Topmost Valence Band Research Articles

The Non-Centrosymmetric Borate Hydride Sr4Ba3(BO3)3.83H2.5.

Sr4Ba3(BO3)3.83H2.5, as the second compound to combine borate and hydride ions, has been synthesized by a mechanochemical synthesis route. The structure has been elucidated by synchrotron X-ray and neutron diffraction and determined to crystallize in the non-centrosymmetric space group P63mc (186) with the cell parameters a=10.87762(15) Å and c=6.98061(11) Å. A detailed investigation of the compound by vibrational spectroscopy in combination with Density Functional Theory calculations reveals the disordered nature of the structure and proves the presence of both borate and hydride ions. Electronic band structure calculations predict a large band gap of 7.1 eV. Hydride states are predicted at the topmost valence band, which agrees well with earlier reported heteroanionic hydrides. We hereby were able to successfully apply previously synthetic and analytical schemes to introduce another member of the rare compounds that contain complex oxoanions simultaneously with hydride ions.

Large valley polarization and the valley-dependent Hall effect in a Janus TiTeBr monolayer.

Ferrovalley materials hold great promise for implementation of logic and memory devices in valleytronics. However, there have so far been limited ferrovalley materials exhibiting significant valley polarization and high Curie temperature (TC). Using first-principles calculations, we predict that the TiTeBr monolayer is a promising ferrovalley candidate. It exhibits intrinsic ferromagnetism with TC as high as 220 K. It is indicated that an out-of-plane alignment of magnetization demonstrates a valley polarization up to 113 meV in the topmost valence band, as further verified by perturbation theory considering both the spin polarization and spin-orbit coupling. Under an in-plane electric field, the valley-dependent Berry curvature results in the anomalous valley Hall effect (AVHE). Moreover, under a suitable in-plane biaxial strain, the TiTeBr monolayer transforms into a Chern insulator with a nonzero Chern number, yet retains its ferrovalley characters and thus the emergent quantum anomalous valley Hall effect (QAVHE). Our study indicates that the TiTeBr monolayer is a promising ferrovalley material, and it provides a platform for investigating the valley-dependent Hall effect.

Enhanced spin Hall conductivity and charge to spin conversion efficiency in strained orthorhombic SnSe through orbital selective hybridization

The realization of the spin Hall effect has opened new frontiers for the design of efficient memory storage devices facilitated by the conversion of charge currents to spin currents. Here, using the Kubo formula, we calculate the intrinsic spin Hall conductivity (SHC) of orthorhombic tin selenide (o-SnSe) under the influence of isotropic compressive strain in the ab-plane. As the strain is gradually increased, we obtain a substantial hybridization between the pz orbitals of Sn and Se atoms of an electron pocket from the lowest conduction band and the topmost valence band, respectively. This hybridization process greatly enhances the SHC at the Fermi level and charge-to-spin conversion efficiency, the latter of which is superior to that of popular transition metals such as Ta and Pt. This makes strained o-SnSe an attractive candidate for use in spintronic devices.

Electronic, spintronic, and piezoelectric properties of new Janus ZnAXY ( A=Si,Ge,Sn , and X,Y=S,Se,Te ) monolayers

