Browsing by Subject "2D materials"
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Item Electron interactions in 2D materials : excitons and quantum hall effect(2016-08) Wu, Fengcheng; MacDonald, Allan H.; Li, Xiaoqin; Fiete, Gregory A.; Niu, Qian; Tutuc, EmanuelThis dissertation presents studies of the electron interaction effects in two-dimensional materials. In particular, excitonic effect in transition metal dichalcogenides and quantum Hall effect in graphene have been investigated. The common thread that passes through the two topics is the interplay between electron interactions and spin and valley degrees of freedom. Chapter 1 is a brief introduction to the thesis. Chapter 2 addresses the energy and wave function of excitons in monolayer MoS$_2$. It reveals several interesting features, which can be important for exciton dynamics. Chapter 3 describes a theory of spatially indirect exciton condensates in transition metal dichalcogenide heterostructures. A systematic approach is developed to construct an effective exciton model with exciton-exciton interactions. The effective exciton model provides a useful guidance to construct the condensate phase diagram of excitons with multiple flavors. Chapter 4 identifies an SO(5) symmetry in the quantum Hall effect in graphene. The enlarged SO(5) symmetry unifies the spin antiferromagnetic order and valley $XY$ order. The physics of the SO(5) symmetry is explored using exact diagonalization and low-energy effective theory. Chapter 5 speculates about possible SU(3) and SU(4) singlet fractional quantum Hall states at a filling factor $\nu=2/3$ based on finite-size exact diagonalization study. These singlets are surprising because they are not captured by the composite fermion approach. The shift quantum number and the pair correlation function of the new states are presented.Item Microwave impedance microscope study of two dimensional materials(2015-05) Liu, Yingnan, M.A.; Lai, Keji, 1978-; Shih, Chih-KangIn this thesis, I will introduce a unique technique, microwave impedance microscope (MIM), which has shown its potential in characterization of local electrical inhomogeneity of materials. I will also discuss some results about the study of In₂Se₃ and MoS₂ electrical properties with MIM.Item Pressure induced structure-property tuning of two dimensional materials(2015-05) Nayak Pradeep, Avinash; Akinwande, Deji; Lin, Jung-Fu; Wang, Yaguo; Wang, Zheng; Hall, Neal; Dodabalapur, AnanthControlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two dimensional (2D) materials. Here we investigate the electronic and lattice vibrational dynamics of the distorted monolayer 1T-MoS₂ (1T') and the monolayer 2H-MoS₂ via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS₂ increases by 11.7% from 1.85 eV to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS₂, an overall complex structure-property relation due to the rich band structure of MoS₂. Remarkably, the metastable 1T'-MoS₂ metallic state remains invariant with pressure, with the J₂, A₁[subscript g], and E₂[subscript g] modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational property of the MoS₂ family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS₂ via pressure, which has vital implications for enhanced device applications.Item Quantum simulation of device physics for the pseudospintronic devices(2015-05) Mou, Xuehao; Register, Leonard F.; Banerjee, Sanjay K; MacDonald, Allan H; Lee, Jack C; Tutuc, EmanuelIt has been predicted that room-temperature electron-hole exciton condensation may be possible in dielectrically separated and exchange-correlation-coupled two-dimensional material bilayer systems, which is the fundamental basis for a family of new device proposals—the pseudospintronic devices. These new devices employ the global interlayer phase coherence accompanying the condensation, with the “which layer” degree of freedom as the “pseudospin” and the coherent phase as the “pseudospin phase.” The interlayer transport is expected to be greatly enhanced by this phase coherence up to a critical interlayer current and the associated critical voltage possibly below thermal temperature k [subscript B] T/q. Beyond that, the DC enhancement of interlayer transport will be lost and a rapid AC oscillation will emerge; the latter could be filtered out by RC constants of circuits. This transition serves as the ON/OFF mechanism, allowing the power crisis faced by traditional CMOS devices be solved via this potential low-voltage and, thus, low-power operation. A highly efficient, highly flexible full-quantum Hartree-Fock numerical simulator is developed in this work to study the device physics of pseudospintronic devices via modeling the interlayer exchange correlations. The dependences of possible room-temperature condensates are explored on orientational misalignments and short-range disorders in bilayer graphene systems and on different materials—preliminarily ideal MoS₂ bilayers belonging to the TMD family—as new potential hosts of such condensates. The possibility of room-temperature condensates is disproved in neither systems, although other issues could make the condensate formation challenging. Not focusing on those challenges, the simulator is modified and applied to study the transport physics in the presence of spatially confined condensates in bilayer graphene systems. The expected behaviors, including not only aforementioned low-voltage switching and greatly enhanced interlayer transmission, but also a near-ideal Coulomb drag and a theoretically expected underlying process much resembling the Andreev reflection, are all demonstrated in these simulations in nanoscale devices at room temperature. A new current-controlled scheme of pseudospintronic device, the BiSJT, is proposed based on exhibited physics, along with an extracted compact model for circuit simulations. Conventional and novel BiSJT-based logic gates are designed, with circuit simulations to illustrate the logic functionality and still low-voltage, low-power operations.