Showing papers on "Lattice constant published in 2010"

Real-space observation of a two-dimensional skyrmion crystal

Abstract: Crystal order is not restricted to the periodic atomic array, but can also be found in electronic systems such as the Wigner crystal or in the form of orbital order, stripe order and magnetic order. In the case of magnetic order, spins align parallel to each other in ferromagnets and antiparallel in antiferromagnets. In other, less conventional, cases, spins can sometimes form highly nontrivial structures called spin textures. Among them is the unusual, topologically stable skyrmion spin texture, in which the spins point in all the directions wrapping a sphere. The skyrmion configuration in a magnetic solid is anticipated to produce unconventional spin-electronic phenomena such as the topological Hall effect. The crystallization of skyrmions as driven by thermal fluctuations has recently been confirmed in a narrow region of the temperature/magnetic field (T-B) phase diagram in neutron scattering studies of the three-dimensional helical magnets MnSi (ref. 17) and Fe(1-x)Co(x)Si (ref. 22). Here we report real-space imaging of a two-dimensional skyrmion lattice in a thin film of Fe(0.5)Co(0.5)Si using Lorentz transmission electron microscopy. With a magnetic field of 50-70 mT applied normal to the film, we observe skyrmions in the form of a hexagonal arrangement of swirling spin textures, with a lattice spacing of 90 nm. The related T-B phase diagram is found to be in good agreement with Monte Carlo simulations. In this two-dimensional case, the skyrmion crystal seems very stable and appears over a wide range of the phase diagram, including near zero temperature. Such a controlled nanometre-scale spin topology in a thin film may be useful in observing unconventional magneto-transport effects.

Assessing the quality of the random phase approximation for lattice constants and atomization energies of solids

Abstract: We present lattice constants, bulk moduli, and atomization energies of solids using the correlation energy evaluated within the adiabatic connection fluctuation-dissipation framework and applying the random-phase approximation. Recently, we have shown [Phys. Rev. Lett. 103, 056401 (2009)] that geometrical properties and heats of formation are well described within this approximation. We extend this study to a larger set of materials and focus on the treatment of metals and the effect introduced by the frozen-core approximation.

Light hadrons from lattice QCD with light (u, d), strange and charm dynamical quarks

Abstract: We present results of lattice QCD simulations with mass-degenerate up and down and mass-split strange and charm (N f = 2 + 1 + 1) dynamical quarks using Wilson twisted mass fermions at maximal twist. The tuning of the strange and charm quark masses is performed at two values of the lattice spacing a ≈ 0:078 fm and a ≈ 0:086 fm with lattice sizes ranging from L ≈ 1:9 fm to L ≈ 2:8 fm. We measure with high statistical precision the light pseudoscalar mass m PS and decay constant f PS in a range 270 ≲ m PS ≲ 510 MeV and determine the low energy parameters f 0 and $ {\bar{l}_{3,4}} $ of SU(2) chiral perturbation theory. We use the two values of the lattice spacing, several lattice sizes as well as different values of the light, strange and charm quark masses to explore the systematic effects. A first study of discretisation effects in light-quark observables and a comparison to N f = 2 results are performed.

Nano-optical investigations of the metal-insulator phase behavior of individual VO(2) microcrystals.

Abstract: Despite the relatively simple stoichiometry and structure of VO(2), many questions regarding the nature of its famous metal-insulator transition (MIT) remain unresolved. This is in part due to the prevailing use of polycrystalline film samples and the limited spatial resolution in most studies, hindering access to and control of the complex phase behavior and its inevitable spatial inhomogeneities. Here, we investigate the MIT and associated nanodomain formation in individual VO(2) microcrystals subject to substrate stress. We employ symmetry-selective polarization Raman spectroscopy to identify crystals that are strain-stabilized in either the monoclinic M1 or M2 insulating phase at room-temperature. Raman measurements are further used to characterize the phase dependence on temperature, identifying the appearance of the M2 phase during the MIT. The associated formation and spatial evolution of rutile (R) metallic domains is studied with nanometer-scale spatial resolution using infrared scattering-scanning near-field optical microscopy (s-SNOM). We deduce that even for small crystals of VO(2), the MIT is influenced by the competition between the R, M1, and M2 crystal phases with their different lattice constants subjected to the external substrate-induced stress. The results have important implications for the interpretation of the investigations of conventional polycrystalline thin films where the mutual interaction of constituent crystallites may affect the nature of the MIT in VO(2).

