3,770 research outputs found
Real-space anisotropy of the superconducting gap in the charge-density wave material 2H-NbSe2
We present a scanning tunneling microscopy (STM) and ab-initio study of the anisotropic superconductivity of 2H-NbSe2 in the charge-density-wave (CDW) phase. Differential-conductance spectra show a clear double-peak structure, which is well reproduced by density functional theory simulations enabling full k- and real-space resolution of the superconducting gap. The hollow-centered (HC) and chalcogen-centered (CC) CDW patterns observed in the experiment are mapped onto separate van der Waals layers with different electronic properties. We identify the CC layer as the high-gap region responsible for the main STM peak. Remarkably, this region belongs to the same Fermi surface sheet that is broken by the CDW gap opening. Simulations reveal a highly anisotropic distribution of the superconducting gap within single Fermi sheets, setting aside the proposed scenario of a two-gap superconductivity. Our results point to a spatially localized competition between superconductivity and CDW involving the HC regions of the crystal
Novel Reconstruction Mechanism for Dangling-Bond Minimization: Combined Method Surface Structure Determination of SiC(111)-(3×3)
The SiC(111)−(3×3) phase was analyzed by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) holography, density functional theory (DFT), and conventional LEED. A single adatom per unit cell found in STM acts as a beam splitter for the holographic inversion of discrete LEED spot intensities. The resulting 3D image guides the detailed analyses by LEED and DFT which find a Si tetramer on a twisted Si adlayer with cloverlike rings. This twist model with one dangling bond left per unit cell represents a novel (n×n)-reconstruction mechanism of group-IV (111) surfaces
About direct Dark Matter detection in Next-to-Minimal Supersymmetric Standard Model
Direct dark matter detection is considered in the Next-to-Minimal
Supersymmetric Standard Model (NMSSM). The effective neutralino-quark
Lagrangian is obtained and event rates are calculated for the Ge-73 isotope.
Accelerator and cosmological constraints on the NMSSM parameter space are
included. By means of scanning the parameter space at the Fermi scale we show
that the lightest neutralino could be detected in dark matter experiments with
sizable event rate.Comment: latex, 12 pages, 2 ps-figures; extra LEP constraint is included,
extra figure is added, recorrected version, resubmitted to Phys.Rev.
Collider signals from slow decays in supersymmetric models with an intermediate-scale solution to the mu problem
The problem of the origin of the mu parameter in the Minimal Supersymmetric
Standard Model can be solved by introducing singlet supermultiplets with
non-renormalizable couplings to the ordinary Higgs supermultiplets. The
Peccei-Quinn symmetry is broken at a scale which is the geometric mean between
the weak scale and the Planck scale, yielding a mu term of the right order of
magnitude and an invisible axion. These models also predict one or more singlet
fermions which have electroweak-scale masses and suppressed couplings to MSSM
states. I consider the case that such a singlet fermion, containing the axino
as an admixture, is the lightest supersymmetric particle. I work out the
relevant couplings in several of the simplest models of this type, and compute
the partial decay widths of the next-to-lightest supersymmetric particle
involving leptons or jets. Although these decays will have an average proper
decay length which is most likely much larger than a typical collider detector,
they can occasionally occur within the detector, providing a striking signal.
