arXiv subjects

Takami Tohyama

Publications and source records attributed to Takami Tohyama.

133 records · Page 8Linked to original sources

arXiv2002

Electronic States and Superconductivity in Multi-layer High-Tc Cuprates

We study electronic states of multilayer cuprates in the normal phases as functions of the number of CuO_2 planes and the doping rate. The resonating valence bond wave function and the Gutzwiller approximation are used for a two-dimensional multilayer t-t'-t''-J model. We calculate the electron-removal spectral functions at (π,0) in the CuO_2 plane next to the surface to understand the angle-resolved photoemission spectroscopy (ARPES) spectra. We find that the trilayer spectrum is narrower than the bilayer spectrum but is wider than the monolayer spectrum. In the tri- and tetralayer systems, the outer CuO_2 plane has different superconducting amplitude from the inner CuO_2 plane, while each layer in the bilayer systems has same amplitude. The recent ARPES and NMR experiments are discussed in the light of the present theory.

cond-mat.str-el↗

arXiv2000

Systematic Study of Magnetic Interactions in Insulating Cuprates

The magnetic interactions in one-dimensional, two-dimensional (2D) and ladder cuprates are evaluated systematically by using small Cu-O clusters. We find that the superexchange interaction J between nearest neighbor Cu spins strongly depends on Cu-O structure through the Madelung potential, and in 2D and ladder cuprates there is a four-spin interaction Jcyc, with magnitude of 10% of J. We show that Jcyc has a strong influence on the magnetic excitation in the high-energy region of 2D cuprates.

cond-mat.str-el↗

arXiv1999

Magnetic Interaction in Insulating Cuprates

We examine two-spin (2S) and cyclic four-spin (4S) magnetic interactions in insulating ladder and two-dimensional (2D) cuprates. By a comparison of eigenstates between d-p and Heisenberg models, we evaluate magnitudes of these interactions. We find that the 4S interaction is $\sim$10% of nearest neighboring 2S interaction, and a diagonal 2S interaction is considerably small. The 4S interaction for a ladder cuprate is larger than that for 2D one, and plays an important role in the low-energy excitation. The Heisenberg ladder with the obtained 2S and 4S interactions reproduces very well the experimental result of the temperature dependence of the magnetic susceptibility.

cond-mat.str-el↗

arXiv1998

Effect of Spin Gap on Single-Hole Excitation Spectrum in the One-Dimensional t-J-J'Model

We examine the effect of spin gap on single-hole spectral function in the one-dimensional t-J-J'model. At J'/J=1/2, where a dimer state is the ground state, an analytic expression is derived for J/t \to 0. At finite values of J/t, the numerical diagonalizations for finite-size clusters are performed. In contrast to the gapless case, we find that (i) the dispersion becomes symmetric around k=π/2, (ii) the singularity of the holon band is strongly suppressed, and (iii) the quasi-particle weight Z_{h} becomes finite at finite J/t. The feature (iii) is related to the presence of a bound state of the doped hole and an unpaired spin.

cond-mat.str-el↗

arXiv1998

Electronic States and Excitation Spectra of Copper Oxides with Ladder and/or Chain

Recently the superconductivity has been reported in the copper oxides (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ with ladders and edge-sharing chains under pressure. In order to understand the mechanism of the superconductivity, it is crucial to clarify the distribution of self-doped holes. From the analysis of the optical conductivity by ionic model and exact diagonalization method, we confirm that with substituting Ca for Sr, the self-doped holes are transferred from the chains to the ladders. We also examine the electronic states of edge-sharing chain in a variety of copper oxides, and derive the effective Hamiltonian. The dependences of the magnetic interactions between Cu ions and of the hopping energies of the Zhang-Rice singlet on the bond angle of Cu-O-Cu are discussed.

cond-mat.str-el↗

arXiv1998

Electronic States and Magnetic Propertis of Edge-sharing Cu-O Chains

The electronic states and magnetic properties for the copper oxides containing edge-sharing Cu-O chains such as Li$_2$CuO$_2$, La$_6$Ca$_8$Cu$_{24}$O$_{41}$ and CuGeO$_3$ are systematically studied. The optical conductivity $σ(ω)$ and the temperature dependence of the magnetic susceptibility $χ(T)$ for single crystalline samples Li$_2$CuO$_2$ are measured as a reference system and analyzed by using the exact diagonalization method for small Cu-O clusters. It is shown that the spectral distribution of $σ(ω)$ is different between edge-sharing and corner-sharing Cu-O-Cu bonds. The charge transfer gap in edge-sharing chains is larger than that of high-$T_{c}$ cuprates. The exchange interaction between nearest-neighbor copper ions in edge-sharing chains $J_1$ depends sensitively on the Cu-O-Cu bond angles. In addition to $J_1$, the exchange interaction between next-nearest-neighbor copper ions $J_2$ has sufficient contribution to the magnetic properties. We calculate $J_1$ and $J_2$ for all the copper oxides containing edge-sharing Cu-O chains and discuss the magnetic properties.

cond-mat.str-el↗

arXiv1996

Electronic States of doped spin ladders (Sr,Ca)14Cu24O41

We examine the electronic states of Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ by the ionic and cluster model approach. It is found that self-doped holes are likely to stay on the chain at $x=0$ and the Ca substitution drives the holes to move to the ladder. This feature is caused by the change of the positions of (Sr,Ca) layers, which enhances the electrostatic potentials in the chain. The optical conductivity is calculated applying the exact diagonalization method to small Cu-O clusters. It is shown that the excitations below 2 eV are governed by the ladder, giving rise to the spectral weight transfer from the charge-transfer excitation around 2 eV to low energy Drude excitation, while the contribution from the chain mainly emerges in higher energy region showing large weight around 3 eV. These provide consistent explanation to recent experiment.

cond-mat↗