[超导国家重点实验室学术报告]Go beyond the performance of Nb cavity: potential of alternative materials in SRF

Superconducting radio-frequency (SRF) resonant cavities for particle accelerators regularly achieve a very high qualityfactor, Q0~1010, and high accelerating electric field Eacc~30-40MV/m. Improving these two parameters even more has been the main challenge for SRF researchers. Cavities must be operated in Meissner state to keep Q0 very high, so naively, the maximum field inside the cavity is limited by Hc1 of the cavity material. Thus, the modern SRF cavities are made from pure Nb with Hc1~170mT at 2K, which is the largest Hc1 among materials available.This corresponds to 40MV/mof the present standard cavity shape for electron accelerators. To achieve much higher Eacc, a simple bulk Nb is not enough, but Nblooks the best because of its Hc1. Then, how to go beyond Nb cavity?In this talk, the superconductor–insulator–superconductor (S’IS) multilayer structure forapplication in SRF cavities for higher Eaccis reviewed. S’IS multilayer consists of a thin superconducting layer with a higher thermodynamic critical field formed on a bulk superconductor with a higher Hc1 (i.e., Nb).The theoretical field limit,optimum layer thicknesses and material combination are discussed for theS’IS structure and are also reviewed for the superconductor–superconductor (S’S) bilayer structure.

[超导国家重点实验室学术报告] Statistics of thermomagnetic breakdown in Nb superconducting films

As dissimilar as they might appear, lightning during a dielectric breakdown, failure ofinterconnects in integrated circuits, snow avalanches on mountain slopes, or popcornexplosion share a common physics ground corresponding to the triggering of acatastrophic event when a threshold stress is exceeded. Interestingly, even if the sameexperience is repeated under identical environmental conditions, the event will nothappen at exactly the same threshold stress. The question as to whether the statisticaldistribution of this threshold stress follows a universal law for all phenomena isexperimentally difficult to answer, since exceeding the threshold point brings alongdramatic consequences leading to irreversible changes in the system. Remarkably,superconducting materials offer a unique opportunity to investigate the statisticaldistribution of similar catastrophic events consisting of non-destructive magnetic fluxavalanches in the same sample and thus ruling out the spreading factors associated tounavoidable uncontrollable variations in the replicas of the system.

[超导国家重点实验室学术报告] Controlling Electronic Properties of Correlated Two-Dimensional Materials

Vast opportunities exist in correlated Two-dimensional (2D) materials: they feature a number of exotic phenomena such as charge density waves (CDWs), high-temperature superconductivity (HTS) and magnetism. However, experimentally extracting monolayers from bulk correlated materials and control their electronic properties turned out to be extremely challenging. In this talk, I will introduce the fabrication and control technique we developed in the past years and intriguing results we obtained in several 2D systems. In the model CDW material 1T-TaS2, we are able to control the phase transitions by thinning it down to the 2D limit or electrochemical gating. In itinerant magnet Fe3GeTe2, we observe intrinsic ferromagnetism in the 2D limit and the Curie temperature can be turned up to room temperature. In monolayer high-temperature superconductor Bi2Sr2CaCu2O8+d, we obtain a phase diagram lack of dimensionality effect, which indicates that monolayer cuprate contains all essential physics of HTS.

[超导国家重点实验室学术报告] Interplay between CDW and superconducting fluctuations in high-Tc cuprates

The phase diagram of underdoped superconducting cuprates is known to be extremely rich and complex. In addition to a transition from an antiferromagnetic insulator at half-doping to a strange metal at optimal doping, and the opening of a pseudogap in the density of states, the normal state is also the location for a charge ordering and a Fermi surface reconstruction. By analyzing La1-xSrxCuO4 thin films resistivity under high pulsed magnetic field for various doping, I will carefully inspect the behavior of superconducting thermal and quantum fluctuations and the existence of quantum critical points. I will extract a systematic (H,T) phase diagram and discuss the interplay between charge ordering and superconductivity. In particular I will relate our findings to a theoretical model proposed by J. Lorenzana and A. Attanasi, which enables to demonstrate that disorder can induce a charge-density-wave to become superconducting. This paradoxical effect occurs close to the phase boundary between CDW and SC and is analogous to the occurrence of supersolid effects in 4He. It originates from the topological stabilization of filamentary superconductivity at the interfaces of different CDW domains at low temperature and intermediate/large magnetic fields. By comparing experimental and theoretical phase diagrams I will show that the previously observed two-step phase transition in under-doped films is related to the interplay between superconducting fluctuations and CDW. I will then analyze the doping dependence of this diagram and show that p=1/8 is specific for this CDW/SC competition at least in LSCO, since the coexistence takes place at zero magnetic field for this specific doping.

