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arXiv 0903.3454cond-mat.supr-con
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准二维超导体中从涡旋态到Fulde-Ferrell-Larkin-Ovchinnikov态的渡越

Crossover from the vortex state to the Fulde-Ferrell-Larkin-Ovchinnikov state in quasi-two-dimensional superconductors

Hiroshi Shimahara

摘要:

研究了准二维Ⅱ类超导体中涡旋态与Fulde-Ferrell-Larkin-Ovchinnikov(FFLO)态的共存以及当Maki参数α增大时从共存态到纯FFLO态的过渡.当磁场平行于导电平面时,在二维极限下,出现了由库珀对的有限质心动量q \ne 0表征的纯FFLO态.矢量q由费米面结构和配对各向异性决定,在温度T^* 以下为有限。在准二维情况下,由于轨道对破缺效应,出现了(n,q//)共存态,其中n和q//分别表示涡旋态的朗道能级指数和沿磁场的附加FFLO调制波数.通过数值计算得到了上临界场与α的关系。在H-T相图中,上临界场呈现级联曲线。分析表明,n在低于T^* 的二维极限α \to \infty中发散。在此极限下,共存态的上临界场方程简化为FFLO态的上临界场方程。得到了共存态的n与纯FFLO态的q_{\perp}之间的关系,其中q_{\perp}表示q的垂直于磁场的分量.我们发现,在倾斜磁场中,纯FFLO态只不过是二维超导体中已知的具有无限大n的涡旋态。在三维空间中,具有较大n的涡旋态可以看作是具有非零q_{\perp}的FFLO态.

英文摘要:

We examine the coexistence of the vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in quasi-two-dimensional type-II superconductors and the crossover from the coexistence state to the pure FFLO state when the Maki parameter αincreases. The pure FFLO state, characterized by finite center-of-mass momenta q \ne 0 of Cooper pairs occurs in the two-dimensional limit, when the magnetic field is parallel to the conductive plane. The vectors q are determined from the Fermi-surface structure and pairing anisotropy, and become finite below a temperature T^*. In quasi-two-dimensions, because of the orbital pair-breaking effect, the coexistence state characterized by (n,q//) occurs, where n and q// denote the Landau level index of the vortex state and the wave number of the additional FFLO modulation along the magnetic field. We obtain the αdependence of the upper critical field by numerical calculations. The upper critical field exhibits a cascade curve in the H-T phase diagram. It is analytically shown that n diverges in the two-dimensional limit α\to \infty below T^*. In this limit, the upper critical field equation of the coexistence state is reduced to that of the FFLO state. A relation between n of the coexistence state and q_{\perp} of the pure FFLO state is obtained, where q_{\perp} denotes the component of q perpendicular to the magnetic field. It is found that the pure FFLO state is nothing but the vortex state with infinitely large n as is known in two-dimensional superconductors in a tilted magnetic field. The vortex state with large n can be regarded as the FFLO state with non-zero q_{\perp} in three dimensions.

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