Browsing by Subject "Berry curvature"
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Item Berry curvature and orbital magnetic moment effects on magnetoresistance in two-dimensional valley systems(2021-08-13) Zhou, Hailong; Niu, Qian; MacDonald, Allan H; Shi, Li; Tutuc, EmanuelThe mechanisms of negative magnetoresistance in various topological materials have been the center of transport study from the rise of two-dimensional and three-dimensional topological semimetals and metals, especially when the chiral anomaly induced negative magnetoresistance was predicted and observed. The role played by the Berry curvature has been studied in many aspects, including chiral magnetic effect, chiral anomaly, and other intrinsic geometric effects. Here we revisit the effects of the Berry curvature on the negative longitudinal magnetoresistance in Weyl metals, and then extend our study to include the effect of orbital magnetic moment of Bloch electrons in semiclassical Boltzmann transport theory. By examining the two-dimensional multivalley model with the corrected Boltzmann equation, we find that the valley-contrasting orbital magnetic moment allows the lifting of valley degeneracy by an out-of-plane magnetic field. We demonstrate that this leads to negative magnetoresistance, utilizing a gapped graphene model as an example. An intuitive physical picture in terms of the increased carrier density from a magnetic gating effect is proposed for this negative transverse magnetoresistance. In particular, giant negative transverse magnetoresistance is achieved after one of the two valleys is depleted by the magnetic field. This mechanism of negative magnetoresistance is argued to be relevant in ionic-liquid gated gapped graphene with a small effective mass.Item Effect of molecular Berry curvature on the dynamics of phonons(2022-08-31) Saparov, Daniyar; Niu, Qian; MacDonald, Allan H; Tutuc, Emanuel; Shi, LiUnder the Born-Oppenheimer approximation, the electronic ground state evolves adiabatically and can accumulate geometrical phases characterized by the molecular Berry curvature. In this work, we study the effect of the molecular Berry curvature on the lattice dynamics in a system with broken time-reversal symmetry. The molecular Berry curvature is formulated based on the single-particle electronic Bloch states. It manifests as a non-local effective magnetic field in the equations of motion of the ions that are beyond the widely adopted Raman spin-lattice coupling model. We employ the Bogoliubov transformation to solve the quantized equations of motion and to obtain phonon polarization vectors. We apply our formula to the Haldane model on a honeycomb lattice and find a large molecular Berry curvature around the Brillouin zone center. As a result, the degeneracy of the optical branches at this point is lifted intrinsically. The lifted optical phonons show circular polarizations, possess large phonon Berry curvature, and have a nearly quantized angular momentum that modifies the Einstein-de Haas effect.Item Second order semiclassical theory of Bloch electrons in uniform electromagnetic fields(2012-05) Gao, Yang 1987-; Niu, QianBerry curvature appears in the semi-classical theory of Bloch electrons already to first order in electromagnetic fields, resulting in profound modification of the carrier velocity and phase space density of states. Here we derive the equations of motion for the physical position and crystal momentum to second order in the fields. The dynamics still has a Hamiltonian structure, albeit with noncanonical Poisson brackets between the physical variables. We are able to expand both the carrier energy and the Poisson brackets to second order in the fields with terms of clear physical meaning. To demonstrate the utility of our theory, we obtain with much ease the electromagnetic response and orbital magnetic susceptibility.