Two-dimensional (2D) Janus materials due to their asymmetric structures show fascinating spintronic and piezoelectric properties making them a research hot spot in recent years. In this work, inspired by Janus group-III monochalcogenides, we propose $\mathrm{Zn}AXY$ ($A=\mathrm{Si}$, Ge, Sn, and $X/Y=\mathrm{S}$, Se, Te, $X<Y$) monolayers as a novel structure with broken inversion and mirror symmetry. By calculating cohesive energy and phonon dispersion, eight of nine possible $\mathrm{Zn}AXY$ monolayers are proved to be dynamically stable. In addition, thermal stability of these eight structures is confirmed by ab initio molecular dynamics simulations. The electronic band structures of $\mathrm{Zn}AXY$ monolayers indicate that all of them are indirect semiconductors with strong spin-orbit coupling effects. Lack of inversion symmetry gives rise to Zeeman-type spin splitting at the $K$ point of the conduction band with the highest value of 136 meV for ZnSiSeTe. Furthermore, out-of-plane asymmetry results in Rashba spin splitting (RSS) at the \ensuremath{\Gamma} point of the valence and conduction bands in the most compositions of Janus $\mathrm{Zn}AXY$ monolayers. Among them, ZnGeSTe with ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{V}}$ of $1.79\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{c}}$ of $0.862\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ZnSiSTe with ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{V}}$ of $1.537\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{c}}$ of $0.756\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ are found to be great materials for future spintronic applications. Interestingly, in addition to large RSS, Mexican hat dispersion is observed at the \ensuremath{\Gamma} point of the topmost valence band in these materials. Moreover, the calculated elastic coefficients for the hexagonal $\mathrm{Zn}AXY$ monolayers confirm the mechanical stability of the predicted structures. Finally, Janus $\mathrm{Zn}AXY$ monolayers possess high in-plane (up to 7.46 pm/V) and out-of-plane (up to 0.67 pm/V) piezoelectric coefficients making them appealing alternatives for prevalent piezoelectric materials.

Polarization anisotropy and valence band ordering in semipolar (112¯2) AlInN/GaN heterostructures

Semipolar $(11\overline{2}2)$ AlInN/GaN heterostructures on GaN templates were studied using photoluminescence (PL) spectroscopy both at 15 K and at room temperature. The polarization-resolved PL measurements revealed a dominant polarization along the $[11\overline{2}\overline{3}] {c}^{\ensuremath{'}}$ and a weaker signal along the $[1\overline{1}00] m$-direction, i.e., the two in-plane directions of the semipolar $(11\overline{2}2)$ growth plane. We observed slightly different polarization degrees of $0.34\ifmmode\pm\else\textpm\fi{}0.01, 0.25\ifmmode\pm\else\textpm\fi{}0.02$, and $0.20\ifmmode\pm\else\textpm\fi{}0.01$ at room temperature, respectively, depending on the degree of strain relaxation in the $[1\overline{1}00]$ direction. From a theoretical model based on a $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ calculation, we find that the transition from the conduction band (CB) to the uppermost valence band (VB) is C for AlInN similar to AlN, followed by the transition A from the CB to the second VB, for a wide range of compositions. Thus, the in-plane transition matrix elements from the CB to the two topmost VBs near the $\mathrm{\ensuremath{\Gamma}}$ point of the Brillouin zone are dominated by ${M}_{[11\overline{2}\overline{3}]}$ for the C transition and by ${M}_{[1\overline{1}00]}$ for the A transition. For the samples under consideration with an energy splitting of about $18\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, there is sizable thermal occupation of the second VB at room temperature, reasonably explaining the experimental results. The results show that AlInN possesses a band structure similar to AlN, which might explain the strong Stokes shift and the large variation in the band-gap values reported previously.

Surface enhanced electron correlation on the trivial quasi-two-dimensional bulk insulator 1T−TaS2

While the prototypical quasi-two-dimensional charge density wave system of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ has been known as a Mott insulator with a possibility of quantum spin liquid, recent band-structure calculations and spectroscopic works in parallel suggested a metallic system or a spin-singlet insulator due to the interlayer coupling. Here, we carefully reinvestigate the out-of-plane electron dispersion, which reflects the interlayer electronic coupling, with angle-resolved photoelectron spectroscopy. We identify two distinct branches for the topmost valence band, which can be unambiguously related to the surface and the bulk layers with different band gaps. Density functional theory calculations clearly indicate a trivial band insulator due to the interlayer coupling for the bulk but the surface band gap affected substantially by the electron correlation. The surface-bulk electronic dichotomy consistently incorporates most of the theoretical and spectroscopic results reported so far and has wide implications for van der Waals materials with nontrivial interlayer interactions.

Enhanced Light Emission from Type-II Red InGaN/GaNSb/GaN Quantum-Well Structures

Electronic and optical properties of type-II InGaN/GaNSb/GaN quantum-well (QW) structures are investigated by using the multiband effective mass theory for potential applications in red light-emitting diodes. The heavy-hole effective mass around the topmost valence band is not affected much by the insertion of the GaNSb layer, and the optical matrix elements are greatly increased by the inclusion of the GaNSb layer in the InGaN/GaN QW structure. As a result, the type-II InGaN/GaNSb/GaN QW structure shows a much larger emission peak than the conventional type-I QW structure owing to the decrease in spatial separation between electron and hole wavefunctions, in addition to the reduction of the effective well width. It is also observed that the In content in InGaN well can be significantly reduced for the type-II QW structure with a large Sb content, compared to that for the type-I QW structure.

