田島 裕之

The transverse magnetoresistance of α-(BEDT-TTF)2KHg(SCN)4 was investigated with the magnetic field rotated within a conducting ac-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance in the weak-field limit, ΔR(B,θ), has the form ΔR(B,θ) = B2(psin2(θ - θmin) + qcos2(θ - θmin)), where θ is the angle between a-axis and magnetic field direction and (p, q, θmin) are temperature-dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical anisotropy within bc-plane is 3.5∼4.5 and 2.5∼3.0 above and below the phase transition at 10 K, respectively. © 1995.

Crystals of TPP-Ni[(dmit)(2)](3) (TPP = tetraphenylphosphonium, dmit = 2-thioxo-1,3-dithiol-4,5-dithiolate) were prepared. The structure of the crystal was determined as follows: monoclinic C2/c; a = 18.086(3) Angstrom, b = 7.184(2) Angstrom, c = 45.838(6) Angstrom, beta = 92.59(1)degrees, V = 5950(2) Angstrom(3), and Z = 4. In the crystal, the large counteractions separate the conducting sheet of Ni(dmit)(2) anions. Within the conducting sheet, two of the three Ni(dmit)(2) anions form columns, and remaining the Ni(dmit)(2) anion fills the spaces between the columns. The molecular plane of the Ni(dmit)(2) anions in the column is almost perpendicular to that of isolated Ni(dmit)(2). The electrical conductivity was nearly one-dimensional with the highest conductivity being observed parallel to the column direction. The inplane anisotropy of the conductivity was ca. 1:100. The crystal exhibited a conductivity of 10 S/cm at room temperature and semiconducting behavior at lower temperatures. Activation energies of 0.035 and 0.036 eV, respectively, were observed below and above 125 K. A deviation from Arrhenius behavior was observed above 160 K. The effect of pressure was relatively small. The crossover point between the two semiconducting regimes shifted to 80 K at 7 kbar and the activation energies decreased to 0.011 and 0.036 eV, respectively. The thermoelectric power was negative at room temperature in the column direction and showed an activation-type behavior with the crossover point around 125 K. Analysis of the reflectance spectra of the salt indicated that the gap was smaller than 0.09 eV, which is consistent with that of conductivity measurements and that the crystal is not exactly one-dimensional. The magnetic susceptibility obeyed the Curie-Weiss law below room temperature and one spin per three Ni(dmit)(2) units was calculated from the Curie constant. The distribution of electrons in the crystal is discussed.

Reflectance spectra, Hall coefficient and magnetoresistance were measured for incommensurate layered compounds (MS)(x)TaS2 in order to elucidate their electronic band structures. The charge transfer from MS to TaS2 is found to be much smaller in Pb and Sn compounds than in the rare earth compounds. The Fermi energy and magnetoresistance have the same M-atom dependence among the rare earth compounds, which is related to the number of electrons transferred from the MS layers to TaS2 layers. The electronic structures ale described in terms of a two-hole carrier system in the compounds with small charge transfer. With increasing the charge transfer rate, it becomes a one-hole carrier system as one of the Fermi surfaces vanishes.

The compounds [NMe4] [Pd(C3S3Se2)2]2 and [PMe4][Pd(C3S5)2]2 [C3S3Se2 = 4,5-bis(hydroseleno)-1,3-dithiole-2-thionate C3S5 = 4,5-dimercapto-1,3-dithiole-2-thionate] have been prepared by electrochemical oxidation of [NBu4]2[Pd(C3S3Se2) 2] and [NBu4] [Pd(C3S5)2] in the presence of a large excess of NMe4PF6 and PMe4I, respectively. Their structures have been determined by X-ray crystallographic methods. Both compounds, with space group C2/c, are isostructural to the superconducting phase β-[NMe4][Pd(C3S5)2] 2. The PdL2(L = C3S3Se2 or C3S5) entities are paired, forming [(PdL2)2] dimmers with a Pd-Pd bond length of 3.174 and 3.177 Å, respectively. The structure consists of [(PdL2)2] dimer layers in the (001) plane separated from each other by sheets of cations. The dimers are stacked along [110] and [110]. Both compounds show similar electrical properties: the room-temperature conductivities are ≈50 and ≈20 S cm-1, respectively at high temperature the conductivity is weakly temperature dependent, but abruptly decreases at low temperatures the conductivity increases when applying pressure. A broad dispersion of optical excitations was observed in the reflectance spectra of [NMe4] [Pd(C3S3Se2)2]2, indicating an overlap of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) bands and a HOMO-type conduction band.

