Hirokazu Sakamoto

Magnetic properties in polypyrrole doped by series of dopants

Abstract Magnetic susceptibility and ESR linewidth are measured as a function of temperature in polypyrrole (PPy) doped with PF 6 −, AsF 6 −, ClO 4 −, BF 4 −, ToS − . Spin susceptibility by Schumacher-Slichter (ESR-NMR) technique shows both the temperature independent Pauli term ranging from 0.7 to 1.4×10 −5 emu/mol-ring and the Curie term of 1 spin per 600~1500 rings, which is consistent with that obtained by SQUID measurement in the same sample. The temperature dependence of ESR linewidth is ascribed to the Elliott mechanism typical for the metallic electron spins, increasing monotonically with the temperature. This interpretation is also consistent with the ESR linewidth in polythiophene doped with ClO 4 − and AsF 6 −. Therefore, both the susceptibility and the ESR linewidth demonstrate the PPys doped with several dopants to have a metallic electronic state.

ESR linewidth in conducting polymers with five-membered ring

Abstract The ESR linewidths in polythiophene, poly(3-methylthiophene) and polypyrrole doped with ClO4− and AsF6− were measured as a function of temperature to study the dynamics of charged carriers with spin. The temperature dependence of the ESR linewidth can be understood in terms of the Elliott mechanism, characteristic for metallic electrons. Such an interpretation leads us to conclude that the electron scattering is not governed by potential of dopants, but by that on polymer backbone. In addition, the temperature dependence of the resistance ratio rVSC obtained by voltage-shorted-compaction method was found to coincide with that of the linewidth qualitatively. We show that a simple one-dimensional model for the resistivity that takes the electron scattering only by 2kF-phonons into consideration can qualitatively well explain both the data.

Theoretical Study of Pressure Effect on TDAE-C60

We have theoretically studied pressure effects on molecular ferromagnet C 60 complexes with tetrakis (dimethylamino) ethylene (TDAE), particularly the pressure-induced depression of the Curie temperature. The observed behavior is well simulated by our model which is based on a charge transfer induced intramolecular Jahn-Teller distortion and an intermolecular cooperative Jahn-Teller interaction. We emphasize that the theoretical simulation is carried out with reasonable parameters known for C 60 - complexes. It is concluded that the enhancement of the crystal field at C 60 sites due to increasing pressure causes the depression of Curie temperature.

Photoemission Studies of Electronic Structures of BaPb1-xBixO3

Photoemission spectra of BaPb 1- x Bi x O 3 are measured for x =0, 0.25, 0.3 and 0.6 by use of synchrotron radiation. A remarkable discrepancy from the results of band calculation is recognized. It is found that the O 2p ⊥ band has a high density of states (DOS) at binding energies ( E B ) of 3.0∼3.5 eV. The DOS at the Fermi level ( E F ) is found to be negligibly small in all samples. The E B of the Pb 5d bands for , x =0.3 and 0.6 are around 0.3 eV smaller than those for x =0. It is also found that the Ba 5p core levels shift to larger E B by ∼0.3 eV with increasing x . The Bi 5d core levels have shown no additional structure besides the spin-orbit splitting, whereas the Pb 5d core spectra are remarkably distorted for x =0.3 and 0.6. These effects are discussed in regard to the charge transfer and the hybridization of electronic states.

AF-like Ground State of Mn-DNA and Charge Transfer from Fe to Base-π-Band in Fe-DNA

The electronic states of M-DNA doped with M = Mg, Ca, Zn, Mn, Fe are investigated mainly with magnetic properties. In the ”wet” condition the Mn ions of Mn-DNA form a 1-D chain in the center of a DNA double helix, as evidenced from the formation of the unnatural base pair combination with M, poly(dA)-M-poly(dC), but in the ”dry” condition they form a3 -D network with the antiferromagnetic ground state around 0.4 Kw ith the superexchange coupling via water molecules. The valence of 3+ is found only in Fe-DNA, from which the base ! -band obtains ! charge carriers.

Electronic States of Metal Ions Incorporated in Mn-DNA

It is known that DNA forms compounds with a variety of divalent metal ions, which are located between the bases of a base pair, and that the net charge transfer from the metal ions to DNA occurs only in the Fe-DNA case. However, the electronic states of the metal ions have not been well established, so far. The findings of this study suggest that the water molecules within the DNA double helix play an important role for the exchange coupling between the neighboring Mn ions and that the electronic states of the Mn ions in Mn-DNA are highly ionic on the basis of the Mn hyperfine splitting parameters in solution ESR spectra. The present result should be highly helpful in further investigation of engineering DNA so that it can be used for nanowires.

The effect of oxygen on the ESR linewidth in polypyrrole doped by PF6

Abstract ESR linewidth ( ΔH pp ) of as-grown polypyrrole, PPy-PF 6 − , was measured as a function of temperature under both helium and oxygen atmospheres to study an effect of oxygen on the ESR linewidth. Houze and Nechtschein reported that an interaction between the charge carriers on a polymer backborn and paramagnetic spins of the oxygen molecules could cause an additional ESR broadening that is proportional or inversely proportional to the carrier diffusion rate D ∥ in the case of storng or weak interaction, respectively. The present experimental data demonstrate that only the ESR linewidth under oxygen atmosphere are not enough, but other information is indispensable to discriminate the above two cases. On the present PPy-PF 6 − case, supplemental data on d.c. conductivity, the ESR linewidth under helium atmosphere, VSC resistance, etc. make it possible to conclude mechanism of oxygen effect.

NMR studies of superconducting Ba8AgxSi46−x (x=0∼6)

Abstract NMR spectra of 29 Si and 137 Ba were measured in silicon clathrate compounds, Ba 8 Ag x Si 46−x (x = 0∼6), to investigate their electronic properties and superconducting mechanism. The measurement of the shift shows that the sample with the higher superconducting transition temperature has the larger density of states at the Si and Ba sites. In Ba 8 Si 46 , the Knight shift of Si sites has activation type plus constant temperature dependence between 50 and 300 K. This demonstrates that the electrons at the Fermi level are thermally excited to the Si 46 conduction band states.

Phase transition in alkali-electro-sodalite studied by ESR

Abstract AES (A= Na, K) is a Mott insulator at room temperature consisting of a bcc sub-lattice of F-centers, supported by a zeolite-like framework. Each sodalite cage contains an s -like electron trapped by a spherical potential due to four tetrahedrally arranged alkali cations. On cooling, SES and KES undergo an AF transition at about 50 and 70 K respectively, providing the first example of an s -electron antiferromagnet, ESR linewidth, lineshape and g-shift were measured, which shows unique feature of these s -electron lattices. The linewidth is narrow above T N due to exchange narrowing and rapidly broadens below T N . Under high pressures AES could be metal.

Effect of Pressure on Antiferromagnetic Transition in Alkali-Electro-Sodalite

Abstract Pressure (0–19 kbar) and temperature (4–300 K) dependent EPR study of Sodium-Electro-Sodalite (SES) is presented. SES, which consists of a bcc sub-lattice of F-centers supported by a zeolite-like framework, is known to be a Mott insulator at room temperature. On cooling, SES undergoes an AF transition at 48±2 K providing the first example of an s-electron antiferromagnet. We find that the width of the EPR resonance above T N is influenced not only by a strong exchange interaction, but also by a fast spin-lattice relaxation. Also, with increasing pressure, T N decreases linearly and extrapolates to 0 K at about 65 kbar. The reason for this seemingly unexpected behavior is briefly discussed.