Band structures and topological properties of twisted bilayer MoTe2 and WSe2

We calculate the electronic band structures and topological properties of twisted homobilayer transition metal dichalcogenides(t-TMDs), in particular, bilayer MoTe2 and WSe2 based on a low-energy effective continuum model. We systematically show how the twist angle, vertical electric field and pressure modify the band structures of t-TMDs, often accompanied by topological transitions.We find the variation of topological transitions mainly take place in a limited range of parameters. The electric field can efficiently tune the energy of the topmost second valence band to motify the Chern numbers of the topmost three valance bands. The topological property of the topmost first valance band can be modified by electric field and pressure, but doesn’t depend on twist angle. We show the band gap between the topmost second and third valance bands that both change from non-trivial to trivial closes at -point of the moiré Brillouin zone.

Reversible spin textures with giant spin splitting in two-dimensional GaXY ( X=Se , Te; Y=Cl , Br, I) compounds for a persistent spin helix

The coexistence of ferroelectricity and spin-orbit coupling (SOC) in noncentrosymmetric systems may allow for a nonvolatile control of spin degrees of freedom by switching the ferroelectric polarization through the well-known ferroelectric Rashba effect (FRE). Although the FER has been widely observed for bulk ferroelectric systems, its existence in two-dimensional (2D) ferroelectric systems is still very rarely discovered. Based on first-principles calculations, supplemented with $\vec{k}\cdot\vec{p}$ analysis, we report the emergence of the FRE in the Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) monolayer compounds, a new class of 2D materials having in-plane ferroelectricity. Due to the large in-plane ferroelectric polarization, a giant out-of-plane Rashba effect is observed in the topmost valence band, producing unidirectional out-of-plane spin textures in the momentum space. Importantly, such out-of-plane spin textures, which can host a long-lived helical spin mode known as a persistent spin helix, can be fully reversed by switching the direction of the in-plane ferroelectric polarization. Thus, our findings can open avenues for interplay between the unidirectional out-of-plane Rashba effect and the in-plane ferroelectricity in 2D materials, which is useful for efficient and non-volatile spintronic devices.

Dynamical polarization function and plasmons in monolayer XSe ( X=In,Ga )

By using the single-band and multiband effective Hamiltonian for monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$), we calculate the density-density response function of monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) and study its plasmon dispersion within the random-phase approximation. At long wavelengths ($q\ensuremath{\rightarrow}0$), plasmon dispersion shows the local classical behavior $\ensuremath{\omega}={\ensuremath{\omega}}_{0}\sqrt{q}$. Although, monolayer $X\mathrm{Se}$ ($X=\mathrm{In},\mathrm{Ga}$) has a nonparabolic ``Mexican hat'' topmost valence band dispersion which is so different from the parabolic band structure in two-dimensional electron gas, the corresponding density dependence of the plasmon energy is still of the form ${\ensuremath{\omega}}_{0}\ensuremath{\propto}\sqrt{n}$ ($n$ is the carrier concentration) as in a conventional two-dimensional electron gas (2DEGs). However, the Fermi energy ($|\ensuremath{\mu}|$) dependence of the plasmon energy is of the form ${\ensuremath{\omega}}_{0}\ensuremath{\propto}{|\ensuremath{\mu}|}^{1/4}$, which is different from the conventional 2DEGs (${\ensuremath{\omega}}_{0}\ensuremath{\propto}\sqrt{|\ensuremath{\mu}|}$).