The donor-acceptor type molecular conductor, alpha-(EDT-TTF)[Ni(dmit)2], was found to be a superconductor under ambient pressure below 1.3 K. This is the first ambient-pressure superconductor in a series of M(dmit)2 salts [M=Ni, Pd, Pt]. The sample dependence and magnetic-field dependence on the superconducting transition are described.

We have measured reflectance spectra of several χ-type BEDT-TTF salts in order to investigate their electronic structures. The line shape of the reflectance spectra are all alike in these salts, regardless of their electrical properties. On the other hand, the low-temperature spectra of these salts show considerable difference. Among all, the spectra of χ-(BEDT-TTF)2Ag(CN)2Ag(CN)2·H2O salt are completely different from those of other χ-type salts. Its plasma frequencies of intra-band transition obtained by the Drude-Lorentz fit of the reflectance spectra are 0.33V for E//b and 0.34eV for E//c, respectively, which are comparable to the half of the plasma frequencies already obtained in other χ-type salt. In this article we present a model which explains the anomalous reflection spectra found in this salt. © 1993.

Resistivity measurement under pressure, X-ray study, DC Hall effect and optical measurement were carried out on (BEDT-TTF)4[Pt(C2O4)2] to investigated the nature of two types of phase transitions, i.e. metal-metal(M-M) transition ( Tc=200K under ambient pressure) and metal-insulator (M-I) transition (Tc=60K, under ambient pressure). The M-M transition temperature was found to increase on applying the pressure, while the M-I transition was found to disappear above 4kbar. It is suggested that the M-M transition is due to CDW formation.

α-BETS2I3 and α-(BEDT-STF)2I3, which are analogous of α-(BEDT-TTF)2I3, exhibit a metal-insulator(M-I) transition. Polarized reflectance spectrum and magnetoresistance under high pressure were studied on these salts in order to investigate the electronic structures and the phase transition. The change of reflectance spectrum accompanying M-I transition was found to be considerably smaller in α-BETS2I3 and α-(BEDT-STF)2I3 than in α-(BEDT-TTF)2I3. Application of pressure suppresses the M-I transition for both salts. α-BETSW2I3 becomes metallic under 9Kbar over the whole temperature region. A large positive magnetoresistance is observed in this metallic state α-BETS2I3 even at relatively high temperature. © 1993.

The angular dependent oscillation of magnetoresistance and Shubunikov-de Haas(SdH) Effect were observed on alpha-Et2Me2N[Ni(dmit)2]2. The former showed magic angles, thetan characterized by tan(thetan-90-degrees)=0.47x(-0.5+n), tan(phim-90-degrees)=0.30xm (n,m:integer), where thetan is the angle between c-axis and magnetic field direction within a*c plane, and where phim is the angle between b-axis and magnetic field direction within a*b plane. The latter revealed the existence of two types of oscillations. From the period of the resistance oscillations vs. the inverse magnetic field, the cross-sectional areas of the Fermi surfaces perpendicular to the a*-axis, associated with these two types of oscillations, are estimated to be 0.23% and 4.5% of the two-dimensional Brillouin zone, respectively. The cyclotron mass and the Dingle temperature of the larger closed orbit were estimated to be 0.16me and 11.2K, respectively.

The transverse magnetoresistance of alpha-EDT-TTF[Ni(dmit)2] was investigated with the magnetic field rotated within a conducting ab-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance, DELTAR(B)/R(0) has the form Psin2(theta-delta)+Qcos2(theta-delta), where theta is the angle between b-axis and magnetic field direction and (P, Q, delta) are temperature dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical conduction in this salt is made by both EDT-TTF and Ni(dmit)2 above 20K, mainly by Ni(dmit)2 below 10K.

Magnetoresistance of organic conductor, α-Et2Me2N[Ni(dmit)2]2, was measured for the magnetic field along a*-axis up to 23 T in the temperature down to 0.5 K. Three different types of closed orbits (α-, β-, γ-orbit) of two-dimensional Fermi surface were found through Shubnikov-de Haas oscillation. The areas of these three closed orbits are estimated to be 0.23% (α-orbit), 4.60% (β-orbit), 87.4% (γ- orbit) of the first Brillouin zone. The cyclotron masses for α-, β-, γ-orbit are estimated to be (0.06±0.01)me, (0.166±0.01)me, (4.3±0.2)me, respectively. © 1993.

