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Berry curvature (BC) governs topological phases of matter and generates anomalous transport. When a magnetic field is applied, phonons can acquire BC indirectly through spin-lattice coupling, leading to a linear phonon Hall effect. Here, we show that polar lattice distortion directly couples to a phonon BC dipole, which causes a switchable non-linear phonon Hall effect. In a SnS monolayer, the in-plane ferroelectricity induces a phonon BC and leads to the phononic version of the non-volatile BC memory effect. As a new type of ferroelectricity-phonon coupling, the phonon Rashba effect emerges and opens a mass-gap in tilted Weyl phonon modes, resulting in a large phonon BC dipole. Furthermore, our ab initio non-equilibrium molecular dynamics simulations reveal that non-linear phonon Hall transport occurs in a controllable manner via ferroelectric switching. The ferroelectricity-driven phonon BC and corresponding non-linear phonon transports provide a novel scheme for constructing topological phononic transport/memory devices., Comment: 12pages, 4 figures

We study electronic properties of a new planar carbon crystal formed through networking biphenylene molecules. Novel electronic features among carbon materials such as zone-center saddle point and peculiar type-II Dirac fermionic states are shown to exist in the low energy electronic spectrum. The type-II state here has a nearly flat branch and is close to a transition to type-I. Possible magnetic instabilities related with low energy bands are discussed. Furthermore, with a moderate uniaxial strain, a pair of Dirac points merge with the zone center saddle point, realizing concurrent Lifshitz transitions of van Hove singularity as well as pair annihilation of the Dirac fermions. A new effective Hamiltonian encompassing all distinctive low energy states is constructed, revealing a finite winding number of the pseudo-spin texture around the Dirac point, quantized Zak phases, and topological grain boundary states., title changed, references updated, 6 pages

The discovery of Berry curvature (BC) has spurred a tremendous surge of research into various quantum phenomena such as the anomalous transport of electrons and the topological phases of matter. In two-dimensional crystalline systems, the conventional definition of the BC lacks the in-plane components and thus it cannot explain the transverse transport along the plane-normal direction. Here, we modify the BC to newly provide in-plane components in two dimensions, giving rise to the vertical Hall effects that describe out-of-plane transports in response to in-plane perturbations and their Onsager reciprocity. Our first-principles calculations show that a large in-plane BC can appear even in an atomic-thick GdAg2 monolayer, and a hexagonal BiAg2 monolayer can host a large BC dipole known to vanish in the conventional BC. The quantum transports driven by the hitherto-hidden BC will become more significant in recently emerging two-dimensional platforms, including van der Waals heterostructures., 16+11 pages

Defect engineering is one of the key technologies in materials science, enriching the modern semiconductor industry and providing good test-beds for solid-state physics. While homogenous doping prevails in conventional defect engineering, various artificial defect distributions have been predicted to induce desired physical properties in host materials, especially associated with symmetry breakings. Here, we show layer-by-layer defect-gradients in two-dimensional PtSe

Topological electronics is a new field that uses topological charges as current-carrying degrees of freedom. For topological electronics applications, systems should host topologically distinct phases to control the topological domain boundary through which the topological charges can flow. Due to their multiple Dirac cones and the π-Berry phase of each Dirac cone, graphene-like electronic structures constitute an ideal platform for topological electronics; graphene can provide various topological phases when incorporated with large spin-orbit coupling and mass-gap tunability via symmetry-breaking. Here, we propose that a (111)-oriented BaBiO3 bilayer (BBL) sandwiched between large-gap perovskite oxides is a promising candidate for topological electronics by realizing a gap-tunable, and consequently a topology-tunable, graphene analogue. Depending on how neighboring perovskite spacers are chosen, the inversion symmetry of the BBL heterostructure can be either conserved or broken, leading to the quantum spin Hall (QSH) and quantum valley Hall (QVH) phases, respectively. BBL sandwiched by ferroelectric compounds enables switching of the QSH and QVH phases and generates the topological domain boundary. Given the abundant order parameters of the sandwiching oxides, the BBL can serve as versatile topological building blocks in oxide heterostructures.

