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Journal of Modern Physics 2013年第6期4卷 33-64页

作者： Takashi Yanagisawa Mitake Miyazaki Kunihiko Yamaji Electronics and Photonics Research Group National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba Central 2 Tsukuba Japan Hakodate National College of Technology Hakodate Japan

We present recent theoretical results on superconductivity in correlated-electron systems, especially in the two-dimensional hubbard model and the three-band d-p model. The mechanism of superconductivity in high-temperature superconductors has been extensively studied on the basis of various electronic models and also electron-phonon models. In this study, we investigate the properties of superconductivity in correlated-electron systems by using numerical methods such as the variational Monte Carlo method and the quantum Monte Carlomethod. The hubbard model is one of basic models for strongly correlated electron systems, and is regarded as the model of cuprate high temperature superconductors. The d-p model is more realistic model for cuprates. The superconducting condensation energy obtained by adopting the Gutzwiller ansatz is in reasonable agreement with the condensation energy estimated for YBa2Cu3O7. We show the phase diagram of the ground state using this method. We have further investigated the stability of striped and checkerboard states in the under-doped region. Holes doped in a half-filled square lattice lead to an incommensurate spin and charge density wave. The relationship of the hole density x and incommensurability δ, δ~x, is satisfied in the lower doping region, as indicated by the variationalMonte Carlocalculations for the two-dimensional hubbard model. A checkerboard-like charge-density modulation with a roughly period has also been observed by scanning tunneling microscopy experiments in Bi2212 and Na-CCOC compounds. We have performed a variational Monte Carlo simulation on a two-dimensional t-t′-t″- U hubbard model with a Bi-2212 type band structure and found that the period checkerboard spin modulation, that is characterized by multi Q vectors, is indeed stabilized. We have further performed an investigation by using a quantumMonte Carlomethod, which is a numerical method that can be used to simulate the behavior of correlated electron systems. We present a new algorithm of the quantum Monte

关键词： High-Temperature Superconductivity Strongly Correlated Electrons Monte Carlo Methods hubbard Model Condensation Energy Pair-Correlation Function

Maximally Entangled States in the Hydrogen Molecule: The Role of Spin and Correlation

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Journal of Modern Physics 2011年第7期2卷 664-668页

作者： Mohammad Ali Vesaghi Mohsen Babamoradi Mehdi Heidari Saani 不详

Many electron calculations on a simplest realistic two electron system i.e. H2molecule was applied and as the consequence correlation effects was reflected accurately in the wavefunctions of H2. Zanardi’s entanglement measurement, demonstrated that the maximum of entanglement for the ground state happens when U =J and this resolved the controversial conclusion of U = 0 for maximum entanglement. It was shown that the ground and third excited states are maximally entangled. These maximally entangled states and also the minimally entangled states are correlated to their spin’s property. The wavefunctions of the not magnetic (S = 0) ground and excited states explicitly depend on correlation parameters whereas the first excited states which is magnetic (S2 = 2 and Sz≠0) is not entangled. The second excited state is not magnetic but its wavefunction does not depend on correlation parameters therefore it is a moderately entangled state. In any case, by switching on a magnetic field an entangled state with Sz = 0 can be extracted from a not entangled degenerate magnetic state. We suggest that in a realistic molecular scale system, there is two criteria for finding maximally entangled electronic states, first the system should be in moderately correlated regime and second the system should have a non-magnetic (Sz = 0) electronic state.

关键词： hubbard Model Entanglement Hydrogen Molecule

A POSSIBLE PICTURE OF HIGH-TC SUPERCONUUCT

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Chinese Physics Letters 1988年第8期5卷 369-372页

作者：冯世平邢定钰 Department of Physics Beijing Normal University Department of Physics Nanjing University

The extended hubbard model proposed by Emery for CuO_(2) basal plane is used to discuss high temperature superconducting oxides.It is shown that oxygen holes may form singlet pairs with neighbouring copper holes and t... 详细信息

关键词： conductivity singlet hubbard

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