報告摘要:
In ordinary superconductors, an external magnetic field is needed to generate magnetic vortices. We discuss two examples where this conventional folklore breaks down in a fundamental way. The first example is about superconductors with strong spin-orbit coupling. We show that topological defect excitations, dubbed quantum anomalous vortices (QAV), can nucleate around magnetic ions spontaneously in the absence of external magnetic fields. When such superconductors have a
nontrivial band structure and superconducting topological surface states, as do certain Fe-based superconductors, the magnetic impurity induced QAV are nonabelian and host Majorana zero mode (MZM) in the integer quantized vortex core states. Recent experimental observations of QAV and MZM will be discussed. The second example concerns the recently discovered kagome superconductorsAV3Sb5(A=K, Cs, Rb) that exhibit a cascade of correlated and topological quantum states triggered by an unconventional charge density wave (CDW) order. We discuss recent experimental evidence for time-reversal symmetry breaking in both the normal and the superconducting state. We argue that the essential phenomenology can be captured by a CDW with loop current order, realizable in models with extended Coulomb interactions on the kagome lattice. The loop-current Chern metal has a partially filled Chern band and Chern Fermi pockets carrying concentrated Berry curvature. We show how Cooper pairing over the Chern Fermi pockets produces an unprecedented roton superconductor, where the internal phases of three pairing components are locked at 120-degree and loop supercurrents circulate around an emergent vortex-antivortex lattice with pair density modulations. We discuss the extraordinary properties of this chiral topological superconductor and the experimental observation of fractionalized flux quantization and charge-6e superconductivity.
報告人簡介：
汪自強，1984年畢業于清華大學，1989年獲哥倫比亞大學物理學博士學位。他曾在羅格斯大學和洛斯阿拉莫斯國家實驗室做博士後，1993年成為波士頓大學助理教授，目前是波士頓學院物理系教授，美國物理學會會士。他于1996年獲得Cottrell學者獎，并于2021年獲得SEED獎。汪自強教授發表論文200餘篇，包括50多篇Physical Review Letters、40餘篇Nature及Nature子刊。汪自強教授目前的研究重點是量子材料中的關聯和拓撲電子态，包括非常規超導體、量子磁性、整數和分數量子反常霍爾系統、拓撲超導體以及其他奇異超導态。
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