报告题目：Multicomponent superconductivity and four-fermion phase in the Ba1-xKxFe2As2 system

报 告 人：Vadim Grinenko, Tsung-Dao Lee Institute, Shanghai Jiao Tong University

报告时间：2024-03-08 10:00

报告地点：理科楼C302

报告摘要：In general, magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors in which superconductivity coexists with magnetism, and even fewer examples are known where the superconductivity itself induces spontaneous magnetism. The nature of superconductivity with broken time-reversal symmetry (BTRS) is controversial in some cases due to a lack of systematic experimental studies, contradictory experimental facts and the complexity of a theoretical description of these states. In this talk, I will present our finding of the BTRS superconducting phase in the centrosymmetric multiband superconductor Ba1−xKxFe2As2 (0.7≲x≲0.85), Fig.1 [1,2]. We observe spontaneous magnetic fields in this doping range using the muon spin relaxation (μSR) technique and spontaneous Nernst effect measurements. BTRS superconductivity implies a complex multicomponent superconducting order parameter. The evidence for multicomponent superconductivity in Ba1 − xKxFe2As2 is provided by observation of fractional vortices using scanning SQUID [3] and STM microscopy. Furthermore, I will present a set of experimental observations that evidence the formation of a fermionic quadrupling condensate above superconducting transition temperature at the summit of the BTRS dome [4,5]. This phase breaks time-reversal symmetry and, therefore, is different in symmetry from superconducting and normal states. We attribute the existence of the phase to the theoretical concept that in materials with broken multiple symmetries, the formation of four-fermion condensates is allowed under certain conditions. Such states can be stabilized by phase fluctuations. In this phase, a long-range order exists not between Cooper pairs but only between pairs of Cooper pairs.

报告人简介：Vadim Grinenko graduated from the National Research Nuclear University (MEPhI), Moscow, in 2004. He got a PhD in the National Research Center “Kurchatov Institute”, Moscow, in 2008. After his PhD, he moved to Germany and joined the Institute for Metallic Materials in IFW Dresden, Germany, as a postdoc. At the end of 2015, Vadim moved to TU Dresden, Germany, as a PI. In 2015 and 2016, he was a visiting Associate Professor at Nagoya University. In 2022, Vadim moved to TD Lee Institute, Shanghai. Now, he is a TD Lee Fellow and Associate Professor at Shanghai Jiao Tong University. He works in the field of superconductivity and magnetism and has more than 60 publications.