Topological Quantum Phase Transition in 5dd Transition Metal Oxide Na2_2IrO3_3

We predict a quantum phase transition from normal to topological insulators in the 5$d$ transition metal oxide Na$_2$IrO$_3$, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na$_2$IrO$_3$ model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na$_2$IrO$_3$ is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations

Topological and magnetic phase transitions in Bi2Se3 thin films with magnetic impurities

When topological insulators meet broken time-reversal symmetry, they bring forth many novel phenomena, such as topological magnetoelectric, half-quantum Hall, and quantum anomalous Hall effects. From the well-known quantum spin Hall state in Bi2Se3 thin films, we predict various topological and magnetic phases when the time-reversal symmetry is broken by magnetic ion doping. As the magnetic ion density increases, the system undergoes successive topological or magnetic phase transitions due to variation of the exchange field and the spin-orbit coupling. In order to identify the topological phases, we vary the spin-orbit coupling strength from zero to the original value of the system and count the number of band crossings between the conduction and valence bands, which directly indicates the change of the topological phase. This method provides a physically intuitive and abstract view to figure out the topological character of each phase and the phase transitions between them.open121

Interfacial Dirac Cones from Alternating Topological Invariant Superlattice Structures of Bi2Se3

When the three-dimensional topological insulators Bi2Se3 and Bi2Te3 have an interface with vacuum, i.e., a surface, they show remarkable features such as topologically protected and spin-momentum locked surface states. However, for practical applications, one often requires multiple interfaces or channels rather than a single surface. Here, for the first time, we show that an interfacial and ideal Dirac cone is realized by alternating band and topological insulators. The multichannel Dirac fermions from the superlattice structures open a new way for applications such as thermoelectric and spintronics devices. Indeed, utilizing the interfacial Dirac fermions, we also demonstrate the possible power factor improvement for thermoelectric applications.open282

Multiple Dirac fermions from a topological insulator and graphene superlattice

Graphene and three-dimensional topological insulators are well-known Dirac materials whose bulk and surface states are governed by Dirac equations. They not only show good transport properties but also carry various quanta related to the geometrical phase such as charge, spin, and valley Hall conductances. Therefore, it is a great challenge to combine the two Dirac materials together, realizing multiple Dirac fermions. By using first-principles density-functional-theory calculations, we demonstrate such a system built from topological insulator-band insulator-graphene superlattice structures. Hexagonal boron nitride is proposed as an ideal band-insulating material in gluing graphene and topological insulators, providing a good substrate for graphene and a sharp interface with a topological insulator. The power factors for p-type doping are largely enhanced due to the charge-conducting channels through multiple Dirac cones. The systems characterized by the coexistence of the topologically protected interfacial and graphene Dirac cones can pave the way for developing integrated devices for electronics, spintronics and valleytronics applications.open5

New Candidates for Topological Insulators : Pb-based chalcogenide series

Here, we theoretically predict that the series of Pb-based layered chalcogenides, Pb$_n$Bi$_2$Se$_{n+3}$ and Pb$_n$Sb$_2$Te$_{n+3}$, are possible new candidates for topological insulators. As $n$ increases, the phase transition from a topological insulator to a band insulator is found to occur between $n=2$ and 3 for both series. Significantly, among the new topological insulators, we found a bulk band gap of 0.40eV in PbBi$_2$Se$_4$ which is one of the largest gap topological insulators, and that Pb$_2$Sb$_2$Te$_5$ is located in the immediate vicinity of the topological phase boundary, making its topological phase easily tunable by changing external parameters such as lattice constants. Due to the three-dimensional Dirac cone at the phase boundary, massless Dirac fermions also may be easily accessible in Pb$_2$Sb$_2$Te$_5$

Dirac cone engineering in Bi2Se3 thin films

In spite of the clear surface-state Dirac cone features in bismuth-based three-dimensional strong topological insulator materials, the Dirac point known as the Kramers point and the topological transport regime are located near the bulk valence band maximum. As a result of a nonisolated Dirac point, the topological transport regime cannot be acquired and there possibly exist scattering channels between surface and bulk states as well. We show that an ideal and isolated Dirac cone is realized in a slab geometry made of Bi2Se3 with appropriate substitutions of surface Se atoms. As an extension of Dirac cone engineering, we also investigate Bi2Se3 ultrathin films with asymmetric or magnetic substitutions of the surface atoms, which can be linked to spintronics applications.open191

Topological insulator phase in halide perovskite structures

Topological insulators are a novel quantum state of matter that reveals their properties and shows exotic phenomena when combined with other phases. Hence, priority has been given to making a good quality topological insulator interface with other compounds. From the applications point of view, the topological insulator phase in perovskite structures could be important to provide the various heterostructure interfaces with multifunctional properties. Here, by performing a tight-binding analysis and first-principles calculations, we predict that cubic-based CsPbI3 and CsSnI3 perovskite compounds under reasonable hydrostatic pressure are feasible candidates for three-dimensional topological insulators. Combined with cubic symmetry, the spin and total angular momentum doublets forming the valence and conduction bands result in a prototype of a continuum model, representing three-dimensional isotropic Dirac fermions, and govern the topological phase transition in halide perovskite materials.close161

Tracing out the Berry curvature dipole and multipoles in second harmonic Hall responses of time-reversal symmetric insulators

Various nonlinear characteristics of solid states, such as the circular photogalvanic effect of time-reversal symmetric insulators, the quantized photogalvanic effect of Weyl semimetals, and the nonlinear Hall effect of time-reversal symmetric metals, have been associated with the Berry curvature dipole (BCD). Here, we explore the question of whether the Berry curvature dipole and multipoles of time-reversal symmetric insulators can be traced in the nonlinear optical responses. We performed real-time time-dependent density functional theory calculations and examined the second harmonic generation susceptibility tensors. The two-band term of the susceptibility tensor is sharply proportional to the interband BCD, dominating over the Hall response once the cancellation effect of the multiple reflection symmetries is lifted. We suggest that the nonlinear Hall component of the second-harmonic spectra of insulators can also be utilized as an effective tool to extract the band structure geometry through Berry curvature dipole and possibly multipoles.Comment: main text: 21 pages with 3 figures; supplementary material: 11 pages with 3 figure

Spin-Orbit Integrated Ground State and Magnetic Anisotropy in Sr2_2IrO4_4

We present a microscopic model for the anisotropic exchange interactions in Sr$_{2}$IrO$_{4}$. A direct construction of Wannier functions from first-principles calculations proves the $j_{\mathrm{eff}}$=1/2 character of the spin-orbit integrated states at the Fermi level. An effective $j_{\mathrm{eff}}$-spin Hamiltonian explains the observed weak ferromagnetism and anisotropy of antiferromagnetically ordered magnetic state, which arise naturally from the $j_{\mathrm{eff}}$=1/2 state with a rotation of IrO$_{6}$ octahedra. It is suggested that Sr$_{2}$IrO$_{4}$ is a unique class of materials with effective exchange interactions in the spin-orbital Hilbert space.Comment: 5 pages, 3 figure