In symmetry-broken crystalline solids, pole structures of Berry curvature (BC) can emerge, and they have been utilized as a versatile tool for controlling transport properties. For example, the monopole component of the BC is induced by the time-reversal symmetry breaking, and the BC dipole arises from a lack of inversion symmetry, leading to the anomalous Hall and nonlinear Hall effects, respectively. Based on first-principles calculations, we show that the ferroelectricity in a tin telluride monolayer produces a unique BC distribution, which offers charge- and spin-controllable photocurrents. Even with the sizable band gap, the ferroelectrically driven BC dipole is comparable to those of small-gap topological materials. By manipulating the photon handedness and the ferroelectric polarization, charge and spin circular photogalvanic currents are generated in a controllable manner. The ferroelectricity in group-IV monochalcogenide monolayers can be a useful tool to control the BC dipole and the nonlinear optoelectronic responses., 28 pages, 5 figures, 14 pages for SI

A non-vanishing electric field inside a non-centrosymmetric crystal transforms into a momentum-dependent magnetic field, namely, a spin–orbit field (SOF). SOFs are of great use in spintronics because they enable spin manipulation via the electric field. At the same time, however, spintronic applications are severely limited by the SOF, as electrons traversing the SOF easily lose their spin information. Here, we propose that in-plane ferroelectricity in (001)-oriented SnTe thin films can support both electrical spin controllability and suppression of spin dephasing. The in-plane ferroelectricity produces a unidirectional out-of-plane Rashba SOF that can host a long-lived helical spin mode known as a persistent spin helix (PSH). Through direct coupling between the inversion asymmetry and the SOF, the ferroelectric switching reverses the out-of-plane Rashba SOF, giving rise to a maximally field-tunable PSH. Furthermore, the giant out-of-plane Rashba SOF seen in the SnTe thin films is linked to the nano-sized PSH, potentially reducing spintronic device sizes to the nanoscale. We combine the two ferroelectric-coupled degrees of freedom, longitudinal charge and transverse PSH, to design intersectional electro-spintronic transistors governed by non-volatile ferroelectric switching within nanoscale lateral and atomic-thick vertical dimensions.

Interplay of Fermi surface topology and electron correlation is the quintessential ingredient underlying spontaneous symmetry breaking in itinerant electronic systems. In one-dimensional (1D) systems at half-filling, the inherent Fermi surface nesting makes the translationally invariant metallic state unstable, which is known as Peierls instability. Extending the scope of Peierls instability to two (2D) or three dimensions (3D), however, is not straightforward, since the Fermi surface in higher dimensions is generally not nested. In this work, we show that a perfectly nested Fermi surface can be realized in a class of 2D perovskite oxides, giving rise to 2D Peierls instability. Here the central role is played by the zone boundary Dirac line node (DLN) protected by two orthogonal glide mirrors induced by the rotation of oxygen octahedra. Especially, at a critical angle of the octahedron rotation, the zone-boundary DLN flattens, leading to logarithmically diverging susceptibility. We propose the 2D Peierls instability driven by dispersionless DLN as a principle mechanism for spontaneous symmetry breaking in various layered perovskite oxides including the antiferromagnetism of Sr$_2$IrO$_4$. As a clear signature of the 2D Peierls instability, we predict that the magnetic domain wall in Sr$_2$IrO$_4$ hosts localized soliton modes., 26+31 pages, 6+8 figures

Two-dimensional materials equipped with strong spin–orbit coupling can display novel electronic, spintronic, and topological properties originating from the breaking of time or inversion symmetry. A lot of interest has focused on the valley degrees of freedom that can be used to encode binary information. By performing ab initio time-dependent density functional simulation on MoS2, here we show that the spin is not only locked to the valley momenta but strongly coupled to the optical E″ phonon that lifts the lattice mirror symmetry. Once the phonon is pumped so as to break time-reversal symmetry, the resulting Floquet spectra of the phonon-dressed spins carry a net out-of-plane magnetization (≈0.024μB for single-phonon quantum) even though the original system is non-magnetic. This dichroic magnetic response of the valley states is general for all 2H semiconducting transition-metal dichalcogenides and can be probed and controlled by infrared coherent laser excitation.