[超导国家重点实验室学术报告] Quasi-periodic magnetic flux avalanches in doubly-connected superconductors

     Magneto-optical imaging (MOI) has been established as a powerful tool for studying magnetic flux distributions in superconductors. In particular, this technique has been extensively utilized in the study of flux penetration in form of dendritic avalanches in a variety of superconducting films. it was found that with increasing field above a certain threshold, , dendrites penetrate the samples at random fields and propagate along paths that terminate inside the sample. In this talk I will focus on our recent findings, revealing a dramatic effect of the sample topology on the dendritic phenomenon. Specifically, MOI of Nb rings, open rings and strips, reveal that that dendrites in a ring can cross the entire width of the sample. Such dendrites appear when the difference between the applied field and the average field inside the central hole reaches a certain threshold level, . With increasing applied field, this condition is reached quasi-periodically as a crossing dendrite creates momentarily a hot channel through which flux flows into the central hole, balancing the field inside the hole with that outside the ring. The threshold  differs in magnitude from the onset field  of magnetic instability, and exhibits qualitatively different dependence on the rim width.

[超导国家重点实验室学术报告] Superconducting materials hosting nontrivial topology

Topological superconductors have attracted tremendous interest for hosting Majorana bound states on their boundaries, which are potential in realizing quantum computations. One promising approach to engineer topological superconductors is to discovery superconductors with nontrivial topology in their electronic band structure. In this talk, I will introduce our recent work on exploration of such materials. Specifically, I will first talk about our high-pressure measurement on the topological nodal-line semimetal SrAs3, which induces a Lifshitz transition below 1 GPa and then superconductivity at 20.6 GPa associated with a structural phase transition. The new cubic SrAs3 phase is found to host not only superconductivity but also a topological crystalline insulator state. I then will introduce our recent high magnetic field magnetotransport measurement on the layered superconductor TaSe3, which reveals the existence of a strong topological insulator state.

[超导国家重点实验室学术报告] 从组合材料学到材料基因组学

结合20多年从事组合材料学的工作经验，从并行合成、高通量表征两个方面阐述组合材料学的思想精髓，并通过两个具体的材料研究，展现组合方法的优越性。同时，也和听众一起回顾从组合材料学到材料基因组学的螺旋发展历程，探讨材料基因组计划孕育的机遇和挑战。

[超导国家重点实验室学术报告] 关联氧化物纳米电子学

关联氧化物纳米电子学结合了关联氧化物物理学与半导体纳米电子学两个领域，为新近发展起来的交叉领域。在这个报告中，我会首先介绍关联氧化物纳米电子学的背景，然后用几个例子来阐述氧化物中的关联性与半导体器件相辅相成的研究特性以及可能应用。在第一个例子中，我会介绍我们在铝酸镧/钛酸锶（LAO/STO）界面上制备超导单电子晶体管，并观测到了超导外电子配对的实验证据[1]，验证了一个悬浮半个世纪的理论。从而阐明纳米器件为研究电子关联性提供了重要工具。在第二个例子中，我会着重介绍氧化物电子/电子对波导中的量子输运，阐明电子关联性丰富了纳米器件的特性[2]。最后我会介绍最近两年来在这个领域中我们取得的一些新进展，包括可调的电子-电子相互作用[2], 氧化物界面量子振荡的一维根源性[3]与氧化物纳米线的一维超导性[4]以及可能的应用。

[超导国家重点实验室学术报告]三角晶格稀土氧族化合物AReCh2中自旋激发的研究进展

阻挫磁性是当今凝聚态物理研究的重要内容。虽已经历长期探索，在自旋阻挫材料中寻找量子自旋液体仍然是一个富有挑战的课题。在许多已知的量子自旋液体候选材料中，磁性杂质、结构缺陷、无序等的存在都会影响人们对其磁基态的判断，从而降低其量子自旋液体基态的可信度。因而，寻找不存在结构缺陷、无序的阻挫磁性材料是实现量子自旋液体的重要前提。2018年出现的具有铜铁矿结构的三角晶格稀土氧族化合物AReCh2 (A为碱金属，Re为稀土元素，Ch为氧族元素)中被证明不易存在结构缺陷和无序，而且很多AReCh2材料在实验中都没有形成磁有序，因此被认为是实现量子自旋液体态的理想材料体系。随后的非弹性中子散射也相继在NaYbO2，NaYbSe2等材料中观测到了连续的自旋激发，被认为是分数化的自旋子激发，为量子自旋液体态的存在提供了重要证据。

[超导国家重点实验室学术报告]Electronic state control in layered quantum materials

The multifunctional behaviors of quantum materials are driven by the intricate interplay between charge, orbital and spin degrees of freedom. Advances in modern field-effect devices allow for unprecedented control of these degrees of freedom, revealing a plethora of exotic physical phenomena such as superconductivity and long-range magnetic order. In this talk, I will provide an overview of our recent progress in controlling the emerging physical phenomena in layered quantum devices and how they offer new insights to the underlying physics. First, I will discuss the observation of a spontaneous anomalous Hall effect, a trademark of time-reversal symmetry breaking, in a doped layered polar semiconductor. Notably, the magnitude of anomalous Hall conductivity can be enhanced by tuning the carrier density, which sheds new light on the interplay of magnetic and ferroelectric-like responses [1]. Next, I will present recent results on the electric field control at the interfaces of ionic gated tellurium thin flake devices, demonstrating a fourfold increase in the Rashba spin-orbit coupling coefficient.