With a large sample of supersymmetric events, there will be an opportunity to
observe these decays, and so gain direct information about physics at very high
energy scales.Comment: 24 pages, LaTeX, 4 figure
Singular del Pezzo surfaces that are equivariant compactifications
We determine which singular del Pezzo surfaces are equivariant
compactifications of G_a^2, to assist with proofs of Manin's conjecture for
such surfaces. Additionally, we give an example of a singular quartic del Pezzo
surface that is an equivariant compactification of a semidirect product of G_a
and G_m.Comment: 14 pages, main result extended to non-closed ground field
HIGGS PHENOMENOLOGY OF THE SUPERSYMMETRIC MODEL WITH A GAUGE SINGLET
We discuss the Higgs sector of the supersymmetric standard model extended by
a gauge singlet for the range of parameters, which is compatible with universal
soft supersymmetry breaking terms at the GUT scale. We present results for the
masses, couplings and decay properties of the lightest Higgs bosons, in
particular with regard to Higgs boson searches at LEP. The prospects differ
significantly from the ones within the MSSM.Comment: 12 pages (Plain Tex), 7 fig
Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-Tetramethoxychalcogenanthrene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1:1 charger-transfer complexes
[EN] 2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-chalcogena-cyclo-diveratrylenes, 'Vn(2)E(2)', E = S, Se) form isotypical 1:1 charge-transfer (CT) complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). X-ray analysis of Vn(2)S(2) . DDQ shows the compound to have a columnar structure with segregated stacks of donors and acceptors. The donors are virtually planar in accordance with a formulation of [Vn(2)E(2)](+)[DDQ](-). Donor cations and acceptor anions are equidistant in their respective stacks, but in each case they inclined to the stacking axis, nevertheless guaranteeing an optimum overlap of the half-filled frontier orbitals which are of pi-type character according to MNDO calculations. Dielectric ac measurements of permittivity epsilon' and loss factor E '' clearly reveal two processes, a dielectric one at low temperatures and a conductive one at high temperatures. The dielectric process can be described by the Havriliak-Negami (HN) and the Kohlrausch-Williams-Watts (KWW) model, and the conductive process by a Debye-type plot. Using these methods, the relevant parameters are evaluated. The de conductivities of polycrystalline samples moulded at 10(8) Pa show a temperature dependence in the plots of ln sigma vs. T-1, which is typical of semiconductors. Two slopes are found; that in the low-temperature region (<285 K) is explained by an easy-path model (intragrain conductivity with low activation energies), whereas in the high-temperature region conduction across the grain boundaries (with higher activation energies) is becoming predominant. The activation energies for the intrinsic conductivities obtained by the ac and de measurements are similar. Despite the columnar structure with segregated stacks, due to stoichiometric oxidation states of the components, the absolute values of conductivity are low ten. 10(-6) S cm(-1) at 293 K), though higher (by a factor of ca. 10(3)) than those of compounds like Vn(2)E(2) . TCNQ with stacks in which donor and acceptor molecules alternate.Behrens, U.; Díaz Calleja, R.; Dötze, M.; Franke, U.; Gunsser, W.; Klar, G.; Kudnig, J.... (1996). Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-Tetramethoxychalcogenanthrene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1:1 charger-transfer complexes. Journal of Materials Chemistry. 