Two-dimensional materials equipped with strong spin–orbit coupling can display novel electronic, spintronic, and topological properties originating from the breaking of time or inversion symmetry. A lot of interest has focused on the valley degrees of freedom that can be used to encode binary information. By performing ab initio time-dependent density functional simulation on MoS2, here we show that the spin is not only locked to the valley momenta but strongly coupled to the optical E″ phonon that lifts the lattice mirror symmetry. Once the phonon is pumped so as to break time-reversal symmetry, the resulting Floquet spectra of the phonon-dressed spins carry a net out-of-plane magnetization (≈0.024μB for single-phonon quantum) even though the original system is non-magnetic. This dichroic magnetic response of the valley states is general for all 2H semiconducting transition-metal dichalcogenides and can be probed and controlled by infrared coherent laser excitation., In 2H semiconducting transition-metal dichalcogenides the valley-selective excitation has been achieved with circularly polarized photons. Here, the authors show that circularly polarized phonons produce a valley-dependent dynamic spin state as a result of strong spin-phonon coupling.

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.

The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1/2 and J = 1/2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices.

Flame-melting rapid-cooling reactions were used to synthesize a number of pure phases of the Cs2MIIMIV3Q8 family (MII = Mg, Zn, Cd, Hg; MIV = Ge, Sn; Q = S, Se, Te) whereas the more toxic members were synthesized using a traditional tube furnace synthesis. All Cs2MIIMIV3Q8 compounds presented here crystallize in the noncentrosymmetric space group P212121, except for Cs2ZnGe3S8, which crystallizes in the centrosymmetric space group P21/n. The structures contain chains of corner-sharing MIIQ4 and MIVQ4 tetrahedra linked by edge-sharing MIV2Q6 dimers to give a two-dimensional structure. All phases are structurally similar to the AMIIIMIVQ4 (A = alkali metal, Tl; MIII = Al, Ga, In; MIV = Si, Ge, Sn; Q = S, Se) phases; however, the members of this family have completely ordered MII and MIV sites as opposed to the occupational disorder of MIII and MIV over all tetrahedral sites present in AMIIIMIVQ4. The structural trends of the Cs2MIIMIV3Q8 family are discussed, along with a systematic study of their optical p...

The chemical concept of lattice hybridization was applied to identify new chalcohalide compounds as candidates for X-ray and γ-ray detection. Per this approach, compound semiconductor materials with high density and wide band gaps can be produced that can absorb and detect hard radiation. Here, we show that the mixed chalcogenide–halide compound Tl6SI4 is a congruently melting, mechanically robust chalcohalide material with strong photoconductivity response and an impressive room-temperature figure of merit. Tl6SI4 crystallizes in the tetragonal P4/mnc space group, with a = 9.1758(13) A, c = 9.5879(19) A, V = 807.3(2) A3, and a calculated density of 7.265 g·cm–3. The new material requires a more simplified crystal growth compared to the leading system Cd0.9Zn0.1Te, which is the benchmark room-temperature hard radiation detector material. We successfully synthesized Tl6SI4 crystals to produce detector-grade wafers with high resistivity values (∼1010 Ω·cm) and high-resolution detection of X-ray spectra from...

