K. R. Zhdanov

Spin transition and thermal expansion in the layered cobaltite GdBaCo2O5.5

The electrical conductivity and thermal expansion coefficient of GdBaCo2O5.5 samples have been measured in order to reveal the mechanism of the metal-insulator transition in cation-ordered cobaltites RBaCo2O5.5 (where R is a rare-earth element) and its relation to the change in the spin state of cobalt ions. It has been established that the unit cell volume considerably increases upon the transition to the metallic state at TMI ≈ 360–365 K and that the thermal expansion exhibits anomalies (which are two orders of magnitude weaker) due to the ferromagnetic and antiferromagnetic orderings. The data obtained confirm that the spin transition in Co3+ ions actually proceeds simultaneously with the metal-insulator transition and excludes the possibility of stepwise spin transitions occurring at lower temperatures.

Boundary line of the transition into the pseudogap state in thulium cuprates

Precision measurements of the heat capacity of 1-2-3 thulium cuprates with oxygen content ranging from 6.3 to 6.92 were performed in the temperature range 6–300 K. Analysis of the experimentl data showed anomalies in the temperature dependence of the electronic heat capacity. It is conjectured that the anomalies are due to a transition from the normal metallic into the pseudogap state.

Anomalies of the electronic heat capacity of thulium cuprates in the pseudogap phase region

Precision measurements of the heat capacity of thulium 1-2-3 cuprate with oxygen concentrations x=6.92 and x=6.7 are made in the temperature range 6–300 K. After the lattice components of the heat capacity of the samples are subtracted off, a comparison of the results makes it possible to determine the contribution to the heat capacity due to the formation of the pseudogap phase for the sample with x=6.7. The temperature dependence of this contribution has an anomaly near the boundary of the transition from the pseudogap phase to the normal metallic phase. The experimental results correlate with the theoretical ideas about the rearrangement of the electronic structure upon the transition of the system to the pseudogap phase region.

Magnetoresistance of lightly doped TmBa2Cu3Ox crystals. Reorientation of the antiferromagnetic structure in a magnetic field

The magnetoresistance of lightly doped TmBa2Cu3Ox single crystals is investigated in the temperature range 4.2–300 K for magnetic fields up to 12 T. For the antiferromagnetic sample (x=6.3), when the current and field lie in the ab plane, the magnetoresistance is the sum of an anisotropic and a background component. The existence of the anisotropic component is attributed to the restructuring of the antiferromagnetic domain structure in a magnetic field.

Anomalies of thermal expansion and electrical resistivity of layered cobaltates YBaCo2O5 + x: The role of oxygen chain ordering

Layered cobaltates YBaCo2O5 + x have been investigated in the oxygen concentration range 0.23 ≤ x ≤ 0.52. It has been revealed that the oxygen ordering plays the key role in the appearance of anomalies in temperature dependences of structural parameters and electron transport. It has been shown that the orthorhombic lattice distortion caused by oxygen chain ordering is a necessary “trigger” for the phase transition from the insulating state to the metallic state at T ≈ 290–295 K, after which the orthorhombic distortion is significantly more pronounced. In the boundary region of the cobaltate compositions, where the oxygen ordering has a partial or local character, there are additional low-temperature (100–240 K) structural and resistive features with a large hysteresis. The observed anomalies can be explained by a change in the spin state of the cobalt ions, which is extremely sensitive to parameters of the crystal field acting on the ions, as well as by the spin-transition-induced delocalization of electrons.