Theory of Weyl orbital semimetals and predictions of several materials classes

Graphene, topological insulators, and Weyl semimetals are three widely studied materials classes which possess Dirac or Weyl cones arising from either sublattice symmetry or spin-orbit coupling. In this work, we present a theory of a new class of bulk Dirac and Weyl cones, dubbed Weyl orbital semimetals, where the orbital polarization and texture inversion between two electronic states at discrete momenta lend itself into protected Dirac or Weyl cones without spin-orbit coupling. We also predict several families of Weyl orbital semimetals including V$_3$S$_4$, NiTi3S6, BLi, and PbO$_2$ via first-principle band structure calculations. We find that the highest Fermi velocity predicted in some of these materials is even larger than that of the existing Dirac materials. The synthesis of Weyl orbital semimetals will not only expand the territory of Dirac materials beyond the quintessential spin-orbit coupled systems and hexagonal lattice to the entire periodic table, but it may also open up new possibilities for orbital controlled electronics or `orbitronics'.Comment: 11 pages, 7 figures, 2 tables; v2: 2D band structure are provided with discrete Weyl node

Dimensionality driven charge density wave instability in TiS2_2

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS$_2$. Within the local density approximation the material is a semi-metal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS$_2$ is thinned to less than four monolayers. Such charge density wave phase can be tuned by compressive strain, which appears to be the control parameter of the instability

Quantum-confinement and Structural Anisotropy result in Electrically-Tunable Dirac Cone in Few-layer Black Phosphorous

2D materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E_ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, E_c. Increasing E_ext beyond E_c can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semi-metal transition.Comment: 8 Pages and 8 figures in the main text + 8 supplementary figure

Spatially regularized compressed sensing of diffusion MRI data

The present paper introduces a method for substantial reduction of the number of diffusion encoding gradients required for reliable reconstruction of HARDI signals. The method exploits the theory of compressed sensing (CS), which establishes conditions on which a signal of interest can be recovered from its under-sampled measurements, provided that the signal admits a sparse representation in the domain of a linear transform. In the case at hand, the latter is defined to be spherical ridgelet transformation, which excels in sparsifying HARDI signals. What makes the resulting reconstruction procedure even more accurate is a combination of the sparsity constraints in the diffusion domain with additional constraints imposed on the estimated diffusion field in the spatial domain. Accordingly, the present paper describes a novel way to combine the diffusion- and spatial-domain constraints to achieve a maximal reduction in the number of diffusion measurements, while sacrificing little in terms of reconstruction accuracy. Finally, details are provided on a particularly efficient numerical scheme which can be used to solve the aforementioned reconstruction problem by means of standard and readily available estimation tools. The paper is concluded with experimental results which support the practical value of the proposed reconstruction methodology.Comment: 10 figure

Spin-memory loss due to spin-orbit coupling at ferromagnet/heavy-metal interfaces: Ab initio spin-density matrix approach

Spin-memory loss (SML) of electrons traversing ferromagnetic-metal/heavy-metal (FM/HM), FM/normal-metal (FM/NM) and HM/NM interfaces is a fundamental phenomenon that must be invoked to explain consistently large number of spintronic experiments. However, its strength extracted by fitting experimental data to phenomenological semiclassical theory, which replaces each interface by a fictitious bulk diffusive layer, is poorly understood from a microscopic quantum framework and/or materials properties. Here we describe an ensemble of flowing spin quantum states using spin-density matrix, so that SML is measured like any decoherence process by the decay of its off-diagonal elements or, equivalently, by the reduction of the magnitude of polarization vector. By combining this framework with density functional theory (DFT), we examine how all three components of the polarization vector change at Co/Ta, Co/Pt, Co/Cu, Pt/Cu and Pt/Au interfaces embedded within Cu/FM/HM/Cu vertical heterostructures. In addition, we use ab initio Green's functions to compute spectral functions and spin textures over FM, HM and NM monolayers around these interfaces which quantify interfacial spin-orbit coupling and explain the microscopic origin of SML in long-standing puzzles, such as why it is nonzero at Co/Cu interface; why it is very large at Pt/Cu interface; and why it occurs even in the absence of disorder, intermixing and magnons at the interface.Comment: 6 pages, 4 figures, PDFLaTeX; published versio

Origin of the n-type and p-type conductivity of MoS2 monolayers on a SiO2 substrate

Ab-initio density functional theory calculations are performed to study the electronic properties of a MoS2 monolayer deposited over a SiO2 substrate in the presence of interface impurities and defects. When MoS2 is placed on a defect-free substrate the oxide plays an insignificant role, since the conduction band top and the valence band minimum of MoS2 are located approximately in the middle of the SiO2 band-gap. However, if Na impurities and O dangling bonds are introduced at the SiO2 surface, these lead to localized states, which modulate the conductivity of the MoS2 monolayer from n- to p-type. Our results show that the conductive properties of MoS2 deposited on SiO2 are mainly determined by the detailed structure of the MoS2 /SiO2 interface, and suggest that doping the substrate can represent a viable strategy for engineering MoS2 -based devices.Comment: 8 pages, 7 figure

