Spin correlations in the paramagnetic phase of La2CuO4 have been studied using polarized neutron scattering, with two important results. First, the temperature dependence of the characteristic energy scale of the fluctuations and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum non-linear sigma model. Secondly, comparison of a high-temperature series expansion of the equal-time spin correlations with the diffuse neutron intensity provides definitive experimental evidence for ring exchange.Heisenberg was the first to realize that strong effective spin interactions arise from the principle of the indistinguishability of the particles [1]. Dirac generalized this concept, in the context of Group Theory, to include higher order interactions[2]. Multi-particle exchange dominates the physics of the quantum solid 3 He[3] but, surprisingly, is generally not taken into account for electronic magnetic materials. The most powerful technique for exploring exchange interactions is the study of excitations from the ordered phase using inelastic neutron scattering. However, ambiguities in the interpretation of magnon dispersion curves sometimes mean that higher order terms remain hidden. By employing the independent approach of studying the instantaneous spin correlations in the paramagnetic phase, we obtain complementary information that enables a better understanding of the exchange mechanism. In this letter we describe studies of the diffuse magnetic scattering from La 2 CuO 4 which provide compelling, quantitative evidence for the existence of four-particle cyclic exchange.La 2 CuO 4 is of great intrinsic interest both as the parent compound of a canonical high-temperature superconductor, and as a very good realization of a two-dimensional quantum Heisenberg antiferromagnet (2DQHAF). Magnetic Raman experiments [4], infrared absorption studies [5,6] and inelastic neutron scattering measurements [7] show definitively the inadequacy of the nearest-neighbour Heisenberg model, and suggest the possibility that four-particle exchange may be significant.In an important series of experiments to study the diffuse magnetic scattering from La 2 CuO 4 using unpolarized neutrons[8] the temperature dependence of the magnetic correlation length was found to agree with the predictions of the quantum non-linear sigma model (QNLσM) [9]. However, the observed amplitude shows dramatic deviations from the predictions of this theory [8]. The QNLσM is the simplest possible effective action for a 2DQHAF that is compatible with the long-wavelength spin waves and that does not assume a spontaneously broken symmetry. Moreover, its predictions should hold even in the presence of four-spin exchange, as discussed below. Here we study the dynamical spin correlations above the Néel temperature using polarized neutrons, and find complete agreement of the observed 2D critical fluctuations with the predictions of the QNLσM.Dirac's approach provides the most transparent theoretical fram...

Anomalous x-ray diffraction studies show that the charge ordering in α ′ -NaV2O5 is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along 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.

Abstract:Mitoxantrone is a synthetic anticancer drug used clinically in the treatment of different types of cancer. It was developed as a doxorubicin analogue in a program to find drugs with improved antitumor activity and decreased cardiotoxicity compared with the anthracyclines. As the cell membrane is the first barrier encountered by anticancer drugs before reaching the DNA sites inside the cells and as surfactant micelles are known as simple model systems for biological membranes, the drugs-surfactant interaction has been the subject of great research interest. Further, quantitative understanding of the interactions of drugs with biomimicking structures like surfactant micelles may provide helpful information for the control of physicochemical properties and bioactivities of encapsulated drugs in order to design better delivery systems with possible biomedical applications. The present review describes the physicochemical aspects of the interactions between the anticancer drug mitoxantrone and different surfactants. Mitoxantrone-micelle binding constants, partitions coefficient of the drug between aqueous and micellar phases and the corresponding Gibbs free energy for the above processes, and the probable location of drug molecules in the micelles are discussed.

We present computational analyses, methodological advances, and heuristic conclusions applied on a series of polyaromatic systems condensed in the shape of regular triangles, carrying spin, because of topological reasons. A new clue about the classification of title systems in three equivalency classes is presented. A conjugated hydrocarbon having n-hexagonal rings at one edge, carrying n − 1 unpaired electrons, will be called n-triangulene in the generalization of the experimentally known structures with n = 2 (phenalenyl) and n = 3 (triangulene). To be distinguished from most of the previous computational approaches, done by density functional theory, we challenged the problem in the key of valence bond (VB) paradigm in both ab initio and phenomenological manners. The Heisenberg spin Hamiltonian was used to simulate the computed spectrum of VB states for the phenalenyl radical (n = 2), predicting with the fitted parameters the effective VB description of n = 3 triangulene and other related systems. The outcome has practical importance in the prospects of spin chemistry because the VB ab initio calculations are prohibitive beyond the n = 2 case. The results are made transparent to the chemical intuition using the language of resonance structures.

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Toader et al. Reply: In their inelastic neutron scattering study of the spin-wave dispersion of the square-lattice quantum antiferromagnet La 2 CuO 4 , Coldea et al. [1] definitively rule out the simple nearest-neighbor Heisenberg model. They offer two alternative explanations of the observed dispersion: (1) the existence of an ad hoc ferromagnetic next-nearest-neighbor interactionJ 2 2 , and (2) an extended Heisenberg model containing four-spin cyclic exchange J 4 , as arising in large-U expansions of Hubbard models.Unfortunately, it is impossible to distinguish between (1) and (2) on the basis of the spin-wave dispersion alone, in the magnetically ordered phase of La 2 CuO 4 since J 4 does not contribute to the spin-wave spectrum at lowest order in 1=S. In our recent Letter [2], we argued that it is possible to distinguish between these alternatives on the basis of spin fluctuations in its paramagnetic phase, and presented diffuse neutron scattering data which clearly favor (2)-the model with four-spin exchange.When plotted as a function ofJ 1 2 , the quantum Monte Carlo (QMC) simulations of the diffuse scattering for a one-band Hubbard model made by Raymond et al.show very little dependence on the ratio t=U and by implication on the value of J 4 [3]. These authors also point out that since we found the universal scaling behavior predicted by the quantum nonlinear sigma model (QNLM) this means that we are insensitive to microscopic details of the Hamiltonian. We agree that several Hamiltonians can lead to the same scaling. However, using the value of J 2 2 =J 1 2 found in [1] by spin-wave analysis of the T 295 K data, it is then possible to distinguish between models (1) and (2).

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