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  • 1
    Electronic Resource
    Electronic Resource
    Analysis of the ground-state wave function of theJ 1-J 2 quantum Heisenberg antiferromagnet (1993)
    Springer
    The European physical journal 93 (1993), S. 21-31 
    Details
    ISSN: 1434-6036
    Keywords: 75.10.Jm ; 75.40.Cx ; 75.50.Ee
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We investigate the ground-state wave function $$|\psi \rangle = \sum\limits_n {\alpha _n } |n\rangle $$ of the spin-1/2J 1-J 2 model on finite square lattices ofN=16(4×4) andN=24(4×6) sites. We find that the Marshall-Peierls phase rule for coefficientsα n , which was derived for unfrustrated bipartite lattices (J 2=0), holds exactly for comparably large frustration up toJ 2/J 1=0.28 (N=16) and up toJ 2/J 1=0.20 (N=24). But even for strong frustration up toJ 2/J 1≈0.45 the Marshall-Peierls rule describes the phase relationships in the ground state excellently. In the region of dominatingJ 2 a phase rule can be formulated as a product of the rules for two independent antiferromagnets. We find that the violation of the Marshall-Peierls sign rule does not dramatically affect the order parameters up toJ 2/J 1≈0.6. To calculate the magnitude of the coefficientsα n by a variational procedure we search for a represantative relationship between theα n and (a few) parametersP 1,n ...P k, n classifying the Ising basis states |n〉. By comparison with the exact ground state we analyze classification schemes based on pair correlations (Jastrow type wave functions) as well as schemes taking into account cluster parameters. While for small frustrationJ 2/J 1〈0.2 a short-range Jastrow description (nearest-neighbour and next nearestneighbour pairing) seems to be sufficient for the adequate description of the ground state one definitly needs longrange pairing and/or cluster parameters to construct a reasonable trial wave function for strong frustration. As an example for a special Jastrow type wave function we discuss an ansatz coming from the spin-wave theory. Finally, we consider the influence of the anisotropic exchangeJ zz ≠J xx ,J yy on the quality of the short-range Jastrow wave function and find and excellent agreement with the exact ground state already for Ising exchange anisotropiesJ xx =J yy ≈0.5J zz .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01308803
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  • 2
    Electronic Resource
    Electronic Resource
    Quantum fluctuations in structurally disordered antiferrromagnets (1995)
    Springer
    Journal of low temperature physics 99 (1995), S. 359-361 
    Details
    ISSN: 1573-7357
    Keywords: 75.10.Jm ; 75.50.Ee ; 71.55.Jv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present exact diagonalisation data for the ground state of finite two-dimensional spin 1/2 Heisenberg antiferromagnets with structural disorder. The combination of antiferromagnetic coupling and structural disorder creates frustation and a local singlet formation which may strongly influence the magnetic ordering. In detail we study the magnetic radial distribution function, the magnetic order parameter M2 as well as a parameter measuring the spin misfit and present these quantities in dependence on the strength of the structural disorder. We estimate the critical disorder where the antiferromagnetic long-range order breaks down.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00752309
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  • 3
    Electronic Resource
    Electronic Resource
    Morphometric evaluation of volume shifts between intra- and extra-cellular space before and during global ischemia (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    The @Anatomical Record 241 (1995), S. 319-327 
    Details
    ISSN: 0003-276X
    Keywords: Heart ; Dog ; Cardioplegia ; Cardiac arrest ; Ischemia ; Morphometry ; Interstitial space ; Contraction state ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Background: It is well known that all forms of cardiac arrest lead to global ischemia combined with alterations in cellular and interstitial volume. The aim of this study was to investigate the nature of these alterations with respect to different methods of cardiac arrest and establish the extent of their mutual influence at the onset as well as during the course of global ischemia.Methods: Three tested clinical methods were employed to induce cardiac arrest by a) aortic cross clamping, b) coronary perfusion with the cardioplegic solution St. Thomas, and c) coronary perfusion with the cardioplegic solution histidine-tryptophane-ketoglutarate (HTK). The arrested hearts were subjected to global ischemia at 25°C. The size of the myocytes, as well as the interstitial space of myocytes, was determined morphometrically. The contraction state of myocytes was evaluated according to a score.Results: We found that the degree of contraction, as well as nature of alterations in the cellular and interstitial volumes, depended both on the form of cardiac arrest and on the duration of ischemia. The following relationships were established. High contraction at the onset of ischemia leads to expulsion of fluid from the interstitium between bundles of myocytes into the tissue clefts increasing their size. The decrease in contraction during ischemia leads to narrower tissue clefts. Cellular swelling at the onset of and during ischemia is caused by volume shifts between intracellular and interstitial space. An increase in cellular volume during global ischemia and/or additional contraction reduce the interstitium within bundles of myocytes. Sufficient relaxation and/or interstitial edema enlarge the interstitium.Conclusions: Cellular and intersticial alterations seen at the onset and during the course of ischemia are dependent upon the method of cardiac arrest. Furthermore, a considerable mutual influence is exerted by the alterations in cellular and interstitial spaces. © 1995 Wiley-Liss, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ar.1092410305
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  • 4
    Electronic Resource
    Electronic Resource
    Cellular distribution patterns of lanthanum and morphometry of rat hearts exposed to different degrees of ischemic stress (1995)
    New York, NY [u.a.] : Wiley-Blackwell
    The @Anatomical Record 243 (1995), S. 496-508 
    Details
    ISSN: 0003-276X
    Keywords: Heart ; Ischemia ; Lanthanum ; Tracer ; ESI ; EELS ; Morphometry ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Background: The element lanthanum (La) can be used as a tracer for verification of membrane permeability. The aim of this study was to establish whether (1) distribution of La in the myocardium of rat hearts depends on the degree of ischemic stress and (2) morphometrically determined cell and mitochondrial swelling correlates with the La distribution.Materials and Methods: Isolated beating rat hearts were arrested by coronary perfusion with the cardioplegic solution Custodiol® (controls) or by aortic cross clamping followed by exposur to different degrees of ischemic stress. The solutions for perfusion-and postfixation as well as for rinsing contained 1.1% La(NO3)3. Cellular and mitochondrial swelling were determined morphometrically and myocytes exhibiting intracellular La were quantified and stated as percentage of test fields.Results: Immediately after cardiac arrest La was present as precipitates only in a few myocytes adjacent to the outer mitochondrial membrane as seen by cTEM and ESI. In such cells La was also detected by EELS in mitochondrial matrix and myofibrils. Advanced ischemic stress led to an increase of the percentage of myocytes containing detectable intracellular La. After 45 min ischemia at 30°C, myocytes and mitochondria showed a remarkable edema and different intracellular distribution patterns of La. After 90 min of ischemia at 20°C interruptions of sarcolemma could only be detected in a few of the swollen myocytes. Roundish La granules were seen in the myofibrils. The percentage of myocytes containing intracellular La and the extent of cellular and mitochondrial swelling showed a significant correlation.Conclusions: Patterns of intracellular La distribution depend on the degree of ischemic stress and correspond to the degree of cellular as well as mitochondrial edema. These results point at a direct relation between alterations of membrane permeability and development of edema. © 1995 Wiley-Liss, Inc.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ar.1092430412
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