We study the thermal and electric transport of a fluid of interacting Dirac fermions as they arise in single-layer graphene. We include Coulomb interactions, a dilute density of charged impurities and the presence of a magnetic field to describe both the static and the low frequency response as a function of temperature T and chemical potential mu. In the critical regime mu << T where both bands above and below the Dirac point contribute to transport we find pronounced deviations from Fermi liquid behavior, universal, collision-dominated values for transport coefficients and a cyclotron resonance of collective nature. In the collision-dominated high temperature regime the linear thermoelectric transport coefficients are shown to obey the constraints of relativistic magnetohydrodynamics which we derive microscopically from Boltzmann theory. The latter also allows us to describe the crossover to disorder-dominated Fermi liquid behavior at large doping and low temperatures, as well as the crossover to the ballistic regime at high fields.
We report that a perpendicular magnetic field introduces an anomalous interaction correction, $δσ$, to the static conductivity of doped graphene in the ballistic regime. The correction implies that the magnetoresistance, $δρ_{xx}$ scales inversely with temperature $δρ_{xx}(T) \propto 1/T$ in a parametrically large interval. When the disorder is scalar-like, the $\propto 1/T$ behavior is the leading contribution in the crossover between diffusive regime exhibiting weak localization and quantum magnetooscillations. The behavior originates from the field-induced breaking of the chiral symmetry of…
Non linear magnetotransport theory and Hall induced resistance oscillations in graphene
研究了直流偏压下石墨烯非线性磁电阻的量子振荡。我们提出了一个理论模型的非线性磁输运的石墨烯载流子。该模型是基于有效的狄拉克方程的精确解在交叉的电场和磁场，而随机分布的杂质的影响微扰添加。为了计算非线性电流，我们开发了一个协变公式的迁移中心理论。对大磁场区微分电阻率的分析表明，当直流电流超过阈值时，Shubnikov de Hass振荡的极值反转。这一结果与实验观察结果符合得很好。在小磁场下，超净石墨烯样品存在霍尔感应电阻振荡。这些振荡起源于朗道能级之间的朗道-齐纳跃迁，朗道能级被强霍尔电场倾斜。
Confinement and edge structures are known to play significant roles in electronic and transport properties of two-dimensional materials. Here, we report on low-temperature magnetotransport measurements of lithographically patterned graphene cavity nanodevices. It is found that the evolution of the low-field magnetoconductance characteristics with varying carrier density exhibits different behaviors in graphene cavity and bulk graphene devices. In the graphene cavity devices, we have observed that intravalley scattering becomes dominant as the Fermi level gets close to the Dirac point. We assoc…
We report fabrication of graphene devices in a Corbino geometry consisting of concentric circular electrodes with no physical edge connecting the inner and outer electrodes. High device mobility is realized using boron nitride encapsulation together with a dual-graphite gate structure. Bulk conductance measurement in the quantum Hall effect (QHE) regime outperforms previously reported Hall bar measurements, with improved resolution observed for both the integer and fractional QHE states. We identify apparent phase transitions in the fractional sequence in both the lowest and first excited Land…
We present transport measurements of bilayer graphene with 1.38° interlayer twist and apparent additional alignment to its hexagonal boron nitride cladding. As with other devices with twist angles substantially larger than the magic angle of 1.1°, we do not observe correlated insulating states or band reorganization. However, we do observe several highly unusual behaviors in magnetotransport. For a large range of densities around half filling of the moiré bands, magnetoresistance is large and quadratic. Over these same densities, the magnetoresistance minima corresponding to gaps between Landa…
Quantum coherence and its dephasing in the giant spin Hall effect and nonlocal voltage generated by magnetotransport through multiterminal graphene bars
受近期实验观察的启发[D. A. Abanin等人，Science 323，328（2011）]中的六端石墨烯霍尔棒中的狄拉克点附近的磁输运中的非定域性，对于宽范围的温度和磁场，我们开发了该现象的非平衡格林函数（NEGF）理论。在相位相干区和强磁场中，我们发现四端桥中存在大的自旋霍尔（SH）电导，其中SH电流仅在狄拉克点（DP）处是纯的，而六端棒中的正、逆SH效应同时作用，在远处的位置处存在非局域电压.“动量松弛”的失相减少了两个数量级的DP的值，同时洗出任何功能远离DP。我们的理论是基于Meir-Wingreen公式，通过现象学的多体自能，然后线性化的多端几何形状，以提取电流和电压的失相。
Reply to arXiv:1212.3831v1 "Comment on: Magnetotransport through graphene spin valves and its following works" by Y. Zhou and M.W. Wu
在他们的评论arXiv：1212.3831v1 Y. Zhou和M.W.吴声称，基本的输运方程与当前的传输函数，使用我们和事实上由许多其他研究人员，是无效的扩展系统，应该得到纠正。它们为扩展系统中的输运提供了一个“正确的”新公式。这确实是量子输运理论的一个令人惊讶的新方面。然而，在这里，我们从数学上证明了新公式是一种误解，它是由于增加了一个能量和动量相关的函数，而这个函数由于根本原因而必须消失。添加此功能所产生的结果和结论无关紧要。已知的量子输运公式在有据可查的条件下是一致的。
Landau levels of AB-stacked four- to six-layer graphene were studied experimentally in the presence of a perpendicular electric field. The Landau levels at a low magnetic field showed a characteristic structure that originated from the mini-Dirac cones created by the perpendicular electric fields. Six-fold or twelve-fold degenerate Landau levels arising from the mini-Dirac cones were observed in the vicinity of the charge neutrality point as a radial structure in a plot of their Landau fan diagrams. The structure of four-layer graphene had approximate electron-hole symmetry near the charge neu…
Ultra-small-angle graphene twistronics with monolayer spacers: From interlayer hybridisation and moiré effects to high-temperature magnetotransport oscillations
University of Manchester（曼彻斯特大学）；National Graphene Institute, University of Manchester（国家石墨烯研究所，曼彻斯特大学）；City University of Hong Kong（香港城市大学）；National University of Singapore（新加坡国立大学）；Institute for Functional Intelligent Materials, National University of Singapore（功能智能材料研究所，新加坡国立大学）
The polymorph of ${\rm 8-Pmmn}$ borophene is an anisotropic Dirac material with tilted Dirac cones at two valleys. The tilting of the Dirac cones at two valleys are in opposite direction, which manifests itself via valley dependent Landau levels in presence of an in-plane electric field (Hall field). The sensitivity of the Landau levels on valley index causes valley polarized magnetotransport properties in presence of a Hall field, which is in contrast to the monolayer graphene with isotropic non-tilted Dirac cones. The longitudinal conductivity and Hall conductivity are evaluated by using lin…
Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here, we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry.
The complex nature of filling factor $ν$=0 of monolayer graphene is studied in magnetotransport experiments. As function of perpendicular magnetic field a metal-insulator transition is observed, which is attributed to disorder-induced Landau level broadening in the canted antiferromagnetic phase. In the metallic regime a separation of the zeroth Landau level appears and signs of the quantum spin Hall effect are seen near $ν$=0. In addition to local transport, nonlocal transport experiments show results being consistent with helical edge transport.
