Nonequilibrium superconducting thin films with sub-gap and pair-breaking photon illumination
Nonequilibrium superconducting thin films with sub-gap and pair-breaking photon illumination
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DOI：
10.1088/0953-2048/28/5/054002
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发表时间：
2015-01
影响因子：
3.6
通讯作者：
T. Guruswamy;D. Goldie;S. Withington
中科院分区：
文献类型：
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作者：
T. Guruswamy;D. Goldie;S. Withington
我们计算了几种常用的低转变温度薄膜超导体在恒定均匀照射下的准粒子和声子的非平衡分布。根据这些分布，我们计算了所有材料(Mo、Al、Ta、Nb和NbN)的所有光子能量的材料特征参数，这些参数允许使用简单的解析表达式快速评估有效的准粒子温度。我们还通过对分裂信号光子发现，在与材料无关的厚膜极限中，温度和能量对低能准粒子产生效率η的影响。考虑到过剩准粒子的能量分布，我们发现当槽温接近相变温度时，→1？gt；与非平衡响应的双温度模型的假设相一致。信号频率受超导能隙控制的η的行为也与材料无关，并与最近的实验结果定性一致。在超导带隙频率附近的信号频率处，子能隙(探测)光子的存在对η的增强最为显著，这是由于多个光子吸收事件使过剩准粒子的平均能量高于没有探测时的平均能量而引起的。
We calculate nonequilibrium quasiparticle and phonon distributions for a number of widely-used low transition temperature thin-film superconductors under constant, uniform illumination by sub-gap probe and pair-breaking signal photons simultaneously. From these distributions we calculate material-characteristic parameters that allow rapid evaluation of an effective quasiparticle temperature using a simple analytical expression, for all materials studied (Mo, Al, Ta, Nb, and NbN) for all photon energies. We also explore the temperature and energy-dependence of the low-energy quasiparticle generation efficiency η by pair-breaking signal photons finding ≈ 0.6 ?> in the limit of thick films at low bath temperatures that is material-independent. Taking the energy distribution of excess quasiparticles into account, we find → 1 ?> as the bath temperature approaches the transition temperature in agreement with the assumption of the two-temperature model of the nonequilibrium response that is appropriate in that regime. The behaviour of η with signal frequency scaled by the superconducting energy gap is also shown to be material-independent, and is in qualitative agreement with recent experimental results. An enhancement of η in the presence of sub-gap (probe) photons is shown to be most significant at signal frequencies near the superconducting gap frequency and arises due to multiple photon absorption events that increase the average energy of excess quasiparticles above that in the absence of the probe.