Search for novel low-energy states in strongly correlated electron systems

在强相关电子系统中寻找新的低能态

• 批准号: 15540333
• 负责人: OGATA Masao
• 金额: $ 2.24万
• 依托单位: University of Tokyo
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2003
• 资助国家: 日本
• 起止时间: 2003 至 2005
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-15540333/
• 关键词: high-temperature superconductivity; variational Monte Carlo; high-temperature expansion; t-J model; optical sum rule; 時間反転対称性を破る超伝導; Gutzwiller近似; 凝集エネルギー; ハバードモデル; グッツウィラー近似

In this study we planned to study novel low-energy states in strongly correlated electron systems using variational Monte Carlo method and analytical methods. In particular, we take account of the effects of strong correlation which cannot be treated in the mean-field theories and perturbative approaches.We studied the mechanism of high-temperature superconductivity from a viewpoint of condensation energy. We evaluate the ground state energy by variational Monte Carlo method and study the condensation energy by comparing the energies of superconducting state and normal state. As a result, we conclude that the superconductivity is realized by gaining the kinetic energy in the strong correlation regime, which is opposite to the conventional BCS theory. In contrast, in the weak correlation regime, the system behaves similarly to the BCS theory.As another method, we study the microscopic mechanism of the energy gain below Tc in fluctuation-exchange approximation, and obtain similar results. This is a characteristic phenomenon in high-temperature superconductors where the superconducting gap is very large and is due to the change of the renormalization parameters of quasiparticles by the gap. This result also explains the breakdown of the optical sum rule observed experimentally.About the t-J model which is a typical model for strongly correlated electron systems, we study the superconducting correlation functions using high-temperature expansion method which is alternative to the variational Monte Carlo. In this method, the extrapolation to low temperatures is the most difficult part, but we carry out higher order calculations and succeed to evaluate physical quantities in low temperatures by combining a new extrapolation scheme. As a result, we show that the d-wave superconductivity correlation develops in the parameter space corresponding to the actual materials. This supports the results obtained in the variational Monte Carlo method.

本研究计划利用变分蒙特卡罗方法和解析方法研究强关联电子系统中的新低能态。特别地，我们考虑了在平均场理论和微扰方法中不能处理的强关联效应，从凝聚能的角度研究了高温超导的机制。我们用变分蒙特卡罗方法计算了基态能量，并通过比较超导态和正常态的能量来研究凝聚能。结果表明，超导是通过获得强关联区的动能来实现的，这与传统的BCS理论相反。作为另一种方法，我们在涨落交换近似下研究了Tc以下能量增益的微观机制，得到了类似的结果。这是高温超导体中超导能隙很大的特征现象，是由于差距改变了准粒子的重整化参数。对于强关联电子系统的典型模型t-J模型，我们采用高温展开方法代替变分Monte Carlo方法研究了超导关联函数。在该方法中，外推到低温是最困难的部分，但我们进行了高阶计算，并通过结合一种新的外推方案，成功地计算了低温下的物理量。结果表明，d波超导关联是在与实际材料相对应的参数空间中发展的。这支持在变分蒙特卡罗方法中得到的结果。

项目成果

Variational Monte Carlo Studies of Pairing Symmetry for the t-J Model on a Triangular Lattice

• DOI: 10.1143/jpsj.73.3404
• 发表时间: 2004-08
• 期刊: Journal of the Physical Society of Japan
• 影响因子: 1.7
• 作者: Tsutomu Watanabe;H. Yokoyama;Yukio Tanaka;J. Inoue;M. Ogata

Kinetic Energy, Condensation Energy, Optical Sum Rule and Pairing Mechanisms in High-T_c Cuprates

高温铜氧化物中的动能、凝聚能、光和法则及配对机制

• 发表时间: 2005
• 期刊: J.Phys.Soc.Japan 74
• 作者: Y.Yanase;M.Ogata
• 通讯作者: M.Ogata

Crossover of Superconducting Properties and Kinetic-Energy Gain in Two-Dimensional Hubbard Model

二维哈伯德模型中超导特性与动能增益的交叉

• 发表时间: 2004
• 期刊: J. Phys. Soc. Japan 73
• 作者: H.Yokoyama;Y.Tanaka;M.Ogata;H.Tsuchiura
• 通讯作者: H.Tsuchiura

Development of Superconducting Correlation at Low Temperatures in the Two-Dimensional t-J Model

二维t-J模型中低温超导相关性的发展

• 发表时间: 2005
• 期刊: J.Phys.Soc.Japan 74
• 作者: T.Koretsune;M.Ogata
• 通讯作者: M.Ogata

Crossover of Superconducting Properties, Kinetic-Energy Gain in Two-Dimensional Hubbard Model

二维哈伯德模型中超导特性与动能增益的交叉

• 发表时间: 2004
• 期刊: J.Phys.Soc.Japan 73
• 作者: H.Yokoyama;Y.Tanaka;M.Ogata;H.Tsuchiura
• 通讯作者: H.Tsuchiura