Yukihiro Shimoi

Spin- and charge-polarized states in nanographene ribbons with zigzag edges

Effects of the nearest-neighbor Coulomb interaction on nanographene ribbons with zigzag edges are investigated using the extended Hubbard model within the unrestricted Hartree-Fock approximation. The nearest Coulomb interaction stabilizes an electronic state with the opposite electric charges separated and localized along both edges, resulting in a finite electric dipole moment pointing from one edge to the other. This charge-polarized state competes with the peculiar spin-polarized state caused by the on-site Coulomb interaction and is stabilized by an external electric field.

Electron Spin Resonance of Field-Induced Polarons in Regioregular Poly(3-alkylthiophene) Using Metal–Insulator–Semiconductor Diode Structures

We report electron spin resonance (ESR) studies on field-induced charge carriers in regioregular poly(3-hexylthiophene) (RR-P3HT) and regioregular poly(3-octylthiophene) (RR-P3OT) for metal–insulator–semiconductor (MIS) diode structures with Al 2 O 3 as an insulating layer. The field-induced ESR signals ( g ∼2.002) were clearly observed; their intensities monotonically increased as the absolute value of the gate bias increased in the accumulation mode with a saturation behavior at higher voltages. The ESR signals were consistent with those of the photogenerated positive polarons in RR-P3HT and RR-P3OT, demonstrating that the carriers are polarons. The transient responses of the field-induced ESR intensity show fast and slow components. The self-organized lamellar molecular orientation was confirmed by the anisotropic ESR signals due to π-electrons, which is semiquantitatively reproduced by an ESR simulation.

Superconducting transition of the two-chain Hubbard model indicated by diagonalization calculations

Abstract In the two-chain Hubbard system the on-site Coulumb interaction at each site generates a pair-transfer interaction between the two bands consisting of bonding and antibonding orbitals for the two sites located on the opposite chains and linked by the transverse transfer energy. It is shown to be as strong as the intraband repulsive interaction generated in the same way so that it can give rise to superconductivity due to the two-band effect. Verifying this reasoning, exact-and also variational-diagonalization calculations clearly indicate that a superconducting state arises in this system in an appropriate region of band and filling parameters for a wide range of Coulomb strengths. A scaling analysis with systems of electron number equal to 6, 10 and 14 supports the assertion that the superconductivity survives in the limit of infinite size. The essence of this mechanism looks relevant to a wide variety of systems including D[M(dmit)2]2 superconductors and high-Tc oxides.

Electronic and optical properties of neutral and charged poly (p-phenylene vinylene )

Abstract Electronic and optical properties of a conjugated polymer, poly ( p -phenylene vinylene) (PPV), are theoretically studied. Long-range interactions among π electrons as well as electron-lattice coupling are taken into account in both neutral and charged states of a PPV chain by applying the Pariser-Parr-Pople model with intermediate interaction strength. This model coupled with the single-excitation configuration interaction method gives an excitation energy diagram for neutral PPV, consistent with experiment. The optical absorption spectrum is calculated in a wide frequency range, and its polarization dependence and spectral features are analyzed in detail. A polaron state in doped PPV is investigated, and its spatial pattern and absorption spectrum calculated. Its spin density gives an electron-nuclear double resonance spectrum, which is in good agreement with experiment. The present model provides a unified description of the electronic properties of neutral and doped PPV.

Spin distributions and excitation spectra of optically generated polarons in poly(p-phenylenevinylene) derivatives

Abstract Light-induced electron spin resonance (LESR) measurements have been performed on two dialkoxy derivatives (MEH-PPV and CN-PPV) of poly(p-phenylenevinylene) where cyano groups are attached at the vinylic carbon sites in CN-PPV. The transient response and excitation spectra of the LESR signals indicate that the observed spins are optically generated polarons. A clear reduction of ESR linewidth has been observed in CN-PPV compared with MEH-PPV, providing direct evidence that the polaron spin density resides predominantly on the vinylic carbons. Linewidth analysis based on a theoretical model for the spin distribution yields an extension of the polaron over approximately four phenyl rings.

Polarons and their ENDOR spectra in poly(p-phenylene vinylene)

Abstract Polaron state in poly( p -phenylene vinylene) (PPV) is studied theoretically by means of the Pariser-Parr-Pople model for π electrons on a PPV chain. The model contains a long range eletron-electron (e-e) interaction as well as an electron-lattice coupling. The electronic structures of polarons are calculated within the unrestricted Hartree-Fock approximation. Based on the calculated spin density of the polaron, we compute electron-nuclear double-resonance (ENDOR) spectra which explain satisfactorily the experimental spectra of undoped PPV measured in a dark state. When the e-e interaction is neglected, the reproduction of the observed spectra becomes poor, demonstrating the importance of the e-e interaction.

Superconductivity indications of the two-chain Hubbard model due to the two-band effect

Abstract Corroborating our assertion worked out in preceding papers that the two-chain Hubbard model has a superconducting (SC) parameter region, we report new results obtained by the Lanczos diagonalization method. The SC region indicated by SC pair correlation functions show the tendency to converge to a considerable definite region in the parameter space with increase of the system size. A certain SC correlation function reveals that the SC order parameters pertinent to the two bands have different signs. The SC pair operators at two rungs keep strong spatial correlation in our SC region in striking contrast to the non-SC region. Calculation of correlation exponent K ϱ allows us to get an SC parameter region which proves to be in a qualitative agreement with the above-mentioned one.

PHOTOGENERATED POLARONS IN POLY(PARAPHENYLENE VINYLENE)

Abstract Photo-induced spin species in poly(paraphenylene vinylene) (PPV) and its derivative have been investigated by means of the light-induced ESR (LESR) technique using variable excitation energy. LESR spectra at the X and K bands for stretch-oriented PPV samples exhibit clear anisotropy, which is well explained in terms of the anisotropic g -value and hyperfine coupling of unpaired π electron on the conjugated chain. The X band spectra were well reproduced by theoretical ESR spectra using a polaron spin density obtained from the Pariser-Parr-Pople model. The LESR signal increases significantly above around 3 eV. This threshold is interpreted as the interband gap above which free electron-hole pairs are created efficiently. From these results, we conclude that the observed light-induced spin species are polarons.

EXACT RESULTS IN STRONGLY CORRELATED ELECTRONS - SPIN-REFLECTION POSITIVITY AND THE PERRON-FROBENIUS THEOREM

This article is devoted to discuss two exact arguments in strongly-correlated electron systems: one is the method of reflection positivity in the spin space and the other is the Perron-Frobenius theorem in matrix theory. For several specific systems, we can show that the ground state is unique and we can predict the total spin of the ground state.

ESR spectroscopy of polarons in conjugated electroluminescent polymers

Polarons are non-linear excitations that carry spin and charge in conjugated polymers. Spin distribution of polarons, or their spatial extension in other words, is one of the essential physical quantities that can be used to identify polarons. In this paper, we discuss the ESR and electron-nuclear double-resonance (ENDOR) observation of polarons in an electroluminescent polymer, poly(paraphenylene vinylene), PPV and its derivatives. Observed spin distribution extend over approximately four phenyl rings in half width, with its maximum density on the carbons of vinyl sites, and is well described by a theoretical calculation in the case of finite electron correlation. We also discuss the light-induced ESR (LESR) observation of the excitation spectra of photogenerated polarons, that compare well with those of photocurrents in PPV derivatives.