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Time-Resolved Measurement of the Photoinduced Change in the Electrical Conductivity of the Organic Superconductor K-(BEDT-TTF)2Cu[N(CN)2]Br Toshifumi Iimori,† Toshio Naito,‡ and Nobuhiro Ohta*,† Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 001-0020, Japan, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: December 28, 2009; ReVised Manuscript ReceiVed: April 7, 2010
Photoinduced change in the electrical conductivity of a single crystal of κ-(BEDT-TTF)2Cu[N(CN)2]Br (κ-Br) has been examined at various temperatures in the vicinity of the metal-superconductor phase-transition temperature (Tc) with a nanosecond visible laser pulse. The transient change of the electric potential difference was measured following photoirradiation, and the time profile of the electrical conductivity was obtained in the current-biased sample. Upon photoirradiation, the bulk resistance increases at all the temperatures under the present study. However, the decay profile shows a marked temperature dependence, and a prolongation of the decay time is observed at temperatures just below Tc. The relaxation times below and above Tc are different from each other, and unconventional asymmetry of the critical slowing down about Tc is found. The presence of a nonthermal (nonbolometric) photoirradiation effect is confirmed in the superconducting phase of κ-Br based on the analysis of the temperature dependence of the photoresponse in the conductivity. Introduction Time-resolved measurements of the relaxation process of nonequilibrium states in superconductors enable us to understand fundamental electrical properties of superconductors. In conventional superconductors, such as Pb or Al, time-resolved experiments were performed with various techniques, including visible and far-infrared pump-probe experiments and transient optoelectronic measurements.1–3 In the pump-probe spectroscopic experiments, for example, the dynamics of electrons has been detected as the transient change in the optical properties, such as optical reflectivity or far-infrared transmittance. Electrical measurements have also been performed with the direct measurements of photoinduced change in the resistivity. Transient photoresponse of resistance in superconducting Pb films was first studied by Testardi using microsecond visible laser pulses.1 In thick films, the observed response was ascribed to the thermal effects induced by the light absorption. However, a nonthermal resistive state was identified in films with a thickness comparable to or less than the optical penetration depth or the superconducting coherence length at temperatures below Tc. A fast response of the resistance in current-biased superconducting thin films has also been investigated by several groups with short laser pulse excitation. In Nb thin films, for example, a biexponential decay of resistance was observed,3 and the deduced pair recombination time was in agreement with a theoretical prediction. In organic superconductors, the photoresponse of the conductivity as well as the photoexcitation dynamics has been scarcely investigated so far.4 In contrast with the lack of such a research on organic superconductors, the photoresponse of the conductivity has been intensively studied in high-Tc superconducting copper oxide films.5–14 In most of the investigations of the photoresponse of the high-Tc superconducting films, the * To whom correspondence should be addressed. E-mail: nohta@ es.hokudai.ac.jp. † Research Institute for Electronic Science. ‡ Division of Chemistry, Graduate School of Science.
response of the conductivity to incident light was measured in current-biased films as a transient voltage signal at temperatures near Tc. Experiments were performed in various time scales ranging from steady-state to subpicosecond with various light sources, including a chopped cw laser or femtosecond pulsed laser. A few papers ascribed the observed photoresponses to the temperature rise of the samples due to the absorption of incident light, namely, a bolometric effect.8,9 Other authors argued that a nonbolometric response could generate the photoinduced change.10–12 Organic superconductors composed of the charge-transfer salt including bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) have been intensively studied because of their exotic electronic properties and high critical temperatures above 10 K.15–19 The charge-transfer salts of BEDT-TTF show extensive polymorphisms of the crystal structure, and they are classified on the basis of the stacking pattern of BEDT-TTF molecules.20 On κ-phase crystals, a lot of studies have been reported and revealed unconventional properties, such as Mott transition or pseudogap behavior.18 The ground electronic state of κ-(BEDTTTF)2Cu[N(CN)2]Cl (hereafter denoted as κ-Cl) or κ-(BEDTTTF)2Cu[N(CN)2]Br (κ-Br) is located in the vicinity of the Mott boundary in the pressure-temperature phase diagram. The κ-Cl crystal is an antiferromagnetic insulator below ∼25 K,21 whereas the κ-Br crystal is a superconductor with a critical temperature Tc ) 11.8 K.22 A similar proximity between the antiferromagnetic insulator phase and the superconducting phase was found in high-Tc copper oxide superconductors. Moreover, both material systems show an analogous phase diagram, although phase transitions are induced by carrier doping in the copper oxides and by applying a pressure in the κ-phase crystals.23 These similarities of the physical properties between high-Tc copper oxides and organic superconductors have also stimulated the study of the κ-phase crystals as a model system of the organic conductors for the understanding of the physics of strongly correlated electronic systems. As already reported, we have studied the photoirradiation effect on the electrical conductivity of R-(BEDT-TTF)2I3 (R-I3),
10.1021/jp912204n 2010 American Chemical Society Published on Web 04/26/2010
Electrical Conductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br
Figure 1. Experimental setup for time-resolved electrical conductivity measurement with laser pulse irradiation in a superconducting crystal. The time profile of the electric potential difference, V(t), was measured by an oscilloscope.
