3,324 research outputs found
In situ observation of shrinking and swelling of normal and compression Chinese fir wood at the tissue, cell and cell wall level
The shrinking and swelling of wood due to moisture changes are intrinsic material properties that control and limit the use of wood in many applications. Herein, hygroscopic deformations of normal and compression wood of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) were measured during desorption and absorption processes. The dimensional changes were observed in situ by an environmental scanning electron microscope and analyzed at different hierarchical levels (tissue, cell and cell wall). The relationship between moisture variation and hygroscopic deformation was measured. During initial desorption periods from 95 to 90 or 75% RH, an expansion of the lumen and a shrinkage of the cell wall were observed, revealing a non-uniform and directional deformation of single wood cells. The variation of shrinking or swelling at different hierarchical levels (tissue, cell and cell wall) indicates that the hygroscopic middle lamella plays a role in the deformation at the tissue level. Higher microfibril angles and helical cavities on the cell wall in compression wood correlate with a lower shrinking/swelling ratio. Normal wood showed a more pronounced swelling hysteresis than compression wood, while the sorption hysteresis was almost the same for both wood types. This finding is helpful to elucidate effects of micro- and ultrastructure on sorption. The present findings suggest that the sophisticated system of wood has the abilities to adjust the hygroscopic deformations by fine-tuning its hierarchical structures
Dynamics of an SO(5) symmetric ladder model
We discuss properties of an exactly SO(5) symmetric ladder model. In the
strong coupling limit we demonstrate how the SO(3)-symmetric description of
spin ladders in terms of bond Bosons can be upgraded to an SO(5)-symmetric
bond-Boson model, which provides a particularly simple example for the concept
of SO(5) symmetry. Based on this representation we show that antiferro-
magnetism on one hand and superconductivity on the other hand can be understood
as condensation of either magnetic or charged Bosons into an RVB vacuum. We
identify exact eigenstates of a finite cluster with general multiplets of the
SO(5) group, and present numerical results for the single particle spectra and
spin/charge correlation functions of the SO(5)-symmetric model and identify
`fingerprints' of SO(5) symmetry in these. In particluar we show that SO(5)
symmetry implies a `generalized rigid band behavior' of the photoemission
spectrum, i.e. spectra for the doped case are rigorously identical to spectra
for spin-polarized states at half-filling. We discuss the problem of adiabatic
continuity between the SO(5) symmetric ladder and the actual t-J ladder and
demonstrate the feasibility of a `Landau mapping' between the two models.Comment: Revtex-file, 16 pages with 15 eps-figures. Hardcopies of Figures (or
the entire manuscript) obtainable by e-mail request to
[email protected]
Dynamics of spin ladders
We derive an approximate theory for Heisenberg spin ladders with two legs by
mapping the spin dynamics onto the problem of hard-core `bond-Bosons'. The
parameters of the Bosonic Hamiltonian are obtained by matching anomalous
Green's functions to Lanczos results and we find evidence for a strong
renormalization due to quantum fluctuations. Various dynamical spin correlation
functions are calculated and found to be in good agreement with Lanczos
results. We then enlarge the effective Hamiltonian to describe the coupling of
the bond-Bosons to a single hole injected into the system and treat the
hole-dynamics within the `rainbow-diagram' approximation by Schmidt-Rink et.
