Optimum ground states for spin-32\frac{3}{2} chains

We present a set of {\em optimum ground states} for a large class of spin-$\frac{3}{2}$ chains. Such global ground states are simultaneously ground states of the local Hamiltonian, i.e. the nearest neighbour interaction in the present case. They are constructed in the form of a matrix product. We find three types of phases, namely a {\em weak antiferromagnet}, a {\em weak ferromagnet}, and a {\em dimerized antiferromagnet}. The main physical properties of these phases are calculated exactly by using a transfer matrix technique, in particular magnetization and two spin correlations. Depending on the model parameters, they show a surprisingly rich structure.Comment: LaTeX, 22 pages, 6 embedded Postscript figure

Spin-3/2 models on the Cayley tree -- optimum ground state approach

We present a class of optimum ground states for spin-3/2 models on the Cayley tree with coordination number 3. The interaction is restricted to nearest neighbours and contains 5 continuous parameters. For all values of these parameters the Hamiltonian has parity invariance, spin-flip invariance, and rotational symmetry in the xy-plane of spin space. The global ground states are constructed in terms of a 1-parametric vertex state model, which is a direct generalization of the well-known matrix product ground state approach. By using recursion relations and the transfer matrix technique we derive exact analytical expressions for local fluctuations and longitudinal and transversal two-point correlation functions.Comment: LaTeX 2e, 8 embedded eps figures, 14 page

Mixed Heisenberg Chains. II. Thermodynamics

We consider thermodynamic properties, e.g. specific heat, magnetic susceptibility, of alternating Heisenberg spin chains. Due to a hidden Ising symmetry these chains can be decomposed into a set of finite chain fragments. The problem of finding the thermodynamic quantities is effectively separated into two parts. First we deal with finite objects, secondly we can incorporate the fragments into a statistical ensemble. As functions of the coupling constants, the models exhibit special features in the thermodynamic quantities, e.g. the specific heat displays double peaks at low enough temperatures. These features stem from first order quantum phase transitions at zero temperature, which have been investigated in the first part of this work.Comment: 12 pages, RevTeX, 12 embedded eps figures, cf. cond-mat/9703206, minor modification

Exact ground states for two new spin-1 quantum chains, new features of matrix product states

We use the matrix product formalism to find exact ground states of two new spin-1 quantum chains with nearest neighbor interactions. One of the models, model I, describes a one-parameter family of quantum chains for which the ground state can be found exactly. In certain limit of the parameter, the Hamiltonian turns into the interesting case $H=\sum_i ({\bf S}_i\cdot {\bf S}_{i+1})^2$. The other model which we label as model II, corresponds to a family of solvable three-state vertex models on square two dimensional lattices. The ground state of this model is highly degenerate and the matrix product states is a generating state of such degenerate states. The simple structure of the matrix product state allows us to determine the properties of degenerate states which are otherwise difficult to determine. For both models we find exact expressions for correlation functions.Comment: 22 pages, references added, accepted for publication in European Physics Journal

The square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond

We consider the spin-1/2 antiferromagnetic Heisenberg model on the two-dimensional square-kagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate low-energy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the low-energy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) square-lattice spin-1/2 $XXZ$ model in a $z$-aligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the low-temperature properties of the square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magnetic-field driven Berezinskii-Kosterlitz-Thouless phase transition which occurs at low temperatures.Comment: 6 figure

A classification of spin 1/2 matrix product states with two dimensional auxiliary matrices

e classify the matrix product states having only spin-flip and parity symmetries, which can be constructed from two dimensional auxiliary matrices. We show that there are three distinct classes of such states and in each case, we determine the parent Hamiltonian and the points of possible quantum phase transitions. For two of the models, the interactions are three-body and for one the interaction is two-bodyComment: 17 pages, 3 figure

What makes the Difference between Unsuccessful and Successful Firms in the German Mechanical Engineering Industry?

Against a background of rising costs and increasing competition, it is besoming more and more difficult for the small and medium-sized firms of the German mechanical engineering industry to be economically successful. The thesis that rapidly changing markets, products and production processes cause serious economic problems for these firms is, however, a proposition on an average trend. A substantial number of firms are not only capable of coping with these conditions and challenges, but are even able to expand their business activities, including employment. We may hypothesize that their product and market strategies as well as their internal mode of operation and organization differs significantly from those firms doing economically less well. In order to test the significance of factors which could lead to different levels of success, operationalized with data of the NIFA panel the method of static microsimulation is applied using the program MICSIM. This particular method offers the possibility of reweighting the information contained in micro datasets according to restrictions given by aggregated data (i.e. marginal distributions). The latter will be chosen in such a way that the number of firms with properties (strategies), hypothetically leading to success in terms of lower excess capacity, are 'artificially', increased in the sample. The research goal is to find out whether such hypothetical strategies are supported by the data. The basic finding that certain complex strategies are more often successful demonstrates that unidimensional approaches to modernize production are of less value. Only in those strategies wehere organization of production, technical equipment, degree of vertical integration, products and customers are part of an intergrated innovational strategy, is success most likely to be fuelled.economic succes, NIFA PANEL, microsimulation, engineering

New Renaissance (The)

Les sages de ce comité ont procédé à l\u27étude du projet de numérisation de l\u27ensemble du patrimoine culturel européen et proposent dans ce rapport une série de recommandations visant à encadrer cet ambitieux programme afin de : -partager notre patrimoine commun, dans toute sa richesse et sa diversité ; - relier notre passé à notre présent ; - préserver cet héritage pour les générations futures ; - protéger les intérêts des créateurs européens ; - favoriser la créativité, celles des professionnels comme celles des amateur