Charge-density wave order is now understood to be a widespread feature of underdoped cuprate high-temperature superconductors, although its origins remain unclear. While experiments suggest that the charge-ordering wavevector is determined by Fermi-surface nesting, the relevant sections of the Fermi surface are featureless and provide no clue as to the underlying mechanism. Here, focusing on underdoped YBa₂Cu₃O_6+x, we propose that charge-density waves form from the incipient softening of a bond-buckling phonon. The momentum dependence of its coupling to itinerant electrons favourably selects the wavevector found in experiments. But, it requires quasiparticle renormalization by strong electronic correlations to enable a unique enhancement of the charge susceptibility near the B_1g-phonon selected wavevector. The B_1g phonon frequency softens by a few percent, and finite-range charge-density wave correlations will form locally, if nucleated by defects or dopant disorder. These results suggest that underdoped cuprates cannot be understood in the context of strong electronic correlations alone.

电荷密度波阶现在被认为是掺杂不足的铜酸盐高温超导体的普遍特征，尽管其起源尚不清楚。尽管实验表明电荷排序波矢量由费米表面嵌套确定，但费米表面的相关部分没有特征，也没有提供有关潜在机理的线索。在这里，重点介绍掺杂不足的YBa ₂ Cu ₃ O _{6+ x}，我们提出，电荷屈曲声子的初期软化会形成电荷密度波。其耦合到流动电子的动量依赖性有利地选择了实验中发现的波矢。但是，这需要通过强电子相关性对准粒子进行重新归一化，以使B _{1 g-}声子选择波矢附近的电荷磁化率唯一提高。B _{1 g}声子频率会软化百分之几，并且如果缺陷或掺杂剂紊乱形核，则会在局部形成有限范围的电荷密度波相关性。这些结果表明，仅在强电子相关性的背景下不能理解掺杂不足的铜酸盐。