Lowest Unoccupied Band Research Articles

Topological nodal surface semimetal states in Sr5X3 compounds (X=As,Sb,Bi)

Topological nodal surface semimetals are a class of materials characterized by degenerate conduction and valence bands on closed nodal surfaces in the Brillouin zone that are protected by crystalline symmetries. These materials attract considerable attention for exhibiting intriguing, topologically driven quantum phenomena that are fundamentally interesting and practically useful. Here we identify by systematic ab initio calculations and topological quantum chemistry analysis prominent topological nodal surface semimetal states in ${\mathrm{Sr}}_{5}{X}_{3}(X$ = As, Sb, Bi) compounds in hexagonal $P{6}_{3}/mcm$ (${D}_{6h}^{3}$) symmetry. These nodal surface states comprise degenerate the highest occupied band and the lowest unoccupied band that overlap in a single band at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane and are protected by time-reversal and nonsymmorphic twofold screw rotation symmetry. Topological quantum chemistry theory analysis reveals a twofold band degeneracy without spin-orbit coupling at the high-symmetry and generic $\mathbit{k}$ points located at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane, which is formed by the bands of (${\mathrm{A}}_{1}\ensuremath{\bigoplus}{\mathrm{B}}_{2})@6g$ band representations. Since the compounds studied here have been already experimentally synthesized, the present results have an immediate impact on the experimental detection and characterization of the theoretically predicted topological physics.

Analysis of band structures of phosphorene and bismuthene based on the double group theory

We study band structures of group-V two-dimensional materials, i.e. phosphorene and bismuthene, by carrying out first-principles calculations including spin–orbit coupling (SOC). We propose a method to identify irreducible representations (IR) of both symmorphic and nonsymmorphic systems. We find for the α structures that all the non-SOC bands are doubly degenerated on the first Brillouin zone edge due to sticking or pairing of bands and that the SOC slightly splits the bands in most of the cases. We evaluate Z 2 invariants based on identified IR. We find that the Z 2 invariant of 1 in the case of β bismuthene is due to the strong SOC that reverses the highest occupied and the lowest unoccupied bands at the Γ point.

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations.

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti2CO2 monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti2CO2 monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti2CO2. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti2CO2 is calculated to be 1.76–1.67 mA cm−2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti2CO2, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

Topological and superconducting properties in YD3 ( D=In , Sn, Tl, Pb)

Using first-principles calculations, we explore the electron properties of ${\text{Y}D}_{3}$ ($D=\text{In}$, Sn, Tl, Pb) and predict that ${\mathrm{YIn}}_{3}$, ${\mathrm{YTl}}_{3}$, and ${\mathrm{YPb}}_{3}$ are topological superconductor candidates. In the presence of spin-orbit coupling effect, continuous band gaps for ${\mathrm{YIn}}_{3}$, ${\mathrm{YSn}}_{3}$, and ${\mathrm{YPb}}_{3}$ are opened between their highest occupied bands ($N$) and the lowest unoccupied bands ($N+1$), where the different ${Z}_{2}$ invariants are obtained. Differently and specially, there are type-II Dirac points (DPs) at the high-symmetry lines in ${\mathrm{YTl}}_{3}$, indicating one possibility of topological Dirac semimetal. Furthermore, the nontrivial Rashba-like topological surface states are achieved at the $\overline{\mathrm{X}}$ point on the (001) surface for ${\mathrm{YIn}}_{3}$, ${\mathrm{YTl}}_{3}$, and ${\mathrm{YPb}}_{3}$, as well as the Fermi arcs in ${\mathrm{YTl}}_{3}$ connecting the DPs. In addition to the topological properties, our electron-phonon coupling calculations indicate clearly that these four intermetallics are all phonon-mediated superconductors. The calculated superconducting transition temperatures of ${T}_{c}=0.96$, 6.34, 2.17, and 4.37 K respectively for ${\mathrm{YIn}}_{3}$, ${\mathrm{YSn}}_{3}$, ${\mathrm{YPb}}_{3}$, and ${\mathrm{YTl}}_{3}$ agree well with experiments.

