Aleksander I. Golovashkin

Thermal expansion in the Ba1?x K x BiO3 system

A negative thermal expansion was measured for Ba1−xKxBiO3. This anomaly decreases with increasingx, still exists atx=0.4, and disappears atx=0.5.

Hole localization, electronic structure and high-Tc superconductivity

Abstract It is shown that the dielectric gap in the electronic spectrum of oxide high- T c systems appears because of ordering arrays of localized (heavy) holes or the charge density waves. Dielectric-metal (DM) phase transitions in these systems are the consequence of impurity bands appearing at doping, delocalization of the ordered heavy holes located on the metal-apical ion bonds. The free carriers introduced by doping can couple through excitation of these localized holes. This leads to high- T c superconductivity with small coherence length and anomalous temperature dependence of H c2 ( T ). These results were obtained on the basis of experimental studies of specific heat, magnetic properties, thermal expansion and other characteristics. The physical nature of the observed DM transitions and negative thermal expansion in HTSC systems are discussed.

Anomalous thermal expansion in HTS

The data about anomalous thermal expansion α(T) at low temperatures are presented for number of oxide HTS system. The anomaly of thermal expansion in such systems is explained in the terms of charge density waves instability.

Low-temperature anomalies in the specific heat and thermal conductivity of MgB2

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB2 were measured at low temperatures and in the neighborhood of Tc. In addition to the well-known superconducting transition at Tc≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB2 was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB2 of a second group of carriers and its transition to the superconducting state at Tc2≈10−12 K.

Upper critical magnetic fields of NbC from clean to dirty limit

For exceptionally low-defect films of superconducting compound NbC with critical temperature Tc=12 K we have researched the upper critical field Hc2(T) and Tc change caused by high-energy He+ ion irradiation. From the dependence of Hc2 on resistivity ρ the electron characteristics have been evaluated. The parallel critical field H‖(T) of the thin films in a vortex-free state and the Tc degradation mechanism were discussed.

The anomalous magnetic properties of high-TC Ba–K–Bi–O in superconducting state

Abstract The magnetic response of HTSC Ba–K–Bi–O single crystals in superconducting state was investigated with the help of a small Hall detector. The magnetic moment of the sample was measured as a function of the magnetic field up to 650 Oe and as a function of temperature while heating in the external magnetic field. Distinct paramagnetic response (PMR) was observed. In contrast to the other works where “paramagnetic Meissner effect” was noted these experiments were carried out on zero field cooled samples and the paramagnetic response was exhibited both at very low (

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ, HoBa2Cu3O7−δ, and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa2Cu3O7−δ, HoBa2Cu3O7−δ, and Nd1.85Ce0.15CuO4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in Hc2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering.

EPR observations of anomalous triplet states in Ba1−xKxBiO3 and BaPbyBi1− yO3

Electron paramagnetic resonance (EPR) spectra of samples of the systems Ba1−xKxBiO3 and BaPbyBi1−yO3 are investigated over wide ranges of composition and temperature. Two main lines in the EPR spectrum with factors g1≈2.1 and g2≈4.2 are found for all compositions. It is shown that the observed EPR line with g2≈4.2 is due to oxygen ions. This probably indicates the presence of oxygen ions with different effective charges, i.e., the existence of charge density waves in the oxygen-ion sublattice in addition to charge density waves in the bismuth sublattice.

The dc-current destruction of superconducting state and hysteresis effects in Ba−K−Bi−O

The dc-current destruction of superconducting state in Ba−K−Bi−O was registered simultaneously both by measuring V–I characteristic and magnetic susceptibility χ. The experiments were carried out on the polycrystall samples in the temperature interval 1.5–4.2 K. The hysteresis of both characteristics was observed, the temperature dependencies of hysteresis value of V–I and c-I being quiet opposite. The nonmonotonic temperature dependence of I has been found. The observed effects can be explained in the framework of the model, according to which the special nonhomogeneous state can arise—the space structure of the alternate superconducting and insulating regions with Josephson tunnelling between the former.

Low-temperature anomalies in MgB2 thermal expansion

The thermal expansion coefficient α(T) of MgB2 was measured at low temperatures both in a zero magnetic field and at H=36 kOe. As in the oxide HTSCs, a region of anomalous (negative) thermal expansion and a strong effect of magnetic field on α(T) were revealed. The results obtained indicate the anomalous properties of MgB2 and the oxide HTSCs to follow a common pattern.