Session R26: Complex Structured Materials - Computational

Topological Phonon-Plasmon Modes in Two-Dimensional Ferromagnetic Wigner Crystal of Electrons

We show that a two-dimensional ferromagnetic Wigner crystal of electrons confined in a semiconductor quantum well/heterostructure with spin-orbit coupling and an appropriate sign of $g$-factor could be transformed to a topological phonon-plasmon system by applying a weak perpendicular magnetic field. The competition between the magnetic field and the spin-orbit coupling will drive a topological phase transition, resulting in a topologically trivial phonon-plasmon system in the high magnetic field. We demonstrate the existence of chiral edge phonon-plasmon modes in finite size samples for both phases, and the robustness of the chiral edge modes in the topological phase. We estimate parameters for a few commonly used semiconductors,such as GaAs, GaAl, InAs and InSb. Moreover, we rule out the possibility of Wigner crystal of holes as a topological phonon-plasmon system, due to the unfavorable form of spin-orbit coupling in hole bands dictated by symmetry.   

Electron transport calculations with Wannier functions in van der Waals heterostructures

The vertical stacking of 2D materials forming van der Waals heterostructures (vdWHs) exhibits a wide range of interesting properties. A combined approach based on the Green's function formalism and a mean-field description of the electronic structure is used to calculate vertical electron transport in vdWHs. Tight-binding parameters obtained from Maximally Localized Wannier Functions enable us to model quantum electron transport at low computational costs. Our analysis of electron transport efficiencies provides the foundation and motivation for experimental works.   

Interlayer coupling in few-layers transition metal dichalcogenides

The vertically heterostructured transition metal dichalcogenides (TMD) few-layers may display a wide range of lattice registry. By density functional theory and numerical structural analysis, we examined both the atomic and electronic structures of arbitrarily stacked TMD few-layers. It is shown [1] that the variation of indirect band gap in MoS$_2$ bilayers is mainly attributed to the interlayer sulfur-sulfur (S-S) interaction. We developed a structural model that allows understanding such interaction under an arbitrary stacking sequence. It is shown the arbitrarily stacked MoS$_2$ bilayers should exhibit a weak twist angle dependence on the magnitude of its indirect band gap, except for those special twist angles that recover high symmetry stacking sequences. Our analysis provides a thorough theoretical explanation to the recently measured photoluminescence spectroscopy and can form the basis for understanding the coupling in other vertically heterostructured TMD few-layers. For example, through a close experimental collaboration, we have recently identified the electronic origin for the metallization of WS$_2$ under high hydrostatic pressure (up to 35 GPa) [2]. \\ 1. B. Cao and T. Li, J. Phys. Chem. C 119, 1247 (2015) \\ 2. A. Nayak, {\em et. al.}, ACS Nano 9, 9117 (2015)   

First-principles study of electric and magnetic properties of an F4TCNQ ribbon on graphene

We present density functional calculations on the electrical and magnetic properties of a ribbon of tetrafluoro-tetracyanoquinodimethan (F4TCNQ) molecules deposited on a graphene sheet. We find that doping the system with electrons results in a spatial variation of the Dirac point energy along the direction perpendicular to the ribbon, which makes a p-n junction configuration in the graphene sheet. In addition, ferromagnetism appears in the ribbon and the ferromagnetic moments can be controlled by the electron doping. This work was supported by NSF Grant No. DMR10-1006184 and the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the DOE at Lawrence Berkeley National Laboratory's NERSC facility.   

RKKY interaction in triangular MoS$_{2}$ nanoflakes

Transition-metal dichalcogenides (TMDs), such as MoS$_2$, possess unique electronic and optical properties, making them promising for optospintronics. Exfoliation and CVD growth processes produce nanoflakes of different shapes, often triangular with zigzag edges [1]. Magnetic impurities in this material interact indirectly through the TMD conduction electrons/holes. Using an effective 3-orbital tight-binding model [2], we study the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities in p-doped triangular flakes with zigzag termination. We analyze the interaction as function of impurity separation along high symmetry directions in the nanoflake, considering hybridization to different Mo orbitals, and different fillings. The interaction is anisotropic for impurities in the interior of the flake. However, when impurities lie on the edges of the crystallite, the effective exchange is Ising-like, reflecting the presence of z$^2$-orbitals associated with edge states. Other interactions are possible by selecting impurity positions and orbital character of the states in their neighborhood. Our results can be tested with local probes, such as spin-polarized STM. [1] A. M. van der Zande et al., Nat. Mat. 12, 554 (2013). [2] G. B. Liu et al., PRB 88, 085433 (2013).   

