Xi-Lai Li

Surface polar phonon dominated electron transport in graphene

The effects of surface polar phonons on the electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene’s electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.

Influence of electron-electron scattering on transport characteristics in monolayer graphene

The influence of electron-electron scattering on the distribution function and transport characteristics of intrinsic monolayer graphene is investigated via an ensemble Monte Carlo simulation. Due to the linear dispersion relation in the vicinity of the Dirac points, it is found that pair-wise collisions in graphene do not conserve the ensemble average velocity in contrast to conventional semiconductors with parabolic energy bands. Numerical results indicate that electron-electron scattering can lead to a decrease in the low field mobility by more than a factor of 2 for moderate electron densities. The corresponding degradation in the saturation velocity is more modest at around 15%. At high densities, the impact gradually diminishes due to increased degeneracy.

Electron transport properties of bilayer graphene

Electron transport in bilayer graphene is studied by using a first principles analysis and theMonte Carlo simulation under conditions relevant to potential applications. While the intrinsic properties are found to be much less desirable in bilayer than in monolayer graphene, with significantly reduced mobilities and saturation velocities, the calculation also reveals the dominant influence of extrinsic factors such as the substrate and impurities. Accordingly, the difference between two graphene forms are more muted in realistic settings although the velocity-field characteristics remain substantially lower in the bilayer. When bilayer graphene is subject to an interlayer bias, the resulting changes in the energy dispersion lead to stronger electron scattering at the bottom of the conduction band. The mobility decreases significantly with the size of the generated bandgap, whereas the saturation velocity remains largely unaffected.

Strong substrate effects of Joule heating in graphene electronics

The effect of Joule heating on graphene electronic properties is investigated by self-consistent use of full-band Monte Carlo electron dynamics and three-dimensional heat transfer simulations. Several technologically important substrate materials are examined: SiO2, SiC, hexagonal BN, and diamond. Results illustrate that the choice of substrate has a major impact via heat conduction and surface polar phonon scattering. Particularly, the poor thermal conductivity of SiO2 leads to significant Joule heating and saturation velocity degradation in graphene characterized by the 1/n decay. Considering the overall characteristics, BN appears to compare favorably against the other substrate choices for graphene electronic applications.

Electrical switching of antiferromagnets via strongly spin-orbit coupled materials

Electrically controlled ultra-fast switching of an antiferromagnet (AFM) is shown to be realizable by interfacing it with a material of strong spin-orbit coupling. The proximity interaction between the sublattice magnetic moments of a layered AFM and the spin-polarized free electrons at the interface offers an efficient way to manipulate antiferromagnetic states. A quantitative analysis, using the combination with a topological insulator as an example, demonstrates highly reliable 90° and 180° rotations of AFM magnetic states under two different mechanisms of effective torque generation at the interface. The estimated switching speed and energy requirement are in the ps and aJ ranges, respectively, which are about two-three orders of magnitude better than the ferromagnetic counterparts. The observed differences in the magnetization dynamics may explain the disparate characteristic responses. Unlike the usual precessional/chiral motions in the ferromagnets, those of the AFMs can essentially be described as...

Currentless reversal of N\'{e}el vector in antiferromagnets

The bias driven perpendicular magnetic anisotropy is a magneto-electric effect that can realize 90

Electrically controlled switching of antiferromagnets via proximity interaction induced by topological insulator

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Highly efficient conductance control in a topological insulator based magnetoelectric transistor

magnetization rotation and even 180

Electron-phonon interactions in bilayer graphene

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Nonlinear magnetic dynamics in a nanomagnet–topological insulator heterostructure

flip along the easy axis in the ferromagnets with a minimal energy consumption. This study theoretically demonstrates a similar phenomenon of the Neel vector reversal via a short electrical pulse that can mediate perpendicular magnetic anisotropy in the antiferromagnets. The analysis based on the dynamical equations as well as the micro-magnetic simulations reveals the important role of the inertial behavior in the antiferromagnets that facilitates the Neel vector to overcome the barrier between two free-energy minima of the bistable states along the easy axis. In contrast to the ferromagnets, this Neel vector reversal does not accompany angular moment transfer to the environment, leading to acceleration in the dynamical response by a few orders of magnitude. Further, a small switching energy requirement of a few attojoules illustrates an added advantage of the phenomenon in low-power spintronic applications.