which shows a metal-insulator phase transition, with a timeresolved measurement technique.24,25 In the present study, the transient photoresponse of the electrical conductivity has been examined in an organic superconductor, κ-Br, based on the timeresolved measurements of the electric potential difference following laser pulse irradiation at low temperatures. Experimental Methods Single crystals of κ-Br were prepared by galvanostatic anodic oxidation of BEDT-TTF in 1,1,2-trichloroethane solution.22 The dimensions of a typical sample used in the present study were 1 × 0.8 × 0.2 mm3. The sample was placed in a cryostat using temperature-controlled helium buffer gas. Coaxial cables were used as signal wires. Gold wire (25 or 10 µm in diameter) and gold paste were used as the electrodes on the largest surface (c-a plane) of the sample crystal. The resistance of the sample was measured using a standard four-probe technique (Figure 1). The distance between the inner two electrodes was ∼0.5 mm. In the measurements of the temperature dependence of the resistance, a combination of a current source (Keithley 2400) and a nanovoltmeter (Keithley 2182) was used. Hereafter, the
J. Phys. Chem. C, Vol. 114, No. 19, 2010 9071 resistance at temperature T is denoted by R(T). In the timeresolved measurements of the resistance, constant current pulses that were generated by a pulse generator assembled in our laboratory were used. The electric potential difference (i.e., voltage) between two electrodes on the single crystal surface of κ-Br was amplified and detected following photoirradiation by a digital oscilloscope (Tektronix, 2440) at various temperatures. To photoirradiate the sample, we employed the second harmonic of an output from a Nd:YAG laser (QuantaRay, DCR11), which had a pulse width of ∼10 ns and wavelength of 532 nm. An optical fiber was used to illuminate the center of the crystal surface between the two electrodes. The diameter of the illuminated area on the sample was ∼0.4 mm. The intensity of the irradiation light was adjusted with a variable neutral-density filter. The direction of propagation of the laser light was normal to the conducting c-a plane of the crystal. The polarization direction of the laser light was accordingly parallel to the c-a plane, although it was not intentionally specified. A single shot of the laser pulse was used to obtain the transient voltage signal. The repetition rate of the laser pulse was 2 Hz, and we verified that the current pulse did not induce a steady rise of the temperature of the sample at this repetition rate. The transient change of the resistance was obtained from the transient profile of the voltage divided by the bias current. The current pulse had a rectangular form, and hence, a spike noise synchronized with the edge of the pulse was observed. This electrical noise showed a large fluctuation of the height at each shot. Consequently, it was inevitable for this noise to appear, even in some of the time profiles presented in this paper. Results Time profiles of the change in resistance (∆R) following photoirradiation were measured at various temperatures across Tc ()11.8 K). The results obtained with a laser light intensity
Figure 2. Time profiles of the change in the electrical resistance of κ-Br in the time range of 0 - 20 ms.