Electronic structures of quasi-one-dimensional cuprate superconductors Ba2CuO3+δ

An intact CuO$_2$ plane is widely believed to be a prerequisite for the high-$T_c$ superconductivity in cuprate superconductors. However, an exception may exist in the superconducting Ba$_2$CuO$_{3+\delta}$ materials where CuO chains play a more important role. From first-principles density functional theory calculations, we have studied the electronic and magnetic structures of Ba$_2$CuO$_{3+\delta}$. The stoichiometric Ba$_2$CuO$_3$ and Ba$_2$CuO$_4$ contain quasi-one-dimensional CuO chains and intact two-dimensional CuO$_2$ planes, respectively. In comparison with the nonmagnetic metal Ba$_2$CuO$_4$, Ba$_2$CuO$_3$ is found to be an antiferromagnetic (AFM) Mott insulator. It possesses a nearest-neighbor intra-chain antiferromagnetic (AFM) coupling and a weak inter-chain interaction, and its lowest unoccupied band and highest occupied band are contributed by Cu 3$d_{b^2-c^2}$-orbital (or $d_{x^2-y^2}$-orbital if we denote the $bc$-plane as the $xy$-plane) and O 2$p$-orbitals, respectively. Total energy calculations indicate that the oxygen vacancies in Ba$_2$CuO$_{3+\delta}$ prefer to reside in the planar sites rather than the apical oxygens in the CuO chains, in agreement with the experimental observation. Furthermore, we find that the magnetic frustrations or spin fluctuations can be effectively induced by moderate charge doping. This suggests that the superconducting pairing in oxygen-enriched Ba$_2$CuO$_{3+\delta}$ or oxygen-deficient Ba$_2$CuO$_{4-\delta}$ is likely to be mainly driven by the AFM fluctuations within CuO chains.

Characterization of AlPO4(110) Surface in Adsorption of Rh Dimer and Its Comparison with γ-Al2O3(100) Surface: A Theoretical Study

Adsorption of Rh dimer on AlPO4(110) and γ-Al2O3(100) surfaces was theoretically investigated by periodic DFT calculation with a slab model to elucidate characteristic features of the AlPO4 surface in comparison with the γ-Al2O3 surface. The adsorption at the PO site is the most favorable in both nonhydrated and hydrated AlPO4 surfaces, which is consistent with the experimental finding. The adsorption at the AlO site is the least favorable. The adsorption energy at the PO site of the AlPO4 surface is considerably larger than that at the γ-Al2O3 surface. One important reason is that the deformation energy of the γ-Al2O3 surface is much larger than that of the AlPO4 surface. Bader charge analysis, difference electron density map, and projected density of states (p-DOS) clearly disclose that the charge transfer (CT) occurs from the Rh dimer to the AlPO4 surface. This CT is stronger than in the adsorption on the γ-Al2O3 surface. The lowest unoccupied band (LU band in conduction band) plays a crucial role as a...

Crystal orbital study on the double walls made of nanotubes encapsulated inside zigzag carbon nanotubes

The structure stabilities and electronic properties are investigated by using ab initio self-consistent-field crystal orbital method based on density functional theory for the one-dimensional (1D) double-wall nanotubes made of n-gon SiO2 nanotubes encapsulated inside zigzag carbon nanotubes. It is found that formation of the combined systems is energetically favorable when the distance between the two constituents is around the Van der Waals scope. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. The frontier energy bands (the highest occupied band and the lowest unoccupied band) of double-wall nanotubes are mainly derived from the corresponding carbon nanotubes. The mobilities of charge carriers are calculated to be within the range of 102–104cm2V−1s−1 for the hybrid double-wall nanotubes. Young’s moduli are also calculated for the combined systems. For the comparison, geometrical and electronic properties of n-gon SiO2 nanotubes are also calculated and discussed.

Effects of charge-orbital order-disorder phenomena on the unoccupied electronic states in the single-layered half-doped Pr0.5Ca1.5MnO4

Here we report a study on the unoccupied states of the half-doped Pr${}_{0.5}$Ca${}_{1.5}$MnO${}_{4}$ (PCMO). Our investigation, based on temperature dependent x-ray absorption linear dichroism (XLD) and density-functional theory discloses the role of the charge-orbital ordering-disordering mechanisms on the unoccupied density of states. In particular, the lowest unoccupied band has a Mn ${e}_{g}$ ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ character, proving that the physical properties of the two-dimensional (2D) PCMO are also determined by the out-of-plane orbital. Yet, the difference in energy between the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states is observed to increase when a charge-orbital ordering is established, hence revealing that the Mn $3d$ electronic hopping is frustrated when the MnO${}_{6}$ cluster orthorhombic strain is increased. This finding addresses the question concerning the complex interplay between the in-plane and out-of plane orbitals in these 2D half-doped single layered manganites.

Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduc- tion states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.

Is Auger-free luminescence present in CeF 3?

It is well known that Auger-free luminescence (AFL) is observable when the condition E g> E VC is satisfied, where E g is the band-gap energy between the lowest unoccupied band and the highest occupied band and E VC the energy difference between the top of the highest occupied band and the top of the next lower occupied band. From measurements of reflection and X-ray photoelectron spectra, CeF 3 is demonstrated to really satisfy this condition. No evidence for AFL is found, nevertheless. The absence of AFL in CeF 3 is related to a characteristic nature of its highest and next lower occupied bands, which are quite different from those of previously studied AFL-materials.

Electronic structures of ferrocene-containing polymers

Electronic structures of polymers, ferrocene (Cp 2Fe, Cp = η 5-C 5H 5) in the main chain, were examined using the tight-binding crystal orbital method. Attention was focused on the manner in which the band structures of the polymers were influenced by the bridging moieties between the ferrocenes. Ferrocene-containing polymers with unsaturated carbon bridges possessed relatively wide bandwidths of both the highest occupied band and the lowest unoccupied band. Although the π conjugation was broken at the iron atoms, the unsaturated carbon bridges could form limited delocalization of the π electrons between the neighbouring Cp rings via the bridging moieties.

Correlation between height selection and electronic structure of the uniform height Pb/Si(111) islands

Uniform-height islands, with preferred heights differing by bilayer height increments, can be grown on Pb/Si(111) at low temperatures most likely as a result of quantum size effects. With scanning-tunneling-microscope spectroscopy we have determined how the electronic structure of individual islands is related to their stability. Differences between preferred vs nonpreferred island heights are seen at the position of the Fermi level with respect to the highest occupied band and lowest unoccupied band. This difference is supported from oscillations of the measured apparent barrier $\ensuremath{\Delta}(\mathrm{ln}I)/\ensuremath{\Delta}z$ with island height.

Ab Initio Study of Carbon Nanotubes in Electric Fields

Electronic structures of four types of carbon nanotubes in an electric field have been investigated by the ab initio pseudopotential plane wave method. The local field enhancement at the tips of nanotubes and the screening of the electric field inside the nanotubes are considerable for nanotubes with a small gap between the highest occupied band (HOB) and the lowest unoccupied band (LUB) in the electronic states. The dangling bond (DB) states near the HOB localized at the edge of nanotubes are significantly influenced by the electric field. Thus, the present results strongly suggest an important role of the localized states of conducting nanotubes with small electronic gaps in field emissions.

Ti2Nb6O12, a Novel Niobium Oxide Cluster Compound with “Chevrel Phase” Intercluster Connectivity Type

The niobium oxide cluster compound Ti2Nb6O12 was synthesized from NbO, NbO2, and TiO2 in the presence of NaF melt at 950 °C. It crystallizes in an original structure type based on octahedral cluster units (Nb6 ) . The clusters connect to each other through outer−inner linkages to form a three-dimensional framework containing cubeoctahedral cavities. The cluster connectivity pattern in Ti2Nb6O12 and intercluster metal−metal distances are similar to those found in the “Chevrel phases”. The Ti4+ ions have a distorted octahedral environment formed by the oxide ligands belonging to the clusters. Band structure calculations indicate that the lowest unoccupied band in Ti2Nb6O12 resembles the conduction band in the superconducting “Chevrel phases”.

Application of the Recursion Method to the Electronic Structures of Simple-Cubic Na2CsC60 and Body-Centered-Cubic K6C60

Electronic structures of simple cubic (sc) Na2CsC60 and body-centered-cubic (bcc) K6C60 are studied using the tight-binding recursion method. We find that the orientational disorder modifies the electronic states of sc Na2CsC60 significantly. The ground state of K6C60 is found to be insulating with a gap of 0.20 eV. The valence electrons of the alkali-metal atoms in both Na2CsC60 and K6C60 are almost completely transferred to the lowest unoccupied bands of the C 60 molecular solid. The strength of the interaction between the alkali-metal atom and the C 60 molecule is estimated.

