中科院:稳态强磁场装置助力发现超高电导率材料

<正>稳态强磁场实验装置(SHMFF)用户复旦大学物理系教授修发贤领衔的研究团队在砷化铌纳米带中观测到其表面态具有超高电导率,这也是目前二维非超导体系中的最高电导率,其低电子散射几率的机制源自外尔半金属特有的费米     

中国科大在新型拓扑材料外尔半导体研究中取得进展

正在新型量子材料中,具有特殊能带结构的拓扑材料也兼具新奇电子输运特性。相关研究不仅可以加深对于拓扑物态的理解,更有望推动新型高性能电子学器件的发展。一个典型的代表是目前引起广泛关注的外尔半金属体系,其输运研究往往表现出超大非饱和磁阻、平行磁场下的     

纳米粒子的生物安全性研究进展

纳米材料具有表面效应、小尺寸效应、量子尺寸效应、宏观量子效应等特殊性质,在社会生产和生活中有广阔的应用前景,其对人体健康及环境的潜在影响已引起科学界及政府部门的关注。综述了大量常见的人工纳米粒子,包括碳纳米材料、纳米氧化物、纳米金属单质等的生物和毒理学国内外研究成果,比较了这些材料的毒理行为,分析了其生物毒性的产生机理,并展望了纳米粒子生物安全性研究的可能方向。     

金纳米粒子的最新研究进展

金纳米粒子是最稳定的金属纳米粒子之一，由于其具有优良的稳定性和光学性质，使其在许多领域有着广阔的应用前景。本文主要对金纳米粒子和表面修饰金纳米粒子的制备方法进行分析总结，指出各种方法的制备原理及特点。同时，阐述了金纳米粒子的一些特殊性能，如：表面吸收带效应、荧光效应、量子尺寸效应、单电子跃迁等。并对金纳米粒子的应用进行了展望。     

石墨烯中Dirac费米子量子输运特性研究

利用紧束缚模型的方法研究了石墨烯中Di-rac费米子的势垒相关输运特性,分析了石墨烯中Di-rac费米子的势垒透射率与其入射角度、入射能量、势垒高度和势垒厚度等对应关系。通过理论分析和数值计算表明,石墨烯中Dirac费米子的隧穿系数随矩形势垒的高度和厚度的变化都呈现出明显的振荡效应,隧穿系数与入射角度依赖关系证实了Klein效应。在一定参数下,Dirac费米子的势垒透射率随Dirac费米子入射能量的变化经历从"0"～"1"的突变,显示良好的"电导开关效应",该效应在微电子器件中将有很好的应用前景。     

我国科学家首次观测到Weyl半金属手征磁效应

正Weyl(外尔)半金属的物性研究一直是凝聚态物理学的前沿热点之一,其中Weyl半金属材料的手征磁效应只存在于理论预言中,由于材料制备的困难和表征手段的缺乏,至今尚未在实验室直接观测到手征磁效应。近日,南京大学于扬课题组与香港课题组合作,利用超导量子电路首次模拟出外尔半金属能     

一般费米量子信道的经典容量

文章在费米子平均占据数受限的情况下 ,利用费米型量子高斯加性噪声的球谐函数展开方法 ,给出单模费米子在一般费米量子加性信道上传输经典信息的经典容量     

重新认识重费米子超导

正在重费米子超导体中,正常态重电子的有效质量可以达到自由电子质量的上百倍,其特征费米能量也相应削减,只有meV的量级。1979年,德国科学家Frank Steglich等首先在中发现了重费米子超导,其超导转变温度约为0.6K,为重电子费米能的5%,远大于一般的元素超导体,堪称"高温超导体"。超导的发现不仅仅具有开创的意义,还确立了以磁性量子临界涨落为配对胶水,诱导重电子发生d波配对的基本理论图像,成为描述重费米子超导的标准范     

电磁场调制下石墨烯中Hartman效应的研究

从理论上研究了电磁场调制下非本征石墨烯中的Hartman效应。利用相位移动及传递矩阵的方法计算粒子隧穿的群延迟时间τg,系统地分析群延迟时间与不同参数(如粒子能量E、入射角度θ、电磁场强度U和A珤等)之间的关系。石墨烯中的载流子是狄拉克费米子,故用狄拉克方程表征。研究发现在势垒宽度足够大时,石墨烯中的狄拉克费米子隧穿通过势垒的群延迟时间τg与势垒宽度无关,证明Hartman效应不仅存在于静电场下零带隙的石墨烯中,还存在于电磁场调制下非本征(非零带隙)的石墨烯中。     

