233 research outputs found
Comparative chromosome band mapping in primates byin situ suppression hybridization of band specific DNA microlibraries
A DNA-library established from microdissected bands 8q23 to 8q24.1 of normal human chromosomes 8 (Lüdecke et al., 1989) was used as a probe for chromosomal in situ suppression (CISS-) hybridization to metaphase chromosomes of man and primates including Hylobates lar and Macaca fuscata. Comparative band mapping as first applied in this study shows the specific visualization of a single subchromosomal region in all three species and thus demonstrates that synteny of the bulk sequences of a specific human chromosome subregion has been conserved for more than 20 million years
Superexchange in the quarter- filled two- leg ladder system NaV2O5
A theory of superexchange in the mixed valent layer compound NaV2O5 is
presented which provides a consistent description of exchange both in the
disordered and charge ordered state. Starting from results of band structure
calculations for NaV2O5 first an underlying electronic model for a ladder unit
in the Trellis lattice is formulated. By using the molecular orbital
representation for intra-rung electronic states a second-order perturbation
procedure is developed and an effective spin-chain model for a ladder is
derived. Variation of the resulting superexchange integral J is examined
numerically as the ladder system evolves from a charge disordered to the
extreme ('zig-zag') charge ordered state. It is found that the effective intra-
ladder superexchange is always antiferromagnetic.Comment: 18 pages Revtex, 7 Postscript figure
Zigzag Charge Ordering in alpha'-NaV2O5
23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline
sample are reported. In the charge-ordered phase, the number of inequivalent Na
sites observed is more than that expected from the low-temperature structures
of space group Fmm2 reported so far. This disagreement indicates that the real
structure including both atomic displacement and charge disproportionation is
of lower symmetry. It is suggested that zigzag ordering is the most probable.
The temperature variation of the NMR spectra near the transition temperature is
incompatible with that of second-order transitions. It is thus concluded that
the charge ordering transition is first-order.Comment: 4 pages, 5 eps figures, submitted to J. Phys. Soc. Jp
Discussion of a spin-cluster model for the low temperature phase of NaV_2O_5
We discuss magnetic excitations of a spin-cluster model which has been
suggested to describe the low temperature phase of alpha'-NaV_2O_5. This model
fulfills all symmetry criteria proposed by recent x-ray investigations. We find
that this model is not able to describe the occurence of two well separated
magnon lines perpendicular to the ladder direction as observed in INS
experiments. We suggest further experimental analysis to generally distinguish
between models with double reflection or inversion symmetry.Comment: 4 pages, 4 figures, added a calculation of level repulsio
Low-energy excitations in NaV2O5
In the (ab) polarized Raman scattering spectra of NaV2O5 single crystals,
measured with 647.1 nm laser line at T < Tc, we found two modes at 86, and 126
cm-1 not previously reported. These two modes, together with 66, and 106 cm-1
modes, make an array of four low-energy equidistant modes below the energy
onset of the continuum at about 132 cm-1. All four modes are strongly
suppressed by increasing Na deficiency, indicating their nonvibrational origin
and the existence of a quantum phase transition at critical Na deficiency
between 3 and 4%. These results question current understanding of NaV2O5 as
quasi one-dimensional Heisenberg antiferromagnet.Comment: 6 pages, 3 figure
Frustrated H-Induced Instability of Mo(110)
Using helium atom scattering Hulpke and L"udecke recently observed a giant
phonon anomaly for the hydrogen covered W(110) and Mo(110) surfaces. An
explanation which is able to account for this and other experiments is still
lacking. Below we present density-functional theory calculations of the atomic
and electronic structure of the clean and hydrogen-covered Mo(110) surfaces.
For the full adsorbate monolayer the calculations provide evidence for a strong
Fermi surface nesting instability. This explains the observed anomalies and
resolves the apparent inconsistencies of different experiments.Comment: 4 pages, 2 figures, submitted to PR
Low-Temperature Structure of the Quarter-Filled Ladder Compound alpha'-NaV2O5
The low-temperature (LT) superstructure of -NaVO was
determined by synchrotron radiation x-ray diffraction. Below the phase
transition temperature associated with atomic displacement and charge ordering
at 34K, we observed the Bragg peak splittings, which evidence that the LT
structure is monoclinic. It was determined that the LT structure is
with the space group where and
represent the high temperature orthorhombic unit cell. The valence estimation
of V ions according to the bond valence sum method shows that the V sites are
clearly separated into two groups of V and V with a
charge ordering pattern. This LT structure is consistent with resonant x-ray
and NMR measurements, and strikingly contrasts to the LT structure previously
reported, which includes V sites.Comment: 4 pages, 3 figures, 1 tabl
X-ray anomalous scattering investigations on the charge order in -NaVO
Anomalous x-ray diffraction studies show that the charge ordering in
-NaVO is of zig-zag type in all vanadium ladders. We
have found that there are two models of the stacking of layers along
\emph{c-}direction, each of them consisting of 2 degenerated patterns, and that
the experimental data is well reproduced if the 2 patterns appears
simultaneously. We believe that the low temperature structure contains stacking
faults separating regions corresponding to the four possible patterns.Comment: Submitted to Phys. Rev. Lett., 4 pages, 4 eps figures inserted in the
tex
Exact diagonalisation study of charge order in the quarter-filled two-leg ladder system NaV2O5
The charge ordering transition in the layer compound NaV2O5 is studied by
means of exact diagonalization methods for finite systems. The 2-leg ladders of
the V-Trellis lattice are associated with one spin variable of the vanadium
3d-electron in the rung and a pseudospin variable that describes its positional
degree of freedom. The charge ordering (CO) due to intersite Coulomb
interactions is described by an effective Ising-like Hamiltonian for the
pseudo-spins that are coupled to the spin fluctuations along the ladder. We
employ a Lanczos algortihm on 2D lattice to compute charge (pseudo-spin) and
spin-correlation functions and the energies of the low lying excited states. A
CO-phase diagram is constructed and the effect of intra-ladder exchange on the
CO transition is studied. It is shown that a phase with no-longe range order
(no-LRO) exists between the in-line and zig-zag ordered structures. We provide
a finite-size scaling analysis for the spin excitation gap and also discuss the
type of excitations. In addition we studied the effect of bond-alternation of
spin exchange and derived a scaling form for the spin gap in terms of the
dimerization parameter.Comment: 9 pages with 9 EPS figures and 1 table, To be appeared in Phys. Rev.
B (2001
Orthorhombic versus monoclinic symmetry of the charge-ordered state of NaV2O5
High-resolution X-ray diffraction data show that the low-temperature
superstructure of alpha-NaV2O5 has an F-centered orthorhombic 2a x 2b x 4c
superlattice. A structure model is proposed, that is characterized by layers
with zigzag charge order on all ladders and stacking disorder, such that the
averaged structure has space group Fmm2. This model is in accordance with both
X-ray scattering and NMR data. Variations in the stacking order and disorder
offer an explanation for the recently observed devils staircase of the
superlattice period along c.Comment: REVTEX, 4 pages including 2 figures, shortened, submitted to PR
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