Karlubíková Paulína, Růžičková Hana, Chaloupka Jiří, Munzar Dominik
Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic.
J Phys Condens Matter. 2019 Apr 3;31(13):135502. doi: 10.1088/1361-648X/aafd10. Epub 2019 Jan 9.
Motivated by similarities between cuprate superconductors and two-leg ladder copper-oxide compounds and in order to obtain a better understanding of optical properties of cuprate superconductors we have studied the c-axis (along the ladder) optical conductivity [Formula: see text] of a doped [Formula: see text] two-leg ladder. Using exact diagonalization, we have calculated the conductivity and related quantities for cyclic ladders of up to 13 rungs. In agreement with results of an early study by Hayward and coworkers (Hayward et al 1996 Phys. Rev. B 53 8863) we find that [Formula: see text] consists of a Drude peak at zero frequency and an absorption band in the infrared region that is separated from the former by a pseudogap. The width of the pseudogap [Formula: see text] increases with increasing J/t, in parallel with an increase of the magnitude [Formula: see text] of the gap in the quasiparticle excitation spectra. Our central finding is that [Formula: see text], where [Formula: see text] is the magnitude of the gap in the spin excitation spectra. We demonstrate that this approximate relation can be understood in terms of a phenomenological model involving a superconducting ladder and a coupling between charged quasiparticles and spin excitations. The relation is remarkably similar to the one between experimental values of the energy scale of a dip in the in-plane optical conductivity, the superconducting gap [Formula: see text] and the energy of the spin-resonance in cuprate superconductors (for a recent discussion of the optical data, see Šopík et al 2015 New J. Phys. 17 053022). Our findings support the point of view that low energy infrared active excited states of cuprate superconductors can be viewed as consisting of two charged quasiparticles connected with pair-breaking and a spin excitation.
受铜酸盐超导体与两腿梯形铜氧化物化合物之间相似性的启发，为了更好地理解铜酸盐超导体的光学性质，我们研究了掺杂的(t-J)两腿梯形的(c)轴（沿梯形方向）光导率(\sigma_{c}(\omega))。通过精确对角化，我们计算了多达13个梯级的循环梯形的电导率及相关量。与Hayward及其同事早期的研究结果（Hayward等人，1996年，《物理评论B》53卷，8863页）一致，我们发现(\sigma_{c}(\omega))由零频率处的德鲁德峰和红外区域的吸收带组成，该吸收带与前者由一个赝能隙隔开。赝能隙(\Delta_{\text{pg}})的宽度随(J/t)的增加而增加，与准粒子激发谱中能隙(\Delta)的大小增加并行。我们的核心发现是(\Delta_{\text{pg}}\approx\Delta/2)，其中(\Delta)是自旋激发谱中的能隙大小。我们证明，这种近似关系可以通过一个涉及超导梯形以及带电准粒子与自旋激发之间耦合的唯象模型来理解。该关系与铜酸盐超导体中面内光导率下降的能量尺度的实验值、超导能隙(\Delta)和自旋共振能量之间的关系非常相似（关于光学数据的最新讨论，见Šopík等人，2015年，《新物理学杂志》17卷，053022页）。我们的发现支持这样一种观点，即铜酸盐超导体的低能红外活性激发态可以看作是由两个与配对破坏相关的带电准粒子和一个自旋激发组成。