Electric Field Effects on Armchair MoS2 Nanoribbons

{\it Ab initio} density functional theory calculations are performed to investigate the electronic structure of MoS$_2$ armchair nanoribbons in the presence of an external static electric field. Such nanoribbons, which are nonmagnetic and semiconducting, exhibit a set of weakly interacting edge states whose energy position determines the band-gap of the system. We show that, by applying an external transverse electric field, $E_\mathrm{ext}$, the nanoribbons band-gap can be significantly reduced, leading to a metal-insulator transition beyond a certain critical value. Moreover, the presence of a sufficiently high density of states at the Fermi level in the vicinity of the metal-insulator transition leads to the onset of Stoner ferromagnetism that can be modulated, and even extinguished, by $E_\mathrm{ext}$. In the case of bi-layer nanoribbons we further show that the band-gap can be changed from indirect to direct by applying a transverse field, an effect which might be of significance for opto-electronics applications.Comment: 12 pages, 15 figure

Efficient spin injection and giant magnetoresistance in Fe/MoS2_2/Fe junctions

We demonstrate giant magnetoresistance in Fe/MoS$_2$/Fe junctions by means of \textit{ab-initio} transport calculations. We show that junctions incorporating either a mono- or a bi-layer of MoS$_2$ are metallic and that Fe acts as an efficient spin injector into MoS$_2$ with an efficiency of about 45\%. This is the result of the strong coupling between the Fe and S atoms at the interface. For junctions of greater thickness a maximum magnetoresistance of $\sim$300\% is obtained, which remains robust with the applied bias as long as transport is in the tunneling limit. A general recipe for improving the magnetoresistance in spin valves incorporating layered transition metal dichalcogenides is proposed.Comment: Updated text, published versio

Effective spin-mixing conductance of topological-insulator/ferromagnet and heavy-metal/ferromagnet spin-orbit-coupled interfaces: A first-principles Floquet-nonequilibrium-Green-function approach

The spin mixing conductance (SMC) is a key quantity determining efficiency of spin transport across interfaces. Thus, knowledge of its precise value is required for accurate measurement of parameters quantifying numerous effects in spintronics, such as spin-orbit torque, spin Hall magnetoresistance, spin Hall effect and spin pumping. However, the standard expression for SMC, provided by the scattering theory in terms of the reflection probability amplitudes, is inapplicable when strong spin-orbit coupling (SOC) is present directly at the interface. This is the precisely the case of topological-insulator/ferromagnet and heavy-metal/ferromagnet interfaces of great contemporary interest. We introduce an approach where first-principles Hamiltonian of these interfaces, obtained from noncollinear density functional theory (ncDFT) calculations, is combined with charge conserving Floquet-nonequilibrium-Green-function formalism to compute {\em directly} the pumped spin current $I^{S_z}$ into semi-infinite left lead of two-terminal heterostructures Cu/X/Co/Cu or Y/Co/Cu---where X=Bi$_2$Se$_3$ and Y=Pt or W---due to microwave-driven steadily precessing magnetization of the Co layer. This allows us extract an effective SMC as a prefactor in $I^{S_z}$ vs. precession cone angle $\theta$ dependence, as long as it remains the same, $I^{S_z} \propto \sin^2 \theta$, as in the case where SOC is absent. By comparing calculations where SOC in switched off vs. switched on in ncDFT calculations, we find that SOC consistently reduces the pumped spin current and, therefore, the effective SMC.Comment: 5 pages, 3 figure

Superconducting dome in MoS2 and TiSe2 generated by quasiparticle-phonon coupling

We use a first-principles based self-consistent momentum-resolved density fluctuation (MRDF) model to compute the combined effects of electron-electron and electron-phonon interactions to describe the superconducting dome in the correlated MoS2 thin flake and TiSe2. We find that without including the electron-electron interaction, the electron-phonon coupling and the superconducting transition temperature (Tc) are overestimated in both these materials. However, once the full angular and dynamical fluctuations of the spin and charge density induced quasiparticle self-energy effects are included, the electron-phonon coupling and Tc are reduced to the experimental value. With doping, both electronic correlation and electron-phonon coupling grows, and above some doping value, the former becomes so large that it starts to reduce the quasiparticle-phonon coupling constant and Tc, creating a superconducting dome, in agreement with experiments.Comment: 8 figures, 9 pages; (v2) Method section is expanded, references added (v3) published version, typos correcte