106 research outputs found
LiBC by polarized Raman spectroscopy: Evidence for lower crystal symmetry ?
The paper presents polarized Raman scattering study on a few-micron-size
crystallite of LiBC with natural faces. The experiment on as grown sample has
revealed a four lattice modes with frequencies at 1276 cm^-1, 830 cm^-1, 546
cm^-1 and 170 cm^-1, respectively. The number of observed Raman lines and their
selection rules are incompatible with the assumed D6h symmetry. The modes at
1276 cm^-1 and 170 cm^-1 correspond to the expected Raman active modes. In
contrast with the superconducting compound MgB2, the B-C bond stretching mode
(at 1276 cm^-1) has rather small damping. The two "forbidden" modes (at 830
cm^-1 and 546 cm^-1) disappeared after subsequent thermal treatment.Comment: 4 pages, LaTeX, complementary experimental resul
Electronic and optical properties of LiBC
LiBC, a semiconducting ternary borocarbide constituted of the lightest
elements only, has been synthesized and characterized by x-ray powder
diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing
an infrared microscope the phonon spectrum has been investigated in single
crystals. The in-plane B-C stretching mode has been detected at 150 meV,
noticeably higher than in AlB2, a non-superconducting isostructural analog of
MgB2. It is this stretching mode, which reveals a strong electron-phonon
coupling in MgB2, driving it into a superconducting state below 40 K, and is
believed to mediate predicted high-temperature superconductivity in hole-doped
LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88,
127001 (2002)].Comment: 4 pages, 4 figure
Born Effective Charges and Infrared Response of LiBC
Calculations of the zone center optical mode frequencies (including LO-TO
splitting), Born effective charges Z for each atom,
dielectric constants and , and the dielectric
response in the infrared, using density functional linear response theory, are
reported. Calculated Raman modes are in excellent agreement with experimental
values (170 cm and 1170 cm), while it will require better
experimental data to clarify the infrared active mode frequencies. The Born
effective charges Z (i) have surprisingly different values
for B and C, and (ii) show considerable anisotropy. Relationships between the
effective charges and LO-TO splitting are discussed, and the predicted
reflectivity in the range 0 -- 1400 cm is presented. These results hold
possible implications for Li removal in LiBC, and C substition for B in
MgB.Comment: 6 pages, 3 figure
Detailed electronic structure studies on superconducting MgB and related compounds
In order to understand the unexpected superconducting behavior of MgB
compound we have made electronic structure calculations for MgB and closely
related systems. Our calculated Debye temperature from the elastic properties
indicate that the average phonon frequency is very large in MgB compared
with other superconducting intermetallics and the exceptionally high in
this material can be explained through BCS mechanism only if phonon softening
occurs or the phonon modes are highly anisotropic. We identified a
doubly-degenerate quasi-two dimensional key-energy band in the vicinity of
along -A direction of BZ which play an important role in
deciding the superconducting behavior of this material. Based on this result,
we have searched for similar kinds of electronic feature in a series of
isoelectronic compounds such as BeB, CaB, SrB, LiBC and
MgBC and found that MgBC is one potential material from the
superconductivity point of view. There are contradictory experimental results
regarding the anisotropy in the elastic properties of MgB ranging from
isotropic, moderately anisotropic to highly anisotropic. In order to settle
this issue we have calculated the single crystal elastic constants for MgB
by the accurate full-potential method and derived the directional dependent
linear compressibility, Young's modulus, shear modulus and relevant elastic
properties. We have observed large anisotropy in the elastic properties. Our
calculated polarized optical dielectric tensor shows highly anisotropic
behavior even though it possesses isotropic transport property. MgB
possesses a mixed bonding character and this has been verified from density of
states, charge density and crystal orbital Hamiltonian population analyses
Anionic 1-Aza-3,4-diphospholides as redox active ligands
ISSN:0020-1693ISSN:1873-325
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Fischer-type gold(I) carbene complexes stabilized by aurophilic interactions