al. Theoretical predictions for the single hole spectral function are obtained
and found to be in good agreement with Lanczos results.Comment: RevTex-file, 10 PRB pages with 7 eps files. Hardcopies of figures (or
the entire manuscript) can be obtained by e-mail request to:
[email protected]
Evidence for vivianite formation and its contribution to long-term phosphorus retention in a recent lake sediment: a novel analytical approach
Vivianite, Fe3(PO4)2 · 8 H2O, is a ferrous iron
phosphate mineral which forms in waterlogged soils and
sediments. The phosphorus (P) bound in its crystal lattice is
considered to be immobilised because vivianite is stable under
anoxic, reducing, sedimentary conditions. Thus, vivianite formation
can make a major contribution to P retention during early
diagenesis. Much remains unknown about vivianite in sediments,
because technical challenges have rendered direct identification and
quantification difficult. To identify vivianite and assess its
significance for P burial during early diagenesis we studied the
consequences of a 1992/1993 in-lake application of FeCl3 and
Fe(OH)3 aimed at restoring Lake Groß-Glienicke (Berlin,
Germany). In a novel approach, we firstly applied a heavy-liquid
separation to the iron-rich surface sediments which allowed direct
identification of vivianite by X-ray diffraction in the high-density
(ρ > 2.3 g cm−3) sediment fraction. Secondly, we
assessed the contribution of vivianite to P retention, combining
results from chemical digestion with magnetic susceptibility data
derived from magnetic hysteresis measurements. Scanning electron
microscopy revealed that the dark blue spherical vivianite nodules
were 40–180 μm in diameter, and formed of platy- and
needle-shaped crystal aggregates. Although equilibrium calculations
indicated supersaturation of vivianite throughout the upper
30 cm of the sediment, the vivianite deposits were
homogeneously distributed within, and restricted to, the upper
23 cm only. Thus, supersaturated pore water alone cannot
serve as a reliable predictor for the in situ formation of
vivianite. In Lake Groß -Glienicke, vivianite formation continues
to be triggered by the artificial iron amendment more than
20 yr ago, significantly contributing to P retention in
surface sediments
Validity of the rigid band picture for the t-J model
We present an exact diagonalization study of the doping dependence of the
single particle Green's function in 16, 18 and 20 site clusters of t-J model.
We find evidence for rigid-band behaviour starting from the half-filled case:
upon doping, the topmost states of the quasiparticle band observed in the
photoemisson spectrum at half-filling cross the chemical potential and reappear
as the lowermost states of the inverse photoemission spectrum. Features in the
inverse photoemission spectra which are inconsistent with rigid-band behaviour
are shown to originate from the nontrivial point group symmetry of the ground
state with two holes, which enforces different selection rules than at
half-filling. Deviations from rigid band behaviour which lead to the formation
of the `large Fermi surface' in the momentum distribution occur only at
energies far from the chemical potential. A Luttinger Fermi surface and a
nearest neighbor hopping band do not exist.Comment: Remarks: Revtex file + 7 figures attached as compressed postscript
files Figures can also be obtained by ordinary mail on reques
Spin bags in the doped t-J model
We present a nonperturbative method for deriving a quasiparticle description
of the low-energy excitations in the t-J model for strongly correlated
electrons. Using the exact diagonalization technique we evaluated exactly the
spectral functions of composite operators which describe an electron or hole
dressed by antiferromagnetic spin fluctuations as expected in the string or
spin bag picture. For hole doping up to , use of the composite operators
leads to a drastic simplification of the single particle spectral function: at
half-filling it takes free-particle form, for the doped case it resembles a
system of weakly interacting Fermions corresponding to the doped holes. We
conclude that for all doping levels under study, the elementary electronic
excitations next to the Fermi level are adequately described by the
antiferromagnetic spin fluctuation picture and show that the dressing of the
holes leads to formation of a bound state with d(x^2-y^2) symmetry.Comment: Remarks: Revtex file + 4 figures attached as compressed postscript
files Figures can also be obtained by ordinary mail on reques
Ground state properties and dynamics of the bilayer t-J model
We present an exact diagonalization study of bilayer clusters of t-J model.
Our results indicate a crossover between two markedly different regimes which
occurs when the ratio J_perp/J between inter-layer and intra-layer exchange
constants increases: for small J_perp/J the data suggest the development of 3D
antiferromagnetic correlations without appreciable degradation of the
intra-layer spin order and the d_(x2-y2) hole pairs within the planes persist.
For larger values of J_perp/J local singlets along the inter-layer bonds
dominate, leading to an almost complete suppression of the intra-layer spin
correlation and the breaking of the intra-layer pairs. The ground state with
two holes in this regime has s-like symmetry. The data suggest that the
crossover may occur for values of J_perp/J as small as 0.2. We present data for
static spin correlations, spin gap, and electron momentum distribution and
spectral function of the `inter-layer RVB state' realized for large J_perp/J.
The latter deviates from the single layer ground state, making it an
implausible candidate for modelling high-temperature superconductors.Comment: Revtex-file, 6 PRB pages, figures appended as uu-encoded postscript.
Hardcopies of figures (or the entire manuscript) can be obtained by e-mailing
to: [email protected]
Excitation spectrum of the homogeneous spin liquid
We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivityComment: RevTex-file, 16 PRB pages with 13 embedded eps figures. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to:
[email protected]
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