Tailoring of carrier concentration and engineering of band gap for Sn-doped In2O3 films by postirradiation of negatively charged oxygen ions

We demonstrate that the state-of-the-art postirradiation technology for negatively charged oxygen (O−) ions is effective for tailoring carrier concentration (n e ), electrical resistivity (ρ), and optical band gap (E g) in a wide range for polycrystalline 50-nm-thick Sn-doped In2O3 (ITO) films on glass substrates by reactive plasma deposition with direct-current arc discharge. As-deposited ITO films showed n e of 9.2 × 1020 cm−3, ρ of 1.5 × 10−4 Ω cm, and E g of 3.50 eV. The postirradiation of O− ions for 180 min at 250 °C decreased n e to 2.4 × 1018 cm−3. This resulted in a significant increase in ρ to 3.5 × 10−1 Ω cm while retaining the bixbyite crystal structure and the spatial distribution of Sn dopant atoms. The postirradiation of O− ions led to the continuous decrease in the optical E g ranging from 3.50 to 3.02 eV, which is smaller than that of undoped In2O3. For degenerate ITO films, conventional theories about the broadening and narrowing of the optical E g explain the experimental results well. On the other hand, for nondegenerate ITO films, the optical E g shrinkage would be mainly caused by an upward energy shift attributable to the generation of the anti-bonding π* states between O 2p and In 4d orbitals within the topmost valence band owing to the lattice disorder associated with incorporated interstitial oxygen atoms that fill structural vacancy sites. On the basis of the Ioffe–Regel criterion utilizing the electron mean free path, Fermi momentum, and their product, we determined the critical n e at which degenerate ITO films transform to nondegenerate ones.

Crossover from weakly indirect to direct excitons in atomically thin films of InSe

We perform a $\mathbf{k \cdot p}$ theory analysis of the spectra of the lowest energy and excited states of the excitons in few-layer atomically thin films of InSe taking into account in-plane electric polarizability of the film and the influence of the encapsulation environment. For the thinner films, the lowest-energy state of the exciton is weakly indirect in momentum space, with its dispersion showing minima at a layer-number-dependent wave number, due to an inverted edge of a relatively flat topmost valence band branch of the InSe film spectrum and we compute the activation energy from the momentum dark exciton ground state into the bright state. For the films with more than seven In$_2$Se$_2$ layers, the exciton dispersion minimum shifts to $\Gamma$-point.

Bulk and surface electronic states in the dosed semimetallic HfTe2

The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T-${\mathrm{HfTe}}_{2}$, a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of potassium (K) deposition. From the ${k}_{z}$ maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wave-function confinement at the topmost layer. In our highest-dosing cases, a monolayerlike electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below ${E}_{F}$, we can directly measure a band overlap of $\ensuremath{\sim}0.2$ eV. However, 3D bulklike states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

Electronic properties and crystal structures of double-perovskites, Ba2BiIIIBiVO6,Ba2PrBiO6, and Ba2PrSbO6: First-principles study

In recent experiments, a significant band gap widening was observed when Sb was substituted for Bi in the double-perovskite Ba2PrBiO6. In this work, we study a series of double-perovskites, Ba2BiIIIBiVO6, Ba2PrBiO6, and Ba2PrSbO6 using the first-principles density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional to investigate the substitution effect on the structural and electronic properties. We find that the two topmost valence bands are disappeared on the substitution of PrIII for BiIII, and the two bottommost conduction bands are disappeared on the substitution of SbV for BiV, causing the significant band gap widening. Further, our calculation suggests that the Ba2PrPrBiO6 is a possible candidate as a source of the PrIV signal observed in the experiment. We find that the B-site disordering atomic configuration, Ba2B″VB′IIIO6, are restored to those of the original structures. On the other hand, our results also suggest the importance of the partial B-site disorder to explain the experimentally observed band gaps.

Influence of magnetic ordering on the spectral properties of binary transition metal oxides

Using the ab initio embedded DMFT (eDMFT) approach, we study the effect of long-range magnetic ordering on the spectral properties in the binary transition metal oxides, and find that the most significant changes appear in the momentum resolved spectral functions, which sharpen into quite well-defined bands in the antiferromagnetic (AFM) phase. The strongest change across the transition is found at the topmost valence band edge (VBE), which is commonly associated with the Zhang-Rice bound state. This VBE state strengthens in the AFM phase, but only for the minority spin component, which is subject to stronger fluctuations. A similar hybridized VBE state also appears in the DFT single-particle description of the AFM phase, but gets much stronger and acquires a well-defined energy in the eDMFT description.