Effect of topological defects in graphite

The simplest topological defect in a graphite sheet can be introduced by switching one bond with the movement of two carbon atoms, which transforms four hexagons to two pentagons and two heptagons. Putting the topological defects periodically in a graphite sheet, we study how the electronic state changes based on the tight binding model. Depending on the combination of geometrical parameters which specify the arrangement of the defects, we find that the electronic states can be classified into the zero-gap semiconducting, semimetallic and metallic cases. The localized charge density states at the defect sites emerge just above the Fermi level. Curious states having a flat band at the Fermi level appear in some particular geometrical arrangements of the defects. In addition, we propose to design a series of graphitic polymers with a flat lowest-unoccupied band.

Molecular engineering of small band gap polymers: heteroaromatic bicyclic polymers

A brief account is given of the various routes currently pursued to engineer novel electrically conducting polymers. Using these routes, the results of an investigation of the electronic structure and conduction properties of the nitrogen and oxygen containing analogues of polythieno[3,4-C]thiophene (PTTP), i.e. polypyrrolo[3,4-C]pyrrole (PPPY) and polyfurano[3,4-C]furan (PFFU), respectively, obtained on the basis of ab initio Hartree-Fock crystal orbital method, are reported. The energy band structures of all three polymers were found to be similar, both the highest occupied and the lowest unoccupied bands being of π character. The calculated band gap values were found to decrease in the order PTTP < PFFU <PPPY. PTTP and PPPY are predicted to be the best candidates for forming highly conducting materials on doping with electron donors (n-doping) and electron acceptors (p-doping), respectively. On the basis of their band alignments, the copolymer pairs PTTP-PPPY, PTTP-PFFU and PPPY-PFFU are found to belong to the category of type II staggered superlattices. The electronic density of states of these three pairs of quasi-one-dimensional copolymers were calculated using the negative factor counting technique within the tight binding approximation. The trends in the electronic structure and conduction properties of the pairs are discussed.

A Hypothetical Dense 3,4-Connected Carbon Net and Related B2C and CN2 Nets Built from 1,4-Cyclohexadienoid Units

