Functional Theory Band Structure Calculations Research Articles

Bandgap tuning of mono- and bilayer graphene doped with group IV elements

We report density functional theory band structure calculations of graphene doped with group IV elements. A bandgap as high as 2.13 eV is calculated for a single layer of graphene doped with Si while Ge and Sn doping reduce this bandgap for equal doping concentrations. Bilayers of doped graphene are also studied and it is found that the bandgap of these materials is less than that of the single layer counterparts. Finally, a transverse electric field is applied to the doped bilayers and it is found that the bandgap is inversely proportional to the electric field strength in contrast to what has been observed in the case of pristine bilayers. Carrier effective masses were calculated and in general the effective masses of electrons and holes are found to be similar.

Role ofp-dands-dinteractions in the electronic structure and band gap of Zn1−xMxO (M=Cr, Mn, Fe, Co, Ni, and Cu): Photoelectron and optical spectroscopy and first-principles band structure calculations

We report an investigation on the effect of $p$-$d$ and $s$-$d$ interactions in the electronic structure, and especially in the band-gap value, of wurtzite wide-gap diluted magnetic semiconductors Zn${}_{1\ensuremath{-}x}$${M}_{x}$O ($M=\mathrm{Cr}$, Mn, Fe, Co, Ni, Cu). Thin films prepared by pulsed laser deposition are investigated by means of optical absorption at low-temperature and photoelectron spectroscopy. Pure wurzite phase is shown to be maintained for Co and Mn concentrations up to 25$%$ and for Cr up to 10$%$, while in the case of Fe, Ni, and Cu, other phases are present for concentrations higher than 5, 2, and 1$%$, respectively. The band gap of the Zn${}_{1\ensuremath{-}x}$${M}_{x}$O alloy increases at a rate of 9, 22, 4, and 23 meV/$%$$M$ for $M=\mathrm{Cr}$, Mn, Fe, and Co, respectively, and decreases at a rate of about $\ensuremath{-}$14 and $\ensuremath{-}$10 meV/$%$$M$ for $M=\mathrm{Ni}$ and Cu. Photoelectron spectroscopy of the Zn${}_{1\ensuremath{-}x}$${M}_{x}$O valence band for $M=\mathrm{Mn}$ and Co shows that the emergence of the transition metal-related photoemission peak is clearly correlated to a larger binding energy of the O 2$p$ valence-band peaks. A simple model of $p$-$d$ and $s$-$d$ interaction is proposed in which the decrease of Zn 3$d$ electron density below the valence band and the increase of $M$ 3$d$ electron density for $M=\mathrm{Cr}$ to Co lead to higher binding energies of the valence-band maximum and, thus, to a larger band gap. In contrast, for Ni and Cu the 3$d$ electrons lie below the valence-band maximum and push it to lower binding energies, thus decreasing the band gap. This simple model is basically confirmed by first-principles density functional theory band structure calculations. Detailed analyses of the band structures and densities of states show that the $p$-$d$ interaction leads to an increase of the band gap for $M=\mathrm{Mn}$ to Co but a decrease for $M=\mathrm{Ni}$ and Cu. They also suggest that the $s$-$d$ interaction plays the major role or contributes as much as the $p$-$d$ interaction in leading to the increase of the band gap for $M=\mathrm{Cr}$ and Mn, respectively.

Quantum electron confinement in closely matched metals: Au films on Ag(111)

We examined by two-dimensional photoemission band mapping the electronic structure of Au films epitaxially grown on an Ag(111) substrate. The very similar structural and electronic properties of the two metals make this system extremely unfavorable for the occurrence and observation of electron confinement effects. At variance with previous spectroscopic studies, we show that the electron reflectivity at the interface sustains the formation of well-defined $sp$-derived quantum well states (QWS) and weak quantum well resonance states (QWRS) in the Au layers. The character and degree of confinement of these states are analyzed and quantitatively related to the Au/Ag interface reflectivity on the basis of density functional theory (DFT) band structure calculations.

Hidden magnetic order in CuNCN

We report a comprehensive experimental and theoretical study of the quasi-one-dimensional quantum magnet CuNCN. Based on magnetization measurements above room temperature as well as muon spin rotation and electron spin resonance measurements, we unequivocally establish the localized Cu+2-based magnetism and the magnetic transition around 70 K, both controversially discussed in the previous literature. Thermodynamic data conform to the uniform-spin-chain model with a nearest-neighbor intrachain coupling of about 2300 K, in remarkable agreement with the microscopic magnetic model based on density functional theory band-structure calculations. Using exact diagonalization and the coupled-cluster method, we derive a collinear antiferromagnetic order with a strongly reduced ordered moment of about 0.4 mu_B, indicating strong quantum fluctuations inherent to this quasi-one-dimensional spin system. We re-analyze the available neutron-scattering data, and conclude that they are not sufficient to resolve or disprove the magnetic order in CuNCN. By contrast, spectroscopic techniques indeed show signatures of long-range magnetic order below 70 K, yet with a rather broad distribution of internal field probed by implanted muons. We contemplate the possible structural origin of this effect and emphasize peculiar features of the microstructure studied with synchrotron powder x-ray diffraction.

