A. N. Lavrov

Dendritic growth of TmBa2Cu3O6+x single crystals

Abstract We report on the growth of whisker-like TmBa2Cu3O6+x (Tm-123) single crystals which are especially suitable for studying the anisotropic electron transport. We have found that the formation of Tm-123 crystals from the flux starts with the growth of skeletons (dendrites), and their familiar plate habit develops through the following skeleton overgrowth. Incomplete growth results in large clear dendrites, whose branches are exotic whisker-like single crystals with sizes up to 50×1200×600 μ m3 with the bc-face being the largest one.

Effect of Oxygen Nonstoichiometry on the Magnetic Phase Transitions in Frustrated YBaCo4O7 + x (x = 0, 0.1, 0.2) Cobaltites

The structural and elastic characteristics of YBaCo4O7 + x (x = 0, 0.1, 0.2) cobaltites synthesized by various technologies and having various excess oxygen contents x are experimentally studied. The distortion of the crystal structure in stoichiometric samples is found to remove frustrations and to bring about a longrange magnetic order in the cobalt subsystem, which is accompanied by well-pronounced anomalies in the elastic properties in the temperature range of a magnetic phase transition TN. At a weak deviation from oxygen stoichiometry, the structure distortion disappears, frustrations are retained, and the further development of a long-range magnetic order is hindered. As a result of an absent long-range magnetic order, the anomalies of the elastic characteristics at TN smooth rapidly and disappear. This finding points to the suppression of structural and magnetic transitions in nonstoichiometric samples and to the conservation of only short-range correlations of order parameter. It is found that nonstoichiometric samples can be separated into two phases depending on the ceramic synthesis conditions.

Peculiarity of interrelation between electronic and magnetic properties of HTSC cuprates associated with short-range antiferromagnetic order

We report on the results of measurements of anisotropic resistivity of RBa2Cu3O6 + x (R = Tm, Lu) high-temperature superconducting single crystals in a wide range of doping levels, indicating a nontrivial effect of magnetic order on the electronic properties of cuprates. In particular, our results visually demonstrate the crossover from the state with moderate anisotropy of resistivity ρc/ρab ∼ 30 to a strongly anisotropic state with ρc/ρab ∼ 7 × 103 upon cooling as well as upon a decrease in the hole concentration in the CuO2 planes. It is also shown that anisotropy is sensitive to the magnetic state of CuO2 planes and attains its maximum value after the establishment of the long-range antiferromagnetic order. The results are discussed in the framework of the theory based on the t-t′-t″-J model of CuO2 layers taking into account strong electron correlations and short-range magnetic order. In this theory, anomalies of spin correlators and Fermi surface topology for a critical hole concentration of p* ≈ 0.24 are demonstrated. The concentration dependence of the charge carrier energy indicates partial suppression of energy due to the emergence of a pseudogap at p < p*. This theory explains both the experimentally observed sensitivity of anisotropy in conductivity to the establishment of the antiferromagnetic order and the absence of anomalies in the temperature dependence of resistivity ρab(T) in the vicinity of the Néel temperature.