Order and disorder in the ensemble of Cu-O chain fragments in oxygen-deficient planes ofYBa2Cu3<

Abstract

In connection with numerous X-ray and neutron investigations of some high temperature superconductors (YBa 2 Cu 3 O 6+x and related compounds) a nontrivial part of the structure factor, coming from partly disordered Cu-O-. . .-O-Cu chain fragments, situated within basal planes, CuO x , can be a subject of theoretical interest. Closely connected to such a diffusive part of the structure factor are the correlation lengths, which are also available in neutron and Xray diffraction studies and depend on a degree of oxygen disorder in a basal plane. The quantitative measure of such a disorder can be associated with temperature of a sample anneal, T q , at which oxygen in a basal plane remains frozen-in high temperature equilibrium after a fast quench of a sample to room or lower temperature. The structure factor evolution with x is vizualized in figures after the numerical calculations. The theoretical approach employed in the paper has been developed for the orthorhombic state of YBCO. 74.70.Vy, 64.60.Cn, 64.80.Eb

Spin glass state and its correlations with the magnetic and electrical properties as well as with the defectiveness in the spinels CuCr2X4 (X=S, Se) doped with Sb

“…It means that both the Cu + ions and the 2+ vacancies in the tetrahedral subarray do not change the Cr 3+ ions into Cr 4+ ions, but create the holes either in the sulphur band or in the selenium band. However, it would be combined with both the increase of the electrical conductivity and with the increase of the molecular magnetic moment, comparing with the corresponding values of these quantities in the matrix [17,18]. Because for the samples under study the state of the magnetic saturation can not be reached even at 14 T [19], one has to take into account instead the effective magnetic moment.…”

Metamagnetism, giant negative magnetoresistance and spin glass state in the Sm1−xSrxMnO3 (x=0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55=Eu0.397Nd0.153, Tb0.25Nd0.30) manganites as well as in the CuCr2X4 (X=S, Se) spinels doped with Sb

“…It results in the strengthening of the antiferromagnetic interactions. Such a picture of the electronic structure of the chromium ions recalls the situation in the high temperature type YBa 2 Cu 3 O 6+x superconductors, because in the latter compounds the O vacancies cause the appearance of holes both in the anion band 2p via the change of a certain amount of O 2− ions into O 1− ions and in the copper band 3d via the change of a certain amount of Cu + ions into Cu 2+ ions [10,11]. The simultaneous appearance of holes in the oxygen anion band and in the copper cation band causes that the chains created from both copper and oxygen atoms (where some oxygen atoms in the chain happen to be as O 1− ions) present the paths which ease the transport of the electric charge carriers.…”

Chain length probability distribution — equivalence of ASYNNNI and 1d Ising model

“…It is also important to note that, from expression (6) and the definition of the variable r, we can derive the relationship, = 1/ . Since the variable is, in the case of the 1d Ising chain with ferromagnetic interaction J > 0, bounded from above by 1/2, it follows that the ASYNNNI model can be considered equivalent to the ferromagnetically coupled Ising model only when the condition > 2 is met.…”

“…Since the YBCO material becomes superconducting only after a certain oxygen stoichiometry is achieved, it was assumed that this coincides with oxygen chains reaching some critical length . The theory of critical chain length and the microscopic properties of the chain fragments were investigated in several theoretical studies [5][6][7][8][9]. The general conclusion of these studies was that only chains longer than a critical length, whose value was argued [8,10] to be 3 or 4, can transfer charge out of the basal planes, thus providing the increase in the number of the mobile charge carriers in the CuO 2 sheets which leads to the appearance of superconductivity [11].…”