The Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218 USA.
Brookhaven National Laboratory, Upton, New York 11973, USA.
Phys Rev Lett. 2019 Jan 18;122(2):027003. doi: 10.1103/PhysRevLett.122.027003.
Overdoped high-temperature cuprate superconductors have often been understood within the standard BCS framework of superconductivity. However, measurements in a variety of overdoped cuprates indicate that the superfluid density is much smaller than expected from BCS theory and decreases smoothly to zero as the doping is increased. Here, we combine time-domain THz spectroscopy with kHz range mutual inductance measurements on the same overdoped La_{2-x}Sr_{x}CuO_{4} films to determine the total, superfluid, and uncondensed spectral weight as a function of doping. A significant fraction of the carriers remains uncondensed in a wide Drude-like peak as T→0, while the superfluid density remains linear in temperature. These observations are seemingly inconsistent with existing, realistic theories of impurity scattering suppressing the superfluid density in a BCS-like d-wave superconductor. Our large measurement frequency range gives us a unique look at the low frequency spectral weight distribution, which may suggest the presence of quantum phase fluctuations as the critical doping is approached.
过度掺杂高温铜酸盐超导体通常在超导的标准 BCS 框架内得到理解。然而，在各种过度掺杂的铜酸盐中的测量表明，超流密度远小于 BCS 理论所预期的，并且随着掺杂的增加而平滑地降至零。在这里，我们将时域太赫兹光谱学与 kHz 范围内的互感测量相结合，对相同的过度掺杂 La_{2-x}Sr_{x}CuO_{4} 薄膜进行测量，以确定总、超流和未凝聚的光谱权重作为掺杂的函数。在 T→0 时，相当一部分载流子仍保持在类似 Drude 的宽峰中未凝聚，而超流密度仍呈线性温度关系。这些观察结果似乎与现有的、现实的杂质散射抑制 BCS 类似的 d 波超导体中超流密度的理论不一致。我们的大测量频率范围使我们能够独特地观察低频光谱权重分布，这可能表明在接近临界掺杂时存在量子相位涨落。
