135 research outputs found
Thermodynamic stabilities of ternary metal borides: An ab initio guide for synthesizing layered superconductors
Density functional theory calculations have been used to identify stable
layered Li--B crystal structure phases derived from a recently proposed
binary metal-sandwich (MS) lithium monoboride superconductor. We show that the
MS lithium monoboride gains in stability when alloyed with electron-rich metal
diborides; the resulting ordered LiB ternary phases may form
under normal synthesis conditions in a wide concentration range of for a
number of group-III-V metals . In an effort to pre-select compounds with the
strongest electron-phonon coupling we examine the softening of the in-plane
boron phonon mode at in a large class of metal borides. Our results
reveal interesting general trends for the frequency of the in-plane boron
phonon modes as a function of the boron-boron bond length and the valence of
the metal. One of the candidates with a promise to be an MgB-type
superconductor, LiAlB, has been examined in more detail: according to
our {\it ab initio} calculations of the phonon dispersion and the
electron-phonon coupling , the compound should have a critical
temperature of K.Comment: 10 pages, 9 figures, submitted to PR
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
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
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
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
Magnetic correlations in the triangular antiferromagnet FeGa2 S4
The crystal structure and magnetic correlations in triangular antiferromagnet FeGa2S4 are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction, and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at 0.77Ã…-1 correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30-50 K. The minor sites contribute to the magnetic peak at 0.6Ã…-1, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modeling of the inelastic neutron spectrum within linear spin-wave theory results in the set of exchange couplings J1=1.7,J2=0.9,J3=0.8meV for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model.Fil: Guratinder, K.. Universidad de Ginebra; SuizaFil: Schmidt, M.. Max-Planck-Institut für Chemische Physik fester Stoffe; AlemaniaFil: Walker, H. C.. Rutherford Appleton Laboratory; Reino UnidoFil: Bewley, R.. Rutherford Appleton Laboratory; Reino UnidoFil: Wörle, M.. Laboratorium für Anorganische Chemie; SuizaFil: Cabra, Daniel Carlos. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - La Plata. Instituto de FÃsica de LÃquidos y Sistemas Biológicos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FÃsica de LÃquidos y Sistemas Biológicos; ArgentinaFil: Osorio, Santiago Antonio. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y NanotecnologÃa. Unidad Ejecutora Instituto de Nanociencia y NanotecnologÃa - Nodo Bariloche | Comisión Nacional de EnergÃa Atómica. Unidad Ejecutora Instituto de Nanociencia y NanotecnologÃa. Unidad Ejecutora Instituto de Nanociencia y NanotecnologÃa - Nodo Bariloche; ArgentinaFil: Villalba, Maria Luisa. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - La Plata. Instituto de FÃsica de LÃquidos y Sistemas Biológicos. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FÃsica de LÃquidos y Sistemas Biológicos; ArgentinaFil: Madsen, A. K.. Paul Scherrer Institute; SuizaFil: Keller, L.. Paul Scherrer Institute; SuizaFil: Wildes, A.. Institut Laue Langevin; FranciaFil: Puphal, P.. Institut Laue Langevin; FranciaFil: Cervellino, A.. Institut Laue Langevin; FranciaFil: Rüegg, Ch.. Universidad de Ginebra; SuizaFil: Zaharko, O.. Universidad de Ginebra; Suiz
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
Water-Soluble Fullerene (C60) Derivatives as Nonviral Gene-Delivery Vectors
A new class of water-soluble C60 transfecting agents has been prepared using Hirsch-Bingel chemistry and assessed for their ability to act as gene-delivery vectors in vitro. In an effort to elucidate the relationship between the hydrophobicity of the fullerene core, the hydrophilicity of the water-solubilizing groups, and the overall charge state of the C60 vectors in gene delivery and expression, several different C60 derivatives were synthesized to yield either positively charged, negatively charged, or neutral chemical functionalities under physiological conditions. These fullerene derivatives were then tested for their ability to transfect cells grown in culture with DNA carrying the green fluorescent protein (GFP) reporter gene. Statistically significant expression of GFP was observed for all forms of the C60 derivatives when used as DNA vectors and compared to the ability of naked DNA alone to transfect cells. However, efficient in vitro transfection was only achieved with the two positively charged C60 derivatives, namely, an octa-amino derivatized C60 and a dodeca-amino derivatized C60 vector. All C60 vectors showed an increase in toxicity in a dose-dependent manner. Increased levels of cellular toxicity were observed for positively charged C60 vectors relative to the negatively charged and neutral vectors. Structural analyses using dynamic light scattering and optical microscopy offered further insights into possible correlations between the various derivatized C60 compounds, the C60 vector/DNA complexes, their physical attributes (aggregation, charge) and their transfection efficiencies. Recently, similar Gd@C60-based compounds have demonstrated potential as advanced contrast agents for magnetic resonance imaging (MRI). Thus, the successful demonstration of intracellular DNA uptake, intracellular transport, and gene expression from DNA using C60 vectors suggests the possibility of developing analogous Gd@C60-based vectors to serve simultaneously as both therapeutic and diagnostic agents
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