Optical properties of exotic low-Tc superconductors

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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.