Keita Kishigi

Spin-Density-Wave Transition in Quasi-One-Dimensional System Under Uniaxial Pressure

We have investigated the effect of uniaxial pressure on the transition temperature of spin-density-wave (SDW) in quasi-one-dimensional organic conductors. Although the imperfectness of the Fermi surface nesting is reduced when the hopping matrix element t a increases by uniaxial pressure along the most conducting a axis, which is favorable for the SDW state, the decrease of density of the state due to the increase of t a suppresses the SDW transition. As a result, T SDW decreases as t a increases. This is consistent with the recent experimental result for (TMTSF) 2 PF 6 under uniaxial strain along the a axis.

Two Kinds of the Coexistent States in One-Dimensional Quarter-Filled Systems under Magnetic Fields and Temperatures

The coexistent state of the spin density wave (SDW) and the charge density wave (CDW) in the one-dimensional quarter-filled system with the Coulomb interaction up to the next-nearest sites under magnetic fields is studied. It is found that under high magnetic fields, in the coexistent state of 2 k F -SDW and 2 k F -CDW, the charge order is suppressed, whereas, in the coexistent state of 2 k F -SDW and 4 k F -CDW, 4 k F -CDW still remains and 2 k F -SDW is suppressed. The critical temperature of the charge order is higher than that of the spin order when the inter-site Coulomb interaction is strong. These will be observed in experiment such as the X-ray scattering measurement.

High Field Phase Diagram in (TMTSF)2ClO4 under Anion Ordering

We study the ordered states of the field induced spin density wave (FISDW) in the presence of an anion ordering potential at zero and finite temperature self-consistently. We obtain a new FISDW state stabilized in a strong potential region, by which we can explain recently obtained phase diagram of (TMTSF) 2 ClO 4 . The anomalous behavior of the so-called rapid oscillation in the transport and thermodynamics quantities is attributable to the new FISDW state.

Field-Induced Spin Density Wave in Periodic Potential

We study the Field-Induced Spin Density Wave in the presence of the periodic potential which is treated non-perturbatively. We obtain that both even and odd subphases weakly depend on the magnetic field in the strong potential case, while in the perturbation only even subphases strongly depend on the magnetic field. We discuss the recently reported phase diagram in (TMTSF) 2 ClO 4 .

Coexistent State of Charge Density Wave and Spin Density Wave in One-Dimensional Quarter Filled Band Systems under Magnetic Fields

We theoretically study how the coexistent state of the charge density wave and the spin density wave in the one-dimensional quarter filled band is affected by magnetic fields. We found that when the correlation between electrons is strong the spin density wave state is suppressed under high magnetic fields, whereas the charge density wave state still remains. This will be observed in experiments such as the X-ray scattering measurement.

Enhancement of de Haas-van Alphen Oscillation due to Spin in the Magnetic Breakdown System

The effects of the Zeeman term on the de Haas-van Alphen oscillation are studied in the magnetic breakdown system. We find that the amplitudes of the oscillations with the frequencies of f β + f α and f β +2 f α are enhanced by the Zeeman term, while they are expected to be reduced according to semiclassical theory. A possible interpretation of the experiments in organic conductors is discussed.

Superconductivity of Quasi-One- and Quasi-Two-Dimensional Tight-Binding Electrons in a Magnetic Field

The upper critical field H c2 ( T ) of tight-binding electrons in a three-dimensional lattice is investigated. The electrons form Cooper pairs between eigenstates with the same energy in a strong magnetic field. The transition lines in the quasi-one-dimensional case are shown to deviate from the previously obtained results when the hopping matrix elements along the magnetic field are neglected. In the absence of Pauli pair breaking the transition temperature T c ( H ) of the quasi-two-dimensional electrons is found to increase in an oscillatory manner as the magnetic field becomes large and to reach T c (0) in the strong field, as in the quasi-one-dimensional case.

Theory of the High Field Phase in (TMTSF)2ClO4 under Anion Ordering

Stimulated by a recent experiment on (TMTSF) 2 ClO 4 , reporting a new phase diagram with a first order transition line at the high field, the field induced spin density wave (FISDW) states are reexamined under the anion ordering. It is found that a new type of FISDW state, which differs from that described by the so-called standard theory, is stabilized. We interpret the first order line as arising from a transition between the new state and the standard one when H is varied.

Spin Splitting Effect on de Haas-van Alphen Oscillation in Two-Dimensional Two-Band Systems

We study the effects of the spin splitting due to the Zeeman term on the de Haas van Alphen (dHvA) oscillation in two-dimensional two-band systems. Both systems with and without the magnetic breakdown are investigated. We found that the Fourier transform amplitudes of the oscillations are not described by the spin reduction factor as in the Lifshitz-Kosevich formalism in two-dimensional systems. The anomalous dependence of the Fourier transform amplitudes on the effective g -factor can be observed by tilting-angle dependence of the dHvA oscillation in Sr 2 RuO 4 and quasi-two-dimensional organic conductors.We study the effects of the spin splitting due to the Zeeman term on the de Haas van Alphen (dHvA) oscillation in two-dimensional two-band systems. Both systems with and without the magnetic breakdown are investigated. We found that the Fourier transform amplitudes of the oscillations are not described by the spin reduction factor as in the Lifshitz-Kosevich formalism in two-dimensional systems. The anomalous dependence of the Fourier transform amplitudes on the effective g -factor can be observed by tilting-angle dependence of the dHvA oscillation in Sr 2 RuO 4 and quasi-two-dimensional organic conductors.

Superconductivity of Quasi-Two-Dimensional Tight-Binding Electrons in a Strong Magnetic Field

We have investigated the transition temperature T c ( H ) of superconductivity in quasi-two-dimensional (Q2D) tight-binding electrons in a strong magnetic field. When the magnetic field is parallel to 2D conducting plane, T c ( H ) of the Q2D superconductor is shown to increase in an oscillatory manner as the magnetic field becomes large and to reach T c (0) in a strong magnetic field limit for the spin-triplet superconductor. We consider the cases of on-site and nearest sites attractive interaction, and calculate the magnetic field dependences of the transition temperature for various types of symmetry. The first order transition from p y -wave to p x -wave is shown to occur at H ∼100xa0T when the magnetic field is parallel to the y direction, which will be observed in a triplet superconductor, Sr 2 RuO 4 .