104 research outputs found
Bulk and surface electronic structure of rare earth metals.
The basic properties of the rare earth metals, including single crystal growth, crystal
and magnetic structures, and the relationship between electronic and magnetic structure, are
reviewed. The problems encountered by the theoretical treatment of the partially occupied, but
highly localised, lanthanide 4f levels as bands are discussed, and bandstructure calculations
presented for the hexagonal close-packed rare earths. These are compared with available
experimental and theoretical data. It is suggested that the exchange-splitting of the lanthanide
valence bands may well persist in the paramagnetic state, and that account should be taken of
the localised 4f moments in future calculations. The difficulties associated with the preparation
of clean single crystal rare earth surfaces are described. The origin of the surface-orderdependent
state seen in angle-resolved UV photoemission (ARUPS) spectra from rare earth
(0/001) surfaces is discussed. (7 x 1) reconstructions of the (1120) surfaces of Ho, Er and Y
are reported, with the resulting surface geometric and electronic structure being
indistinguishable from those of the ideal (0001) structure. Momentum-resolved inverse
photoernission measurements are presented for Y(000l), with results in good agreement with
the calculated bandstructure. A comprehensive ARUPS study of the valence band of Ho(OOOl)
is reported, and the results demonstrated to be entirely explicable in terms of emission from
one-electron states. ARUPS data from Y(000l), Gd(000l) and Tb(000l) are presented,
discussed in the light of the Ho results, and the conclusions of previous ARUPS studies of
these surfaces revealed to be in error. Essentially similar ARUPS features are seen on all hcp
rare earth (0001) surfaces so far studied and it is suggested that all other such surfaces will
show the same features. The Ho(000l) 5p levels are shown to have significant band character,
suggesting that further refinements to the band structure calculations are required
Surface morphology and optical properties of thin films of thiophene-based binary blends
We report on the relationship between optical and morphological properties of thin films of a blend of thiophene-based monomer and a diamine derivative. This system is of interest as it gives rise to white emission, due to the formation of exciplex energy levels in the solid state. The photoluminescence emission has been collected for different relative concentrations of the two blend components, and the film topography has been mapped correspondingly by means of atomic force microscopy. Along with a complex evolution of the film surface morphology, a significant variation of the emission properties has been observed upon change of the blend composition. In particular, the intensity of the broad low-energy exciplex band turns out to be affected by a balance of two factors, namely, the extension of homogeneous blend film areas, and their effective concentration
White light emission from blends of blue-emitting organic molecules: A general route to the white organic light-emitting diode?
We show that all possible binary combinations of molecules from four different families of organics - a diamine derivative, N,N′-bis(3methylphenyl)-N,N′-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole) - produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes
Self-assembled monolayers of cobalt(II)- (4-tert-butylphenyl)-porphyrins: the influence of the electronic dipole on scanning tunneling microscopy images.
Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-teit-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction. but with bias-dependent contrast images and asymmetric W characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70degrees with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model
X-ray excited visible luminescence spectroscopy of organic materials using a portable optical spectrometer
The use of a portable video telescope, mounted externally to a beamline endstation, to obtain synchrotron-radiation-excited visible luminescence, is described. Real-time video monitoring permits simple and quick alignment, and allows a visual record of the luminescence experiment. The telescope is fibre-optic-coupled to an optical spectrometer. Examples are given of X-ray excited optical spectroscopy from organic materials for light-emitting-diode applications
Zinc Porphyrin‐Driven Assembly of Gold Nanofingers
Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges
Limits on Active to Sterile Neutrino Oscillations from Disappearance Searches in the MINOS, Daya Bay, and Bugey-3 Experiments
Searches for a light sterile neutrino have been performed independently by the MINOS and the Daya Bay experiments using the muon (anti) neutrino and electron antineutrino disappearance channels, respectively. In this Letter, results from both experiments are combined with those from the Bugey-3 reactor neutrino experiment to constrain oscillations into light sterile neutrinos. The three experiments are sensitive to complementary regions of parameter space, enabling the combined analysis to probe regions allowed by the Liquid Scintillator Neutrino Detector (LSND) and MiniBooNE experiments in a minimally extended four-neutrino flavor framework. Stringent limits on sin(2) 2 theta(mu e) are set over 6 orders of magnitude in the sterile mass-squared splitting Delta m(41)(2). The sterile-neutrino mixing phase space allowed by the LSND and MiniBooNE experiments is excluded for Delta m(41)(2) \u3c 0.8 eV(2) at 95% CLs
Measurements of meson production in relativistic heavy-ion collisions at RHIC
We present results for the measurement of meson production via its
charged kaon decay channel in Au+Au collisions at
, 130, and 200 GeV, and in and +Au collisions
at GeV from the STAR experiment at the BNL Relativistic
Heavy Ion Collider (RHIC). The midrapidity () meson transverse
momentum () spectra in central Au+Au collisions are found to be well
described by a single exponential distribution. On the other hand, the
spectra from , +Au and peripheral Au+Au collisions show power-law tails
at intermediate and high and are described better by Levy
distributions. The constant yield ratio vs beam species, collision
centrality and colliding energy is in contradiction with expectations from
models having kaon coalescence as the dominant mechanism for production
at RHIC. The yield ratio as a function of is consistent
with a model based on the recombination of thermal quarks up to GeV/, but disagrees at higher transverse momenta. The measured nuclear
modification factor, , for the meson increases above unity at
intermediate , similar to that for pions and protons, while is
suppressed due to the energy loss effect in central Au+Au collisions. Number of
constituent quark scaling of both and for the meson
with respect to other hadrons in Au+Au collisions at =200 GeV
at intermediate is observed. These observations support quark
coalescence as being the dominant mechanism of hadronization in the
intermediate region at RHIC.Comment: 22 pages, 21 figures, 4 table
Strangelet Search in AuAu Collisions at 200 GeV
We have searched for strangelets in a triggered sample of 61 million central
(top 4%) Au+Au collisions at \sNN = 200 GeV near beam rapidities at the STAR
detector. We have sensitivity to metastable strangelets with lifetimes of order
, in contrast to limits over ten times longer in AGS studies and
longer still at the SPS. Upper limits of a few 10^{-6} to 10^{-7} per central
Au+Au collision are set for strangelets with mass GeV/c^{2}.Comment: As publishe
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