Experimental Methods

In this chapter, several of the most important experimental techniques are described. These have been used to probe the most fundamental properties of the superconducting state: the energy gap and the pairing interaction. These methods have played a crucial role in validating the mechanism of superconductivity in conventional superconductors and are key to a fundamental understanding of superconductivity in more recently discovered novel superconductors like cuprates, Fe-based superconductors, and so on. The techniques that are described are all spectroscopic: tunnelling of quasiparticles through an insulating barrier or through a point contact ,Josephson tunnelling, the interaction of photons with a superconducting film or surface, the attenuation of ultrasonic waves,, the relaxation and/or resonance of muons interacting with a superconducting compound, and resonant inelastic X-ray scattering (RIXS). High-pressure techniques and the preparation of thin films and junctions are described. In addition, a state-of-the-art experimental procedure that enables the observation of the Little mechanism is discussed.

Polar nematic state in an iron-based superconductor LaFeAsO1-xHx

High critical temperature (Tc) superconductivity is generally considered to result from a fluctuation-mediated Cooper-pairing derived from a parent phase. The question of what type of fluctuation forms in materials thus plays a key role in understanding the mechanism of superconductivity. The iron-based superconductor LaFeAsO1-xHx possesses bipartite magnetic parent phases with centrosymmetric (x ~ 0) and non-centrosymmetric (x ~ 0.5) structures. The latter is an intriguing polar-metal phase induced by temperature and carrier doping. Here, we investigate average and local structures of LaFeAsO1-xHx using X-ray diffraction and extended X-ray absorption fine-structure measurements. We found lattice C4 symmetry breaking far above the structural transition temperature, and the signature of a tiny split in As–Fe bond distances with broken spatial inversion symmetry in a wide temperature/doping range. The former reveals a nematic state, and the latter highlights a fluctuated state of polar structure which can be appropriately called polar nematic state.

Old puzzle of incommensurate crystal structure of calaverite AuTe2and predicted stability of novel AuTe compound

Gold is a very inert element, which forms relatively few compounds. Among them is a unique material—mineral calaverite,AuTe2. Besides being the only compound in nature from which one can extract gold on an industrial scale, it is a rare example of a natural mineral with incommensurate crystal structure. Moreover, it is one of few systems based on Au, which become superconducting (at elevated pressure or doped by Pd and Pt). Using ab initio calculations we theoretically explain these unusual phenomena in the picture of negative charge-transfer energy and self-doping, with holes being largely in the Te5pbands. This scenario naturally explains incommensurate crystal structure ofAuTe2, and it also suggests a possible mechanism of superconductivity. An ab initio evolutionary search for stable compounds in the Au–Te system confirms stability ofAuTe2andAuTe3and leads to a prediction of an as yet unknown stable compound AuTe, which until now has not been synthesized.