蓝宝石上石墨烯的近程催化生长研究

在半导体或者绝缘衬底上利用化学气相沉积方法生长石墨烯和在金属上生长石墨烯相比具有很明显的优势,它克服了前期转移石墨烯时引入的杂质污染和缺陷产生的问题。本文提出了一种新的近程催化的方法可以在蓝宝石衬底上直接进行石墨烯的生长。该方法利用了金属作为催化剂的优势,又避免了转移石墨烯的过程。该方法能制备出完全覆盖衬底的石墨烯,同时石墨烯畴的大小接近1μm。基于生长过程中石墨烯的成核和畴的生长,本文提出了近程催化的生长机理。

Asymmetric Fraunhofer pattern in Josephson junctions from heterodimensional superlattice V_(5)S_(8)

Introduction of spin-orbit coupling(SOC)in a Josephson junction(JJ)gives rise to unusual Josephson effects.We investigate JJs based on a newly discovered heterodimensional superlattice V_(5)S_(8) with a special form of SOC.The unique homointerface of our JJs enables elimination of extrinsic effects due to interfaces and disorder.We observe asymmetric Fraunhofer patterns with respect to both the perpendicular magnetic field and the current.The asymmetry is influenced by an in-plane magnetic field.Analysis of the pattern points to a nontrivial spatial distribution of the Josephson current that is intrinsic to the SOC in V_(5)S_(8).

Finite superconducting square wire-network based on two-dimensional crystalline Mo_(2)C

Superconducting wire-networks are paradigms to study Cooper pairing issues,vortex dynamics and arrangements.Recently,emergent low-dimensional crystalline superconductors were reported in the minimal-disorder limit,providing novel platforms to reveal vortices-related physics.Study on superconducting loops with high-crystallinity is thus currently demanded.Here,we report fabrication and transport measurement of finite square-network based on two-dimensional crystalline superconductor Mo_(2)C.We observe oscillations in the resistance as a function of the magnetic flux through the loops.Resistance dips at both matching field and fractional fillings are revealed.Temperature and current evolutions are carried out in magnetoresistance to study vortex dynamics.The amplitude of oscillation is enhanced due to the interaction between thermally activated vortices and the currents induced in the loops.The driving current reduces the effective activation energy for vortex,giving rise to stronger vortex interaction.Moreover,by the thermally activated vortex creep model,we derive the effective potential barrier for vortex dissipation,which shows well-defined correspondence with structures in magnetoresistance.Our work shows that low-dimensional crystalline superconducting network based on Mo_(2)C possesses pronounced potential in studying the modulation of vortex arrangements and dynamics,paving the way for further investigations on crystalline superconducting network with various configurations.

Electrical and thermoelectric study of two-dimensional crystal of NbSe2

We report experimental investigation of the resistivity and Nernst effect in two-dimensional(2D)NbSe2 crystals.A strongly enhanced Nernst effect,100 times larger than that in bulk NbSe2,caused by moving vortices is observed in thin film.It is found that in the low temperature,high magnetic field regime,pinning effects show little dependence on the thickness and resistivity of the superconductor films.Strong Nernst signals persist above the superconducting transition,suggesting that the Nernst effect is a sensitive probe to superconducting fluctuations.A magnetic field induced superconductor-insulator transition(SIT)is evident,which is surprising in that such a SIT usually takes place in disordered dirty superconductors,while our samples are highly crystalline and close to the clean limit.Hence,our results expand the scope of SIT into 2D crystal clean superconductors.

Intercalation of van der Waals layered materials: A route towards engineering of electron correlation

Being parent materials of two-dimensional (2D) crystals, van der Waals layered materials have received revived interest. In most 2D materials, the interaction between electrons is negligible. Introducing the interaction can give rise to a variety of exotic properties. Here, via intercalating a van der Waals layered compound VS2, we find evidence for electron correlation by extensive magnetic, thermal, electrical, and thermoelectric characterizations. The low temperature Sommerfeld coefficient is 64 mJ·K-2·mol-1 and the Kadowaki-Woods ratio rKW^0.20a0. Both supports an enhancement of the electron correlation. The temperature dependences of the resistivity and thermopower indicate an important role played by the Kondo effect. The Kondo temperature TK is estimated to be around 8 K. Our results suggest intercalation as a potential means to engineer the electron correlation in van der Waals materials, as well as 2D materials.

Facile and fast growth of high mobility nanoribbons of ZrTe5

Recently, ZrTe5 has received a lot of attention as it exhibits various topological phases, such as weak and strong topological insulators, a Dirac semimetal, a three-dimensional quantum Hall state, and a quantum spin Hall insulator in the monolayer limit. While most of studies have been focused on the three-dimensional bulk material, it is highly desired to obtain nanostructured materials due to their advantages in device applications. We report the synthesis and characterizations of ZrTe5 nanoribbons. Via a silicon-assisted chemical vapor transport method, long nanoribbons with thickness as thin as 20 nm can be grown. The growth rate is over an order of magnitude faster than the previous method for the bulk crystals.Moreover, transport studies show that the nanoribbons are of low unintentional doping and high carrier mobility, over30000 cm2/V·s, which enable reliable determination of the Berry phase of π in the ac plane from quantum oscillations. Our method holds great potential in growth of high quality ultra-thin nanostructures of ZrTe5.

On the Onsager-Casimir reciprocal relations in a tilted Weyl semimetal

The Onsager-Casimir reciprocal relations are a fundamental symmetry of nonequilibrium statistical systems.Here we study an unusual chirality-dependent Hall effect in a tilted Weyl semimetal Co_(3)Sn_(2)S_(2) with broken time-reversal symmetry.It is confirmed that the reciprocal relations are satisfied.Since two Berry curvature effects,an anomalous velocity and a chiral chemical potential,contribute to the observed Hall effect,the reciprocal relations suggest their intriguing connection.

外尔费米子的奇特热电性质

世界由各种微观粒子组成。这些粒子可以分为两大类,玻色子和费米子。费米子组成了物质,而玻色子传递物质之间的相互作用。1928年,为了精确计算原子的能谱,狄拉克基于量子力学的原理,同时考虑电子的相对论性运动,提出一个波函数方程来描述电子的行为。这个方程后来被称为狄拉克方程。该方程的意义远远超出了狄拉克的初衷。

碳化硅表面的外延Graphene

Graphene具有优异的电学性质,是非常有前途的纳米电子材料,有希望替代硅成为下一代集成电路材料.尽管目前有很多种制备Graphene的方法,但就Graphene在将来集成电路方面的应用而言,在碳化硅上的外延生长法最具潜力.文章首先从Graphene的能带结构开始,简单介绍为什么Graphene具有诸多优异的电学性质,比如异常霍尔效应、室温下的高迁移率、碳纳米管的弹道输运等.然后介绍这种外延生长方法及其发展现状.通过比较在不同碳化硅晶面和在不同条件下生长的Graphene的表面形貌,得出结论,在加热炉内,生长在碳化硅晶体碳面的Graphene拥有特别高的质量.最后文章着重讨论对碳面Graphene的电学表征实验.这些实验证明这种材料中的电子是狄拉克电子,同时也发现材料具有优异的电学性质.