The transverse magnetoresistance of α-EDT-TTF[Ni(dmit) 2 ] was investigated with the magnetic field rotated within a conducting ab-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance, δR(B)/R(0) has the form Psin 2 (θ-δ)+Qcos 2 (θ-δ), where θ is the angle between b-axis and magnetic field direction and (P, Q, δ) are temperature dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical conduction in this salt is made by both EDT-TTF and Ni(dmit) 2 above 20K, mainly by Ni(dmit) 2 below 10K. © 1993.

We report observations of various 'saw-tooth' dHvA oscillation waveforms in organic conductors. In θ-(BEDT-TTF)2I3 and α-(BEDT-TTF)2KHg(SCN)4, we have observed 'saw-tooth' and 'inverse saw-tooth' waveforms, respectively, both characteristic for two-dimensional electron systems. The difference between the two is discussed. In β-(BEDT- TTF)2AuBr2, we observed pseudo 'saw-tooth' magnetization jumps due to the magnetic interaction effect. © 1993.

The angular dependent magnetoresistance oscillation and Shubnikov-de Haas (SdH) effect were observed in α -Et 2 Me 2 N[Ni(dmit) 2 ] 2 . The former data showed magic angles, θ n characterized by tan(θ n-90)=(-0.5+n) × 0.47 (n:integer). The data of the SdH effect revealed the existence of two types of oscillations. From the period of the resistance oscillation vs. the inverse magnetic field, the cross-sectional areas of the Fermi surfaces perpendicular to the a * -axis, associated with these two types of oscillations, are estimated to be ∼ 0.23% and ∼ 4.5% of the two dimensional Brillouin zone, respectively. The angular dependence of the SdH fundamental field shows two-dimensional nature of the band structure. © 1992.

Polarized reflectance spectra of single crystals of (CH3)4As[Pd(dmit)2]2 and Cs[Pd(dmit)2]2 were measured and compared with those of (CH3)4N[Ni(dmit)2]2. Prominent absorption bands, which do not exist in the reflectance spectra of (CH3)4N[Ni(dmit)2]2 were observed around 10000 cm-1 in the spectra of [Pd(dmit)2] salts. Analysis of the reflectance spectra reveals the anomalous electronic structure in these salts where the energy levels derived from HOMO and LUMO of one molecule are partly inverted due to their strongly dimerized structure.

In order to probe the conduction band structure and the inter-molecular interactions, we have investigated the polarized infrared reflectance spectra of single crystals of theta- and kappa-(BEDT-TTF)2I3, (BEDT-TTF)2KHg(SCN)4, Me4N[Ni(dmit)2]2 and alpha-(Et2Me2N)[Ni(dmit)2]2. From the low temperature spectra, the Drude parameters are estimated by a Drude-Lorentz curve-fitting. Analyses of the plasma frequencies based on tight-binding band model afford the estimation of transfer integrals of these salts. The shape of Fermi surface and the conduction band structure are deduced by using these parameters.

Visible and infrared reflectance spectra were observed on single crystals of [M(dmit)2] salts. The band parameters of Me4N[Ni(dmit)2]2 were obtained from the reflectance spectra and compared with the results of thermoelectric power measurements. The direct experimental evidences showing the "LUMO-HOMO energy level inversion" are presented for Cs[Pd(dmit)2]2, Me4As[Pd(dmit)2]2, and Me4N[Pt(dmit)2]2.

Observation of Shubnikov-de Haas(SdH) and de Haas-van Alphen(dHvA) oscillations in organic metals (BEDT-TTF)2X, with X = KHg(SCN)4, theta-I3 and beta"-AuBr2, are reported. In KHg(SCN)4 salt, in addition to the SdH oscillations with fundamental frequency of 670 T corresponding to about 16% of the first Brillouin zone(FBZ), we observed splitting of each SdH peak which we ascribed to "spin-splitting". We have also found that the ground state of this salt is not a simple metal but has some magnetic character. In theta-I3 salt we have succeeded in an observation of dHvA oscillations for the first time. We observed a "saw-tooth" dHvA oscillation characteristic to a highly two-dimensional and extraordinary clean electronic system. In addition to the fundamental frequency of 4170 T corresponding to 50.4% of the FBZ and its higher harmonics, we observed an oscillation with lower frequency of 730 T corresponding to about 8.8% of the FBZ. A new Fermi surface topology for theta-13 salt is proposed based on the analysis of the dHvA effect. In beta"-AuBr2, we observed complex dHvA oscillations, which can be explained in terms of the mixing of two fundamental frequencies of 47 and 268 T, suggesting the presence of very small pockets corresponding to 0.6 and 2.9% of the FBZ.

