3,408 research outputs found
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]
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
Computer-assisted ex vivo, normothermic small bowel perfusion
Background: In the present study, a technique for computer-assisted, normothermic, oxygenated, ex vivo, recirculating small bowel perfusion was established as a tool to investigate organ pretreatment protocols and ischemia/reperfusion phenomena. A prerequisite for the desired setup was an organ chamber for ex vivo perfusion and the use of syngeneic whole blood as perfusate. Methods: The entire small bowel was harvested from Lewis rats and perfused in an organ chamber ex vivo for at least 2 h. The temperature was kept at 37 degrees C in a water bath. Three experimental groups were explored, characterized by different perfusion solutions. The basic perfusate consisted of syngeneic whole blood diluted with either NaCl, Krebs' solution or Krebs' solution and norepinephrine to a hematocrit of 30%. In addition, in each group l-glutamine was administered intraluminally. The desired perfusion pressure was 100 mm Hg which was kept constant with a computer-assisted data acquisition software, which measured an-line pressure, oxygenation, flow, temperature and pH and adjusted the pressure by changing the flow via a peristaltic pump. The viability of the preparation was tested by measuring oxygen consumption and maltose absorption, which requires intact enzymes of the mucosal brush border to break down maltose into glucose. Results: Organ perfusion in group 1 (dilution with NaCl) revealed problems such as hypersecretion into the bowel lumen, low vascular resistance and no maltose uptake. In contrast a viable organ could be demonstrated using Krebs' solution as dilution solution. The addition of norepinephrine led to an improved perfusion over the entire perfusion period. Maltose absorption was comparable to tests conducted with native small bower. Oxygen consumption was stable during the 2-hour perfusion period. Conclusions: The ex vivo perfusion system established enables small bowel perfusion for at least 2 h. The viability of the graft could be demonstrated. The perfusion time achieved is sufficient to study leukocyte/lymphocyte interaction with the endothelium of the graft vessels. In addition, a viable small bowel, after 2 h of ex vivo perfusion, facilitates testing of pretreatment protocols for the reduction of the immunogenicity of small bowel allografts. Copyright (C) 2000 S. Karger AG, Basel
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
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]
Interrelation of Superconducting and Antiferromagnetic Gaps in High-Tc Compounds: a Test Case for a Microscopic Theory
Recent angle resolved photoemission (ARPES) data, which found evidence for a
d-wave-like modulation of the antiferromagnetic gap, suggest an intimate
interrelation between the antiferromagnetic insulator and the superconductor
with its d-wave gap. This poses a new challenge to microscopic descriptions,
which should account for this correlation between, at first sight, very
different states of matter. Here, we propose a microscopic mechanism which
provides a definite correlation between these two different gap structures: it
is shown that a projected SO(5) theory, which aims at unifying
antiferromagnetism and d-wave superconductivity via a common symmetry principle
while explicitly taking the Mott-Hubbard gap into account, correctly describes
the observed gap characteristics. Specifically, it accounts for both the
dispersion and the order of magnitude difference between the antiferromagnetic
gap modulation and the superconducting gap.Comment: 8 pages, 5 figure
Hole photoproduction in insulating copper oxide
Basing on t-J model we calculate the k-dependence of a single hole
photoproduction probability for CuO2 plane at zero doping. We also discuss the
radiation of spin-waves which can substantially deform the shape of
photoemission spectra.Comment: latex 8 pages, 3 figure
Renormalized SO(5) symmetry in ladders with next-nearest-neighbor hopping
We study the occurrence of SO(5) symmetry in the low-energy sector of
two-chain Hubbard-like systems by analyzing the flow of the running couplings
(-ology) under renormalization group in the weak-interaction limit. It is
shown that SO(5) is asymptotically restored for low energies for rather general
parameters of the bare Hamiltonian. This holds also with inclusion of a
next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry
provided one accounts for a different single-particle weight for the
quasiparticles of the two bands of the system. The physical significance of
this renormalized SO(5) symmetry is discussed.Comment: Final version: to appear in Phys. Rev. Lett., sched. Mar. 9
- …