NMR compared to band structure calculations of the quaternary superconductor La3Ni2B2N3−x

We measured susceptibility and NMR spectra, spin-lattice, and spin-spin relaxation of ${}^{11}$B and ${}^{139}$La in the quaternary superconductor La${}_{3}$Ni${}_{2}$B${}_{2}$N${}_{3\ensuremath{-}x}$ and compare these experimental results to full potential density functional theory band structure calculations. The electric field gradients (EFGs) at B and La sites clearly show that the N vacancies are restricted to the N(2$b$) sublattice. Temperature-dependent susceptibility, Knight shift, and nuclear spin-lattice relaxation ${T}_{1}$ are successfully described within the Fermi-liquid model. The Hebel-Slichter peak is suppressed in the mixed state and in the presence of paramagnetic impurities.

Na2Ge2Se5: A highly nonlinear optical material

We report an integrated experimental and theoretical study of the two-dimensional polar selenogermanate compound Na2Ge2Se5, which exhibits strong nonlinear optical (NLO) second harmonic generation response in the visible and infrared region. The compound is type-I phase-matchable with a large SHG coefficient χ(2)≈290pmV−1, which is the second highest among the phase-matchable NLO materials. It also performs as an NLO frequency mixer to produce radiation via difference frequency generation. The compound is optically transparent from the visible (0.521μm) to the far IR (18.2μm) and melts congruently. Ab initio density functional theory band structure calculations show that the unusually large second-order optical nonlinearity is attributed to the two-dimensional character of the crystal structure. Raman, IR and optical absorption spectroscopy, differential thermal analysis, NLO properties of derivatives of Na2Ge2Se4.55Te0.45 and Na2Ge1.64Sn0.36Se5 and thermal expansion behavior studies are also reported.

(Si)5−2y(AlP)y alloys assembled on Si(100) from Al-P-Si3 building units

An original class of IV/III-V hybrid (Si)5−2y(AlP)y/Si(100) semiconductors have been produced via tailored interactions of molecular P(SiH3)3 and atomic Al yielding tetrahedral “Al-P-Si3” building blocks. Extensive structural, optical, and vibrational characterization corroborates that these units condense to assemble single-phase, monocrystalline alloys containing 60%-90% Si (y = 0.3-1.0) as nearly defect-free layers lattice-matched to Si. Spectroscopic ellipsometry and density functional theory band structure calculations indicate mild compositional bowing of the band gaps, suggesting that the tuning needed for optoelectronic applications should be feasible.

Superposition of ferromagnetic and antiferromagnetic spin chains in the quantum magnet BaAg2Cu[VO4]2

Based on density functional theory band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements, we address the microscopic magnetic model of BaAg2Cu[VO4]2 that was recently proposed as a spin-1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of Ja(1) ~ -19 K and Ja(2) ~ 9.5 K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic 'a' direction, and slightly differ in the mutual arrangement of the magnetic CuO4 plaquettes and non-magnetic VO4 tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds.

Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal–Organic Polymers [Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n

The reaction of Hg(OAc)(2) with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC(6)H(4)S)(en)](n), while the reaction of Pb(OAc)(2) with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb(SC(6)H(4)S)(dien)](n). Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal-organic polymers with -M-SC(6)H(4)S- repeat units. Diffuse reflectance UV-visible spectroscopy indicates band gaps of 2.89 eV for [Hg(SC(6)H(4)S)(en)](n) and 2.54 eV for [Pb(SC(6)H(4)S)(dien)](n), while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb(SC(6)H(4)S)(dien)](n) calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg(SC(6)H(4)S)(en)](n) does have moderate dispersion and a calculated electron effective mass of 0.29·m(e). Hybridization of the empty Hg 6s orbital with SC(6)H(4)S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.