Two new compounds CsCdInQ3 (Q = Se, Te) have been synthesized using a polychalcogenide flux. CsCdInQ3 (Q = Se, Te) crystals are promising candidates for X-ray and γ-ray detection. The compounds crystallize in the monoclinic C2/c space group with a layered structure, which is related to the CsInQ2 (Q = Se, Te) ternary compounds. The cell parameters are: a = 11.708(2) A, b = 11.712(2) A, c = 23.051(5) A, β = 97.28(3)° for CsCdInSe3 and a = 12.523(3) A, b = 12.517(3) A, c = 24.441(5) A, β = 97.38(3)° for CsCdInTe3. Both the Se and Te analogues are wide-band-gap semiconductors with optical band gaps of 2.4 and 1.78 eV for CsCdInSe3 and CsCdInTe3, respectively. High-purity polycrystalline raw material for crystal growth was synthesized by the vapor transfer method for CsCdInQ3. Large single crystals up to 1 cm have been grown using the vertical Bridgman method and exhibit photoconductive response. The electrical resistivity of the crystals is highly anisotropic. The electronic structure calculation within the ...

The new layered compound CsHgInS3 was synthesized using solid state and flux synthesis techniques. The compound is a semiconductor and shows promising properties for X-ray and γ-ray detection. It features a layered structure that crystallizes in the monoclinic space group C2/c with cell parameters: a = 11.2499(7) Ǻ, b = 11.2565(6) Ǻ, c = 22.146(1) Ǻ, β = 97.30(5)°, V = 2781.8(4) Ǻ3, and Z = 8. CsHgInS3 is isostructural to Rb2Cu2Sn2S6, where the Hg, In, and Cs atoms occupy the Cu, Sn, and Rb sites, respectively. Large single crystals with dimension up to 5 mm were grown with a vertical Bridgman method as well as a horizontal traveling heater method. CsHgInS3 has a γ-ray attenuation length comparable to commercial Cd1–xZnxTe and a band gap value of 2.30 eV. The electrical resistivity of CsHgInS3 is anisotropic with values of 98 GΩ cm and 0.33 GΩ cm perpendicular and parallel to the (001) plane, respectively. The mobility-lifetime product (μτ) of electrons and holes estimated from photoconductivity measureme...

We present the synthesis, crystal structures, and physical properties of Tl2Hg3Q4 (Q = S, Se, and Te). The incongruently melting Tl2Hg3Q4 crystals were grown in a TlxQ flux. These compounds are isostructural and crystallize in a monoclinic cell with a layered structure, adopting the space group C2/c with a = 11.493(2) A, b = 6.6953(13) A, c = 12.937(3) A, β = 114.98(3)° for Tl2Hg3S4, a = 11.977(2) A, b = 6.9264(14) A, c = 13.203(3) A, β = 116.36(3)° for Tl2Hg3Se4 and a = 12.648(3) A, b = 7.3574(15) A, c = 13.701(3) A, β = 117.48(3)° for Tl2Hg3Te4. The structures feature infinite chains of [Hg3Q4]2–, which are linked into layers by charge balancing Tl atoms. The compounds have very high densities (>8.3 g/cm3) with experimentally determined band gaps of 2.05, 1.57, and 0.90 eV for Q = S, Se, and Te, respectively. Using the refined crystal structures, we performed detailed band structure calculations at the density functional theory (DFT) level, using the screened-exchange local density approximation (sx-LDA...

John Androulakis , Sebastian C. Peter , Hao Li , Christos D. Malliakas , John A. Peters , Zhifu Liu , Bruce W W. essels , Jung-Hwan Song , Hosub Jin , Arthur J. reeman , F and Mercouri G. Kanatzidis *

The new semiconductor LiPbSb3S6 crystallizes in the space group P21/c. The structure is a member of the lillianite homologous series and is composed of layers of PbS archetype Sb/Li-S separated by trigonal-prismatic-coordinated Pb/Li. Electronic band structure calculations indicate an indirect band gap, with direct gaps lying very close in energy. LiPbSb3S6 has one of the lowest thermal conductivities seen in a crystalline material, ∼0.24 W m(-1) K(-1) at room temperature, and a high resistivity, ∼4 × 10(9) Ω·cm, and exhibits strong light absorption with a nearly direct band gap of 1.6 eV.

