沈志勋 教授
斯坦福大学，南京大学名誉教授

时间：2014年12月18日（星期四）下午4:20
地点：科技馆一期二楼报告厅

摘要：Quantum systems in confined geometries have been a very rich ground for discoveries. In this talk, I will discuss recent progresses in uncovering novel physics at ultra-thin limit, with focus on mono-unit-cell superconductor FeSe and semiconductor MoSe2 respectively. Time permits, pathways to make superconducting graphene will also be discussed.
The first example is mono-unit-cell (UC) superconductor FeSe grown on SrTiO3, where the Cooper pairing temperature is reported to have dramatically enhanced from its bulk value of 8K to ~ 60K. A challenge is to understand the cause of the enhanced pairing strength, and possibly increase superconducting Tc. We show angle-resolved photoemission spectroscopy (ARPES) data that provide clear evidence for strong cross-interface electron-phonon coupling in single UC limit, suggesting that pairing is significantly enhanced by the strong interface mode coupling. [1] This suggests a pathway of "integrated functional components" approach to boost superconducting properties.
The second example is the recent discovery of large enhancement in photo-luminescence quantum efficiency and a potential route to "valleytronics" in atomically thin layered transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect to direct band gap transition in the single layer limit. Using angle-resolved photoemission spectroscopy (ARPES) on high quality thin film samples of MoSe2 grown by molecular beam epitaxy (MBE), we have made a direct observation of a distinct transition from indirect to direct band gap as the thickness of the sample is reduced to a monolayer [2]. The experimental band structure indicates a stronger tendency of monolayer MoSe2 towards direct band gap with larger gap size than theoretical prediction. A comparison of directly measured ARPES band gap and optical data led to new insights on enhanced many-body effect of semiconductor in 2D [3].
If time permits, I will also discuss the superconductivity in CaC6 and its implication on a possible pathway for superconducting graphene [4].
References:
[1]JJ Lee et al., Nature 515, 7526(2014)
[2]Y. Zhang, et al., Nature NanoTechnology, 9/2 (2014)
[3]M. Ugeda et al., arXiv:1404.2331 (2014), Nature Materials (2014)
[4]S.L. Yang et al., Nature Communication, 5, 3493 (2014)

沈志勋教授现为美国斯坦福大学Paul Pigott物理学教授。他于1983年毕业于复旦大学物理学系，1989年于美国斯坦福大学物理学系获理学博士学位。沈志勋教授在凝聚态物理和复杂材料研究中做出了开创性工作，尤其在超导和氧化物研究，以及同步辐射和光电子能谱发展与应用中取得了令人瞩目的成绩，是学术界公认的凝聚态物理领域国际一流科学家。他在国际权威学术杂志"自然"、"科学"、"物理评论快报"上发表80余篇论文，其中6篇被国际ISI数据统计列为被引用最多和最有影响论文。他的论文被SCI引用近2万次，其中单篇引用超过100次的有52篇，H因子为71。 他应邀在重要学术会议上作邀请报告300多次。其中美国物理学会年会8次，包括美国物理学会百年特邀演讲。获得物理领域一些重要的国际奖项：2000年第一个获得世界超导实验物理最重要大奖：卡梅琳-昂内斯奖（H.Kamerlingh Onnes Prize）；2009年获美国能源部代表总统颁发的科学大奖：欧内斯特.奥兰多.劳伦斯奖；2011年获美国物理学会凝聚态物理最高奖：奥利弗.伯克莱奖（Oliver Buckley）。