Ab Initio Probing of the Magnetic and Electronic Properties of ThCr2Si2-Like Charge-Balanced KFeAgTe2

Based on density functional theory band structure calculations, we address the magnetic and electronic properties of KFeAgTe2 system. This phase belongs to the newest family of ThCr2Si2-type ternary iron-chalcogenide materials AxFe2−yCh2 (A= alkali metals or Tl, Ch= Se or S, discovered in 2010) which have aroused recently a tremendous interest and triggered intense studies owing to intriguing coexistence of metal vacancy ordering, antiferromagnetism, and superconductivity. But, unlike AxFe2−yCh2, KFeAgTe2 is a Te-containing ThCr2Si2-type phase, includes a large amount of the noble metal and is free of vacancies. Here, by means of first-principles FLAPW-GGA calculations, the spin ordering of the magnetic ground state, total and partial densities of states for KFeAgTe2 were obtained for the first time and analyzed in comparison with the related AxFe2−yCh2 systems.

Unusual ferromagnetic superexchange in CdVO3: The role of Cd

A microscopic magnetic model of the low-pressure modification of CdVO3 is established, based on density functional theory (DFT) band-structure calculations, magnetization measurements, and quantum Monte-Carlo simulations. This compound is a rare example of a quasi-one-dimensional spin-1/2 system showing exclusively ferromagnetic exchange. The spin lattice of CdVO3 entails zigzag chains with an effective intrachain coupling J ~ -90 K and interchain couplings of Jc ~ -18 K and Ja ~ -3 K. Quantum fluctuations are partially suppressed by the sizable interchain coupling Jc that leads to an intermediate regime between one-dimensional and two-dimensional ferromagnetic systems. Apart from the peculiar spin model, CdVO3 features an unusual mechanism of ferromagnetic superexchange. The couplings largely originate from Cd 5s states mediating hoppings between half-filled and empty 3d states of V+4.

Volume contraction in CeB4induced by delocalized Ce4felectrons

We have investigated the delocalized nature of Ce 4$f$ electrons in CeB${}_{4}$, employing the first-principles total energy electronic structure calculations. We have shown that CeB${}_{4}$ has a paramagnetic ground state in agreement with experiment, and that the valence state of Ce is rather close to trivalent (${f}^{1}$), but far from tetravalent (${f}^{0}$). The anomalous lattice constant of CeB${}_{4}$, which strongly deviates from the Lanthanides contraction curve, is well explained by the delocalized nature of Ce $4f$ electrons. We have also examined the charge densities and the Fermi surfaces of CeB${}_{4}$, based on density functional theory band structure calculations.

Band structure calculations of CuAlO2, CuGaO2, CuInO2, and CuCrO2by screened exchange

We report density functional theory (DFT) band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2 and CuCrO2. The use of the hybrid functional sX-LDA leads to considerably improved electronic properties compared to standard local density approximation (LDA) and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations and show the correct trends with respect to the atomic number of the cation (Al, Ga, In). The resulting energetic depths of Cu d and O p levels and the valence band widths are considerable improvements compared to LDA and GGA and in good agreement with available x-ray photoelectron spectroscopy (XPS) data. Lastly, we show the calculated imaginary part of the dielectric function for all four systems.

Spiral ground state against ferroelectricity in the frustrated magnet BiMnFe2O6

The spiral magnetic structure and underlying spin lattice of BiMnFe${}_{2}$O${}_{6}$ are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn${}^{3+}$ and Fe${}^{3+}$ cations, this centrosymmetric compound undergoes a transition into an incommensurate antiferromagnetically ordered state below ${T}_{N}\ensuremath{\simeq}220$ K. The magnetic structure is characterized by the propagation vector $\mathbf{k}=[0,\ensuremath{\beta},0]$ with $\ensuremath{\beta}\ensuremath{\simeq}0.14$ and the $P{22}_{1}{2}_{1}{1}^{\ensuremath{'}}(0\ensuremath{\beta}0)0s0s$ magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the $b$ axis. The magnetic moments lie in the $\mathit{ac}$ plane and rotate about $\ensuremath{\pi}(1+\ensuremath{\beta})\ensuremath{\simeq}204.8$-deg angle between the adjacent magnetic atoms along $b$. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the $\mathit{ab}$ plane. The antiferromagnetic coupling along the $c$ axis cancels the possible electric polarization and prevents ferroelectricity in BiMnFe${}_{2}$O${}_{6}$.

