一维纳米线系统中的马约拉纳零模 | 本周物理学术讲座

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一维纳米线系统中的马约拉纳零模 | 本周物理学术讲座

2017 年 10 月 24 日 中国物理学会期刊网

1一维纳米线系统中的马约拉纳零模

报告人：沈洁，荷兰代尔夫特理工大学

时间：10月24日（周二）10:30

单位：中科院物理研究所

地点：M楼253

2012年,马约拉纳零模首次在一维纳米线与超导的复合系统中被发现。之后，基于这个一维拓扑超导系统如何实现拓扑量子比特（Topological Qubit）一直是大家关注的重点。多种拓扑量子比特的理论方案被提出，但大部分都是基于一个或两个马约拉纳岛（Majorana Island）——一定长度的一维拓扑超导构成的量子点，两端各有一个马约拉纳零模。当岛在超导态并且没有准粒子中毒时，通过岛的电输运只能是库珀对（非单电子），因此随着门电压调控呈现2e的周期振荡性；当加磁场使岛进入拓扑态时，由于马约拉纳零模的出现，允许单电子通过，因此震荡周期变成单电子的1e。所以，这种2e/1e的奇偶性振荡周期的转变可作为马约拉纳零模的判据。但零能态可以是由平庸的Andreev 束缚态（ABS）造成的。那么，2e/1e的转变也可以由ABS造成。研究2e/1e的转变如何随着岛和两边势垒的化学势变化，可以获得关键的区分证据。由于这种转变不受势垒高度（隧道谱实验）的限制，可在很大化学势区间探测。在我们的大范围的变化门电压（化学势）的2e/1e转变数据中，我们同时发现了局域的由平庸的 ABS 造成的振荡——这个振荡对于门电压非常敏感且很快就消失，和比较规律的只在大范围门电压有变化的转变。重要的是，后者不随势垒的化学势变化而变化，但随岛的化学势变化，与马约拉纳零模的行为相符。结合不同样品中的可重复性，这些证据都指向2e/1e的转变是很有可能和马约拉纳零模相关的。基于纳米线系统的拓扑量子比特也正在制备和研究中。

2Berry Phases and Potential Energy Surfaces beyond the Born-Oppenheimer Approximation

报告人：E.K.U. Gross，Max Planck Institute of Microstructure Physics

时间：10月24日（周二）14:00

单位：中科院物理研究所

地点：M楼236

The starting point of essentially all modern electronic-structure techniques is the Born-Oppenheimer approximation. It not only makes calculations feasible, it also provides us with an intuitive picture of chemical reactions. Yet it is an approximation, and some of the most fascinating phenomena, such as photovoltaic dynamics, the process of vision, as well as phonon-driven superconductivity occur in the regime where the Born-Oppenheimer approximation breaks down. To tackle such situations one has to face the full Hamiltonian of the complete system of electrons and nuclei. We deduce an exact factorization [1] of the full electron-nuclear wavefunction into a purely nuclear part and a many-electron wavefunction which parametrically depends on the nuclear configuration and which has the meaning of a conditional probability amplitude.

3Controlling symmetry and localization with an artificial gauge field in a disordered quantum system

报告人：Radu Chicireanu，Université de Lille

时间：10月24日（周四）15:30

单位：中科院理论物理研究所

地点：Room 322, ITP

Anderson localization, the absence of diffusion in disordered media, draws its origins from the destructive interference between multiple scattering paths. The localization properties of disordered systems are expected to be dramatically sensitive to their symmetry characteristics. So far however, this question has been little explored experimentally. Here, we investigate the realization of an artificial gauge field in a synthetic (temporal) dimension of a disordered, periodically-driven (Floquet) quantum system. Tuning the strength of this gauge field allows us to control the time-reversal symmetry properties of the system, which we probe through the experimental observation of three symmetry-sensitive ‘smoking-gun’ signatures of localization. The first two are the coherent backscattering, marker of weak localization, and the coherent forward scattering, genuine interferential signature of Anderson localization, observed here for the first time. The third is the direct measurement of the \beta(g) scaling function in two different symmetry classes, allowing to demonstrate its universality and the one-parameter scaling hypothesis.

4Spontaneous ferromagnetic order at high temperatures without d-electrons? — The CeO2 conundrum

报告人：J.M.D. Coey，Trinity College

时间：10月25日（周三）10:30

单位：清华大学物理系

地点：物理系理科楼C302

Cerium dioxide raises the question of whether spontaneous ferromagnetic order is possible at high temperatures without d-electrons. There are many reports in the literature of a ferromagnetic-like response at room temperature to an applied magnetic field, for bulk, nanocrystalline or thin film samples, with or without cation doping. Typical values of the saturation magnetization are very small, of order 100 Am-1, but reports range from zero up to 1,000 kAm-1. The effect is somehow related to lattice defects — Ce3+ cations or oxygen vacancies — but it is a challenge to understand how electrons associated with these defects could order ferromagnetically at room temperature and above.

5Quantum Seiberg-Witten periods at strong coupling

报告人：Katsushi Ito，Tokyo Institute of Technology

时间：10月25日（周三）15:30

单位：中科院理论物理研究所

地点：Room 322, ITP

We study N=2 SU(2) supersymmetric QCD with massive hypermultiplets deformed in the Nekrasov-Shatashvili limit of the Omega-background. The prepotential of the low-energy effective theory is determined by the WKB solution of the quantum Seiberg-Witten curve. We calculate the deformed Seiberg-Witten periods around the massless monoplole point explicitly up to the fourth order in the deformation parameter. We also discss the quantum periods of the Argyres-Douglas theories from the viewpoint of the ODE/IM correspondence.

6Dark matter indirect signals with a long-lived mediator

报告人：储晓勇，奥地利科学院高能物理研究所

时间：10月26日（周四）10:10

单位：清华大学物理系

地点：物理系理科楼B315

In hidden sector models, dark matter particles could annihilate into light and metastable mediators, which subsequently decay far away from where they are produced. I will talk about how the indirect signatures in this scenario are altered with respect to the situation of conventional WIMP, as well as how this scenario may (or may not) be able to explain the observed PAMALA/AMS-02 positron excess in cosmic rays.

7Intrinsic Time Quantum Gravity and the Unique Initial State of the Universe

报告人：Hoi-Lai Yu，台湾中央研究院物理研究所

时间：10月27日（周五）10:00

单位：中科院理论物理研究所

地点：Room 6420, ITP NEW BUILDING

The precise matching between theory of the expanding Universe and the CMB/Planck data are usually believed a great triumph of the ΛCDM model and General Relativity(GR). The homogenous and isotropic sliced ΛCDM model with inflation is quantitatively backed up by GR, which, however, is attractive(to lump things together) and covariant. These are fundamentally conflicting properties. In line with the fact that the Universe is expanding and future is yet to come. As paradoxical as it may seem: only to quest for paradigm shift in both GR and inflationary models may resolve these obvious intrinsic contradictions. Of course, suspicious may seem if inflation also need a paradigm shift; a dynamic of spatial evolution w.r.t to intrinsic time driven by the Cotton-York tensor automatically replaces inflationary paradigm. A successful solution to the initial condition of the Universe will naturally deliver an inflationary epoch, i.e. de Sitter spacetime expansion of the Universe in the early epoch. In this talk, we shall elaborate how trivial one can sew together both paradigm shift in GR and inflation by just identifying the correct symmetries in GR.　