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd(2)Os(2)O(7) using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd(2)Os(2)O(7) is not close to a Slater type but instead to a Lifshitz type.

In diverse classes of organic optoelectronic devices, controlling charge injection, extraction, and blocking across organic semiconductor–inorganic electrode interfaces is crucial for enhancing quantum efficiency and output voltage. To this end, the strategy of inserting engineered interfacial layers (IFLs) between electrical contacts and organic semiconductors has significantly advanced organic light-emitting diode and organic thin film transistor performance. For organic photovoltaic (OPV) devices, an electronically flexible IFL design strategy to incrementally tune energy level matching between the inorganic electrode system and the organic photoactive components without varying the surface chemistry would permit OPV cells to adapt to ever-changing generations of photoactive materials. Here we report the implementation of chemically/environmentally robust, low-temperature solution-processed amorphous transparent semiconducting oxide alloys, In-Ga-O and Ga-Zn-Sn-O, as IFLs for inverted OPVs. Continuous variation of the IFL compositions tunes the conduction band minima over a broad range, affording optimized OPV power conversion efficiencies for multiple classes of organic active layer materials and establishing clear correlations between IFL/photoactive layer energetics and device performance.

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd(2)Os(2)O(7) using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd(2)Os(2)O(7) is not close to a Slater type but instead to a Lifshitz type.

The entanglement of the spin and orbital degrees of freedom through the spin-orbit coupling has been actively studied in condensed matter physics. In several iridium-oxide systems, the spin-orbital entangled state, identified by the effective angular momentum $j_{\rm eff}$, can host novel quantum phases with the help of electron correlations. Here, we show that a series of lacunar spinel compounds, Ga$M_4X_8$ ($M$ = Nb, Mo, Ta, and W and $X$ = S, Se, and Te), gives rise to a $\textit{molecular}$ $j_{\rm eff}$ state as a new spin-orbital composite on which the low energy effective Hamiltonian is based. A wide range of electron correlations is accessible by tuning the bandwidth under external and/or chemical pressure, enabling us to investigate the interesting cooperation between spin-orbit coupling and electron correlations. As illustrative examples, a two-dimensional topological insulating phase and an anisotropic spin Hamiltonian are investigated in the weak and strong coupling regimes, respectively. Our finding can provide an ideal platform for exploring $j_{\rm eff}$ physics and the resulting emergent phenomena., 17 pages and 4 figures, and with supplementary information (14 pages, 6 figures, and 5 tables). Details of the DFT+U results on the magnetism are included in the supplementary material

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}$.

LiPbSb3S6 is prepared by stoichiometric solid state reaction of Li2S, Pb, Sb, and S (evacuated carbon-coated silica ampule, 800 °C, 2 h, quenching; 98% yield).

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

The title compounds (MII: Mg, Zn, Cd, Hg; MIV: Ge, Sn), prepared by flame-melting rapid cooling reactions or by a traditional tube furnace synthesis with

The title compound is prepared from a mixture of Cu, Na2S, BaS, and S (Pyrex tube, 500 °C, 4 d, 75% yield) and characterized by single crystal and powder XRD, SEM, Raman and IR spectroscopy, electrical and magnetic measurements, and DFT band structure calculations.

Mixed S(2-/)S(1-) oxidation states have been discovered in the new quaternary compound NaBa2Cu3S5. Synthesized from the reaction of Cu in a molten alkali metal/polysulfide flux, the compound crystallizes in monoclinic space group C2/m with a = 16.5363(7) A, b = 5.5374(5) A, c = 10.3717(10) A, β = 98.535(8)°. The Na(+) Ba2(+2) [Cu(+)3S3](3-)S2(2-) crystal structure contains layers of edge sharing CuS4 tetrahedra and sheets of S2(2-) dimers. These layers are separated by mixed Ba/Na cation layers. The conductivity of the single crystals of NaBa2Cu3S5 is ∼450 S cm(-1) at room temperature, and increasing conductivity with decreasing temperature is observed, indicating metallic behavior despite the optical band gap of 0.45 eV. A small positive thermopower (45-55 μV K(-1) from 300 K to 500 K) and Hall effect measurements also confirm p-type conductivity with carrier concentration at 200 K of ∼1.6 × 10(21) cm(-3) and a hole mobility of ∼2 cm(2) V(-1) s(-1). NaBa2Cu3S5 exhibits temperature-independent Pauli paramagnetism.