We report a series of measurements of the structural and electronic properties of the molecular conductor Cs[Pd(dmit)2]2 (dmit = isotrithionedithiolate). This material has a C2/c structure with stacks of the acceptor groups arranged in sheets separated by the cations, with appreciable dimerization of the acceptor groups along the stack direction. It shows metallic properties at room temperature. We have characterized this material with measurements of conductivity, thermopower, magnetic susceptibility and polarized optical reflectivity, and we find that the electronic structure is quite isotropic in the plane of the acceptor sheets. On cooling, there is a metal-insulator transition at 56.5 K, and this opens an energy gap at the Fermi energy. This transition is associated with a complex periodic lattice distortion comprising an incommensurate modulation and also a commensurate distortion, which breaks the C centring symmetry. We present a calculation of the electronic band structure using an extended Huckel formalism, and show that, owing to the strong dimerization along the stack direction, the Fermi energy lies halfway in a band of highest occupied molecular orbital character. We consider also how the observed structural instability is able to produce an energy gap over the whole Fermi surface, and propose a model that requires coupling between the two types of modulation.

Polarized reflectance spectra of a single crystal of k-(BEDT-TTF)2 I3 were measured at 295 K and 15 K over the spectral range from 500 cm-1 to 25000 cm-1. The infrared property was two-dimensional. Non-Drude mid-infrared features were observed in spite of the metallic dc conductivity. A comparison with β- and θ-(BEDT-TTF)2I3 shows that this behavior has a close connection with the dimeric structure of K-salts. We analyzed the observed line shape by the curve-fitting analysis, and obtained optical effective masses. On the basis of the perturbation theory, an effective tight-binding band model for K-salt was derived. We estimated the transfer integrals using this model. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

Temperature dependence of the shielding and Meissner effect and anisotropy of the magnetization curve are investigated. The superconductivity of θ-(BEDT-TTF)2I3 is found to be a bulk one, while in some samples the superconducting volume are significantly suppressed. Anisotropy of the lower critical field, Hc1, is estimated; Hc1,b is 40 G, while Hc1,a and Hc1,c are less than 2 G. The in-plane anisotropy of Hc1 is compared with the result of an optical study. © 1990, The Physical Society of Japan. All rights reserved.

A "saw-tooth" de Haas-van Alphen oscillation was observed in an organic superconductor θ-(BEDT-TTF)2I3 under the magnetic field H>15 T at T=0.5 K. The fundamental frequency of the oscillation (H0=4170 T) corresponds to the closed Fermi surface area of 50% of the first Brillouin zone, consistent with one positive charge carrier per I3- anion. An issue about the Fermi surface topology is resolved based on the analysis of the dH-vA oscillation frequencies. The "saw-tooth" variation of magnetization demonstrates that organic metals can provide an ideal example of highly two-dimensional and extraordinary clean electron system with a cylindrical Fermi surface along the direction perpendicular to the conducting plane. © 1990.

The reflectance spectra of (BEDT-TTF)5Hg3Br11 and (BEDT-TTF)HgBr3 were measured by using the microspectrophotometric technique in the spectral range from 450 cm-1 to 25000 cm-1. In the former salt, the gap formation due to a metal-insulator transition was observed in the reflectance spectrum at 100 K. In the latter salt, the effective Coulomb interaction and the intra-dimer transfer integral were estimated and compared with the corresponding values of BMDT-TTF and TTF.

