A. P. Rusakov

Anomalous thermal expansion of high-Tc superconducting oxide system

Abstract We present the experimental resalts of Ba1−xKxBiO3 and La2−xSrxCuO4 thermal expansion in the wide temperature and concentration intervals. The nature of observed thermal expansion anomalies in these and other HTSC systems is discussed.

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.

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.

Strong influence of magnetic field on the thermal expansion anomaly in La2−xSrxCuO4 and Ba1−xKxBiO3 systems

Abstract The strong magnetic field influence on the anomalous (negative) thermal expansion was observed for La 1.9 Sr 0.1 CuO 4 single crystals and Ba 0.6 K 0.4 BiO 3 polycrystals with cubic lattice symmetry. The observed magnetic field influence was the most prominent in the area of negative thermal expansion α at low temperatures. With magnetic field growing (up to 4T) the temperature interval with negative α was decreasing.

Thermal expansion anomaly of MgB2 at low temperatures and magnetic field influence

Abstract The measurements of thermal expansion α ( T ) of MgB 2 were carried out at low temperatures both in zero magnetic field H and at H =36 kOe. The anomalous (negative) thermal expansion and strong magnetic field influence on the α ( T ) was observed. The results indicate the similarity of the anomalous properties of MgB 2 and oxide high temperature superconducting systems.

Superstructural ordering and phase transitions in oxide HTSC

Abstract From the experimental data analysis it follows that in oxide high-temperature superconductor (HTSC) systems there is the superstructural ordering which includes both spin and charge density waves. As a result the lattice period doubles and electron spectrum reconstructs with the appearing of dielectric state. At low doping (underdoped samples) for such reconstructed spectrum the Fermi surface arises and dielectric to metal transition takes place. At the higher doping level (optimal doping) the nearly square Fermi surface in the k x – k y plane appears around points (± π , ± π ). The changes of the Fermi surface with doping coincided with the experimental photospectroscopy data (ARPES). These findings explain the anomalous thermal, magnetic and other properties of the oxide HTSC.

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.

Anomalous low-temperature behavior of the thermal characteristics of MgB2

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB2) in the vicinity of Tc and at lower temperatures. It was established that MgB2, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at Tc≈40 K, MgB2 exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB2 and to an increase in the density of the Bose condensate corresponding to these carriers at Tc2≈10–12 K.

The effect of superstructural ordering on the properties of high-temperature oxide superconductor systems

The effect of superstructural ordering in the oxygen sublattice (in addition to the influence of the antiferromagnetic interaction of copper ions) on the electron and phonon characteristics of oxide high-temperature superconductor (HTSC) systems has been studied. Taking into account this ordering effect, it is possible to explain a wide range of experimental data, including doping-induced changes in shape of the Fermi surface, features of the phonon spectra, the existence of stripes, the presence of a pseudogap and its coexistence with the superconducting gap, and some peculiarities in the phase diagrams of HTSCs.

Magnetostriction and thermal expansion of BaPbyBi1−yO3 and Ba1−xKxBiO3 at low temperatures

Abstract The effect of magnetic field influence on the thermal expansion anomaly (negativity) was observed for Ba1−xKxBiO3, La2−xSrxCuO4 and BaPbyBi1−yO3 systems. With growing magnetic field H the temperature T ∗ of thermal expansion ΔL/L minimum decreases (L – sample dimension). For BaPbyBi1−yO3 samples it was revealed the influence of H on relative change of ΔL/L measured through the magnetostriction curves both in superconducting and normal metal phases. The observed effect is explained by the influence of the superstructure ordering in the oxygen sublattice supplementing the ordering in metal (Bi or Cu) sublattice.