Pt/La0.67Sr0.33MnO3异质结中的反常霍尔效应

非磁/铁磁异质结构中存在很多有趣的演生现象,特别是,铂/铁磁异质结构中的反常霍尔效应是一个研究热点.采用脉冲激光沉积技术和射频磁控溅射技术制备出具有原子级接触界面的铂/锰酸锶镧异质结,并对异质结的电输运性能进行了系统的研究.实验发现,铂/锰酸锶镧异质结中存在由铂贡献的反常霍尔效应,这是由磁近邻效应诱导铂表现出铁磁性造成的.反常霍尔电阻随着温度的降低而急剧增加,并且在低于40 K时改变符号.反常霍尔电阻随铂厚度的增加而急剧降低,证实了铂的铁磁性起源于异质结界面.此外,异质结在低外加磁场下可能产生了拓扑霍尔效应,这是由异质结界面处的Dzyaloshinskii-Moriya相互作用诱导产生手性磁畴壁结构引起的.上述研究结果为进一步理解非磁/铁磁异质结构中的电子自旋和电荷输运之间的相互作用提供了实验基础.     

铁磁石墨烯体系的CT不变量子自旋霍尔效应

文章作者在垂直磁场作用下的铁磁石墨烯体系里预言了一种新类型的量子自旋霍尔效应.这量子自旋霍尔效应与自旋轨道耦合无关,体系也不具有时间反演不变性;但是有CT不变(C为电子-空穴变换、T为时间反演变换).由于量子自旋霍尔效应,体系的纵向电阻和自旋霍尔阻出现量子化平台.特别是,自旋霍尔阻的量子化平台有很强的抗杂质干扰能力.     

拓扑声子与声子霍尔效应

拓扑学与物理的结合是近几十年物理学蓬勃发展的一个新领域,它不仅活跃在量子场理论以及高能物理中,更广泛地存在于凝聚态物理体系中,包括量子(反常、自旋)霍尔效应和拓扑绝缘体(超导体)等.声子是凝聚态体系中热输运的主要载体;最近由于各种声子器件的发现,声子学得到了广泛的关注.本文介绍了声子的拓扑性质以及声子的霍尔效应现象,分别评述了在破坏时间反演对称、破坏空间反演对称、以及同时破坏时间和空间反演对称三种情况下所产生的声子霍尔效应、声子谷霍尔效应等相关物理研究进展.最后对拓扑学在其他声学体系中的应用做了简单介绍,并进一步讨论了其未来的发展方向.     

二维拓扑绝缘体退相干效应研究

拓扑绝缘体是当前凝聚态物理研究的热点.退相干效应对该体系的影响的研究不仅有重要的理论意义,而且也是实现未来量子器件的不可或缺的前期工作.文章作者从理论上研究了退相干对二维拓扑绝缘体特别是量子自旋霍尔效应的影响.研究结果表明,作为量子自旋霍尔效应的标志的量子化纵向电阻平台对不破坏自旋记忆的退相干效应(普通退相干)不敏感,但却对破坏自旋记忆的退相干效应(自旋退相干)非常敏感.因此,该量子化平台只能在尺寸小于自旋退相干长度的介观样品中存在,从而解释了量子自旋霍尔效应实验中所观测到的结果(见Science,2007,318:766).同时,文章作者还定义了一个新的物理量,即自旋霍尔电阻,并发现该自旋霍尔电阻也有量子化平台.特别是该量子化平台对两种类型的退相干都不敏感.这说明在宏观样品中也能观测到自旋霍尔电阻的量子化平台,因此更能全面地反映量子自旋霍尔效应的拓扑特性.     

分数量子霍尔效应的新理论

在经典霍尔效应中 ,霍尔电压ＶＨ 线性正比于垂直方向的外场Ｂ⊥ ,并且沿电流方向的纵向电压Ｖ∥ 也随Ｂ⊥ 的增加而连续上升 .1980年 ,冯·克里青 (ＫｌａｕｓｖｏｎＫｌｉｔｚｉｎｇ)用半导体场效应晶体管进行霍尔测量 ,研究被限制在二维平面内的电子运动 .他发现 ,在极低温和强磁场的条件下 ,霍尔电压ＶＨ 不再随外场的增加而线性增加 ,而是 (在ＶＨ-Ｂ⊥ 图上 )表现为一连串ＶＨ =常数的阶跃平台 .与一个个平台相对应的霍尔电阻ＶＨ Ｉ(Ｉ是纵向电流 )恰好等于物理常数ｈ ｅ2 除以一个整数ｉ(ｉ=1,2 ,3,4 ,… ) .克里青的发现后来被称为…     

超构材料中的光学量子自旋霍尔效应

电子的量子自旋霍尔效应的发现推进了当今凝聚态物理学的发展,它是一种电子自旋依赖的具有量子行为的输运效应.近年来,大量的理论和实验研究表明,描述电磁波场运动规律的麦克斯韦方程组内禀了光的量子自旋霍尔效应,存在于界面的倏逝波表现出强烈的自旋与动量关联性.得益于新兴的光学材料:超构材料(metamaterials)的发展,不仅能够任意设定光学参数,同时也能引入很多复杂的自旋-轨道耦合机理,让我们能够更加清晰地了解和验证其中的物理机理.本文对超构材料中量子自旋霍尔效应做了简要的介绍,内容主要包括真空中光的量子自旋霍尔效应的物理本质、电单负和磁单负超构材料能带反转导致的不同拓扑相的界面态、拓扑电路系统中光量子自旋霍尔效应等.     