Light hadrons from lattice QCD with light (u,d), strange and charm dynamical quarks

Abstract: We present results of lattice QCD simulations with mass-degenerate up and down and mass-split strange and charm (N_f = 2+1+1) dynamical quarks using Wilson twisted mass fermions at maximal twist. The tuning of the strange and charm quark masses is performed at two values of the lattice spacing a~0.078 fm and a~0.086 fm with lattice sizes ranging from L~1.9 fm to L~2.8 fm. We measure with high statistical precision the light pseudoscalar mass m_PS and decay constant f_PS in a range 270 < m_PS < 510 MeV and determine the low energy parameters f_0, l_3 and l_4 of SU(2) chiral perturbation theory. We use the two values of the lattice spacing, several lattice sizes as well as different values of the light, strange and charm quark masses to explore the systematic effects. A first study of discretisation effects in light-quark observables and a comparison to N_f=2 results are performed.

On-chip beam-steering photonic-crystal lasers

Abstract: Using a composite photonic-crystal structure composed of both a square and rectangular lattice, scientists successfully realize an on-chip semiconductor laser whose emitted beams can be dynamically controlled by varying their relative lattice constants.

Electronic band gaps and transport properties in graphene superlattices with one-dimensional periodic potentials of square barriers

Abstract: The electronic transport properties and band structures for the graphene-based one-dimensional (1D) superlattices with periodic potentials of square barriers are investigated. It is found that a new Dirac point is formed, which is exactly located at the energy which corresponds to the zero (volume) averaged wave number inside the 1D periodic potentials. The location of such a new Dirac point is robust against variations in the lattice constant, and it is only dependent on the ratio of widths of the potential barriers. The zero-averaged wave-number gap associated with the new Dirac point is insensitive to both the lattice constant and the structural disorder and the defect mode in the zero-averaged wave-number gap is weakly dependent on the incident angles of carriers.

Update: Precision D_s decay constant from full lattice QCD using very fine lattices

Abstract: We update our previous determination of both the decay constant and the mass of the D s meson using the highly improved staggered quark formalism. We include additional results at two finer values of the lattice spacing along with improved determinations of the lattice spacing and improved tuning of the charm and strange quark masses. We obtain m Ds = 1.9691(32) GeV, in good agreement with experiment, and f Ds = 0.2480(25) GeV. Our result for f Ds is 1.6σ lower than the most recent experimental average determined from the D s leptonic decay rate and using V cs from Cabibbo-Kobayashi-Maskawa unitarity. Combining our f Ds with the experimental rate we obtain a direct determination of V cs = 1.010(22), or alternatively 0.990 +0.013 -0.016 using a probability distribution for statistical errors for this quantity which vanishes above 1. We also include an accurate prediction of the decay constant of the η c , f ηc = 0.3947(24) GeV, as a calibration point for other lattice calculations.

I-II-V half-Heusler compounds for optoelectronics: Ab initio calculations

Abstract: Half-Heusler compounds $XYZ$ crystallize in the space group $F\overline{4}3m$ and can be viewed as a zinc-blende-like ${(YZ)}^{\ensuremath{-}}$ lattice partially filled with He-like ${X}^{+}$ interstitials. In this work, we investigated I-II-V (eight-electrons) half-Heusler compounds by first-principles calculations in order to find suitable semiconductors for optoelectronics such as Cd-free buffer layer materials for chalcopyrite-based thin-film solar-cell devices. We report a systematic examination of band gaps and lattice parameters, depending on the electronegativities and the ion radii of the involved elements. Half-Heusler buffer materials should have a band gap of more than 2 eV to avoid absorption losses and a lattice constant of about $5.9\text{ }\text{\AA{}}$ to match the crystal structure of the absorber material. With these criteria we selected seven half-Heusler compounds as candidates for a buffer layer material.

Second-order Møller-Plesset perturbation theory applied to extended systems. II. Structural and energetic properties

Abstract: Results for the lattice constants, atomization energies, and band gaps of typical semiconductors and insulators are presented for Hartree–Fock and second-order Moller–Plesset perturbation theory (MP2). We find that MP2 tends to undercorrelate weakly polarizable systems and overcorrelates strongly polarizable systems. As a result, lattice constants are overestimated for large gap systems and underestimated for small gap systems. The volume dependence of the MP2 correlation energy and the dependence of the MP2 band gaps on the static dielectric screening properties are discussed in detail. Moreover, the relationship between MP2 and the G0W0 quasiparticle energies is elucidated and discussed. Finally, we demonstrate explicitly that the correlation energy diverges with decreasing k-point spacing for metals.

