Recent Advances in β-FeSe̳1̳-̳x and Related Superconductors

Abstract

[evdate 2012-12-15] IAS Asia Pacific Workshop on Condensed Matter Physics. Talk no. 13 "̳1̳-̳x" is subscript. Host: Institute for Advanced Study. Sponsor: The Collaborative Research Fund (CRF), The Research Grants Council (RGC).

Abstract: It has been more than 4 years since the discovery of β-FeSe̳1̳-̳x superconductor. Through the efforts of many outstanding research groups unprecedented advancement in the field has been achieved. High quality single crystals of β-FeSe̳1̳-̳x and related compounds have been successfully prepared by various techniques, thus allowing us to explore in details the physical properties of this class of materials. Detailed structure and properties characterizations of these crystals have provided critical information for better understanding the origin of superconductivity in β-FeSe̳1̳-̳x. Recently, our high-resolution Transmission Electron Microscopy (HRTEM) study demonstrates the presence and ordering of iron vacancies in high-pressure synthesized tetragonal β-FeSe. Combining analytical electron microscopy and multi-slice simulation, and show that the new phase, called β-FeSe, exhibits an iron-vacancy ordering of 2x2 d110 with 1/2 d100 shift every other (001) plane (the Fe-Se layer). Preliminary measurement of this new phase indicates the existence of a magnetic order at low temperature. It is believed that this new phase is more likely the parent compound of the superconducting phase. On the other hand, it has also been clearly demonstrated that the occurrence of superconductivity is directly associated with a low temperature structure distortion. Several anomalous behaviors are also found to accompany the structural distortion. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in β-FeSe̳1̳-̳x suggested the opening of an energy gap below 130 - 140 K, accompanying with a coincident transfer of the optical spectral weight in the visible range and alterations in the transport properties. These observations provide convincing evidence that the modification of the electronic structure is prior to the lattice distortion. These results further suggest that the high-T gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders. Implication of these results to the occurrence of superconductivity in β-FeSe will be discussed.

Duration: 29 min.