高性能石墨烯霍尔传感器

本文回顾了石墨烯霍尔传感器的相关研究工作.通过改善石墨烯生长转移和霍尔元件的微加工工艺,石墨烯霍尔元件和霍尔集成电路都展示出超越传统霍尔传感器的优异性能.石墨烯霍尔元件的灵敏度、分辨率、线性度和温度稳定性等重要指标都优于传统商用霍尔元件.通过开发一套钝化工艺,霍尔元件的稳定性有了明显提升.结合石墨烯材料的特点,展示了石墨烯在柔性磁传感和多功能传感领域的新颖应用.此外,成功实现了石墨烯/硅互补型金属-氧化物-半导体(CMOS)混合霍尔集成电路,并进行了应用展示.通过发展一套低温加工工艺(不超过180℃),将石墨烯霍尔元件制备在硅基CMOS芯片的钝化层上,从而与硅基CMOS电路实现了单片集成.本文的研究结果表明石墨烯在霍尔磁探测方向拥有重大的性能优势,在产业化应用中有巨大发展潜力.     

赋予科学家更多的自由和责任——访量子霍尔效应发现者Klaus von Klitzing教授

Klaus von Klitzing,德国物理学家,1943年生.他在1980年研究二维电子气在强磁场中的输运性质时发现了霍尔电阻高度精确的量子化效应.这一发现为他赢得了1985年的诺贝尔物理学奖.国际计量标准化组织在1990年决定使用量子霍尔电阻(RK=h/e2=25812.807欧姆)作为电阻的国际标准,并用他的名字命名.他的发现的重要性还在于量子霍尔电阻的精确性与普适性引发了规范不变性、几何相位和拓扑不变量等重要理论概     

碳化硅衬底上外延双层石墨烯的电输运性质

石墨烯是低维材料领域研究的热点,在这一体系中研究发现了诸多新奇的量子现象,深入理解石墨烯的电输运性质对于其在未来电子学器件中的应用具有重要的意义.本文通过热分解的方法在SiC单晶衬底上获得外延的双层石墨烯,并系统研究了其电输运性质.在小磁场范围内观测到弱局域化效应,并在较大的磁场区间发现了不饱和线性磁阻.通过角度依赖的磁阻测量,发现该线性磁阻现象符合二维体系的磁输运特征.还在平行场下观测到了负磁阻效应,可能是由双层石墨烯的转角莫尔条纹导致的局部晶格起伏导致的.本文工作加深了对于外延生长的层间具有一定转角的双层石墨烯的电输运性质的认识.     

光子石墨烯中赝磁场作用下的谷霍尔效应

将石墨烯中赝磁场的产生机理运用于光子石墨烯,通过在光子石墨烯中引入晶格有规律单轴形变的方式,理论分析得到了谷依赖的均匀赝磁场,并通过数值模拟的方法观察到明显的谷霍尔效应.这种谷霍尔效应的显著程度随晶格形变度的增加而加强.在具有一定损耗的电介质材料构成的形变光子石墨烯中仍可观察到明显的谷霍尔效应.随着电介质材料损耗的增加,谷霍尔效应导致的波束转弯效果依然能够保持,只是强度逐渐变弱.类似于自旋电子学中的自旋霍尔效应,这种光子石墨烯中等效赝磁场作用下的谷霍尔效应在未来谷极化器件的设计和应用中具有重要意义.     

Graphene-like physics in optical lattices

Graphene has attracted enormous attention over the past years in condensed matter physics. The most interesting feature of graphene is that its low-energy excitations are relativistic Dirac fermions. Such feature is the origin of many topological properties in graphene-like physics. On the other hand, ultracold quantum gas trapped in an optical lattice has become a unique setting for quantum simulation of condensed matter physics. Here, we mainly review our recent work on quantum simulation of graphene-like physics with ultracold atoms trapped in a honeycomb or square optical lattice, including the simulation of Dirac fermions and quantum Hall effect with and without Landau levels. We also present the related experimental advances.     