拓扑量子材料与量子反常霍尔效应

简要综述自从2005年前后发展起来的一类新的材料体系-"拓扑"量子材料,它包含拓扑绝缘体、拓扑晶体绝缘体、拓扑超导体和拓扑半金属等。这类材料中较强的自旋轨道耦合作用导致了包括量子反常霍尔效应在内的丰富多彩的量子现象,有可能对未来低能耗电子学、拓扑量子计算和清洁能源等技术的发展具有重大的推动作用。     

Unconventional Charge–Spin Conversion in Weyl-Semimetal WTe2

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe₂ and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba–Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe₂, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe₂ due to a reduced crystal symmetry combined with its large spin Berry curvature, spin–orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.     

Weyl Semimetals as Hydrogen Evolution Catalysts

The search for highly efficient and low‐cost catalysts is one of the main driving forces in catalytic chemistry. Current strategies for the catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfides in the hydrogen evolution reaction (HER). Here, the study proposes and demonstrates a different principle that goes beyond local site optimization by utilizing topological electronic states to spur catalytic activity. For HER, excellent catalysts have been found among the transition‐metal monopnictides—NbP, TaP, NbAs, and TaAs—which are recently discovered to be topological Weyl semimetals. Here the study shows that the combination of robust topological surface states and large room temperature carrier mobility, both of which originate from bulk Dirac bands of the Weyl semimetal, is a recipe for high activity HER catalysts. This approach has the potential to go beyond graphene based composite photocatalysts where graphene simply provides a high mobility medium without any active catalytic sites that have been found in these topological materials. Thus, the work provides a guiding principle for the discovery of novel catalysts from the emerging field of topological materials.     

Lifshitz Transitions,Type-II Dirac and Weyl Fermions,Event Horizon and All That

The type-II Weyl and type-II Dirac points emerge in semimetals and also in relativistic systems. In particular, the type-II Weyl fermions may emerge behind the event horizon of black holes. In this case the horizon with Painlevé–Gullstrand metric serves as the surface of the Lifshitz transition. This relativistic analogy allows us to simulate the black hole horizon and Hawking radiation using the fermionic superfluid with supercritical velocity, and the Dirac and Weyl semimetals with the interface separating the type-I and type-II states. The difference between such type of the artificial event horizon and that which arises in acoustic metric is discussed. At the Lifshitz transition between type-I and type-II fermions the Dirac lines may also emerge, which are supported by the combined action of topology and symmetry. The type-II Weyl and Dirac points also emerge as the intermediate states of the topological Lifshitz transitions. Different configurations of the Fermi surfaces, involved in such Lifshitz transition, are discussed. In one case the type-II Weyl point connects the Fermi pockets and the Lifshitz transition corresponds to the transfer of the Berry flux between the Fermi pockets. In the other case the type-II Weyl point connects the outer and inner Fermi surfaces. At the Lifshitz transition the Weyl point is released from both Fermi surfaces. They loose their Berry flux, which guarantees the global stability, and without the topological support the inner surface disappears after shrinking to a point at the second Lifshitz transition. These examples reveal the complexity and universality of topological Lifshitz transitions, which originate from the ubiquitous interplay of a variety of topological characters of the momentum-space manifolds. For the interacting electrons, the Lifshitz transitions may lead to the formation of the dispersionless (flat) band with zero energy and singular density of states, which opens the route to room-temperature superconductivity. Originally, the idea of the enhancement of \(T_\mathrm{c}\) due to flat band has been put forward by the nuclear physics community, and this also demonstrates the close connections between different areas of physics.     

Flat Band in Topological Matter

Topological media are systems whose properties are protected by topology, and thus are robust to deformations of the system. In topological insulators and superconductors, the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac, or Majorana fermions on the surface of the system and inside vortex cores. In gapless topological media, the bulk-surface and bulk-vortex correspondence produce topologically protected gapless fermions without dispersion—the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and this property may give rise in particular to surface superconductivity, which in principle could exist even at room temperature.     

Anisotropic Broadband Photoresponse of Layered Type‐II Weyl Semimetal MoTe2

Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self‐powered operation owing to its extraordinary material properties. Layered Type‐II Weyl semimetal that break Lorentz invariance can be further integrated with other two‐dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two‐dimensional materials. Herein, we report the realization of a broadband self‐powered photodetector based on Type‐II Weyl semimetal T_d‐MoTe₂. The prototype metal–MoTe₂–metal photodetector exhibits a responsivity of 0.40 mA W^?1 and specific directivity of 1.07 × 10⁸ Jones with 43 μs response time at 532 nm. Broadband responses from 532 nm to 10.6 μm are experimentally tested with a potential detection range extendable to far‐infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe₂. The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self‐powered photodetection with decent responsivities.     

