Shunichi Matsuno

Spin-polaron pairing and high-temperature superconductivity

Abstract We show that in superconducting materials with CuO 2 layers spin-polarons associated with itinerant holes in the e v band (Cu dz 2 - Opπ band,) are created by strong intra-atomic exchange interaction (Hunds coupling) with localized dx 2 −y 2 holes. Then the interplay of Hunds coupling and superexchange between Cu 2+ (dx 2 −y 2 ) spins gives rise to an attractive exchange interaction between spin-polarons, which leads to the formation of spin-singlet spin-polarons pairs. This contributes to high temperature superconductivity. By solving an gap equation it is shown that the s-wave pairing reproduces the observed x dependence of T c as well as the right magnitudes of T c in (La 1− x Sr x ) 2 CuO 4 satisfactorily.

Possible mechanism of superconductivity in electron superconducting cuprates

Abstract Based on the recent result of cluster calculations for electron doped superconductor that electrons are accommodated in the 4s level of Cu atoms, we propose a unified treatment of electron- and hole-doped superconductivities, i.e. the spin-polaron pairing mechanism developed by Kamimura et al. . For this purpose we perform a computer simulation of a 2D quantum spin system with two impurity spins and show that the long-ranged attractive interaction exists between two spin-polarons. Thus a spin-polaron system forms Cooper pairs and condensates to a usual BCS type wave function. Our mechanism can explain the observed doping concentration and pressure dependences of Tc.

A Novel Approach to the Polaronic Metallic State of Cuprate Superconductors and the d-Wave Pairing Mechanism

The present novel approach consists of two stages: in the first stage the many-electron states of a CuO6 octahedron or a CuO5 pyramid are calculated by the first-principles variational method, by taking into account the local distortions of a CuO6 octahedron or a CuO5 pyramid. In the second stage a metallic state is constructed in the presence of the local AF ordering constructed by the localized spins. In this metallic state the local distortions are treated in the mean field approximation, so that a “pseudopolaronic effect” is taken into account. Based on this approach, the hole-concentration dependence of Tc and the isotope effect are calculated for LSCO, and compared with experimental results. This approach leads to d-wave pairing mechanism.

Electrostatic energy, potential energy, and energy dissipation for a width-variable capacitor system during adiabatic charging

This paper considers the energy consumed by charging and discharging a width-variable capacitor. The capacitor with plate distance d is coupled with repulsive mechanical potential energy, which is proportional to 1∕dn. In this capacitor model, there is a stable point between attractive electrical force and repulsive mechanical force. All energies, including the electrostatic energy, potential energy, and energy dissipation, are proportional not to the ordinary value V2 but to V2∕(n−1)+2, where V is the abrupt power supply voltage. We apply N-stepwise adiabatic charging to the width-variable capacitor system. It is shown that the energy consumption after charging and discharging (or recycling) can be 1∕N times smaller than that of the conventional abrupt operation. By increasing the step number N, the adiabatic operation can ideally charge and discharge the width-variable capacitor system with absolutely no energy dissipation, although the voltage dependence of energies is quite different from the usual on...

Effective-Mass Anomalies of Strained Silicon Thin Films: Surface and Confinement Effects

Using a first-principles calculation, we systematically investigated the effective-mass anomalies in - and -confined Si thin films. The surface terminators, which correspond to the interface between the Si channel and the insulator in real devices, do not have a significant effect on the effective mass anomalies. The effective-mass behaviors as a function of the substrate thickness and the strain is qualitatively well explained by the extended effective-mass approximation using the bulk effective-mass along the confinement direction.

Electronic structure of Nd2CuO4 and its physical properties

In order to clarify the electronic structure of the electron-doped superconducting material Nd2−xMxCuO4, we have performed a first-principle band structure calculation for the matrix material Nd2CuO4. We find that doped electrons can be accommodated in both the Cu 4s and 4pz conduction bands provided that the Cu 3dx2−y2 band splits into two bands, i.e., the upper and lower Hubbard bands by the strong correlation. Based on this electronic structure, we have calculated the Hall coefficient of the Nd system and have shown for the first time that the sign of the Hall coefficient is negative, coinciding with the experimental result in the low-concentration case. In the overdoped region above x=0.18, the dopant electrons occupy not only 4s and 4pz bands but also the upper Hubbarddx2−y2 so that the Hall coefficient changes its sign from negative to positive.

Clarification of normal state properties of La2−zSrzCuO4

Abstract Within the framework of the spin-polaron model proposed by Kamimura, the anomalous behavior of the spin susceptibility in the normal state of La 2− z Sr z CuO 4 is investigated. It is shown that the interplay of the Zeeman interaction in the Cu dz 2 upper Hubbard band, the Hunds coupling between dz 2 holes and Cu dx 2 -y 2 localized spins and the antiferromagnetic interaction between the localized spins plays an essential role in determining the x- and T-dependences of the spin susceptibility.

Spin-Polaron Pairing Mechanism in the High Tc Copper Oxides

On the basis of the spin-polaron pair model with s-wave pairing recently developed by Kamimura, Matsuno and Saito, the origin of the x dependence of Tc and that of a high value of Tc in La2-xSrxCuO4 are clarified. Predictions are also made on (i) the coexistence of the superconducting phase and spin fluid phase and (ii) the simultaneous occurrence of the superconducting phase and the metal-insulator transition. Anomalous behaviour of NMR and Hall effects in high Tc copper oxides are discussed in terms of the spin-polaron pair model.

New theory of superconducting copper oxides—from electronic structure to the d-wave pairing mechanism

Abstract Firstly, a brief review is given on recent theoretical developments made by our group as to the many-body effect including electronic structure of hole-doped cuprates. Then the mechanism of superconductivity is discussed on the basis of the above-mentioned electronic structure of LSCO. In doing so we present the results of quantitative calculation of the electron phonon coupling constants and the k, k′ dependence of the spectral function α2F (Ω, k, k′) made on the basis of the renormalized one-electron-type band. The following important results emerge from this calculation: (1) d-wave pairing occurs even for the phonon mechanism, provided the localized spins form a local antiferromagnetie order: (2) phonon modes in which oxygen ions move vertically for CuO2 plane contribute to the electron phonon interaction in cuprates while those like breathing modes in which oxygen and copper ions vibrate within a CuO2 plane do not contribute significantly: (3) the electron phonon interaction in CuO5 pyramid-type cuprates is stronger than CuO6 octahedral-type cuprates.

On the Interplay of Jahn–Teller Physics and Mott Physics in the Mechanism of High T c Superconductivity

Based on the model proposed by Kamimura and Suwa which bears important characteristics born from the interplay of Jahn–Teller Physics and Mott Physics, it is shown that the feature of Fermi surfaces is the Fermi pockets constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. Then it is discussed that the phonon-involved mechanism based on the Kamimura–Suwa model leads to the d-wave superconductivity. Further it is shown that T c is higher in the cuprates with CuO5 pyramid than those with CuO6 octahedron. Finally a new phase diagram for underdoped cuprates is proposed.