Abstract
The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in YBa2Cu3O7-δ single crystals has been studied. It has been shown that an increase in the critical onset temperature of the transition to the superconducting state during thermocycling annealing procedures is consistent with the decrease of the σс/σаb parameter. This fact indicates a redistribution of the electronic density between the structurally inhomogeneous Cu(2)O2 and Cu(1)O1-δ planes, due to the formation of oxygen long-range order in the O(4)–Cu(1)–O(4) linear groups along the (b) crystal structure axis of the unit cell, and elimination of oxygen defects in the square nets of the Cu(2)O2 planes. The existence of the critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Тс, has been confirmed. In this case an increase in Тс and orthorhombic distortion of the crystal structure during isothermal annealing are caused by the amplification of the “interlayer” interaction between the Cu(2)О2 and Cu(1)О1-δ planes. As a result, the contribution of the Cu(1)О1-δ chain layers to the electron state density at the Fermi level increases. These layers can acquire superconducting properties due to tunneling of Cooper pairs from the Cu(2)О2 planes resulting in the formation of the induced superconductivity in these planes.
Highlights
It was determined that an increase in the critical temperature of the onset of the transition to the superconducting state during step annealing procedures is consistent with decrease of the σс/σаb parameter
This fact indicates the redistribution of the electronic density between the Cu(2)O2 and Cu(1)O1-d structurallyinhomogeneous planes, due to the formation of the oxygen long-range ordering in the O(4)—Cu(1)—O(4) linear groups along the (b) crystal structure axis of the unit cell, and removal of the oxygen defects in the square nets of the Cu(2)O2 planes
The existence of a critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Тс, has been proved
Summary
Установлено, что увеличение значений критической температуры начала перехода в сверхпроводящее состояние при термоциклирующих отжигах согласуется с уменьшениями параметра σс/σаb, что указывает на перераспределение электронной плотности между структурнонеоднородными плоскостями Cu(2)O2 и Cu(1)O1-d за счет формирования длинноцепочечного упорядочения кислорода в линейных группах O(4)—Cu(1)—O(4) вдоль кристаллоструктурной оси (b) элементарной ячейки, и устранению кислородных дефектов в квадратных сетках плоскостей Cu(2)O2. Несмотря на многочисленные работы [8,9,10,11,12,13,14,15], условия изменения упорядочения кислородных вакансий между кристаллоструктурными положениями (0 1/2 0) и (1/2 0 0) в анионной подрешетке YBa2Cu3O7-d, особенно при d → 0, недостаточно изучены. Определение значения пороговой температуры (Тп, К), при достижении которой энергия тепловых колебаний атомов превышает энергию связи кислорода в цепочках —Сu(1)—О(4)—Сu(1)—О(4)— и становится причиной нарушения упорядочения кислородных вакансий в анионной подрешетке, является актуальной задачей
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