Tunable quantum Shubnikov-de Hass oscillations in antiferromagnetic topological semimetal Mn-doped Cd3As2

Three-dimensional Dirac semimetal Cd3As2 has been considered as an excellent candidate for applica-tions of electronic devices owing to its ultrahigh mobility and air-stability.However,current researches are focused mainly on the use of gate-voltage to control its carrier transport tunability,while the manipulation of transport properties by element-doping is quite limited.Here we report the tunable magneto-transport properties by adjusting Mn-doping in the Cd3As2 compound.We find that Mn-element doping has a strong influence on the Fermi level positions,and the Fermi energy approaches to Dirac point with higher Mn-doping.More importantly,the introduction of Mn atoms transforms dia-magnetic Cd3As2 to antiferromagnetic(Cd,Mn)3As2,which provides an approach to control topological protected Dirac materials by manipulating antiferromagnetic order parameters.The Shubnikov-de Hass oscillation originates from the surface states,and the Landau fan diagram yields a nontrivial Berry phase,indicating the existence of massless Dirac fermions in the(Cd1-xMnx)3As2 compounds.Our present results may pave a way for further investigating antiferromagnetic topological Dirac semimetal and expand the potential applications in optoelectronics and spintronics.     

Renormalization group aspects of graphene

Graphene is a two-dimensional crystal of carbon atoms with fascinating electronic and morphological properties. The low-energy excitations of the neutral, clean system are described by a massless Dirac Hamiltonian in (2+1) dimensions, which also captures the main electronic and transport properties. A renormalization group analysis sheds light on the success of the free model: owing to the special form of the Fermi surface that reduces to two single points in momentum space, short-range interactions are irrelevant and only gauge interactions such as long-range Coulomb or effective disorder can play a role in the low-energy physics. We review these features and briefly discuss other aspects related to disorder and to the bilayer material along the same lines.     

Topological Superfluid 3He–B: Fermion Zero Modes on Interfaces and in the Vortex Core

Many quantum condensed matter systems are strongly correlated and strongly interacting fermionic systems, which cannot be treated perturbatively. However, topology allows us to determine generic features of their fermionic spectrum, which are robust to perturbation and interaction. We discuss the nodeless 3D system, such as superfluid ³He–B, vacuum of Dirac fermions, and relativistic singlet and triplet supercondutors which may arise in quark matter. The systems, which have nonzero value of topological invariant, have gapless fermions on the boundary and in the core of quantized vortices. We discuss the index theorem which relates fermion zero modes on vortices with the topological invariants in combined momentum and coordinate space.     

Prospect of Node-Line Semimetal Cu<Subscript>3</Subscript>PdN to Be a Topological Superconductor

We investigate systematically the vibrational and electron–phonon interaction properties of node-line semimetal Cu₃PdN under strain and electron doping by using first-principles calculations. It is found that vibrational modes interact with electrons at the Fermi level isotropically with a three-dimensional character. The phonon frequency and Eliashberg spectral function can be tuned by strain remarkably, and the maximum transition temperature (T _c) predicted is 0.03 K under strain ε = 0.10. The coexistence of superconductivity and topological physics in Cu₃PdN makes it a promising candidate for future quantum computation platform.     

Pressure‐Induced Phase Transition in Weyl Semimetallic WTe2

Tungsten ditelluride (WTe₂) is a semimetal with orthorhombic T_d phase that possesses some unique properties such as Weyl semimetal states, pressure‐induced superconductivity, and giant magnetoresistance. Here, the high‐pressure properties of WTe₂ single crystals are investigated by Raman microspectroscopy and ab initio calculations. WTe₂ shows strong plane‐parallel/plane‐vertical vibrational anisotropy, stemming from its intrinsic Raman tensor. Under pressure, the Raman peaks at ≈120 cm^?1 exhibit redshift, indicating structural instability of the orthorhombic T_d phase. WTe₂ undergoes a phase transition to a monoclinic T′ phase at 8 GPa, where the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry. Such T_d to T′ phase transition provides a feasible method to achieve Weyl state switching in a single material without doping. The new T′ phase also coincides with the appearance of superconductivity reported in the literature.