Size-Related Lattice Parameter Changes and Surface Defects in Ceria Nanocrystals

Abstract: Ceria (CeO2) has many important applications, notably in catalysis. Many of its uses rely on generating nanodimensioned particles. Ceria has important redox chemistry where Ce4+ cations can be reversibly reduced to Ce3+ cations and associated anion vacancies. The significantly larger size of Ce3+ (compared with Ce4+) has been shown to result in lattice expansion. Many authors have observed lattice expansion in nanodimensioned crystals (nanocrystals), and these have been attributed to the presence of stabilized Ce3+−anion vacancy combinations in these systems. Experimental results presented here show (i) that significant, but complex, changes in the lattice parameter with size can occur in 2−500 nm crystallites, (ii) that there is a definitive relationship between defect chemistry and the lattice parameter in ceria nanocrystals, and (iii) that the stabilizing mechanism for the Ce3+−anion vacancy defects at the surface of ceria nanocrystals is determined by the size, the surface status, and the analysis con...

Light meson physics from maximally twisted mass lattice QCD

Abstract: We present a comprehensive investigation of light meson physics using maximally twisted mass fermions for N f = 2 mass-degenerate quark flavours. By employing four values of the lattice spacing, spatial lattice extents ranging from 2.0 fm to 2.5 fm and pseudo scalar masses in the range 280 ≲ m PS ≲ 650MeV we control the major systematic effects of our calculation. This enables us to confront our N f = 2 data with SU(2) chiral perturbation theory and extract low energy constants of the effective chiral Lagrangian and derived quantities, such as the light quark mass.

Lattice, elastic, polarization, and electrostrictive properties of BaTiO3 from first-principles

Abstract: Predicting the domain structures and properties in both bulk single crystal and thin film ferroelectrics using the phase-field approach requires the knowledge of fundamental mechanical, electrical, and electromechanical coupling properties of a single-domain state. In this work, the elastic properties and structural parameters of cubic single crystals as well as tetragonal, orthorhombic, and rhombohedral BaTiO3 single domain states are obtained using first-principles calculations under the local density approximation. The calculated lattice constants, bulk modulus, and elastic constants are in good agreement with experiments for both the cubic paraelectric phase and the low-temperature ferroelectric phases. Spontaneous polarizations for all three ferroelectric phases and the electrostrictive coefficients of cubic BaTiO3 are also computed using the Berry’s phase approach, and the results agree well with existing experimentally measured values.

Materials Science and Engineering B

Abstract: CuInGaSe2 is a I–III–VI2 semiconducting material of tetragonal chalcopyrite structure. It is a very prominent absorber layer for photovoltaic devices. Particle-based coating process for CIGS is considered to be promising technique with relatively simple procedures and low initial investment. In the present work CIGS nanoparticle precursors suitable for screen-printing ink has been prepared by ball milling. High purity elemental copper granules, selenium and indium powders and fine chips of gallium were used as starting materials. First the ball milling was carried out for CuIn1−xGaxSe2 (x = 0.5) with (i) 10 ml of ethyl alcohol (ii) 5 ml of tetra ethylene glycol (wet) and (iii) 1 ml of ethylene diamine (semi-dry) for a milling time of 3 h and the results are not stoichiometric. In order to obtain an improved stoichiometric composition dry ball milling of elemental sources for three different compositions of CuIn1−xGaxSe2 (x = 0.25, 0.5 and 0.75) has been carried out. X-ray diffraction analysis revealed the presence of (1 1 2), (2 2 0)/(2 0 4), (3 1 2)/(1 1 6), (4 0 0) and (3 3 2) reflections for all the milled powders. These reflections correspond to chalcopyrite structure of CIGS. Shift in peaks towards higher value of 2� is observed with the increase in Ga composition. Average grain size calculated by Scherrer’s formula is found to be around 13 nm for the dry samples milled for 1.5 h and 7–8 nm for the samples wet milled for 3 h. Lattice constants ‘a’ and ‘c’ are found to decrease with the increase in concentration of Gallium. FESEM analysis revealed a strong agglomeration of the particles and the particle size varied from 11 to 30 nm for the dry-milled samples. Composition of milled powders has been studied by energy dispersive X-ray analysis. TEM analysis revealed the presence of nanocrystalline particles and SAED pattern corresponds to (1 1 2), (2 2 0)/(2 0 4), (5 1 2)/(4 1 7) and (6 2 0)/(6 0 4) diffraction peaks of CIGS. From the HRTEM analysis the d-spacing values were evaluated and found to be 1.06, 3.33, 2.03 and 0.906 A corresponding to the diffraction pattern. Also the planes corresponding to the nanoparticles have been simulated and matched with the HRTEM pattern. Raman spectra show the intense peak at 168–172 cm −1 , which corresponds to the chalcopyrite structure.

