Optical properties of exotic low-Tc superconductors

超导体在冷却到“转变温度”Tc以下时，能够在没有能量损失的情况下传输电流。“传统”超导体，如铌和锡的合金，已经被很好地理解，并已被用于从电子到医疗扫描仪中使用的高场磁铁的各种应用。超导体的基本特性之一是携带电流的电子成对存在。这与电子独立运动的“正常”金属不同。普通金属的原子排列成一种叫做“晶格”的周期性结构。一些带负电的电子从每个原子中“挣脱”出来，留下带正电的“离子”。电子在离子晶格中移动；由晶格和电子之间的相互作用产生的配对。分解这样一对电子所需的能量，称为“能隙”，是用来表征超导体特性的一个重要参数。它比可见光所含的能量少1000倍，对应于电磁光谱中被称为远红外（FIR）的区域的辐射。入射到样品上的能量小于能隙的FIR辐射被反射回来，类似于可见光从金属表面反射的方式。测量能被吸收和不能被吸收的能量对理解传统超导体的工作原理有重大贡献。还有许多不太为人所知的、具有非常低Tc的“外来”超导体。在这些材料中，磁性相变发生在超导相变附近，甚至与超导相变共存。这被认为对理解超导性是如何产生的很重要。这些材料对光的反应预计将与传统超导体大不相同。在此，我们提议研究这种低Tc超导体的光学性质；到目前为止还没有进行的测量，因为它们必须在具有实验挑战性的低温和能量下进行，而我们的设备是唯一可以处理的。我们打算扩展我们的系统的能力，以便在磁场存在的情况下进行测量。

A superconductor is able to carry an electrical current with no energy losses when cooled below a `transition temperature', Tc. `Conventional' superconductors, such as an alloy of niobium and tin, are well understood and have been utilized for applications ranging from electronics to high field magnets used in medical scanners. One of the fundamental properties of a superconductor is that the electrons which carry the current exist in pairs. This differs from a `normal' metal where the electrons move independently. The atoms of a normal metal are arranged in a periodic structure called a `lattice'. Some negatively charged electrons `break-free' from each atom leaving positively charged `ions'. The electrons move about in this lattice of ions; the pairing resulting from an interaction between the lattice and the electrons. The energy required to break up one such pair of electrons, called the `energy gap', is an important parameter used to characterize superconductors. It is 1000 times less than the amount of energy contained in visible light and corresponds to radiation in a region of the electromagnetic spectrum called the far-infrared (FIR). FIR radiation incident on the sample with energy less than the energy-gap is reflected back, similar to the way visible light is reflected from a metallic surface. Measuring the energies which can and cannot be absorbed has contributed significantly to understanding how conventional superconductors work. There exist many less well-understood, `exotic' superconductors with very low Tc's. In many of these materials a magnetic phase transition occurs in the vicinity of, or even co-exists with the superconducting phase transition. This is thought to be important for understanding how the superconductivity arises. The response of these materials to light is expected to be very different from that of conventional superconductors. Herein we propose to investigate the optical properties of such low Tc superconductors; measurements that have not been carried out so far because they must be conducted at experimentally challenging low temperatures and energies that our apparatus is uniquely set-up to handle. We intend to extend the capabilities of our system to perform measurements in the presence of a magnetic field.