The synthesis and structure of rare acyclic alkoxy- and aminocarbene complexes of gold(I) are reported, including a novel ferrocenophane dinuclear biscarbene complex. X-Ray
diffraction analyses and DFT calculations reveal that these complexes are stabilized by genuine aurophilic interactions.National Research Foundation,South Africa (D.I.B., Grant number 76226), and the Spanish MICINN and CAM (I.F., Grants CTQ2010-20714-CO2-01/BQU, Consolider-Ingenio 2010, CSD2007-00006,S2009/PPQ-1634).http://www.rsc.org/dalton2015-02-28hb201
Short Naphthalene Organophosphonate Linkers to Microporous Frameworks
We report two novel 3D porous metal-organophosphonate metal organic frameworks (MOFs) [{Cu(4, 4’-bpy)0.5(1,4-NDPA-H2)] (1), [{Cu2(4,4’-bpy)0.5}(1,4-NDPA)] (2) and a non-porous [{Cu(4, 4’-bpy)}(2,6-NDPA-H2)] (3) constructed using the structurally rigid 1,4-naphthalenediphosphonic acid (1,4-NDPA-H4) and 2,6-naphthalenediphosphonic acid (2,6-NDPA-H4). 1 and 2 exhibit high surface areas obtained using the structurally rigid and short aromatic organophosphonate linkers with copper. The compound 1 has been further analyzed by TGA and Quantum Design PPMS vibrating sample magnetometer
A novel multiplex detection array revealed systemic complement activation in oral squamous cell carcinoma
Oral squamous cell carcinoma (OSCC) is one of the most common tumors within the oral cavity. Early diagnosis and prognosis tools are urgently needed. This study aimed to investigate the activation of the complement system in OSCC patients as potential biomarker. Therefore, an innovative complement activation array was developed. Characterized antibodies detecting the complement activation specific epitopes C3a, C5a and sC5b-9 along with control antibodies were implemented into a suspension bead array. Human serum from a healthy (n = 46) and OSCC patient (n = 57) cohort were used to investigate the role of complement activation in oral tumor progression. The novel multiplex assay detected C3a, C5a and sC5b-9 from a minimal sample volume of human tears, aqueous humor and blood samples. Limits of detection were 0.04 ng/mL for C3a, 0.03 ng/mL for C5a and 18.9 ng/mL for sC5b-9, respectively. Biological cut-off levels guaranteed specific detections from serum. The mean serum concentration of a healthy control cohort was 680 ng/mL C3a, 70 ng/mL C5a and 2247 ng/mL sC5b-9, respectively. The assay showed an intra-assay precision of 2.9-6.4% and an inter-assay precision of 9.2-18.2%. Increased systemic C5a (p < 0.0001) and sC5b-9 (p = 0.01) concentrations in OSCC patients were determined using the validated multiplex complement assay. Higher C5a concentrations correlated with tumor differentiation and OSCC extension state. Systemic sC5b-9 determination provided a novel biomarker for infiltrating tumor growth and C3a levels were associated with local tumor spreading. Our study suggests that systemic complement activation levels in OSCC patients may be useful to assess disease progression
Emerging methods and tools for environmental risk assessment, decision-making, and policy for nanomaterials: summary of NATO Advanced Research Workshop
Nanomaterials and their associated technologies hold promising opportunities for the development of new materials and applications in a wide variety of disciplines, including medicine, environmental remediation, waste treatment, and energy conservation. However, current information regarding the environmental effects and health risks associated with nanomaterials is limited and sometimes contradictory. This article summarizes the conclusions of a 2008 NATO workshop designed to evaluate the wide-scale implications (e.g., benefits, risks, and costs) of the use of nanomaterials on human health and the environment. A unique feature of this workshop was its interdisciplinary nature and focus on the practical needs of policy decision makers. Workshop presentations and discussion panels were structured along four main themes: technology and benefits, human health risk, environmental risk, and policy implications. Four corresponding working groups (WGs) were formed to develop detailed summaries of the state-of-the-science in their respective areas and to discuss emerging gaps and research needs. The WGs identified gaps between the rapid advances in the types and applications of nanomaterials and the slower pace of human health and environmental risk science, along with strategies to reduce the uncertainties associated with calculating these risks
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