主讲人简介：

Dr. Vadim Grinenko graduated National Research Nuclear University (MEPhI), Moscow, Russia in 2004, with the specialization "Superconductivity and Nanotechnology". In 2008 he got a PhD degree in the Institute of Superconductivity and Solid State Physics, the part of National Research Center “Kurchatov Institute”, Russia. On results of the PhD work, he got a Russian Science Support Foundation prize in the nomination of "The best young doctors of the Russian Academy of Sciences”. At the end of 2008, he moved to Germany and joined IFW Dresden, Germany. He was working with applying the high temperatures superconductors in electrical motor and generators in collaboration with Toyota company. In 2012 he moved for one year to the Institute for Theoretical Solid State Physics, IFW-Dresden, Germany to work in the field of unconventional superconductivity. In 2013 Dr. Grinenko moved to the Institute for Metallic Materials, IFW-Dresden, Germany. He was studying thin-film grows of iron-based superconductors. At the end of 2015 - present: he moved to the Institute of Solid State and Materials Physics, TU Dresden, Germany as principle investigator of DFG-project devoted to studying multiband superconductors that break time-reversal symmetry. This project is performed in strong collaboration with the Paul Scherrer Institut (PSI), Switzerland, where a big part of the research was performed. In 2016 Dr. Grinenko spent one month at Nagoya University (Graduate School of Engineering) as Associate Professor.

讲座内容简介：

尽管经过27年的深入研究，一种著名的化合物Sr2RuO4的超导电性起源仍然是个谜。在其大部分历史中，Sr2RuO4的超导性已被理解为在RuO2平面上具有相等的自旋配对的奇宇称手性序参量：px±ipy。该结论基于以下证据：在零场μ子自旋旋转（ZF-μSR）数据中观察到磁响应后，在超导转变温度（Tc）处出现了时间反转对称性破缺（BTRS）状态的证据，在核磁共振（NMR）数据中，自旋三重态配对没有在Tc以下的Knight移位受到抑制的情况。手性的进一步证据来自极性克尔效应和Sr2RuO4与常规超导体之间的异质结的现象学，这表现了关于超导态结构域的证据。但是，Hc2的应力影响和NMR数据的最新修订的证据表明，与绝大多数已知的超导体一样，偶宇称，自旋-单一态配对。该结果导致考虑了手性两组分dxz±idyz序参数。该序参数的两个分量的简并性受到对称性的保护，自然产生BTRS转变温度TBTRS = Tc，但是在kz = 0时它具有难以解释的水平线节点。因此，有意外简并成分的s±id和d±ig也在考虑范围中。这些因素避免了水平线节点，但需要进行微调以获得TBTRS≈Tc。 

为了区分手性和意外简并的序参数，我们研究了单轴应变，静水压力和化学无序下的系统。基于一般的理论论据和模拟，沿（100）或（110）晶体学方向（在四边形晶格中）施加的单轴压力将所有可能类型的BTRS序参数T_BTRS和Tc分裂。但是，静水压力和化学无序导致手性相Tc = TBTRS，但对于其他类型的两组分序参数则会将它们分开。我们的系统实验研究表明，相变在单轴压力下分裂。相反，静压和无序不会导致分裂，与不含杂质的化合物相比，超导临界温度降低了近50％。这些结果与理论分析相结合，有力地表明了Sr2RuO4中的手性dxz±idyz超导性。这种超导性意味着层间配对，这在像Sr2RuO4这样的强各向异性层状系统中非常罕见，因此可能需要一种新型的配对机制。在本次演讲中，我还将讨论我们可以从μSR实验中学到关于BTRS状态下自发电流的本质的知识。

Zoom Link: https://zoom.com.cn/j/61959573929

Meeting ID: 61959573929      Password: 123456

Abstract：

The origin of the superconductivity in one famous compound, Sr2RuO4, remains a mystery despite 27 years of intense research [1]. For most of its history, the superconductivity of Sr2RuO4 has been understood in terms of an odd-parity chiral order parameter with equal spin pairing in the RuO2 planes: px ± ipy [2]. This conclusion was based on evidence for a broken time-reversal symmetry (BTRS) state at a superconducting transition temperature (Tc) from the observation of a magnetic response in zero-field muon spin rotation (ZF-μSR) data [3] and for spin-triplet pairing from the absence of suppression in Knight shift below Tc in nuclear magnetic resonance (NMR) data [4]. Further evidence for chirality came from the polar-Kerr effect [5] and the phenomenology of junctions between Sr2RuO4 and conventional superconductors, which shows evidence for domains in the superconducting state [6]. However, evidence from the strain dependence of Hc2 [7] and a recent revision of NMR data [8] point to even parity, spin-singlet pairing, as in the vast majority of known superconductors. This result has led to the consideration of a chiral two-component dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding naturally that BTRS transition temperature TBTRS = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters with accidentally degenerated components are also under consideration [9]. These avoid the horizontal line node, but require fine-tuning to obtain TBTRS ≈ Tc. 

To distinguish between chiral and accidentally degenerated order parameters, we studied the system under uniaxial strain [10], hydrostatic pressure and chemical disorder[11]. Based on general theoretical arguments and simulations, the uniaxial pressure applied along (100) or (110) crystallographic directions (in a tetragonal lattice) splits TBTRS and Tc for all possible types of the BTRS order parameters. However, hydrostatic pressure and chemical disorder lead to Tc = TBTRS for the chiral phase but would separate them for other types of two-component order parameters [12]. Our systematic experimental study demonstrates that the transition splits under uniaxial pressure. In contrast, no splitting results from hydrostatic pressure and disorder with a nearly 50% reduced superconducting critical temperature than an impurity-free compound. These results, combined with theoretical analysis, strongly suggest a chiral dxz ± idyz superconductivity in Sr2RuO4. This superconductivity implies interlayer pairing, which is highly unusual in such a strongly anisotropic layered system as Sr2RuO4 and, therefore, may require a new type of pairing mechanism. In this talk, I will also discuss what we can learn from μSR experiments regarding the nature of spontaneous currents in the BTRS state [13].

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11. V. Grinenko et al., https://arxiv.org/pdf/2103.03600.pdf.
12.B. Zinkl and M. Sigrist, Phys. Rev. Research 3, L012004 (2021).
13. V. Grinenko et al., Nat. Phys. 16, 789 - 794 (2020).