Symmetry origins of the 'caldera' valence band distortion in 2D semiconductors

The electronic structures of many two-dimensional van der Waals semiconductors exhibit various fascinating properties distinct from their three-dimensional bulk counterparts. Through an examination of their lattice symmetries, we identify several universal rules dictating their band dispersion in the monolayer limit, where in-plane mirror symmetry and quantum confinement play critical roles. Taking group-III metal monochalcogenides (such as GaSe) as an example, we reveal the origin of the unusual `caldera' shape of the valence band edge (otherwise inelegantly dubbed an `upside down Mexican hat'), which we show is surprisingly common among other 2D semiconductors (such as in phosphorene for {\$}k{\$} along its zigzag direction). Reference: arXiv:1508.06963   

First-principles simulations of Graphene/Transition-metal-Dichalcogenides/Graphene Field-Effect Transistor

A prototype field-effect transistor (FET) with fascinating properties can be made by assembling graphene and two-dimensional insulating crystals into three-dimensional stacks with atomic layer precision. Transition metal dichalcogenides (TMDCs) such as WS$_{\mathrm{2}}$, MoS$_{\mathrm{2}}$ are good candidates for the atomically thin barrier between two layers of graphene in the vertical FET due to their sizable bandgaps. We investigate the electronic properties of the Graphene/TMDCs/Graphene sandwich structure using first-principles method. We find that the effective tunnel barrier height of the TMDC layers in contact with the graphene electrodes has a layer dependence and can be modulated by a gate voltage. Consequently a very high ON/OFF ratio can be achieved with appropriate number of TMDC layers and a suitable range of the gate voltage. The spin-orbit coupling in TMDC layers is also layer dependent but unaffected by the gate voltage. These properties can be important in future nanoelectronic device designs.   

Optical Properties of the $\alpha$-$T_3$ Model

The $\alpha$-$T_3$ model, recently introduced by Raoux et. al [1], provides a continuous evolution between the honeycomb lattice of graphene and the $T_3$ or dice lattice. It is characterized by a variable Berry phase that changes continuously from $\pi$ to $0$. We present our calculations of optical properties of the $\alpha$-$T_3$ model, including the Hall quantization and optical conductivity, with an emphasis on the effect of the variable Berry’s phase of the model. In particular, we describe the continuous evolution of the Hall quantization from a relativistic to a non-relativistic regime. \newline [1] A. Raoux, M. Morigi, J.-N. Fuchs, F. Piechon, and G. Montambaux, Phys. Rev. Lett. 112, 026402 (2014)   

\textbf{Microscopic modeling of nitride intersubband absorbance }

III-nitride intersubband structures have recently attracted much interest because of their potential for a wide variety of applications ranging from electro-optical modulators to terahertz quantum cascade lasers. To overcome present simulation limitations we have developed a microscopic absorbance simulator for nitride intersubband devices. Our simulator calculates the band structure of nitride intersubband systems using a fully coupled 8x8 k.p Hamiltonian and determines the material response of a single period in a density-matrix-formalism by solving the Heisenberg equation including many-body and dephasing contributions. After calculating the polarization due to intersubband transitions in a single period, the resulting absorbance of a superlattice structure including radiative coupling between the different periods is determined using a non-local Green's-function formalism. As a result our simulator allows us to predict intersubband absorbance of superlattice structures with microscopically determined lineshapes and linewidths accounting for both many-body and correlation contributions.   

Enhancement of bulk photovoltaic effect in band inversion topological phase transitions

The bulk photovoltaic effect (BPVE) is the generation of photocurrents in the bulk of a single-phase material. The dominant mechanism for the BPVE is the shift current, a non-linear optical effect which involves the excitation of carriers into current-carrying coherent superpositions. This mechanism has a number of advantages over traditional photovoltaics, such as above-band gap photovoltages, and current generation in the bulk. In this work, we show that the shift current BPVE can be enhanced in materials with band inversion topological phase transitions. Using first-principles calculations, we show that the magnitude of the shift current is sharply increased in the vicinity of the band inversion transition, and that the direction of the shift current changes abruptly at the band inversion transition. Taking as examples layered BiTeI and perovskite CsPbI, we demonstrate that this effect is robust across different materials systems.To understand these results, we analyze our results using a low-energy effective Hamiltonian and derive the functional form of the shift current lineshape near the band gap energy.