity changes probed at 1.77 and 2.07 eV, because they
This work was in part supported by a Grant-in-Aid for Scientific Research (B) no. 20340074 and a Grant-in-Aid for Scientific Research on Innovative Areas no. 21110512 from MEXT Japan, and by the G-COE program of JSPS Japan.
References [1] A. Ota, H. Yamochi, G. Saito, J. Mater. Chem. 12 (2002) 2600. [2] M. Chollet, L. Guerin, N. Uchida, S. Fukaya, H. Shimoda, T. Ishikawa, H. Yamochi, G. Saito, R. Tazaki, S. Adachi, S. Koshihara, Science 307 (2005) 86. [3] K. Onda, T. Ishikawa, M. Chollet, X. Shao, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 21 (2005) 216. [4] K. Onda, S. Ogihara, T. Ishikawa, Y. Okimoto, X.F. Shao, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Cond. Mat. 20 (2008) 224018. [5] K. Onda, S. Ogihara, K. Yonemitsu, N. Maeshima, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, Phys. Rev. Lett. 101 (2008) 067403. [6] K. Onda, S. Ogihara, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 148 (2009) 012002. [7] S. Ogihara, K. Onda, M. Shimizu, T. Ishikawa, Y. Okimoto, X.F. Shao, Y. Nakano, H. Yamochi, G. Saito, S. Koshihara, J. Phys. Conf. Ser. 148 (2009) 012008. [8] S. Iwai, F. Hiramatsu, H. Nakaya, Y. Kawakami, K. Yakushi, Phys. Rev. B 77 (2008) 125131. [9] S. Iwai, K. Yamamoto, A. Kashiwazaki, F. Hiramatsu, H. Nakaya, Y. Kawakami, K. Yakushi, H. Okamoto, H. Mori, Y. Nishio, Phys. Rev. Lett. 98 (2007) 097402. [10] H. Okamoto, K. Ikegami, T. Wakabayashi, Y. Ishige, J. Togo, H. Kishida, H. Matsuzaki, Phys. Rev. Lett. 96 (2006) 37405. [11] O. Drozdova, K. Yakushi, A. Ota, H. Yamochi, G. Saito, Syn. Met. 133–134 (2003) 227. [12] A. Ota, Structures and properties of charge transfer complexes based on TTF derivatives, Ph.D. Thesis, Kyoto University, 2004. [13] A. Ng, P. Celliers, A. Forsman, R.M. More, Y.T. Lee, M.W.C. Dharma-wardana, G.A. Rinker, Phys. Rev. Lett. 72 (1994) 3351. [14] H.M. Milchberg, R.R. Freeman, S.C. Devey, Phys. Rev. Lett. 61 (1988) 2364. [15] N.W. Ashcroft, N.D. Mermin, Solid State Physics, Thomson Learning, 1976.