@misc{oai:ir.soken.ac.jp:00000196,
 author = {欧陽, 建勇 and オウヤン, ジャンヨン and OUYANG, Jianyong},
 month = {2016-02-17, 2016-02-17},
 note = {So many organic donor and acceptor molecules have been synthesized after the discovery of the superconductivity in the charge-transfer salts of BEDT-TTF.  The molecules containing tetrathiapentalene (TTP) skeleton provide very stable organic metals.  BDT-TTP (2, 5-bis(1, 4-dithiol-2-ylidene)-1, 3, 4, 6-tetrathiapentalene) is the most basic molecule among these TTPderivatives.  The 2:1 charge-transfer salts such as (BDT-TTP)2X (X=SbF6, AsF6, CIO4, and ReO4) exhibit metallic behavior down to liquid helium temperature.  This property is regarded as the result of the two-dimensional (2D) network of the conducting path in the solid state.  According to the band calculation with the tight-binding approximation, (BDT-TTP)2SbF6 is predicted to have a closed Fermi surface.  However, no direct experiment has been performed to prove this calculation.  The first part of this thesis is concerned with this experiment.  The charge-transfer salts of DMTSA (2,3-dimethyltetraselenoanthracene) show unusual electrical properties:  The 1:1 salt DMTSA-BF4 shows a metallic conductivity, although a conventional 1:1 organic charge-transfer salt is regarded as a Molt insulator.  For the elucidation of the metallic origin, the followings should be examined:  the dimensionality of the conducting path, the possibility of the multiple bands, and the on-site Coulomb energy.  In the second part this thesis, the author conduct a comparative study of the isostructural salts:  metallic DMTSA-BF4 and non-metallic DMTTA-BF4 (DMTTA, 2, 3-dimethyltetrathioanthracene), and discuss their different electronic structure.
  (BDT-TTP)2X (X=SbF6, AsF6) has β-type crystal structure.  Strong dispersion with a significant anisotropy appears in the infrared region of E//a and E⊥a spectra.  The plasma edges in both directions can be fitted by Drude model very well.  These indicate a two-dimensional electronic structure with significant anisotropy in the crystals.  The transfer integrals estimated from the plasma frequencies are ta=-0.259 and tp=-0.048eV for (BDT-TTP)2SbF6.  The estimated ta is comparable with the calculated by the extended H〓 ckel method, while the estimated tp is almost half of the calculated.  The Fermi surfaces of both compounds are open in the first Brillouin zone, in contrast to the closed Fermi surface of (BDT-TTP)2SbF6 proposed by the theoretical prediction.  The anisotropy of the Fermi surface increases at low temperature.  The CH stretching mode strongly appears at 3084cm-1 in E⊥a spectrum, suggesting that the conjugated HOMO electrons even extend to the hydrogen atoms.
  The crystal structure of BDT-TTP analogue salts (ST-TTP)2AsF6 and (BDS-TTP)2AsF6 (ST-TTP:  2-(1, 3-diselenol-2-ylidene)-5-(1, 3-dithiol-2-ylidene)- 1, 3, 4, 6-tetrathiopentalene, BDS-TTP:  2, 5-bis(1, 3-diselenol-2-ylidene)-1, 3, 4, 6-tetrathiapentalene)) are iso-structural to (BDT-TTP)2X(X=SbF6, AsF6).  The IR polarized reflection spectra resemble those of (BDT-TTP)2X (X=SbF6, AsF6).  The estimated transfer integrals are ta=-0.24 1, tp=-0.042eV for (ST-TTP)2AsF6 and ta=-0.255, tp=-0,044eV for (BDS-TTP)2AsF6, which are smaller than BDT-TTP salts in spite of the introduction of the selenium atoms.  The Fermi surfaces of both compounds are open as well.  The infer-band electronic transition and the charge sensitive vibrational mode in the Raman spectrum shift to lower energy region on increasing selenium atoms in the donor molecule, while the Raman shift between the neutral molecule and the 2:1 salts does not vary.
  The BDT-TTP molecules stack uniformly in (BDT-TTP)2Y (Y=ClO4, ReO4) crystal.  The chemical ratio of BDT-TTP to ReO4 was expected not to be 1:0.5 but 1:0.36.  However, the most charge sensitive vibrational mode of both compounds appears at 1480 cm-1 in the Raman spectrum, the same position as (BDT-TTP)2X (X =SbF6, AsF6), thereby indicating that the ratio of BDT-TTP to the anions is 2:1 in both compounds.  The polarized reflection spectra resemble those of (BDT-TTP)2X (X=SbF6, AsF6).  The Fermi surfaces of (BDT-TTP)2Y (Y =ClO4, ReO4) are open as well.  The weak band at 8420cm-1 of E c spectrum is assigned to the infer-band transition of BDT-TTP monocation radical.
  Different from (BDT-TTP)2X (X=SbF6, AsF6, CIO4 and ReO4) which are metallic down to liquid helium temperature, θ-(BDT-TTP)2Cu(NCS)2 is a semiconductor, although it is expected to be a two-dimensional metal from the calculation of the band structure.  The phase transition is observed at ca. 250K in the electrical resistance measurement.  The activation energy changes from 30-40meV before the phase transition to 100meV.  The polarized reflection spectra suggest two dimensional electronic structure and larger infer-stack transfer integral than the intra-stack transfer integral.  The spectroscopic weight of the CT band shifts to higher energy region below the phase transition temperature.  The onset energy of the E//c CT band at 16K is comparable with the energy gap estimated from the resistance measurement.  The charge sensitive mode in the Raman spectrum splits into several modes at low temperature, suggesting the charge disproportionation through the phase transition.  The spin susceptibility increases on lowering temperature down to IOK, conforming the Curie-Weiss law with the magnetic moment of 0.62μB.  The variations of the g-value and the linewidth of ESR signal with the temperature confirm the phase transition.  All of these results show that θ-(BDT-TTP)2Cu(NCS)2 is a Molt insulator.
  The polarized reflection spectra of DMTSA-BF4 and DMTTA-BF4 demonstrate that both compounds have a quasi-1D electronic structure.  1D metal DMTSA-BF4 shows a metal-insulator transition at ca. l50K.  DMTTA-BF4 is a Mott insulator with U/4t〓0.8-1.2, and a magnetic phase transition takes place at ca. 100 K by the ESR experiment.  The low-temperature reflection spectra of both compounds strongly suggest the breaking of screw-axis symmetry along the conducting axis.  The phase transitions are regarded as the spin Peierls transition for DMTTA-BF4 and Peierls transition for DMTSA-BF4.  Assuming a dimerized stack structure, the transfer integrals t1 and t2 are estimated as 0.25 and 0.21 eV from the 10K spectrum of DMTSA-BF4.
  An interpretation is given to the broad absorption band at ca. 6900cm-1 in the E⊥c spectrum of DMTSA-BF4 single crystal.  It is assigned to the infer-branch transition from the lower to the upper branch produced by folding the HOMO band at the zone boundary.  According to the formulation of this optical transition, the transition probability originates from the zigzag stacking structure of DMTSA molecules in the crystal.  The numerical calculation of the conductivity spectra of this transition agrees well with the observed conductivity spectrum., application/pdf, 総研大甲第423号},
 title = {Spectroscopic Study of the Organic Conductors:Charge-Transfer Salts of BDT-TTP and DMTSA},
 year = {}
}