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Figure 4. Plots of the peak value of ∆R versus temperature.
profile showed the peak shifted toward the larger values, as the temperature decreased. At 9.5 and 6.5 K, for example, the peaks were at ∼0.4 and ∼1.9 ms, respectively. The time profiles obtained at these temperatures cannot be fitted by a simple combination of rise and decay exponential functions. The time that was needed for the resistance to return to the original value became shorter as the temperature decreased. In the time profiles, the temperature dependence was also noticed for the peak height of the change in resistance (see Figure 4). In the temperature range from 15.1 to 10.9 K, the peak height was nearly constant, that is, approximately 0.3 Ω. At temperatures between T ) 10.2 and 8.1 K, at which the decay time showed a marked elongation, the peak height somewhat increased up to 0.5 Ω, At temperatures below 8.1 K, the peak height decreased steeply. Figure 3. Time profiles of the change in the electrical resistance of κ-Br in the time range of 0 - 170 ms.
of 5 µJ/pulse and a bias current of 0.6 mA are shown in Figure 2, where the laser light was irradiated at time t ) 0. In Figure 2, the vertical axis represents the change in resistance, and thus, the positive signal corresponds to the increase of the resistance. The resistance increased due to photoirradiation at all the temperatures, and the shape of the time profiles showed remarkable temperature dependence. The observed photoirradiation effects may be divided into three classes, depending on the temperature range, that is, T > 12 K, T ≈ Tc, and T < 9 K. At T > 12 K, the resistance recovered to the original one within ∼10 ms following photoirradiation by a laser pulse, and time profiles were similar to each other. These decay profiles could be simulated by a biexponential decay function, not by a single exponential decay function. The lifetimes were determined to be approximately 0.1 (0.7) and 1.8 ms (0.3), where each preexponential factor is given in the parentheses. In the temperature range from 12 to 9 K, that is, T ≈ Tc, the time profiles of ∆R(t) were obviously elongated. In particular, ∆R(t) lasted up to ∼150 ms at 10.1 K (see Figure 3). The fast decaying component having a lifetime of 0.1 ms, which was strong at T > 12 K, became weaker with the decrease of temperature, and this component disappeared at temperatures below 10.2 K. The peak intensity in Figure 3 appears to be somewhat higher than that shown in Figure 2. This difference may be ascribed either to the superposition of the electrical noise, as described above, or to an irradiation light intensity fluctuation, which is unavoidable in the single-shot measurement. In addition, the dip having the negative amplitude, which appears at t ) 0, for example, at T ) 10.2 K (see Figure 2), may be also explained in terms of the electrical noise. At temperatures below 9 K, the time profiles showed a distinct rise (see Figure 2). The corresponding time at which the time
Discussion In conventional metals, photoexcited electrons show an ultrafast relaxation arising from electron-electron scattering and electron-phonon coupling. For example, femtosecond timeresolved spectroscopy in Au and Ag has shown that the relaxation of the photoexcited electrons occurs on a time scale faster than 1 ps.26 Owen and Scalapino theoretically suggested that the breakup of Cooper pairs induced by photoexcitation could cause a change of the magnitude of the superconducting gap or the order parameter of the superconducting phase in conventional superconductors.27 In fact, the change of the superconducting gap after a laser pulse excitation has been demonstrated experimentally, and the relaxation time of the superconducting gap has been reported to be ∼200 ps in Pb and hundreds of nanoseconds in Al.2,28 The ultrafast dynamics of electrons in high-Tc copper oxides was also investigated with several different methods.5 The pump-probe spectroscopy using an 800 nm femtosecond laser pulse in YBa2Cu3O7-δ (YBCO) single crystals at temperatures below Tc has shown that the relaxation and recombination of the photoexcited quasiparticles occur on a picosecond time scale.6 As the temperature approaches Tc from lower temperatures, the decay time of the transient signal becomes larger and shows a diverging behavior.6 In the present study of the transient photoresponse of the electrical conductivity of κ-Br, a much slower dynamics that occurred in the time scale from hundreds of microseconds to a hundred of milliseconds was observed. If the relaxation of the quasiparticles occurred on the picosecond time scale, it was very likely that the sample temperature increased because of the thermal equilibrium between electrons and lattice systems, and the transient time profile may arise from the temperature increase of the sample. In such a case, it would be reasonable for the decay profile to be ascribed to the heat diffusion process in the crystals. Accordingly, we have carefully examined such a ther-
Electrical Conductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br
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Figure 5. (solid circle) Temperature rise (∆T) calculated with a thermodynamical model versus the initial sample temperature. (open circle) Maximum temperature rise obtained from the peak value of ∆R versus the initial sample temperature on the assumption of the thermal effect.