单元素类石墨烯二维拓扑材料的研究进展

除了石墨烯外,很多主族元素也可以形成类似石墨烯的稳定单层结构,并且具有丰富的物理化学性质及广阔的应用前景.其中在物理学中最引人注目的是理论预言在这些材料中预言存在的二维拓扑物性.单元素类石墨烯二维材料已经经历了十余年的研究进程,目前已经理论发现和实验制备出了若干新型二维拓扑材料,成为当前凝聚态物理和新材料领域重要研究方向之一.在这篇评述中,我们将详细介绍这一领域的研究历程、研究思路,以及最新研究进展,并对其进一步的研究方向做出展望.     

From magnetically doped topological insulator to the quantum anomalous Hall effect

Quantum Hall effect （QHE）, as a class of quantum phenomena that occur in macroscopic scale, is one of the most important topics in condensed matter physics. It has long been expected that QHE may occur without Landau levels so that neither external magnetic field nor high sample mobility is required for its study and application, Such a QHE free of Landau levels, can appear in topological insulators （TIs） with ferromagnetism as the quantized version of the anomalous Hall effect, i.e., quantum anomalous Hall （QAH） effect. Here we review our recent work on experimental realization of the QAH effect in magnetically doped TIs. With molecular beam epitaxy, we prepare thin films of Cr-doped （Bi,Sb）2Te3 TIs with well- controlled chemical potential and long-range ferromagnetic order that can survive the insulating phase. In such thin films, we eventually observed the quantization of the Hall resistance at h/e2 at zero field, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect provides a foundation for many other novel quantum phenomena predicted in TIs, and opens a route to practical applications of quantum Hall physics in low-power-consumption electronics.     

The enigma of the quantum Hall effect in graphene

We apply Laughlin’s gauge argument to analyze the ν=0 quantum Hall effect observed in graphene when the Fermi energy lies near the Dirac point, and conclude that this necessarily leads to divergent bulk longitudinal resistivity in the zero temperature thermodynamic limit. We further predict that in a Corbino geometry measurement, where edge transport and other mesoscopic effects are unimportant, one should find the longitudinal conductivity vanishing in all graphene samples which have an underlying ν=0 quantized Hall effect. We argue that this ν=0 graphene quantum Hall state is qualitatively similar to the high field insulating phase (also known as the Hall insulator) in the lowest Landau level of ordinary semiconductor two-dimensional electron systems. We establish the necessity of having a high magnetic field and high mobility samples for the observation of the divergent resistivity as arising from the existence of disorder-induced density inhomogeneity at the graphene Dirac point.     

惠斯通电桥测电阻及电阻温度系数的教学探讨

"惠斯通电桥测电阻及电阻温度系数"实验是大学物理实验中一项重要的研究性实验,介绍"惠斯通电桥测电阻及电阻温度系数"的基本原理及实验过程,针对实验教学中经常出现的问题,提出了一些从教学实践中总结出来的解决方法。     

Fractional Quantum Conductance Plateaus in Mosaic‐Like Conductors and Their Similarities to the Fractional Quantum Hall Effect

A simple route to generate magnetotransport data is reported that results in fractional quantum Hall plateaus in the conductance without invoking strongly correlated physics. Ingredients to the generating model are conducting tiles with integer quantum Hall effect and metallic linkers, further Kirchhoff rules. When connecting few identical tiles in a mosaic, fractional steps occur in the conductance values. Richer spectra representing several fractions occur when the tiles are parametrically varied. Parts of the simulation data are supported with purposefully designed graphene mosaics in high magnetic fields. The findings emphasize that the occurrence of fractional conductance values, in particular in two‐terminal measurements, does not necessarily indicate interaction‐driven physics. The importance of an independent determination of charge densities is underscored and similarities with and differences to the fractional quantum Hall effect are critically discussed.     

Detection of hidden localized states by the quantum Hall effect in graphene

We fabricated a monolayer graphene transistor device in the shape of the Hall-bar structure, which produced an exactly symmetric signal following the sample geometry. During electrical characterization, the device showed the standard integer quantum Hall effect of monolayer graphene except for a broader range of several quantum Hall plateaus corresponding to small filling factors in the electron region. We investigated this anomaly on the basis of localized states owing to the presence of possible electron traps, whose energy levels were estimated to be near the Dirac point. In particular, the inequality between the filling of electrons and holes was ascribed to the requirement of excess electrons to fill the trap levels. The relations between the quantum Hall plateau, Landau level, and filling factor were carefully analyzed to reveal the details of the localized states in this graphene device.     

Edge state propagation direction in the fractional quantum Hall regime: multi-terminal magnetocapacitance experiment

The propagation direction of fractional quantum Hall effect (FQHE) edge states has been investigated experimentally via the symmetry properties of the multi-terminal capacitances of a two-dimensional electron gas. Although strong asymmetries with respect to zero magnetic field appear, no asymmetries with respect to even denominator Landau level filling factor ν are seen. This indicates that current-carrying FQHE edge states propagate in the same direction as integer QHE edge states. In addition, anomalous capacitance features, indicative of enhanced bulk conduction, are observed at and .