Tetsufumi Tanamoto

Fermi surface and spin fluctuations in extended t-J model

The spin fluctuations in the extended t - J model have been investigated in the slave-boson mean-field theory by extending the early work by Suzumura et al. [J. Phys. Soc. Jpn. 57 (1988) 2768] based on both uniform and singlet RVB (Resonating Valence Bond). By choosing the transfer integrals to reproduce the Fermi surface of LSCO and YBCO, which is essentially rotated by 45°, it is found that incommensurate spin fluctuations can be expected for the Fermi surface of LSCO type, whereas only commensurate ones for that of YBCO type, in agreement with experiments. Moreover, the Knight shift is shown to decrease as the temperature is lowered in the presence of the singlet RVB, while it is essentially independent of temperature in the uniform RVB state. These two cases correspond to, in our classification, T c =60 K and T c =90 K YBCO systems, which also agrees with the qualitative difference observed experimentally.

Magnetic properties of extended t-J model. II: Dynamical properties

The dynamical spin susceptibility, χ( q , ω), of the extended t - J model, which reproduces Fermi surfaces of LSCO and YBCO type of high- T c cuprates, has been evaluated in the slave-boson mean field approximation based on the uniform (u-) and the d -wave singlet (s-) RVB states. The calculated results show reasonable agreement with neutron scattering data for the incommensurate structure in LSCO. For the commensurate structure in 90 K YBCO, the ω-dependence shows general agreement, but the location of the strongest peak at low temperatures does not. The temperature ( T ) dependences of the rates, T 1 -1 and T G -1 , and the shift of NMR are consistent with experimental data in the high-doping region, while they are not in the low-doping (but superconducting) region. We propose that the sharp drop in ( T 1 T ) -1 above T c in the low-doping region may be due to singlet formation.

Magnetic properties of t-J model

The dynamical spin susceptibility χ( q , ω), of the t-J model in two-dimensional square lattice have been studied based on the mean field approximation to the slave-boson formalism. The static spin susceptibility, χ( q , 0), and Im χ( q , ω) at various energy, ω, have been calculated. We found; 1) χ(0, 0) has only weak temperature dependence 2) T 1 T ∝( T +θ) for \(T\gtrsim0.1{J}\) with θ linearly dependent on the doping rate, δ 3) T 1 T has a minimum at T =(0.1∼0.2) J 4) Im χ( q , ω) at q = Q ≡(π, π) has a gap like structure as a function of ω if the temperature is low. All these are consistent with experimental results.

Magnetic Properties of Extended t-J Model. I. Static Properties

Spin fluctuations in the extended t - J model have been investigated within the slave-boson mean-field approximation based on both uniform and singlet RVB states. By choosing the transfer integrals to reproduce the Fermi surface of LSCO and YBCO, we find that incommensurate spin fluctuations can be expected for the Fermi surface of LSCO-type, whereas only commensurate ones for that of YBCO-type, in agreement with experiments. Moreover effects of singlet RVB pairing have been explored and are shown to lead to the qualitative difference of the temperature dependences of the shift of nuclear magnetic resonance in the high and low doping regions as seen experimentally.

Spin Excitations of the Extended t-J Model: Neutron Scattering

Dynamical spin susceptibility, χ( q , ω), of the t - J model with transfer integrals of the extended spatial range has been evaluated based on the slave-boson mean-field theory for both LSCO- and YBCO-type Fermi surfaces. In the former, the width and height of the incommensurate (IC) peaks are seen to have strong dependences on ω and the temperature, T . Interestingly, and in agreement with experiments, the IC peaks at low energy become sharper in the singlet RVB state which, in our classification, corresponds to the superconducting state in the region of optimal doping. In the case of YBCO, the dependences on ω and T at q = Q ≡(π, π), the antiferromagnetic wave vector, have particular features which share common qualitative characteristics with the experimental findings.

Spin excitations, phonons and phase diagram in high-Tc cuprates

The implications of the slave-boson mean-field theory for the extended t-J model are summarised and compared with various experiments on spin fluctuations and certain phonon modes in high-Tc cuprates, placing special emphasis on the spin-gap phenomenon.

Spin excitations in high Tc cuprates

The results of theoretical studies on the spin excitations of the extended t-J model treated in the slave-boson mean field theory are briefly summarized and compared with the experimental results of neutron scattering and nuclear magnetic resonance both for LSCO and YBCO. It is found that there exists reasonable overall agreement betweern experiments and the theory, which predicts the dx2−y2 superconductivity. The remaining unresolved questions are identified and the possible resolutions for some of these are proposed.

A theoretical model for high-Tc cuprates: The extended t−J model

Abstract The implications of the slave-boson mean-field theory of the t−J model with spatially extended transfer integrals are summarised and compared with experimental results for low-lying excitations of the anomalous metallic state in high-Tc cuprates, specifically for spin excitations and the frequency shifts of certain phonon modes.

Spin dynamics in high- Tc cuprates

Abstract Characteristic features of the spin excitations in high-Tc cuprates revealed by neutron scattering and nuclear magnetic resonance experiments are summarised, and analysed on the basis of the slave-boson mean-field theory for the extended t—J model, placing special emphasis on the spin-gap phenomenon.

Spin excitations and phase diagram of the extended t-J model

Abstract The results of theoretical studies of spin excitations in the extended t-J model treated by the slave-boson mean-field theory are briefly summarized and compared with experimental results of neutron scattering and nuclear magnetic resonance, for both LSCO and YBCO.