6(4):547-553. https://doi.org/10.1039/JM9960600547S54755364Behrens, J., Hinrichs, W., Link, T., Schiffling, C., & Klar, G. (1995). SELFSTACKING SYSTEMS, PART 6.1HOST LATTICE FUNCTION OF 2,3,8,9-TETRAMETHOXYDIBENZO[c,e][1,2]-DICHALCOGENINS IN THEIR ELECTRICALLY CONDUCTING IODINE COMPLEXES. Phosphorus, Sulfur, and Silicon and the Related Elements, 101(1-4), 235-244. doi:10.1080/10426509508042522Berges, P., Kudnig, J., Klar, G., Martínez, E. S., & Calleja, R. D. (1989). Elementorganische Verbindungen mit o-Phenylenresten, XVI . 2:1-Komplexe von 2,3,7,8-Tetramethoxychalcogenanthrenen mit Tetracyanethen / Organometallic Compounds with o-Phenylene Substituents, Part XVI 2:1-Complexes of 2,3,7,8-Tetramethoxychalcogenanthrenes with Tetracyanoethene. Zeitschrift für Naturforschung B, 44(2), 211-219. doi:10.1515/znb-1989-0219Hinrichs, W., Berges, P., Klar, G., Sánchez-Martínez, E., & Gunsser, W. (1987). Structure and electrical conductivity of TCNQ-2,3,7,8-tetramethoxychalcogenanthrene complexes. Synthetic Metals, 20(3), 357-364. doi:10.1016/0379-6779(87)90832-0Sánchez Martínez, E., Díaz Calleja, R., Gunsser, W., Berges, P., & Klar, G. (1989). Structure and dielectric relaxation of 2,3,7,8-tetramethoxychalcogenanthrene-TCNQ complexes. Synthetic Metals, 30(1), 67-78. doi:10.1016/0379-6779(89)90642-5Gunßer, W., Henning, J. H., Klar, G., & Martínez, E. S. (1989). Spin Density and Magnetic Susceptibility of Charge-Transfer Complexes with Chalkogenanthrene Donors. Berichte der Bunsengesellschaft für physikalische Chemie, 93(11), 1370-1373. doi:10.1002/bbpc.19890931148G. M. Sheldrick , SHELXTL-PLUS, Release 4.21/0, Siemens Analytical X-Ray Instruments, 1990.Bock, H., Rauschenbach, A., Näther, C., Havlas, Z., Gavezzotti, A., & Filippini, G. (1995). Orthorhombisches und monoklines 2,3,7,8-Tetramethoxythianthren: kleiner Strukturunterschied – große Gitteränderung. Angewandte Chemie, 107(1), 120-122. doi:10.1002/ange.19951070132Bock, H., Rauschenbach, A., Näther, C., Havlas, Z., Gavezzotti, A., & Filippini, G. (1995). Orthorhombic and Monoclinic 2,3,7,8-Tetramethoxythianthrene: Small Structural Difference–Large Lattice Change. Angewandte Chemie International Edition in English, 34(1), 76-78. doi:10.1002/anie.199500761Hinrichs, W., Berges, P., & Klar, G. (1987). Selbststapelnde Systeme, IV 2,3,7,8-Tetramethoxythianthreniumsalze/Selfstacking Systems, Part IV 2.3.7.8-Tetramethoxythianthrenium Salts. Zeitschrift für Naturforschung B, 42(2), 169-176. doi:10.1515/znb-1987-0209Peover, M. E. (1962). 879. A polarographic investigation into the redox behaviour of quinones: the roles of electron affinity and solvent. Journal of the Chemical Society (Resumed), 4540. doi:10.1039/jr9620004540Wheland, R. C., & Gillson, J. L. (1976). Synthesis of electrically conductive organic solids. Journal of the American Chemical Society, 98(13), 3916-3925. doi:10.1021/ja00429a030Zanotti, G., Del Pra, A., & Bozio, R. (1982). Structure of tetraethylammonium–2,3-dichloro-5,6-dicyano-p-benzoquinone. Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 38(4), 1225-1229. doi:10.1107/s0567740882005330Zanotti, G., Bardi, R., & Del Pra, A. (1980). Structure of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 36(1), 168-171. doi:10.1107/s0567740880002750Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Cleveland, OH, 1977–1978, 58th edn., p. D–178.Sánchez Martínez, E., Díaz Calleja, R., Berges, P., Kudnig, J., & Klar, G. (1989). Structure, electrical conductivity and dielectric relaxation of a 1,2-dimethoxybenzene-tetracyanoethene 1:1 complex. Synthetic Metals, 32(1), 79-89. doi:10.1016/0379-6779(89)90831-xÅsbrink, L., Fridh, C., & Lindholm, E. (1977). HAM/3, a semi-empirical MO theory. I. The SCF method. Chemical Physics Letters, 52(1), 63-68. doi:10.1016/0009-2614(77)85121-xÅsbrink, L., Fridh, C., & Lindholm, E. (1977). HAM/3, a semi-empirical MO theory. III. Unoccupied orbitals. Chemical Physics Letters, 52(1), 72-75. doi:10.1016/0009-2614(77)85123-3Dewar, M. J. S., & Thiel, W. (1977). Ground states of molecules. 38. The MNDO method. Approximations and parameters. Journal of the American Chemical Society, 99(15), 4899-4907. doi:10.1021/ja00457a004Dewar, M. J. S., & Thiel, W. (1977). Ground states of molecules. 