Intersite elastic coupling and invar effect

The invar phenomenon (very small thermal expansion in some iron alloys or compounds) is usually explained by the thermally-induced transitions between different spin states of Fe, having different atomic volumes. We consider these processes taking into account elastic interaction between Fe atoms in different spin states. Inclusion of these interactions explains why thermal expansion may be close to zero in a broad temperature interval and thus gives rise to the invar effect

Charge redistribution and properties of high-temperature superconductors

We show that in high-Tc superconductors (HTSC) with two groups of electrons (e.g., holes in CuO2 planes and in a ‘‘reservoir’’) there should exist a charge redistribution with the temperature: the hole concentration Nh in ‘‘active’’ superconducting CuO2 planes increases below Tc. This effect may explain structural changes such as the shift of the apical oxygen atom, anomalous thermal expansion, the shift of nuclear quadrupole resonance lines, the change of the positron lifetime, and the modification of the ion channeling below Tc. Some other possible consequences of the charge redistribution (the modification of the temperature dependence of a gap Δ and of the ratio 2Δ0/Tc, the phenomena at a contact of HTSC with normal metals and semiconductors) are discussed. We show that in high-Tc superconductors (HTSC) with two groups of electrons (e.g., holes in CuO2 planes and in a ‘‘reservoir’’) there should exist a charge redistribution with the temperature: the hole concentration Nh in ‘‘active’’ superconducting CuO2 planes increases below Tc. This effect may explain structural changes such as the shift of the apical oxygen atom, anomalous thermal expansion, the shift of nuclear quadrupole resonance lines, the change of the positron lifetime, and the modification of the ion channeling below Tc. Some other possible consequences of the charge redistribution (the modification of the temperature dependence of a gap Δ and of the ratio 2Δ0/Tc, the phenomena at a contact of HTSC with normal metals and semiconductors) are discussed