mal effect for the observed photoresponse. Actually, the thermal effect has been analyzed in terms of two different approaches. First, we consider a fundamental thermodynamics of the heat capacity versus the energy. At low temperatures, the heat capacity of solids strongly depends on temperature. The energy supplied at a constant volume should be related to the heat capacity by the following relation
∆E )
∫TT C(T) dT 2
1
(1)
where ∆E is the supplied energy, T1 and T2 are the initial and final temperatures of solids, that is, before and after photoirradiation, respectively, and C(T) is the heat capacity at T. Experimentally, the C(T) of κ-Br was measured at low temperatures by Elsinger et al.29 We obtained the temperature dependence of the C(T) of κ-Br from the polynomial fit to the data points shown in Figure 1 of their paper. Here, ∆E corresponds to the irradiation laser light intensity of 5 µJ used in our measurements. Figure 5 shows the results of the calculation of the temperature rise (∆T ≡ T2 - T1) as a function of the initial sample temperature. In this calculation, we assumed that the conduction of the heat generated by the absorption of the laser light rapidly occurred and that the temperature uniformly rose in the whole volume of the sample via heat diffusion without a loss due to radiation or conduction to the surroundings, such as electrodes or buffer gas. This thermodynamics model predicts a monotonic decrease of ∆T, as the initial temperature increases. If the observed photoresponse is ascribed to the thermal effect, the maximum value of the change in temperature (∆Tmax) that the sample undertakes may be calculated from the peak height of the transient profile of the sample. In fact, the change in temperature induced by photoirradiation was estimated by combining ∆Rpeak shown in Figure 4 with the plots of R as a function of T. The values of ∆Tmax thus obtained are plotted as a function of the initial sample temperature. The results are given in Figure 5, which shows that ∆Tmax has a peak at T ≈ 11 K and a nearly constant value of ≈3 K in the superconducting phase. Such behaviors have not been remarked in the temperature dependence of ∆T resulting from the thermodynamics model. Because the extinction coefficient, that is, optical conductivity, of the κ-Br crystal is quite large at 532 nm, the heating of a thin layer whose thickness is comparable to the optical penetration depth may yield a change in resistance. In such a case, we have to employ the heat capacity determined by the volume of the thin layer, which is different from the one
Figure 6. (a) Plots of resistance versus temperature. (b) Derivative of resistance and the integral of ∆R. (c) Ratio of the integral of ∆R relative to the derivative.
used above. The change in the heat capacity may result in the change in magnitude of ∆T, but it is very unlikely that the shape of the temperature dependence of ∆T changes. The fact that the shape of the temperature dependence of ∆T is very different from the one of ∆Tmax (see Figure 5) seems to show that the observed results cannot be explained in terms of the simple thermodynamics model. Second, we consider the bolometric response, that is, the thermal effect resulting from the temperature dependence of the resistance. The bolometric response in ∆R caused by a small temperature variation (∆T) may be written as
∆R )
dR(T) ∆T dT
(2)
Thus, ∆R should be proportional to the derivative of R(T). Plots both of R(T) and of its derivative calculated numerically as a function of temperature are shown in Figures 6a,b, respectively. The shape of the temperature dependence of ∆Rpeak shown in Figure 4 is different from that of the derivative shown in Figure 6b, indicating that the temperature dependence of ∆Rpeak cannot be explained in terms of the thermal effect presented by eq 2. Because the temperature dependence of the time profile of ∆R may be taken into account by the time-averaging process, the time-averaged photoresponse has been also examined. The timeaveraged ∆R was obtained from the integration of the observed time profile of ∆R with respect to the time, as shown in Figure 6b. The plots of the integral of ∆R, that is, ∆R integrated over the time, as a function of temperature showed a peak at ∼10 K, which is similar to the derivative of R(T), that is, the curve differentiated with respect to T. At ∼10 K, the decay profile showed the most prolonged feature (Figure 3). Consequently,