The new layered title compound is synthesized by solid state reaction of In, HgS, Cs2S, and sulfur (silica tube, 600 °C for 24 h, 800 °C for 8 h; 70% yield).

We demonstrate that mercury and antimony compounds with chalcogens (Q = S, Se, Te) and halogens (X = I, Cl, Br) can be a promising family for radiation detection materials. Chalcogen p-orbitals are usually located near the Fermi level and they are responsible for relative high mobilities but at the same time band gap decreases (from S to Te) due to their extended interactions. Halogens on the other hand have their bands well below the Fermi level and salts between transition metals and halogen are usually insulators. Incorporation of halogen atoms in a mercury or antimony chalcogenide framework can give rise to intermediate properties between the two end members (HgQ and HgX2), i.e. structures composed of heavy elements (Z < 40), wide band gap (1.6 - 2.5 eV), and high carrier mobilities. As a proof of concept, we will present two new chalcohalide families, Hg3Q2X2 and SbQX. Crystal growth of the Hg3Te2Br2 phase (7.8 g/cm3 and 2.5 eV) by a vapor transport method gave mm-sized single crystals with electrical resistivity values more in the GΩ.cm range. Preliminary data for mobility-lifetime products for both electron and hole carriers were around 10-5 cm2/V. SbSeI (5.8 g/cm3 and 1.7 eV) sample grown by relatively fast Bridgman technique showed an MΩ.cm range (2.8 x 106 Ω.cm) resistivity with a similar order of magnitude (10-4 cm2/V) of mobility-lifetime products for both electron and hole carriers.

We report on the characterization of optical and electronic properties of a series of thallium chalcogenide, wide gap ternary semiconductor compounds including Tl6I4Se, Tl6I4S, Tl3SbS3, Tl2SnS3, and Tl7Bi3I16 for ionized radiation detection at the x-ray and γ ray regimes. The semiconductor crystals were grown by the modified Bridgman method. The optical absorption, band gap, mobility-lifetime products for electron and hole carriers were measured at room temperature. For Tl6I4S and Tl6I4Se the mobility-lifetime products are comparable to those of CdZnTe. We measured room temperature detector response to x-ray and γ ray sources. For 137Cs radiation, Tl6I4Se has a well-resolved spectral response comparable to that of CdZnTe.

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.

We predict a quantum phase transition from normal to topological insulators in the $5d$ transition metal oxide ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, 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 ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.

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.

The title compounds are prepared in 2—5 g batches by reactions of Tl2Q (Q: S, Se, and Te) and HgQ in a 1:3 stoichiometry (sealed carbon-coated tubes, 823 K, 2 d; annealing at 623 K, 3 weeks).

We predict a quantum phase transition from normal to topological insulators in the 5d transition metal oxide Na2IrO3, 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 Na2IrO3 model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na2IrO3 is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.

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.

We report that the chalcohalide compound Tl(6)SeI(4) is a promising material for efficient X-ray and γ-ray detection. This material has a higher figure of merit than the current state-of-the-art material for room-temperature operation, Cd(0.9)Zn(0.1)Te (CZT). We have synthesized high-quality single-crystalline wafers of Tl(6)SeI(4) with detector-grade resistivities and good carrier transport of both electrons and holes. We demonstrate that pulse height spectra recorded using Co-57 radiation show an energy resolution matching that of a commercial CZT detector material.