Electrodynamics of the Superconducting State in Ultra-Thin Films at THz Frequencies

In this paper, we report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 3 to 38 <formula formulatype="inline"><tex Notation="TeX">${\hbox{cm}}^{-1}$</tex></formula> (0.1–1.1 THz) by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures <formula formulatype="inline"><tex Notation="TeX">$T_{c}$</tex> </formula>. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. By and large, we find the properties of our NbN and TaN thin films to be well described by the theory. Our results on NbN resemble tendencies towards anomalous behavior of the ratio <formula formulatype="inline"><tex Notation="TeX">$2\Delta(0)/k_{B}T_{c}$</tex> </formula> as a function of <formula formulatype="inline"><tex Notation="TeX">$T_{c}$</tex> </formula>.

在本文中，我们报道了太赫兹频域光谱（THz - FDS）实验，在该实验中我们测量了薄膜超导系统在太赫兹光谱范围内低温下的电荷载流子动力学和激发。对装置的特性和实验过程进行了全面描述。我们讨论了单粒子态密度以及常规超导体的电动力学吸收和光学导率理论。我们以具有不同临界温度$T_{c}$值的弱无序超导体氮化铌（NbN）和氮化钽（TaN）的超薄膜为例，展示了该装置在3到38 ${\hbox{cm}}^{-1}$（0.1 - 1.1太赫兹）的宽光谱范围内低温下的实验性能。此外，我们在常规的巴丁 - 库珀 - 施里弗（BCS）超导理论框架内分析和解释我们的实验数据。总体而言，我们发现我们的NbN和TaN薄膜的特性可以用该理论很好地描述。我们关于NbN的结果类似于$2\Delta(0)/k_{B}T_{c}$比值作为$T_{c}$函数的异常行为趋势。