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The ternary compound Cs2Hg6S7 has shown considerable promise as a wide gap semiconductor for hard radiation detection at room temperature. We report on the measurement of defect levels in Cs2Hg6S7 using photo-induced current transient spectroscopy. We observe a series of defect levels with mean activation energies of 0.053, 0.052, 0.34, 0.35, and 0.46 eV. The defects are attributed to Cs vacancies and Cs and Hg antisite defects. Defect capture cross-sections are in the range 10−20–10−15 cm2.

We examined the temperature ($T$) evolution of the optical conductivity spectra of ${Sr}_{3}$${Ir}_{2}$${O}_{7}$ over a wide range of 10--400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\ensuremath{\le}$0.1 eV. The low-energy features of the optical d-d excitation ($\ensuremath{\hbar}\ensuremath{\omega}$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O $K$-edge x-ray-absorption spectra. This suggests that the $T$ evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating ${Sr}_{3}$${Ir}_{2}$${O}_{7}$.

Silicene is a two-dimensional honeycomb lattice made of silicon atoms, which is considered to be a new Dirac fermion system. Based on first-principles calculations, we examine the possibility of the formation of solitonlike topological domain walls (DWs) in silicene. We show that the DWs between regions of distinct ground states of the buckled geometry should bind electrons when a uniform electric field is applied in the perpendicular direction to the sheet. The topological origin of the electron confinement is demonstrated based on numerical calculations of the valley-specific Hall conductivities, and possible experimental signatures of the quantum valley Hall effects are discussed using simulated scanning tunneling microscopy images. Our results strongly suggest that silicene could be an ideal host for the quantum valley Hall effect, thus providing a pathway to the valleytronics in silicon-based technology.

Until recently, spin-polarization in nonmagnetic materials was the exclusive territory of non- centrosymmetric structures. It was recently shown that a form of hidden spin polarization (named the Rashba-2 or R-2 effect) could exist in globally centrosymmetric crystals provided the individual layers belong to polar point group symmetries. This realization could considerably broaden the range of materials that might be considered for spin-polarization spintronic applications to include the hitherto forbidden spintronic compound that belong to centrosymetric symmetries. Here we take the necessary steps to transition from such general, material-agnostic condensed matter theory arguments to material-specific design principles that could aid future laboratory search of R-2 materials. Specifically, we (i) classify different prototype layered structures that have been broadly studied in the literature in terms of their expected R-2 behavior, including the Bi2Se3-structure type (a prototype topological insulator), MoS2-structure type (a prototype valleytronic compound) and LaBiOS2-structure type (a host of superconductivity upon doping); (ii) formulate the properties that ideal R-2 compounds should have in terms of combination of their global unit cell symmetries with specific point group symmetries of their constituent sectors; (iii) use first-principles band theory to search for compounds from the prototype family of LaOBiS2-type structures that satisfy these R-2 design metrics. We initially consider both stable and hypothetical compounds to establish an understanding of trends of R-2 with composition, and then indicate the predictions that are expected to be stable and synthesizable. We predict large spin splittings (up to ~ 200 meV for holes in LaOBiTe2) as well as surface Rashba states. Experimental testing of such predictions is called for., Comment: 29 pages, 6 figures, 2 tables

We study the general phase diagram of correlated electrons for iridium-based (Ir) compounds on the hyperhoneycomb lattice---a crystal structure where the Ir$^{4+}$ ions form a three-dimensional network with three-fold coordination recently realized in the $\beta$-Li${}_{2}$IrO${}_{3}$ compound. Using a combination of microscopic derivations, symmetry analysis, and density functional calculations, we determine the general model for the electrons occupying the $j_{\text{eff}}=1/2$ orbitals at the Ir$^{4+}$ sites. In the non-interacting limit, we find that this model allows for both topological and trivial electronic band insulators along with metallic states. The effect of Hubbard-type electron-electron repulsion on the above electronic structure in stabilizing $\mathbf{q}=\mathbf{0}$ magnetic order reveals a phase diagram with continuous phase transition between a topological band insulator and a Neel ordered magnetic insulator., Comment: 11 pages, 7 figures

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 CsPbI${}_{3}$ and CsSnI${}_{3}$ 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.

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.

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 Bi${}_{2}$Se${}_{3}$ 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.

Topological insulators are distinguished from normal insulators by their bulk insulating gap and odd number of surface states connecting the inverted conduction and valence bands and showing Dirac cones at the time-reversal invariant points in the Brillouin zone. Bi-based three-dimensional strong topological insulator materials, Bi$_2$Se$_3$ and Bi$_2$Te$_2$, are known as high temperature topological insulators for their relatively large bulk gap and have one simple Dirac cone at the $\Gamma$ point. In spite of their clear surface state Dirac cone features, the Dirac point known as a Kramers point and the topological transport regime is located below the bulk valence band maximum. As a result of a non-isolated Dirac point, the topological transport regime can not be acquired and there possibly exist scattering channels between surface and bulk states as well. In this article we show that an ideal and isolated Dirac cone is realized in a slab geometry made of Bi$_2$Se$_3$ with appropriate substitutions of surface Se atoms. In addition to Dirac cone engineering by surface atom substitutions, we also investigate Bi$_2$Se$_3$ thin films in terms of thickness and magnetic substitutions, which can be linked to applications of spintronics devices.

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$_5$.

We investigated the temperature-dependent evolution of the electronic structure of the ${J}_{\text{eff}}=\frac{1}{2}$ Mott insulator ${\text{Sr}}_{2}{\text{IrO}}_{4}$ using optical spectroscopy. The optical conductivity spectra $\ensuremath{\sigma}(\ensuremath{\omega})$ of this compound has recently been found to exhibit two $d\text{\ensuremath{-}}d$ transitions associated with the transition between the ${J}_{\text{eff}}=\frac{1}{2}$ and ${J}_{\text{eff}}=\frac{3}{2}$ bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change in the electronic structure of ${\text{Sr}}_{2}{\text{IrO}}_{4}$ in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.

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

By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states.

The photoluminescence (PL) from the wide bandgap semiconductor Tl6I4Se, a promising candidate material for gamma ray detection, was studied at low temperature. The Tl6I4Se single crystal was grown by the Bridgman method. For undoped material, we observed a single broad peak at ~1.61 eV with a full width at half maximum of 112 meV at 20 K, which is attributed to donor–acceptor pair (DAP) recombination involving shallow donors and deep acceptors. From the thermal quenching of the integrated PL peak intensity, thermal activation energy of the donor level of 52 meV was obtained. At high excitation intensities a blue-shift of peak emission energy in DAP recombination was observed. From the PL measurements, the ionization energies of the donor and acceptor levels were estimated at 52 and 290 meV, respectively.