标题： 直接可视化由库珀对密度波驱动的向列超导性在NbSe$_2$ 显示英文标题

标题： Directly visualizing nematic superconductivity driven by the pair density wave in NbSe$_2$

摘要： 配对密度波（PDW）是一种独特的超导态，其特征是其序参量在实空间中的周期性调制。 它与电荷密度波（CDW）态的复杂相互作用是凝聚态物理中持续关注的话题。 尽管PDW态已在铜氧化物和其他非常规超导体中被发现，但对各种PDW及其与不同类型的CDW相互作用的理解仍然有限。 在这里，利用扫描隧道显微镜，我们揭示了2H-NbSe$_2$中PDW基态与两种不同的CDW相——即阴离子中心CDW（AC-CDW）和空位中心CDW（HC-CDW）之间的细微关联。 在两个CDW区域中，我们观察到共存的PDW，其结构是共格的，并与底层的CDW相一致。 超导能隙大小，$\Delta(r)$，与配对序参量相关，在两个CDW区域中与电荷密度同相。 同时，相干峰高度，$H(r)$，定性地反映了电子对密度，在相位上相对于CDW有大约$2\pi/3$的差异。 三重旋转对称性在HC-CDW区域中保持不变，但在AC-CDW区域中由于PDW态而自发破坏，导致向列超导性的出现。 显示英文摘要

摘要： Pair density wave (PDW) is a distinct superconducting state characterized by a periodic modulation of its order parameter in real space. Its intricate interplay with the charge density wave (CDW) state is a continuing topic of interest in condensed matter physics. While PDW states have been discovered in cuprates and other unconventional superconductors, the understanding of diverse PDWs and their interactions with different types of CDWs remains limited. Here, utilizing scanning tunneling microscopy, we unveil the subtle correlations between PDW ground states and two distinct CDW phases -- namely, anion-centered-CDW (AC-CDW) and hollow-centered-CDW (HC-CDW) -- in 2H-NbSe$_2$. In both CDW regions, we observe coexisting PDWs with a commensurate structure that aligns with the underlying CDW phase. The superconducting gap size, $\Delta(r)$, related to the pairing order parameter is in phase with the charge density in both CDW regions. Meanwhile, the coherence peak height, $H(r)$, qualitatively reflecting the electron-pair density, exhibits a phase difference of approximately $2\pi/3$ relative to the CDW. The three-fold rotational symmetry is preserved in the HC-CDW region but is spontaneously broken in the AC-CDW region due to the PDW state, leading to the emergence of nematic superconductivity.