skip to main content

Title: Magnetic-Field-Driven Electron Dynamics in Graphene
Graphene exhibits unique optoelectronic properties originating from the band structure at the Dirac points. It is an ideal model structure to study the electronic and optical properties under the influence of the applied magnetic field. In graphene, electric field, laser pulse, and voltage can create electron dynamics which is influenced by momentum dispersion. However, computational modeling of momentum-influenced electron dynamics under the applied magnetic field remains challenging. Here, we perform computational modeling of the photoexcited electron dynamics achieved in graphene under an applied magnetic field. Our results show that magnetic field leads to local deviation from momentum conservation for charge carriers. With the increasing magnetic field, the delocalization of electron probability distribution increases and forms a cyclotron-like trajectory. Our work facilitates understanding of momentum resolved magnetic field effect on non-equilibrium properties of graphene, which is critical for optoelectronic and photovoltaic applications.
; ; ;
Award ID(s):
Publication Date:
Journal Name:
The journal of physical chemistry letters
Page Range or eLocation-ID:
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A ferromagnetic gyroscope (FG) is a ferromagnet whose angular momentum is dominated by electron spin polarization and that will process under the action of an external torque, such as that due to a magnetic field. Here we model and analyze FG dynamics and sensitivity, focusing on practical schemes for experimental realization. In the case of a freely floating FG, we model the transition from dynamics dominated by libration in relatively high externally applied magnetic fields, to those dominated by precession at relatively low applied fields. Measurement of the libration frequency enablesin situdetermination of the magnetic field and a technique to reduce the field below the threshold for which precession dominates the FG dynamics. We note that evidence of gyroscopic behavior is present even at magnetic fields much larger than the threshold field below which precession dominates. We also model the dynamics of an FG levitated above a type-I superconductor via the Meissner effect, and find that for FGs with dimensions larger than about 100 nm the observed precession frequency is reduced compared to that of a freely floating FG. This is due to an effect akin to negative feedback that arises from the distortion of the field from the FGmore »by the superconductor. Finally we assess the sensitivity of an FG levitated above a type-I superconductor to exotic spin-dependent interactions under practical experimental conditions, demonstrating the potential of FGs for tests of fundamental physics.

    « less
  2. In this study, we investigated hierarchical microarchitecture formation of magnetic barium hexaferrite (BF) platelets inside the polydimethylsiloxane (PDMS) matrix using electric and magnetic field colloidal assembly technique. First, external fields were applied to the colloidal solution to form the microstructure before curing the composites. After microstructure formation the composites were cured to freeze the microstructure by the application of heat. We investigated two different cases in this study-(1) magnetic field processed composites and (2) multi-field processed composites which were processed under both magnetic and electric field. We observed that macro-chains formed due to the electric and magnetic field had much higher length compared to the macro-chains formed due to the just magnetic field. For both cases individuals BHF are found to be oriented in the direction of external field. The analysis of SEM microstructures using ImageJ and MATLAB showed that at least two different level of hierarchies are present in the microstructure for both cases which can be named as BHF stacks and micro-chains. From the microstructure analysis, we found that compared to just magnetic field processed composites, the orientation of individual particles, BHF stacks and micro-chains in relation to the external field were found to be higher for themore »multi-field processed composites. Magneto-electro-hydrodynamics modeling of the polymer-particulate mixture predicted similar behavior. Computational simulations were performed wherein particulates, subjected to both DEP forces and additional magnetic dipole interactions, were allowed to form quasi-equilibrium structures before locking in a final structure to represent curing. Results show that dielectrophoretic (DEP) force produced from the local non-uniform electric field facilitates the translation of the platelets towards formation of chain-like structure, while external magnetic field augmented the rotation of particles inside the chain-like structure. Analysis of the simulation of microstructures confirms that multiple level of hierarchies are present in the composites microstructure for both cases, while the case with both electric and magnetic fields produced longer chains. The understanding of the hierarchical microstructure formation using the multi-field processing technique will help in the future to fabricate more complex microarchitectures with resulting multi-material properties.« less
  3. Perovskites have been firmly established as one of the most promising materials for third-generation solar cells. There remain several great and lingering challenges to be addressed regarding device efficiency and stability. The photovoltaic efficiency of perovskite solar cells (PSCs) depends drastically on the charge-carrier dynamics. This complex process includes charge-carrier generation, extraction, transport and collection, each of which needs to be modulated in a favorable manner to achieve high performance. Two-dimensional materials (TDMs) including graphene and its derivatives, transition metal dichalcogenides ( e.g. , MoS 2 , WS 2 ), black phosphorus (BP), metal nanosheets and two-dimensional (2D) perovskite active layers have attracted much attention for application in perovskite solar cells due to their high carrier mobility and tunable work function properties which greatly impact the charge carrier dynamics of PSCs. To date, significant advances have been achieved in the field of TDM-based PSCs. In this review, the recent progress in the development and application of TDMs ( i.e. , graphene, graphdiyne, transition metal dichalcogenides, BP, and others) as electrodes, hole transporting layers, electron transporting layers and buffer layers in PSCs is detailed. 2D perovskites as active absorber materials in PSCs are also summarized. The effect of TDMs and 2Dmore »perovskites on the charge carrier dynamics of PSCs is discussed to provide a comprehensive understanding of their optoelectronic processes. The challenges facing the PSC devices are emphasized with corresponding solutions to these problems provided with the overall goal of improving the efficiency and stability of photovoltaic devices.« less
  4. Abstract

    Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

  5. Over three decades of in-situ observations illustrate that the Kelvin–Helmholtz (KH) instability driven by the sheared flow between the magnetosheath and magnetospheric plasma often occurs on the magnetopause of Earth and other planets under various interplanetary magnetic field (IMF) conditions. It has been well demonstrated that the KH instability plays an important role for energy, momentum, and mass transport during the solar-wind-magnetosphere coupling process. Particularly, the KH instability is an important mechanism to trigger secondary small scale (i.e., often kinetic-scale) physical processes, such as magnetic reconnection, kinetic Alfvén waves, ion-acoustic waves, and turbulence, providing the bridge for the coupling of cross scale physical processes. From the simulation perspective, to fully investigate the role of the KH instability on the cross-scale process requires a numerical modeling that can describe the physical scales from a few Earth radii to a few ion (even electron) inertial lengths in three dimensions, which is often computationally expensive. Thus, different simulation methods are required to explore physical processes on different length scales, and cross validate the physical processes which occur on the overlapping length scales. Test particle simulation provides such a bridge to connect the MHD scale to the kinetic scale. This study applies different testmore »particle approaches and cross validates the different results against one another to investigate the behavior of different ion species (i.e., H+ and O+), which include particle distributions, mixing and heating. It shows that the ion transport rate is about 10 25  particles/s, and mixing diffusion coefficient is about 10 10  m 2  s −1 regardless of the ion species. Magnetic field lines change their topology via the magnetic reconnection process driven by the three-dimensional KH instability, connecting two flux tubes with different temperature, which eventually causes anisotropic temperature in the newly reconnected flux.« less