We examine quasi-two-dimensional superconductors near half-filling under uniaxial pressures perpendicular to conductive layers (hereafter called perpendicular pressures). It is a natural conjecture that the perpendicular pressure decreases Tc because it increases the interlayer electron hopping energy t_z, which weakens the logarithmic enhancement in the density of states due to the two-dimensional Van Hove singularity. It is shown that, contrary to this conjecture, the perpendicular pressure can significantly enhance Tc in systems off half-filling before it decreases Tc, and the strength of the enhancement significantly depends on the pairing symmetry. When the indices d, d', cz, and sz are defined for the basis functions cos k_x - cos k_y, sin k_x sin k_y, cos k_z, and sin k_z, respectively, it is shown that for s-, d-, cz-, and cz-d-wave pairing, Tc steeply increases with increasing t_z near a cusp at a certain value of t_z. On the other hand, for p-, cz-p-, sz-p-, and d'-wave pairing, Tc is almost unaffected by tz. For sz- and sz-d-wave pairing, Tc exhibits a broad and weak peak. The enhancement in Tc is the largest for this state and the second largest for the d-wave pairing and interlayer singlet (cz-wave) pairing. These results may explain recent observations in Sr2RuO4 under perpendicular pressures. A comparison between the theoretical and experimental results indicates that the p-, cz-p-, and sz-p-wave states, including chiral states, and the d'-wave state are the most likely candidates for the intrinsic 1.5-K phase, and the d-, cz-d-, and cz-wave states are the most likely candidates for the 3-K phase induced by the perpendicular pressure. The cz-p- and sz-p-wave states are interlayer triplet and interlayer singlet p-wave states with horizontal line nodes, respectively.

中文翻译：

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准二维超导体中垂直单轴压力引起的残留范霍夫奇点和高温超导相

我们研究了在垂直于导电层的单轴压力（以下称为垂直压力）下接近半填充的准二维超导体。很自然的猜测是，垂直压力降低了 Tc，因为它增加了层间电子跳跃能量 t_z，这削弱了由于二维范霍夫奇点引起的态密度的对数增强。结果表明，与该猜想相反，垂直压力可以在降低 Tc 之前显着提高半填充系统中的 Tc，并且增强的强度显着取决于配对对称性。当分别为基函数 cos k_x - cos k_y、sin k_x sin k_y、cos k_z 和 sin k_z 定义索引 d、d'、cz 和 sz 时，表明对于 s-、d-、cz -, 和 cz-d 波配对，Tc 在 t_z 的某个值处随着 t_z 的增加而急剧增加。另一方面，对于 p-、cz-p-、sz-p- 和 d'-波配对，Tc 几乎不受 tz 影响。对于 sz- 和 sz-d- 波配对，Tc 表现出宽而弱的峰值。Tc 的增强对于这种状态是最大的，对于 d 波配对和层间单线态（cz 波）配对是第二大的。这些结果可以解释最近在垂直压力下 Sr2RuO4 中的观察结果。理论和实验结果之间的比较表明 p-、cz-p- 和 sz-p-波态，包括手征态和 d'-波态是本征 1.5-K 相最可能的候选者, d-, cz-d-, 和 cz- 波状态最有可能是由垂直压力引起的 3-K 相。