Competing energy scales in granular superconductors: phase-governed superfluid and pseudogap determine the superconducting dome

通过探索一个控制良好的介观系统，我们想要推进我们对它们的关联量子态的理解。颗粒铝可以方便地制成薄膜，其超导性能与大块铝的超导性能有很大的不同。特别是临界温度T_c增加了3倍，超过了3K。这种T_c增加的原因几十年来一直不清楚，但我们最近用光学太赫兹光谱表明，当铝颗粒由于薄的氧化层而越来越多地解耦时，其关键成分是超导能隙的增加。然而，对于更多解耦的颗粒，被抑制的超流刚性是T_c的限制因素。这种能隙和超流刚性的相互作用导致了颗粒铝的特征相图，它具有一个广泛的极大值，即所谓的超导穹顶。我们还在相图的高阻/绝缘一侧发现了非常规行为的第一个迹象，例如光谱中的伪隙。现在，我们想要结合太赫兹和微波光谱来详细地探索高阻颗粒铝的特殊超导性质。我们想要回答的一个问题是，随着T_c的降低，由于量子限制而增强的能隙是如何在相图的绝缘一侧演化的。这涉及到假定的超导体到绝缘体的转变以及伪隙制度，这可能是由无法形成超导凝聚体的局域库珀对引起的。这是相图中特别有趣的部分，我们只能通过将我们的实验扩展到光谱范围内的低得多的能量以及温度来获得。我们目前对颗粒铝的特殊相图的理解是以纳米尺度上的电子限制为关键因素，在我们的例子中，铝颗粒的尺寸为2纳米。晶粒度的进一步减小将导致超导能隙和T_c的进一步提高。因此，我们希望通过热蒸发铝并沉积到冷却到4He温度的衬底来生长颗粒状铝薄膜。这种具有更小颗粒的薄膜将使我们能够将我们对超导颗粒铝的理解扩展到更高的能量，它们还将证明在纳米尺度上控制结构性质可以导致故意增强超导电性，包括更高的T_c。这可以作为其他超导材料的范例。因此，我们还希望生长其他元素超导体作为颗粒膜，并表征它们的能量尺度。这一结果对于理解其他介观体系中的新电子态也很重要。

By exploring a well-controlled mesoscopic system we want to advance our understanding of their correlated quantum states. The superconducting properties of granular aluminum, which is conveniently grown as thin films, differ drastically from those of bulk aluminum. In particular there occurs an enhancement of the critical temperature Tc up to a factor of three, exceeding 3 K. The origin for this enhanced Tc remained unclear for decades, but using optical THz spectroscopy we recently showed that the key ingredient is an increase of the superconducting energy gap when the aluminum grains are more and more decoupled due to a thin oxide layer. For yet more decoupled grains, however, the suppressed superfluid stiffness enters as limiting factor for Tc. This interplay of energy gap and superfluid stiffness leads to the characteristic phase diagram ofgranular aluminum that features a broad maximum, the so-called superconducting dome. We also found first signs of unconventional behavior, such as a pseudogap in the optical spectra, on the high resistivity/insulating side of the phase diagram. Now we want to explore the peculiar superconducting properties of high-resistivity granular aluminum in detail by combining THz and microwave spectroscopy. One question that we want to answer is how the energy gap, which is enhanced due to quantum confinement, evolves on the insulating side of the phase diagram as Tc decreases. This concerns the putative superconductor-to-insulator transition as well as thepseudogap regime, which might be caused by localized Cooper pairs that cannot form a superconducting condensate. This particularly interesting part of the phase diagram we can only access by extending our experiments to much lower energies in spectral range as well as temperature. Our present understanding of the extraordinary phase diagram of granular aluminum has as key element the electron confinement on the nm scale, in our case aluminum grains of 2 nm size. Further reduction of grain size should lead to further enhancement of superconducting energy gap and Tc. Therefore, we want to grow granular aluminum thin films by thermal evaporation of aluminum and deposition onto substrates that are cooled to 4He temperatures. Such thin films with even smaller grains will allow us to extend our understanding of superconducting granular aluminum to yet higher energies, and they will also demonstrate that control of structural properties on the nm scale can lead to a deliberate enhancement of superconductivity including a higher Tc. This can then serve as example for other superconducting materials. Therefore, we also want to grow other elemental superconductors as granular films and characterize their energy scales. The results may also prove important in the understanding of novel electronic states in other mesoscopic systems.