The Cu K-edge absorption spectra of high-Tc superconductors and related oxides, (La1-xSrx)2CuO4 (L(S)CO, x=0, 0.075 YBa2Cu3Oy (YBCO, y≈7-6), YBa2 (Cu1-xCox)3Oy (Co substitution for Cu(1), x=0.33, y≈7) (y≈7, ≈6) are interpreted by comparing with those of some divalent Cu complex oxides, Sr2CuO3 and Ca2CuO3 with the 1D CuO chain structure and Nd2CuO4 and Gd2CuO4 with the 2D CuO sheet structure. The twin-peak structure observed in the main absorption band arises from the final-state (core-hole screening) effect through ligand-to-metal charge transfer and can be assigned to the 1s-4pσ1 (3d10L-1) and 4pσ2 (3d9) transitions. In the 1s-4pπ energy region below the 1s-4pσ main absorptio the onset and shoulder structures A-F are observed; the structures C and F are assigned to 1s-4pπ1 and 1s-4pπ2 transitions in the 2D CuO sheets of pyramidally coordinated [Cu(2)O5] in YBCO and octahedrally coordinated [CuO6] in L(S)CO, the structures B and E to those in the 1D CuO chain at the Cu(1) site of oxygen-rich YBCO, and the structures A and D to those in the linearly coordinated [Cu(1)O2] ions of oxygen-deficient YBCO. The twin-peak structure both in the 1s-4pπ and 1s-4pσ energy regions indicates that the valence of Cu in YBCO and LSCO always has both d9 and d10 components; that is, the oxygen atoms coordinated to Cu take an important part in the balance of holes in the systems and in the hole conductivity on the 2D CuO sheet. © 1989.

Artificial superlattice materials composed of alternating layers of europium and manganese metals with a periodicity in the range of 19-227 Å were prepared by means of molecular-beam epitaxy. In situ reflection high-energy electron diffraction observations revealed that the Eu metal layer is amorphous up to a thickness of 23 Å, while a [110]-oriented single-crystal Eu layer grows over the amorphous Eu layer as the thickness exceeds 23 Å. On the other hand, the manganese layers were found to always be in an amorphous state independent of the thickness. The synthesized Eu/Mn systems exhibited a unique magnetic property associated with a magnetic interaction between the amorphous Eu layers.

The polarized reflectance spectra of the single crystals of the title compounds were measured at room and low temperatures. The correlation was found between the appearance of the extra electronic transitions overlapped on the Drude-tail and the dimeric arrangement of BEDT-TTF. From this relation, this extra transition was assigned to the inter-band transition between the split bands due to the strong dimeric interaction. The Drude parameters were determined by analyzing the low-temperature spectra taking account of the inter-band transition. © 1988.

The polarized reflectance spectrum of bis(ethylenedithio)tetrathiafulvalenium perchlorate, (BEDT-TTF)3(ClO4)2, was measured in the infrared and visible regions at room temperature, 200 K and 150 K. The anisotropic ratio of the transfer integrals between the directions parallel and perpendicular to the [012] axis was estimated to be 0.59 on the (100) plane. A drastic change of the reflectance spectrum was observed below the phase transition temperature. © 1988.

Ten YB_a2Cu_3O_<7-y> (YBCO) perovskite samples with varying oxygen deficiency (0.15<y<1) were prepared by changing the quenching temperature or annealing conditions. With increasing quenching temperature, the transition temperature (T_c) for superconductivity decreased. This is accompanied by increasing oxygen deficiency and changes in the lattice constants. The oxygen deficiency of the sample annealed under nitrogen atmosphere at 700℃ was more than that of the sample quenched at 950℃. There were no differences observed between the oxygen deficiency, lattice constants, nor superconductivity of the sample annealed under oxygen atmosphere and that. cooled slowly from 350℃ (0.5℃/min) in air. Annealing under high oxygen pressure caused destruction of the YBCO structure and disappearance of the superconductivity. Considering all the experimental results obtained in this study, YBCO perovskite can be classified into three types : Ortho-I(y<0.2) . Ortho-II(0.2 <y<0.5), and Tetra (0.5 <y<1). Ortho-I type shows superconductivity with a T_c of around 90 K, while the T_c onset of Ortho-II is below 70 K. Although oxygen deficiency and lattice constants change continuously, a discontinuous T_c change is observed when YBCO changes from Ortho-I to Ortho-II. Tetragonal type is semiconductive and is not super conductive above 4 K. The small difference in the oxygen deficiency between Ortho- I and Ortho-II indicates that it is necessary to minimize the oxygen deficiency in order to obtain a uniform material with a T_c of 90 K. However, annealing under_ oxygen does not modify the superconductivity of samples cooled slowly.