39. MNDO results for molecules containing hydrogen, carbon, nitrogen, and oxygen. Journal of the American Chemical Society, 99(15), 4907-4917. doi:10.1021/ja00457a005Åsbrink, L., Fridh, C., & Lindholm, E. (1978). Valence excitation of linear molecules.I. Excitation and UV spectra of N2, Co, acetylene and HCN. Chemical Physics, 27(2), 159-168. doi:10.1016/0301-0104(78)88001-xFridh, C., Åsbrink, L., & Lindholm, E. (1978). Valence excitation of linear molecules. II. Excitation and UV spectra of C2N2, CO2 and N2O. Chemical Physics, 27(2), 169-181. doi:10.1016/0301-0104(78)88002-1Lindholm, E., Bieri, G., Åsbrink, L., & Fridh, C. (1978). Interpretation of electron spectra. III. Spectra of formamide, studied withHAM/3. International Journal of Quantum Chemistry, 14(6), 737-740. doi:10.1002/qua.560140605Starkweather, H. W. (1981). Simple and complex relaxations. Macromolecules, 14(5), 1277-1281. doi:10.1021/ma50006a025Starkweather, H. W. (1990). Distribution of activation enthalpies in viscoelastic relaxations. Macromolecules, 23(1), 328-332. doi:10.1021/ma00203a056Havriliak, S., & Negami, S. (1967). A complex plane representation of dielectric and mechanical relaxation processes in some polymers. Polymer, 8, 161-210. doi:10.1016/0032-3861(67)90021-3J. Ross McDonald , Complex Nonlinear Least Squares Immitance Fitting Program, LEVM6, 1993;Impedance Spectroscopy, Wiley-Interscience, New York, 1987.Williams, G. (1978). Time-correlation functions and molecular motion. Chemical Society Reviews, 7(1), 89. doi:10.1039/cs9780700089Williams, G., & Watts, D. C. (1970). Non-symmetrical dielectric relaxation behaviour arising from a simple empirical decay function. Transactions of the Faraday Society, 66, 80. doi:10.1039/tf9706600080A. R. West , Solid State Chemistry and its Applications, Wiley, Chichester, 1984, ch. 13.Sánchez Martínez, E., Díaz Calleja, R., & Klar, G. (1990). Self-stacking systems 5. Electrical and dielectric properties of 5,5-dibromo-2,3,7,8-tetramethoxyselenanthrene. Synthetic Metals, 38(1), 93-98. doi:10.1016/0379-6779(90)90071-
Production and Decay of Neutralinos in the Next-To-Minimal Supersymmetric Standard Model
Within the framework of the Next-To-Minimal Supersymmetric Standard Model
(NMSSM) we study neutralino production () at center-of-mass energies between 100
and 600 GeV and the decays of the heavier neutralinos into the LSP plus a
fermion pair, a photon or a Higgs boson. For representative gaugino/higgsino
mixing scenarios, where the light neutralinos have significant singlet
components, we find some striking differences between the NMSSM and the minimal
supersymmetric model. Since in the NMSSM neutralino and Higgs sector are
strongly correlated, the decay of the second lightest neutralino into a Higgs
boson and the LSP often is kinematically possible and even dominant in a large
parameter region of typical NMSSM scenarios. Also, the decay rates into final
states with a photon may be enhanced.Comment: 36 pages, latex, 10 uuendcoded figures, complete ps file available at
ftp://ftp.physik.uni-wuerzburg.de/pub/preprint/WUE-ITP-95-021.ps.g
Possibility of spontaneous CP violation in the nonminimal supersymmetric standard model with two neutral Higgs singlets
A supersymmetric standard model with two Higgs doublets and two Higgs
singlets is investigated if it can accommodate the possibility of spontaneous
CP violation. Assuming the degeneracy of the scalar quark masses of the third
generation, we find that spontaneous CP violation in the Higgs sector is viable
in our model. In the case of spontaneous CP violation, the masses of the
lightest two neutral Higgs bosons are estimated to be 80 and 125 GeV for some
parameter values in our model, which, are consistent with LEP2 data.Comment: 18 pages, 3figure
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