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the temperature dependence of the integral of ∆R is mainly ascribed to the temperature dependence of the decay profile, but a difference in the temperature dependence between the derivative of R(T) and the integral of ∆R exists in the superconducting phase, that is, T < 10 K. To show the difference between these two curves more clearly, plots of the integral of ∆R relative to the derivative of R(T) were also obtained as a function of the initial temperature, as shown in Figure 6c. Equation 2 shows that the above-mentioned ratio would be proportional to the temperature rise of the sample if the bolometric response is the origin of the observed photoresponse. In the normal metallic phase at temperatures above Tc, the ratio was constant, as shown in Figure 6c. In the superconducting phase at temperatures below Tc, on the other hand, the ratio increased with a decrease of temperature, and the magnitude of the ratio was much larger than that in the normal phase, especially at low temperatures (see Figure 6c). Such a large change in the ratio, which corresponds to a large change in ∆T (see eq 2), is not consistent with the estimation presented in Figure 5, indicating that the present results cannot be interpreted in terms of eq 2. Thus, it is very likely that a nonbolometric effect of the photoresponse exists in the superconducting phase of κ-Br, though the photoresponse in the normal metallic phase at temperatures above 12 K may be ascribed to a bolometric response. In the thermal mechanism, the surface of the crystal is initially heated by the absorption of the laser pulse, and so there is a possibility that the decay profile of resistance is related to the time evolution of the temperature profile in the crystal, that is, the heat conduction. The thermal conductivity is one of the most essential physical parameters in the heat conduction process, but the thermal conductivity of the κ-Br salt has not been reported yet. We then refer to the thermal conductivity of κ-(BEDT-TTF)2Cu(NCS)2 salt, which is an analogous organic superconductor of the κ-Br salt and has Tc ) 10.4 K.30,31 The thermal conductivity measured along the b axis of the crystal exhibits a peak just below Tc (6-8 K). The origin of this enhancement of the thermal conductivity is believed to be due to the increase of the mean free path of phonons.30 It can be assumed that the κ-Br salt also shows a similar enhancement of the thermal conductivity at temperatures below Tc, and it is expected that the rate of the thermal conduction and the decay rate of the photoresponse would become faster at those temperatures. As mentioned above, however, the decay rate has become slower at temperatures just below Tc. Thus, the analysis of the thermal effect on the basis of the temperature dependence of the thermal conductivity also indicates that the thermal effect is not significant in the observed photoresponse of the electrical conductivity. Because the thermal effect is negligible, one can properly consider that the observed photoresponse arises from the relaxation of the photogenerated carriers. The mechanism of the nonbolometric photoresponse in the superconducting phase of the κ-Br salt is not clear at present, but photoenhanced flux creep10 or quasiparticle recombination5 may be hypothesized as the origin of the observed photoresponse of κ-Br. A rigorous treatment of the bolometric effect on the basis of the solution of the heat-conduction equations would be useful for the extraction of the time profile of the photoresponse originating from the nonbolometric effect from the observed results. Such an analysis would be possible if the requisite properties, including the optical extinction coefficient in the visible
Iimori et al. wavelength region and the anisotropic thermal conductivity of the κ-Br salt, for solving the heat conduction equation were known. In general, the system near the critical point shows a critical slowing down.32 Indeed, the decay rate of the change in the electrical resistance of κ-Br showed a marked decrease and significant temperature dependence at temperatures below the metal-superconductor phase-transition temperature (Tc) (see Figure 3). However, the time profiles observed at temperatures above Tc show an insignificant temperature dependence with faster decay times than those observed at temperatures below Tc. Consequently, the temperature dependence of the relaxation time is not symmetric with respect to the critical temperature Tc. The asymmetry of the critical slowing down about Tc observed in the