Conventional and unconventional superconductivity in chalcogenides under high pressure
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Abstract
A superconductor is a material that can conduct electricity without resistance below a critical temperature Tc. Nowadays, technological applications of superconductors include the design of electromagnets, which are used in MRI/NMR machines, mass spectrometers, particle accelerators, and Josephson junctions, which are the building blocks of the most sensitive magnetometers, particle detectors, including superconducting bolometers and transition edge sensors, as well as low-loss power cables and power storage devices. Therefore, the investigation of high-temperature superconductors is one the most important and challenging problems in the field of solid-state physics and chemistry. Iron chalcogenides are a relatively young and promising family of superconductors. Since the nature of superconductivity in these materials is not fully understood (they are unconventional superconductors), the prospects for the development of their properties are not clear. Applying Mössbauer spectroscopy techniques in combination with magnetic susceptibility and transport measurements under pressure to the simplest systems based mainly on FeSe, we showed how magnetism and superconductivity interact in iron chalcogenides. Magnetic and/or superconducting properties of these materials can be tuned via metal doping, chalcogen substitution or chemical intercalation. Spin fluctuations in high-Tc Lix(NH2)y(NH3)1-yFe2Se2 were shown to be responsible for superconducting pairing at ambient and under applied pressure. For FeSe0.5Te0.5, the electronic phase diagram was investigated, and a structural phase transition associated with disappearance of superconductivity was described. Phase separation in ThCr2Si2-type superconductors was probed by chemical modification using Mössbauer spectroscopy. It was shown that interplay between antiferromagnetic and paramagnetic iron centres, which are responsible for superconducting pairing in RbxFeySe2 series, might be tuned by doping or varying stoichiometry. In contrast to Fe-based materials, metallization of hydrogen sulfide under pressure leaded to the appearance of conventional superconductivity with Tc as high as 203 K, which is 39 K above the previous record in cuprate superconductors. The Meissner effect in H2S under pressure of 155 GPa was demonstrated. Its fundamental parameters, critical field, London penetration depth and coherence length, were found and evidenced that H2S under pressure is a type-II superconductor. A pronounced isotope shift of Tc in D2S suggested an electron-phonon mechanism of superconductivity that is consistent with the Bardeen–Cooper–Schrieffer scenario. The latest says that the presence of hydrogen is a key to the record-high Tc, raising the prospect that even higher transition temperatures – possibly even approaching room temperature – will be discovered in other hydrogen-dominant systems.