This paper describes the geometrical and electronic structure of a hypothetical 3 ,4-co~ected tetragonal allotrope of carbon (space group P42/mmc, No. 131) built from 1,4-cyclohexadiene rings. The three-dimensional net, containing trigonal and tetrahedral atoms in a ratio of 2:1, has a calculated density of 3.12 g/cm3, intermediate between graphite and diamond. Band structure calculations for this net have been performed using the extended Huckel method. One-dimensional substructures of a polyquinoid, polyspiroquinoid, and polycyclophane nature are instructive in approaching the electronic structure of the full net. These substructures point to the importance of through-space interactions of the stacked olefin units in the net, separated by only 2.53 A. It is apparent that interaction leads to substantial dispersion of the n and n* bands, the highest occupied and lowest unoccupied bands in the tetragonal structure, respectively. The resulting density of states profile is that of a metal, with a significant n-n* band overlap at the Fermi level. Related nets formed by substituting boron and nitrogen into the trigonal sites of the lattice are studied as well. The electron count on the atom in the trigonal sites in the lattice significantly affects the band structure about the Fermi level; B2C should be metallic, and CN2 an insulator. Diamond, graphite, and Cml are allotropes of carbon in which space is filled with perfectly tetrahedral, perfectly trigonal, and approximately trigonal carbon atoms, respectively. These structures are not the only ones to fill space in this way,2 at least on paper.3 Of special interest are possible allotropes which would be dense, have interesting properties (e.g. conductivity), and share some of the structural features of the known allotropes, their 3and 4-connectedness4 for instance. In this contribution we suggest one hypothetical carbon structure which combines 3and 4-co~ected atoms, built upon the 1,4-cyclohexadiene motif,5 1, and look at its likely electronic * Author to whom all correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, November 1, 1994. (1) (a) Wells, A. F. Three Dimensional Nets and Polyhedra; Wiley: New York, 1977. (b) Wells, A. F. Further Studies of Three-Dimensional Nets; ACA Monograph No. 8; American Crystallographic Association: Pittsburgh, 1979. (c) Donohue, J. The Structure of the Elements, Wiley: New York, 1974. (d) Kratschmer, W.; Huffmann, H. Nature 1990,347, 354. (e) Kroto, H.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. F. Nature 1985, 318, 162. (f) Osawa, E. Kagaku 1970, 25, 85. (9) Bundy, F. P.; Kasper, J. S. J. Chem. Phys. 1967, 46, 3437. (h) Aust, R. B.; Drickamer, H. G. Science 1963, 140, 817. (2) (a) Zeger, L.; Kaxiras, E. Phys. Rev. Lett. 1993, 70, 2920-2922. (b) Muller, D. A.; Tzou, Y.; Raj, R.; Silcox, J. Nature 1993, 366, 725. (c) Renschler, C. L.; Pouch, J.; Cox, D. M. Eds. Novel Forms of Carbon; Materials Research Society Symposium Proceedings, Vol. 270; Materials Research Society: Pittsburgh, 1992. (d) Moshary, F.; Chen, N. H.; Silvera, I. F.; Brown, C. A.; Dom, H. C.; de Vries, M. S.; Bethune, D. S. Phys. Rev. Lett. 1992, 69, 466-469. (e) Iijima, S. Nature 1991, 354, 56. (f) Utsumi, W.; Yagi, T. Science 1991,252, 1542-1544. (g) Hirai, H.; Kondo, K. Science 1991, 253,772-774. (h) Spear, K. E.; Phelps, A. W.; White, W. B. J . Mater. Res. 1990,5,2277. (i) Angus, J. C.; Hayman, C. C. Science 1988,241, 1988. (i) Stankevich, V.; Nikerov, M. V.; Bochvar, D. A. Russ. Chem. Rev. 1987, 53, 670. (k) Melnitchenko, V. M.; Nikulin, Y. N.; Sladkov, A.M. Carbon 1985,23, 3-7. (1) Stankevich, I. V.; Nikerov, M. V.; Bochvar, D. A. Russ. Chem. Rev. 1984, 53, 640. (m) Whittaker, A. G.; Watts, E. J.; Lewis, R. S.; Anders, E. Science 1980, 209, 1512-1514. (n) Whittaker, A. G.; Tooper, B. J . Am. Ceram. SOC. 1974, 57,443-446. (0) Bochvar, D. A.; Galpem, E. G. Dokl. Akad. Nauk. SSSR 1973, 209, 612. (p) El Gorsey, A.; Donnay, G. Science 1968, 161, 363-364. (9) Ergun, S. Carbon 1968,6, 141-149. (r) Drickamer, H. Science 1967,156, 1183-1188. (s) Bundy, F. P. J. Chem. Phys. 1963, 38, 631. 0002-7863/94/1516-11456$04.50/0 properties. The space-filling structure in question is shown in

Ab Initio study of the electronic structure and conduction properties of nitrogen and oxygen containing analogues of polyisothianaphthene and their copolymers

Results of ab initio band structure calculations aimed at designing polyisobenzopyrrole (PIBP) and polyisobenzofuran (PIBF), two analogues of polyisothianaphthene (PITN), a conjugated organic polymer with the smallest band gap, are presented. The energy band structures of all the three polymers are found to be very similar, with both their highest occupied and lowest unoccupied bands being of π character. The calculated values of the band gap are found to decrease in the order PITN < PIBF < PIBP. Polyisothianaphthene is predicted to be the strongest candidate for forming conducting materials on doping with both electron acceptors (p doping) and electron donors (n doping). It is shown that the electron affinity values of these systems depend more on the heterocyclic substitution whereas their ionization potential values are more influenced by substitution on the backbone. The electronic density of states of the various copolymers of these heterocyclic compounds has been determined using a simple negative-factor counting method. Random copolymers are, in general, found to be intrinsically less insulating and more dopantphilic than their periodic counterparts.

Band structure and limiting behavior of linear polyanilines

AbstractThe band structure of an infinite linear polyaniline is investigated by using the HCO approximation. The degree of modelling of the limiting structure for the series by the linear oligomers so far synthesized is investigated. The upper bound of the wave‐length of the long‐wave ultraviolet absorption band for the limit is found, and the possibility of estimation of the conduction properties of its complex with iodine is indicated. The problem of replacement of the NH group in the chains by a more general group X is dealt with, and the dependence of the energy gap between the highest occupied and the lowest unoccupied band on the parameters of this group is determined.