present study is unusual and of particular interest. For the understanding of the present results, however, further theoretical consideration will be necessary. In the photoexcited κ-Br salt, a slow relaxation dynamics of the resistance has been observed, in contrast with other superconducting systems, including high-Tc cuprates. It is worth mentioning that the slow photoexcitation relaxation has been also reported in the organic conductor R-I3. Time-resolved measurements of the R-I3 salt showed that the decay profile became longer as the temperature approached the metal-insulator phase-transition temperature (TM-I ) 135 K) from lower temperatures.33,34 Time-resolved photocurrent measurement has shown that the photocurrent persists for a few microseconds after the photoexcitation at 115 K.33 In addition, using an ultrafast pump-probe reflection spectroscopy, Iwai et al. have reported a marked slowing down of the photoexcitation relaxation near Tc.34 On the other hand, the study of the κ-Br salt using a similar technique has shown that the slowly decaying component with the lifetime longer than 10 ps exists in the superconducting phase.4 Hence, the presence of the slow photoexcitation relaxation dynamics might be a characteristic of organic conductors, which comprehend complex electronic states. Conclusion Time-resolved measurements of the photoinduced change in electrical conductivity of κ-Br single crystals have been performed with photoirradiation of the nanosecond laser pulse. The transient increase of the resistance is induced by the laser pulse irradiation. In the normal phase at temperatures above Tc, a relatively fast decay profile is observed. At temperatures near Tc, the time profile shows an extremely slow decay. The relaxation rate shows the temperature dependence that was asymmetric with respect to the critical temperature Tc, and asymmetry of the critical slowing down is found. The temperature rise is calculated by using the model in which the temperature dependence of the heat capacity is taken into account, but the analysis shows that this model cannot explain the observed photoresponse. The estimation of the thermal effect using the temperature dependence of the resistance and eq 2 can verify the presence of a nonbolometric photoirradiation effect in the superconducting phase. It is shown that the contribution of the nonbolometric effect to the observed photoresponse becomes larger as the temperature decreases. Acknowledgment. We thank Mr. Tsuyoshi Ushizaka at the Research Institute for Electronic Science at Hokkaido University for designing and assembling the circuit of the constant current pulse generator. This work was supported by a Grant-in-Aid
Electrical Conductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br for Scientific Research (A) (Grant No. 20245001) from the Ministry of Education, Culture, Sports, Science and Technology in Japan. References and Notes (1) Testardi, L. R. Phys. ReV. B 1971, 4, 2189–2196. (2) Carr, G. L.; Lobo, R. P. S. M.; LaVeigne, J.; Reitze, D. H.; Tanner, D. B. Phys. ReV. Lett. 2000, 85, 3001–3004. (3) Johnson, M. Phys. ReV. Lett. 1991, 67, 374–377. (4) Naito, T.; Yamada, Y.; Inabe, T.; Toda, Y. J. Phys. Soc. Jpn. 2008, 77, 064709. (5) Averitt, R. D.; Taylor, A. J. J. Phys.: Condens. Matter 2002, 14, R1357–R1390, and references cited therein. (6) Demsar, J.; Mihailovic, D.; Kabanov, V. V. Proc. SPIE 2002, 4811, 165, and references cited therein. (7) Gedik, N.; Blake, P.; Spitzer, R. C.; Orenstein, J.; Liang, R.; Bonn, D. A.; Hardy, W. N. Phys. ReV. B 2004, 70, 014504. (8) Forrester, M. G.; Gottlieb, M.; Gavaler, J. R.; Braginski, A. I. Appl. Phys. Lett. 1988, 53, 1332–1334. (9) Brocklesby, W. S.; Monroe, D.; Levi, A. F. J.; Hong, M.; Liou, S. H.; Kwo, J.; Rice, C. E.; Mankiewich, P. M.; Howard, R. E. Appl. Phys. Lett. 1989, 54, 1175–1177. (10) Zeldof, E.; Amer, N. M.; Koren, G.; Gupta, A. Phys. ReV. B 1989, 39, 9712–9714. (11) Johnson, M. Appl. Phys. Lett. 1991, 59, 1371–1373. (12) Bluzer, N. J. Appl. Phys. 1992, 71, 1336–1348. (13) Lindgren, M.; Currie, M.; Williams, C. A.; Hsiang, T. Y.; Fauchet, P. M.; Sobolewski, R.; Moffat, S. H.; Hughes, R. A.; Preston, J. S.; Hegmann, F. A. IEEE J. Sel. Top. Quantum Electron. 1996, 2, 668–678, and references cited therein. (14) Kreisler, A. J.; Gaugue, A. Supercond. Sci. Technol. 2000, 13, 1235– 1245. (15) Ishiguro, T.; Yamaji, K.; Saito, G. Organic Superconductors; Springer-Verlag: Heidelberg, Germany, 1998.
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