Relative Abundance of Z(2) Topological Order in Exfoliable Two-Dimensional Insulators

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, Z(2) topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial Z(2) topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuL2 and (platinum) jacutingaite (P2HgS3). We also identify monolayer Pd2HgSe3 (palladium jacutingaite) as a novel Kane-Mele quantum spin Hall insulator and compare it with platinum jacutingaite. A handful of promising materials are mechanically stable and exhibit Z(2) topological order, either unperturbed or driven by small amounts of strain. Such screening highlights a relative abundance of Z(2) topological order of around 1% and provides an optimal set of candidates for experimental efforts.

Electronic Transport in 2D Materials with Strong Spin-orbit Coupling

Artem Pulkin

The thesis describes the computational study of structural, electonic and transport properties of monolayer transition metal dichalcogenides (TMDs) in the stable 2H and the metastable 1T' phases. Several aspects have been covered by the study including the electronic properties of the topological quantum spin Hall (QSH) state in the 1T' monolayer phase as well as the effects of strain, periodic line defects, interfaces and edges of monolayer TMDs. The electronic properties of the bulk monolayer phases were described by the ab-initio density functional theory framework while the electronic and transport properties of 1D defects were calculated using the non-equilibrium Green's function formalism. A specific focus was made on the transport of spin-polarized charge carriers across line defects in the monolayer 2H phase. Subject to energy, pseudomomentum and spin conservation, the size of the transport gap is governed by both bulk properties of a material and symmetries of a line defect. Outside the transport gap energy region, the charge carriers are discriminated with respect to their spin resulting in the spin polarization of the transmitted current. Next, the properties of the metastable monolayer 1T' phase were studied. The presence of a sufficiently large band gap is crucial to observe the QSH phase in the family of materials by probing the topological boundary states. The meV-order band gaps of the 1T' phase of monolayer TMDs were found to be sensitive to materials' lattice constants suggesting the control of the band gap size by strain. In particular, the electronic band structure and the size of the band gap in monolayer 1T'-WSe2 were found to be in agreement with spectroscopy studies. The topologically protected states at the edges of the monolayer 1T' phase as well as at the boundaries between the topological 1T' phase and the trivial 2H phase of monolayer TMDs were studied. Specific atomic structure configurations were suggested to observe experimentally the topological protection of the charge carrier transport against back-scattering. Finally, in the context of lateral semiconducting device engineering, the electronic and transverse transport properties of 2H-1T' phase boundaries as well as the dimerization defects in the 1T' phase were investigated. Both kinds of defects considered exhibit a relatively large transmission probability for the charge carriers crossing the defects. However, the differences between the shapes of bulk bands of the two phases open a sizeable transport gap for charge carriers crossing periodic domain boundaries between the monolayer 2H and 1T' phases. The calculated formation energies of dimerization defects were found to be relatively low suggesting their high concentration in real samples of monolayer 1T'-TMDs. Additionally, the thesis includes studies of magnetic dopants on the surface of Bi2Te3 and atomic vacancies in monolayer 2H-MoSe2 where the electronic properties of point defects were calculated and compared to experimental results. The two possible adsorption sites of Fe on the surface of Bi2Te3 both show a large out-of-plane magnetic anisotropy in agreement with experiments. The calculated local electronic properties of Se vacancies in monolayer 2H-MoSe2 show the presence of in-gap states which are not observed in experiment. Nevertheless, the combination of theoretical and experimental scanning tunneling microscopy images allowed the unambiguous identification of the vacancy defect.

EPFL2017

Mobility of two-dimensional materials from first principles in an accurate and automated framework

Davide Campi, Marco Gibertini, Nicola Marzari, Thibault Daniel Pierre Sohier

We present a first-principles approach to compute the transport properties of 2D materials in an accurate and automated framework. We use density-functional perturbation theory in the appropriate bidimensional setup with open-boundary conditions in the third direction. The materials are charged by field effect via planar countercharges. In this approach, we obtain electron-phonon matrix elements in which dimensionality and doping effects are inherently accounted for, without the need for post-processing corrections. This treatment highlights some unexpected consequences, such as an increase of electron-phonon coupling with doping in transition-metal dichalcogenides. We use symmetries extensively and identify pockets of relevant electronic states to minimize the number of electron-phonon interactions to compute; the integrodifferential Boltzmann transport equation is then linearized and solved beyond the relaxation-time approximation. We apply the entire protocol to a set of much studied materials with diverse electronic and vibrational band structures: electrondoped MoS2, WS2, WSe2, phosphorene, arsenene, and hole-doped phosphorene. Among these, hole-doped phosphorene is found to have the highest mobility, with a room temperature value around 600 cm(2) V-1 s(-1). Last, we identify the factors that affect most phonon-limited mobilities, such as the number and the anisotropy of electron and hole pockets, to provide a broader understanding of the driving forces behind high mobilities in two-dimensional materials.

AMER PHYSICAL SOC2018

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Roberto Car, Matteo Cococcioni, Nicola Marzari, Alfredo Pasquarello, Lorenzo Paulatto, Gabriele Sclauzero

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

2009

Electronic Transport in 2D Materials with Strong Spin-orbit Coupling

Artem Pulkin

EPFL2017