Atomic Layer Deposition of Al-doped ZnO Films: Effect of Grain Orientation on Conductivity

Abstract: Al-doped ZnO (AZO) films were deposited by atomic layer deposition (ALD) on borosilicate glass and sapphire(0001) substrates. The Al composition of the films was varied from 1% to 4% by controlling the ratio of Zn:Al pulses. Film resistivity was measured as a function of Al content and the substrate temperature used for ALD deposition. X-ray diffraction (XRD) was performed on the films, showing a reduction in lattice parameter, as a function of Al concentration, indicating that Al3+ ions occupy substitutional sites in the ZnO lattice. The resistivity of films deposited on sapphire substrates (7.7 × 10−4 Ω cm) was lower than that on glass (3.0 × 10−3 Ω cm), because of the formation of textured grains with the c-axis aligned with respect to the sapphire surface, as confirmed by XRD. The surface morphology of the films on glass and sapphire was compared using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), which showed similar grain sizes on each substrate, suggesting that the dif...

Wurtzite-structure Sc1-xAlxN solid solution films grown by reactive magnetron sputter epitaxy structural characterization and first-principles calculations

Abstract: AlN(0001) was alloyed with ScN with molar fractions up to ∼22%, while retaining a single-crystal wurtzite (w-) structure and with lattice parameters matching calculated values. Material synthesis was realized by magnetron sputter epitaxy of thin films starting from optimal conditions for the formation of w-AlN onto lattice-matched w-AlN seed layers on Al2O3(0001) and MgO(111) substrates. Films with ScN contents between 23% and ∼50% exhibit phase separation into nanocrystalline ScN and AlN, while ScN-rich growth conditions yield a transformation to rocksalt structure Sc1−xAlxN(111) films. The experimental results are analyzed with ion beam analysis, x-ray diffraction, and transmission electron microscopy, together with ab initio calculations of mixing enthalpies and lattice parameters of solid solutions in wurtzite, rocksalt, and layered hexagonal phases.

Systematic Comparison of Eight Substrates in the Growth of FeSe$_{0.5}$Te$_{0.5}$ Superconducting Thin Films

Abstract: We have investigated the crystal structures and superconducting properties of thin films of FeSe$_{0.5}$Te$_{0.5}$ grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch; rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio $c/a$. The best superconducting properties were observed in films on MgO and LaAlO$_3$ ($T_\mathrm{c}^\mathrm{zero}$ of 9.5 K). TEM observation showed that the presence or absence of an amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.

Quasifreestanding single-layer hexagonal boron nitride as a substrate for graphene synthesis

Abstract: We demonstrate that freeing a single-atom thick layer of hexagonal boron nitride (h-BN) from tight chemical bonding to a Ni(111) thin film grown on a W(110) substrate can be achieved by intercalation of Au atoms into the interface. This process has been systematically investigated using angle-resolved photoemission spectroscopy, x-ray photoemission, and absorption techniques. It has been demonstrated that the transition of the h-BN layer from the "rigid" into the "quasifreestanding" state is accompanied by a change in its lattice constant. Using chemical vapor deposition, graphene has been successfully synthesized on the insulating, quasifreestanding h-BN monolayer. We anticipate that the in situ synthesized weakly interacting graphene/h-BN double layered system could be further developed for technological applications and may provide perspectives for further inquiry into the unusual electronic properties of graphene. (Less)

Systematic Comparison of Eight Substrates in the Growth of FeSe0.5Te0.5 Superconducting Thin Films

Abstract: We have investigated the crystal structures and superconducting properties of thin films of FeSe0.5Te0.5 grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch; rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio c/a. The best superconducting properties were observed in films on MgO and LaAlO3 (Tczero of 9.4 K). Transmission electron microscopy observation showed that the presence or absence of an amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.