Single-phase high Tc superconductors, REBa2Cu3O7 - y (RE ≡ Gd, Dy, Ho, Er and Tm) were prepared and their magnetic susceptibilities were measured from 77 to 300 K. The obtained Curie-Weiss temperatures were -1.2, -5.8, -2.1 and -14 K for Dy3+, Ho3+, Er3+ and Tm3+ respectively, indicating that the 4f spins are delocalized to a small extent and the orbital quenching occurs in the normal conducting range. This was supported by the results of electron spin resonance (ESR) measurements. The influence of the magnetic spins on the superconductivity and the relaxation mechanisms responsible for the ESR lineshapes are discussed. © 1988.

Polarized reflectance spectra of (BPDT-TTF)3(PF6)2 and (BPDT-TTF)2I3 were measured at room temperature over the spectral region from 720 cm-1 to 25 000 cm-1. Both of these salts exhibited one-dimensional behaviour in contrast to the BEDT-TTF (bis-(ethylenedithio)tetrathiafulvalenium) salts. The transfer integrals and the on-site Coulomb energy of these materials were estimated by analysing the optical spectra, and the on-site Coulomb energy of (BPDT-TTF)3(PF6)2 was found to be significantly larger than that of TCNQ salts. © 1988.

Ten YBa//2Cu//3O//7// minus //y (YBCO) perovskite samples with varying oxygen deficiency (0. 15 less than y less than 1) were prepared by changing the quenching temperature or annealing conditions. With increasing quenching temperature, the transition temperature (T//c) for superconductivity decreased. This is accompanied by increasing oxygen deficiency and changes in the lattice constants. The oxygen deficiency of the sample annealed under nitrogen atmosphere at 700 degree C was more than that of the sample quenched at 950 degree C. There were no differences observed between the oxygen deficiency, lattice constants, nor superconductivity of the sample annealed under oxygen atmosphere and that cooled slowly from 350 degree C (0. 5 degree C/min) in air. Annealing under high oxygen pressure caused destruction of the YBCO structure and disappearance of the superconductivity.

Reflectance spectra in the infrared and visible regions were observed on (BEDT-TTF)3(ClO4)2 and α-(BEDT-TTF)3(ReO4)2 which exhibit a M-I transition and on β-(BEDT-TTF)2I3 and β-(BEDT-TTF)2IBr2 which exhibit superconductivity at low temperatures. In all these cases, the reflectance spectra at room temperature showed a feature mainly determined by the contribution of inter-band transitions, but a marked difference is found between the two types of the salts in respect to the temperature dependence of reflectance spectrum. © 1987.

The temperature dependence of reflectance spectrum was measured on the organic superconductor, β-(BEDT-TTF)2X (X=I3, IBr2). The obtained reflectance spectra are not Drude-like over the temperature range studied. The calculation of the conductivity spectrum based on the tight-binding approximation reveals the presence of relatively strong contribution of inter-band transitions, which is proposed to be the origin of the non-Drude-like shape of the spectra. In order to separate the contribution of the intra-band transition, the low-temperature reflectance spectrum was transformed into the dielectric function by the Kramers-Kronig transformation and analyzed by the Drude model. The results of this analysis was found to be consistent with the observed d.c. conductivities. © 1987.

Polarized reflectance spectra of α- and β-(BEDT-TTF)2I3 and β-(BEDT-TTF)2IBr2 were measured over the spectral range from 720 cm-1 to 25,000 cm-1 at various low temperature. From these optical spectra, we concluded that the inter-band transition of charge-transfer type overlapped the intra-band transition in the infrared region. Thus the Drude-like dispersion of reflectance were more or less modulated depending on the contribution of inter-band transition. We determined the band parameters of β-(BEDT-TTF)2I3 and β-(BEDT-TTF)2IBr2 by analyzing the reflectance spectra. © 1986.

The temperature dependence of the polarized reflectance spectrum of β-(BEDT-TTF)2I3 was measured over the spectral range from 750 cm-1 to 25,000 cm-1. The plasma frequency and the relaxation rate were obtained by analyzing the low-temperature spectra. The absolute value and the anisotropy of the optical conductivity at low temperature agree with those of the dc conductivity within the experimental error. The dimensionless electron-phonon coupling constant was estimated to be 1.6 from the temperature dependence of the relaxation rate. © 1986.