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arXiv 1308.0664cond-mat.str-elcond-mat.supr-con
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二维吸引哈伯德模型中的BCS-BEC交叉:变分聚类方法

BCS-BEC Crossover in the Two-Dimensional Attractive Hubbard Model: Variational Cluster Approach

Tatsuya Kaneko, Yukinori Ohta

摘要:

利用变分集团近似方法研究了二维吸引哈伯德模型中的超导基态,着重讨论了系统量子涨落的意义.我们首先表明,序参量的抑制相比,使用平均场理论由于在二维系统中的空间波动的影响。然后,我们表明,计算出的Bogoliubov准粒子谱和凝聚振幅清楚地表现出库珀对在动量空间中的字符和对相干长度$ξ$从凝聚振幅评估演示了一个光滑的交叉在实空间中从弱配对BCS状态($ξ\gg a$)的BEC状态的紧密结合对($ξ\ll a$),其中$a$是晶格常数。计算的超导和正常基态的动能和势能表明,弱耦合区的超导态是由势能增益驱动的,而强耦合区的超导态是由动能增益驱动的.

英文摘要:

We use the variational cluster approximation to study the superconducting ground state in the two-dimensional attractive Hubbard model, putting particular emphasis on the significance of quantum fluctuations of the system. We first show that the order parameter is suppressed in comparison with that obtained using the mean-field theory owing to the effects of spatial fluctuations in two-dimensional systems. We then show that the calculated Bogoliubov quasiparticle spectra and condensation amplitude clearly exhibit the character of Cooper pairs in momentum space and that the pair coherence length $ξ$ evaluated from the condensation amplitude demonstrates a smooth crossover in real space from a weakly paired BCS state ($ξ\gg a$) to a BEC state of tightly bound pairs ($ξ\ll a$), where $a$ is the lattice constant. The calculated kinetic and potential energies in the superconducting and normal ground states indicate that the superconducting state in the weak-coupling region is driven by the gain in potential energy, while that in the strongcoupling region is driven by the gain in kinetic energy.

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