Persistent spin helix in Rashba-Dresselhaus ferroelectric CsBiNb2O7

Ferroelectric Rashba semiconductors (FERSC) are a novel class of multifunctional materials showing a giant Rashba spin splitting which can be reversed by switching the electric polarization. Although they are excellent candidates as channels in spin field effect transistors, the experimental research has been limited so far to semiconducting GeTe, in which ferroelectric switching is often prevented by heavy doping and/or large leakage currents. Here, we report that ${\mathrm{CsBiNb}}_{2}{\mathrm{O}}_{7}$, a layered perovskite of Dion-Jacobson type, is a robust ferroelectric with sufficiently strong spin-orbit coupling and spin texture reversible by electric field. Moreover, we reveal that its topmost valence band's spin texture is quasi-independent from the momentum, as a result of the low symmetry of its ferroelectric phase. The peculiar spin-polarization pattern in the momentum space may yield the so-called ``persistent spin helix,'' a specific spin-wave mode which protects the spin from decoherence in the diffusive transport regime, potentially ensuring a very long spin lifetime in this material.

Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu3BiS3 for Photovoltaics.

The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4-1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.

Temperature effects on the electronic band structure of PbTe from first principles

We report a fully {\it ab-initio} calculation of the temperature dependence of the electronic band structure of PbTe. We address two main features relevant for the thermoelectric figure of merit: the temperature variations of the direct gap and the difference in energies of the two topmost valence band maxima located at L and $\Sigma$. We account for the energy shift of the electronic states due to thermal expansion, as well as electron-phonon interaction computed using the non-adiabatic Allen-Heine-Cardona formalism within density functional perturbation theory and the local density approximation. We capture the increase of the direct gap with temperature in very good agreement with experiment. We also predict that the valence band maxima at L and $\Sigma$ become aligned at $\sim 600-700$ K. We find that both thermal expansion and electron-phonon interaction have a considerable effect on these temperature variations. The Fan-Migdal and Debye-Waller terms are of almost equal magnitude but have an opposite sign, and the delicate balance of these terms gives the correct band shifts. The electron-phonon induced renormalization of the direct gap is produced mostly by high-frequency optical phonons, while acoustic phonons are also responsible for the alignment of the valence band maxima at L and $\Sigma$.

Enhanced photocatalytic activity of single-layered Hittorf’s violet phosphorene by isoelectronic doping and mechanical strain: A first-principles research

A systematic theoretically study is performed for violet phosphorus monolayer nanosheet with the objective of improving its photocatalytic activity in water splitting under visible light by isoelectronic elements (N, As, Sb, and Bi) doping and strain engineering. The results show that compared with N-, As-, and Sb-doping, the introduced Bi impurity can effectively enhance visible light absorption efficiency. By applying an in-layer strain for Bi-doping violet phosphorene, the modulation of band edges further narrows the band gap. In particular, for compressive strain, the absorption extends into the overall visible region, greatly increasing the visible light absorption efficiency, which could be attributed to the significant reduced band gap and highly dispersive electronic states around the topmost valence band. Moreover, the band alignments for Bi-doped violet phosphorus monolayer are well positioned for the feasibility of both photo-oxidation and photo-reduction of water within the range of 0 to −3% biaxial strains and 0 to −7% uniaxial strains. Hence, compressive strained Bi-doped violet phosphorus monolayer can be a promising metal-free photocatalyst for water splitting owing to its improved visible light activity as well as the high carrier mobility.

First demonstration of improvement in hole conductivity in c-plane III-Nitrides through application of uniaxial strain

Hole transport in III-Nitrides (III-Ns) is dominantly dependent on the large effective mass of holes and thus, results in a low hole mobility. First principle band structure calculations suggest that under uniaxial strain, the valence band degeneracy can be broken and the light hole band with lower hole effective mass can be obtained as the topmost valence band. In this work, we experimentally demonstrate the improvement in hole conductivity under the application of uniaxial strain in III-Ns. We obtained approximately 25%–50% lower sheet resistance with uniaxially strained InGaN layers compared to planar biaxially strained InGaN layers.