The nature of the ferromagnetic ground state in the Mn4 molecular magnet

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn$_4$ molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn3+ ions (J_{bb}), but along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J_{wb}). The microscopic analysis performed within the perturbation theory allowed to establish the mechanism for this largest ferromagnetic exchange constant. The charge ordering of the Mn ions results in the situation when the energy of the excited state in the exchange process is defined not by the large on-site Coulomb repulsion U, but by much smaller energy V, which stabilizes the charge ordered state. Together with strong Hund's rule coupling and specific orbital order this leads to a large ferromagnetic exchange interaction for two out of four Mn2+ --Mn3+ pairs.Comment: 12 pages, 10 figure

A modified droplet vitrification method for cryopreservation of shoot tips from <i>in vitro</i> potato plants

Collections of common potato maintained in the field genebanks suffer significant losses due to the impact of extreme environmental factors, diseases and pests. The solution of the problem of safe long-term preservation of common potato accessions is to create doublet in vitro and cryo-collections. Cryogenic collections are stored at ultra-low temperatures in cryobanks. Several methods of potato cryoconservation are known, of which the droplet vitrification method developed by B. Panis with colleagues in 2005 is the most widely used in genebanks. This paper provides a detailed description of the modified method of droplet vitrification, which is used for cryopreservation of apexes (shoot tips) of potato in vitro plants at the N.I. Vavilov All-Russian Institute of Plant Genetic Resources (VIR). The method modified at VIR includes the main steps of the original droplet-vitrification method developed by B. Panis and colleagues: 1) preparation of plant material, 2) isolation of shoot tips, 3) treatment of explants with cryoprotector solutions, 4) freezing/immersion in liquid nitrogen, 5) thawing, 6) post-cryogenic recovery and evaluation of viability and regeneration capacity. The modifications of stages 1, 2 and 6 proposed at VIR lead to a significant reduction in the duration of cryopreservation experiments in comparison with the original method of B. Panis. This paper presents the results of cryopreservation of modern potato cultivars and South American landraces which were obtained using the method of droplet vitrification as modified at VIR. The majority (76.7 %) of the studied accessions of cultivated potato were characterized by high rates of postcryogenic recovery (40–95 %) and 23.3 % of the samples had the values of postcryogenic regeneration from 20 to 39 %, which corresponds to the minimal permissible values for long-term storage in a cryobank. Currently the modified droplet-vitrification method is used for further expanding of the VIR potato cryocollection