It is widely believed that carrier-density inhomogeneities ("electron-hole puddles") in single-layer graphene on a substrate like quartz are due to charged impurities located close to the graphene sheet. Here we demonstrate by using a Kohn-Sham-Dirac density-functional scheme that corrugations in a real sample are sufficient to determine electron-hole puddles on length scales that are larger than the spatial resolution of state-of-the-art scanning tunneling microscopy.
We report on spin transport features which are unique to high quality bilayer graphene, in absence of magnetic contaminants and strong intervalley mixing. The time-dependent spin polarization of propagating wavepacket is computed using an efficient quantum transport method. In the limit of vanishing effects of substrate and disorder, the energy-dependence of spin lifetime is similar to monolayer graphene with a M-shape profile and minimum value at the charge neutrality point, but with an electron-hole asymmetry fingerprint. In sharp contrast, the incorporation of substrate-induced electron-hol…
A tight-binding approach based on the Chebyshev-Bogoliubov-de Gennes method is used to describe disordered single-layer graphene Josephson junctions. Scattering by vacancies, ripples or charged impurities is included. We compute the Josephson current and investigate the nature of multiple Andreev reflections, which induce bound states appearing as peaks in the density of states for energies below the superconducting gap. In the presence of single atom vacancies, we observe a strong suppression of the supercurrent that is a consequence of strong inter-valley scattering. Although lattice deforma…
Despite topological protection and the absence of magnetic impurities, two-dimensional topological insulators display quantized conductance only in surprisingly short channels, which can be as short as 100 nm for atomically thin materials. We show that the combined action of short-range nonmagnetic impurities located near the edges and on site electron-electron interactions effectively creates noncollinear magnetic scatterers, and, hence, results in strong backscattering. The mechanism causes deviations from quantization even at zero temperature and for a modest strength of electron-electron i…
研究了低温超高真空下沉积钾时，杂质散射对双层石墨烯（BLG）中载流子输运的影响。带电杂质散射产生的电导率在载流子密度上呈超线性，在2-4 x 10^12 cm^-2的载流子密度测量范围内，其大小与单层石墨烯相似。在添加浓度为n_imp的带电杂质时，最小电导率Sigma_min与n_imp^-1/2成比例地降低，而电子和空穴熔池载流子密度与n_imp ^1/2成比例地增加。这些结果的有意沉积钾的BLG与带电杂质散射的理论预测吻合得很好。然而，我们的研究结果也表明，带电杂质散射单独不能解释所观察到的原始BLG钾掺杂前的SiO2上的输运性质。
Thermoelectric properties of Graphene Nanoribbons doped by magnetic impurities Fe and Co are carried out in room temperature. We report on a study of the band structure dependent properties such as electrical conductivity, charge of carriers and Seebeck coefficients. We investigate the thermoelectric properties using the Semi-classical Boltzmann method. The electronic band structure of doped nanoribbons are evaluated by using density-functional theory in which the Hubbard interaction is considered. In this work we compare the different types of the nanoribbons and their thermoelectric features…
Monolayer graphene is an example of materials with multi-valley electronic structure. In such materials, the valley index is being considered as an information carrier. Consequently, relaxation mechanisms leading to loss of valley information are of interest. Here, we calculate the rate of valley relaxation induced by charged impurities in graphene. A special model of graphene is applied, where the $p_z$ orbitals are two-dimensional Gaussian functions, with a spatial extension characterised by an effective Bohr radius $a_\textrm{eB}$. We obtain the valley relaxation rate by solving the Boltzma…
We report a study of disorder effects on epitaxial graphene in the vicinity of the Dirac point by magneto-transport. Hall effect measurements show that the carrier density increases quadratically with temperature, in good agreement with theoretical predictions which take into account intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. We deduce disorder strengths in the range 10.2 $\sim$ 31.2 meV, depending on the sample treatment. We investigate the scattering mechanisms and estimate the impurity density to be $3.0 \sim 9.1 \times 10^{10}$ cm$^{-2}$…
We explore the problem of atomic collapse in graphene by monopole impurities, both electric and magnetic, within the context of supersymmetric quantum mechanics. For electric impurities, upon factorizing the radial Dirac Hamiltonian and identifying the supercharges, existence of a critical charge that makes the ground state {\em fall-into-the-center} translates into lost of hermicity for the corresponding Hamiltonian and hence lost of unitarity of the theory. For the problem of magnetic monopole impurities, preservation of unitarity for all values of the parameters of the corresponding potenti…
Theory of quasiparticle interference on the surface of a strong topological insulator
强拓扑绝缘体（如Bi 2 Te 3或Bi 1-xSnx）表面的电子形成拓扑保护的螺旋液体，其激发光谱包含奇数个无质量狄拉克费米子。本文对螺旋液体中磁性杂质和非磁性杂质的准粒子散射产生的干涉图样的普遍特征进行了理论分析和分类。我们的研究结果证实了在最近的实验中观察到的非磁性杂质的背散射的情况下，并预测新的干扰功能，独特的螺旋液体的特性，当散射体是磁性的。
In a recent experiment, Young et al. [Nature {\bf 505}, 528 (2014)] observed a metal to insulator transition as well as transport through helical edge states in monolayer graphene under a strong, tilted magnetic field. Under such conditions, the bulk is a magnetic insulator which can exhibit metallic conduction through helical edges. It was found that two-terminal conductance of the helical channels deviates from the expected quantized value at low-temperatures ($=e^2/h$ per edge, at zero temperature). Motivated by this observation, we study the effect of disorder on the conduction through the…
The competition between kinetic energy and Coulomb interactions in electronic systems can lead to complex many-body ground states with competing superconducting, charge density wave, and magnetic orders. Here we study the low temperature phases of a strongly interacting zinc-oxide-based high mobility two dimensional electron system that displays a tunable metal-insulator transition. Through a comprehensive analysis of the dependence of electronic transport on temperature, carrier density, in-plane and perpendicular magnetic fields, and voltage bias, we provide evidence for the existence of com…