Organic Templated Cuprous Cyanide Open Frameworks Based on Cu2(CN)6 Dimer with Strong and Long-Lived Luminescence

By using environmental friendly K3[Fe(CN)6] as a cyanide source, hydrothermal reactions of CuCl2 and diamines resulted in four Cu2(1,1,2-μ3-CN)2(CN)4 dimer-based organic templated cuprous cyanide open frameworks, namely, [H1.5mepip]2[Cu4(CN)7] (1), [H1.5etpip]2[Cu4(CN)7] (2), [H2me2pip][Cu3(CN)5] (3), and [H2dmpa][Cu4(CN)6] (4) (mepip = N-methylpiperazine; etpip = N-ethylpiperazine, me2pip = N,N-dimethylpiperazine, dmpa = 3-dimethyl-aminopropylamine). Isomorphic 1 and 2 show three-dimensional (3-D) (4,6)-connected FeS2-like open frameworks constructed by 4-connected Cu(CN)4 monomers and 6-connected Cu2(CN)6 dimers. Complex 3 shows an unprecedented 3-D (4,6)-connected open framework in which the ratio of 4-connected Cu(CN)4 monomers to 6-connected Cu2(CN)6 dimers is 1:1. Complex 4 shows a 3-D (3,6)-connected AgIn2-like open framework constructed by 3-connected Cu(CN)3 monomers and 6-connected Cu2(CN)6 dimers. Different from CuCN/amine coordination polymers, the protonated organic diamines in 1–4 act as templates in the formation of cuprous cyanide open frameworks. Even at ambient temperature, 1, 2, 3, and 4 show strong blue-green photoluminescence related to Cu2(CN)6 dimer, which is mainly assigned to metal-to-ligand charge-transfer (MLCT) by time-dependent density functional theory (TD-DFT) and electronic band structure calculations.

Spiral ground state in the quasi-two-dimensional spin-12system Cu2GeO4

We apply density functional theory band structure calculations, the coupled cluster method, and exact diagonalization to investigate the microscopic magnetic model of the spin-$\frac{1}{2}$ compound Cu${}_{2}$GeO${}_{4}$. The model is quasi-two-dimensional, with uniform spin chains along one direction and frustrated spin chains along the other direction. The coupling along the uniform chains is antiferromagnetic, $J\ensuremath{\simeq}130$ K. The couplings along the frustrated chains are ${J}_{1}\ensuremath{\simeq}\ensuremath{-}60$ K and ${J}_{2}\ensuremath{\simeq}80$ K between nearest neighbors and next-nearest neighbors, respectively. The ground state of the quantum model is a spiral, with a reduced sublattice magnetization of 0.62 ${\ensuremath{\mu}}_{B}$ and a pitch angle of ${84}^{\ifmmode^\circ\else\textdegree\fi{}}$, both renormalized by quantum effects. The proposed spiral ground state of Cu${}_{2}$GeO${}_{4}$ opens a way to magnetoelectric effects in this compound.

Ab initiomodeling of Bose-Einstein condensation in Pb2V3O9

We apply density functional theory band structure calculations and quantum Monte Carlo simulations to investigate the Bose-Einstein condensation in the spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the one-dimensional nature of this compound, we present a quasi-two-dimensional model of spin dimers with ferromagnetic and antiferromagnetic interdimer couplings. Our model is well justified microscopically and provides a consistent description of the experimental data on the magnetic susceptibility, high-field magnetization, and field vs. temperature phase diagram. The Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9 is largely governed by intralayer interactions, whereas weak interlayer couplings have a moderate effect on the ordering temperature. The proposed computational approach is an efficient tool to analyze and predict high-field properties of quantum magnets.

Substitution-induced superconductivity inSrFe2−xRuxAs2(0≤x≤2)

The magnetism in ${\text{SrFe}}_{2}{\text{As}}_{2}$ can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ and leads to bulk superconductivity for $0.6\ensuremath{\le}x\ensuremath{\le}0.8$. Magnetization, electrical resistivity, and specific-heat data show ${T}_{c}$ up to $\ensuremath{\approx}20\text{ }\text{K}$. Detailed structural investigations reveal a strong decrease in the lattice parameter ratio $c/a$ with increasing $x$. Density functional theory band-structure calculations are in line with the observation that the magnetic order in ${\text{SrFe}}_{2\ensuremath{-}x}{\text{Ru}}_{x}{\text{As}}_{2}$ is only destabilized for large $x$.

Electronic structure studies ofBaFe2As2by angle-resolved photoemission spectroscopy

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of ${\text{BaFe}}_{2}{\text{As}}_{2}$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\ensuremath{\Gamma}$ point and an electron pocket at each of the $X$ points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

First Crystal Structure Studies of CaAlH5

A new member of the aluminum hydride family, CaAlH5, is formed during the decomposition of Ca(AlH4)2. The crystal structure of this new compound was calculated by density functional theory band-structure calculations and confirmed by X-ray powder diffraction analysis. The structure crystallizes in space group P2(1)/n (No. 14), with a = 8.3797(9) angstroms, b = 6.9293(8) angstroms, c = 9.8138(11) angstroms, beta = 93.78(1) degrees, and Z = 8.