M. V. Magnitskaya

Lattice dynamics and melting features of Li and Na

The high-pressure melting of Li and Na has been studied using ab initio calculations of the lattice dynamics. It has been shown that the recently discovered anomalous melting of Na is adequately explained by the phonon spectrum behavior and, accordingly, the thermal vibration amplitudes under compression. In a simple approach using the Lindemann criterion, the nonmonotonic behavior of the melting curve Tm(p) of Na has been quantitatively described within very wide pressure and temperature ranges, and, in particular, the melting temperature drop at p ∼ 1 Mbar down to values lower than those at normal pressure. This approach leads to a nonphysical discontinuity of the melting curve Tm(p) of Li near the bcc-fcc-liquid triple point. This is due to the “softness” of the phonon spectrum of the bcc phase of Li that is the necessary condition for the existence of the high-temperature bcc phase. The melting of Na and Li is used as an example to determine why the Lindemann criterion is efficient in some situations and is inapplicable in the other cases.

New ferromagnetic compound CaCo2 (C15) synthesized at high pressure

The compound CaCo2 with the C15 cubic Laves phase structure and an estimated density of 5.21 g/cm3 has been synthesized at 8.0 GPa pressure. Magnetization measurements showed that the compound CaCo2 is a ferromagnet with Curie temperature 528 K and magnetic moment per Co atom 1.75 μB at T=4.2 K. LMTO calculations of the electronic band structure showed that CaCo2 forms as a result of an s-d electronic transition of Ca and in the ground state it is a ferromagnet with a high magnetic moment per Co atom.

Some problems in the theory of perovskite ferroelectrics

A brief review is given on the studies of ferroelectricity in perovskites. Early phenomenological theories of displacement-type ferroelectrics are considered. Two markedly different approaches based on density-functional theory are described. In one approach, the electron density of crystal is determined as a sum of Bloch functions. In the other, it is constructed as a superposition of the electron densities of individual ions. The latter approach gives a microscopic justification for the old phenomenological theories. We show that some phenomena in perovskites, which seemingly are of the order–disorder origin, can be explained by quasi-one-dimensional peculiarities of lattice dynamics.

Ab initio calculations of the superconducting transition temperature for NbC at various pressures

Ab initio calculations of the superconducting properties have been performed for niobium carbide (NbC) at normal pressure and upon a 15 and 30% compression. Factors accounting for the relatively low values of the transition temperature Tc in transition metal carbides are considered and the possible ways of increasing this parameter are discussed.

Effect of high pressure on the phonon spectra and superconductivity in ZrN and HfN

We report ab initio calculations of the phonon spectra, the electron-phonon interaction, and the superconducting transition temperature T(c) for zirconium and hafnium nitrides under high pressure. The calculated phonon densities of states are qualitatively similar to Raman spectra measured at various pressures up to 32 GPa. The critical temperature T(c) is determined by directly solving the Eliashberg equation using our calculated Eliashberg function. The pressure derivative of T(c) for ZrN is in good agreement with the low pressure experimental result. In the case of HfN, for which there are no relevant experimental data, the available phenomenological estimate of T(c) is significantly different from our first-principles prediction and we discuss the reasons for such a discrepancy. The calculated dependence T(c)(p) becomes nonlinear at p > 10 GPa. We show that in these compounds a decrease in T(c) upon compression mainly occurs because of a decrease in the electron-phonon coupling constant lambda, which is, in turn, due to an increase in the phonon frequencies.

Effect of high pressure on the electron-phonon interaction and superconductivity in ZrN and HfN

Ab initio calculations of the superconducting properties have been performed for zirconium and hafnium nitrides at normal and high pressures. The results for ZrN are in good agreement with the existing data of the tunnel experiments and measurements of the pressure derivative of the critical temperature Tc. It has been shown that the decrease in Tc under compression occurs due primarily to an increase in the phonon frequencies.

Electrophysical properties of calcium at high pressures and temperatures

Electrical resistivity of two crystal phases of shock-compressed calcium and its melt was measured in a range of high pressures (10–50 GPa) and temperatures (800–1600 K). The thermodynamic equilibrium curves were constructed for different calcium phases and the shape of Hugoniot adiabat was determined in the region where it intersects the equilibrium curves. It is shown that sharp kinks observed earlier in the Hugoniot adiabat in shock experiments were caused not by the jumplike electronic transitions but by the intersections of the adiabat and the phase-equilibrium and melting curves. The electronic spectra of the calcium crystal phases were calculated using the electron-density functional method; the computational results are used to explain the observed behavior of the Ca resistivity under compression.

Lattice dynamics and the phase diagram of sodium at high pressures and temperatures from ab initio calculations

The melting of sodium was investigated by two ab initio methods: (i) based on the calculations of the phonon spectra in the quasi-harmonic approximation with the use of the Lindemann criterion and (ii) by molecular-dynamics simulation. It is shown that the anomalous behavior of the melting curve Tm(p) for Na is explained well by the change in the phonon spectrum under compression; in particular, the decrease in Tm(p) at p > 30 GPa is due to the strong softening of transverse-phonon frequencies. The results obtained within both approaches are in good agreement and yield a reasonable quantitative description of the experimental melting curve of sodium at pressures up to ∼1 Mbar and temperatures from 300 to 1000 K. The good agreement between the two approaches indicates the smallness of anharmonicity effects in Na.

Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures P ∼ 5–15 GPa. Data on the thermopower of calcium in the pressure range up to 9 GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at 5–6 GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower (5–12 μV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of 5–7.5 GPa is fairly noticeable (∼10%). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.

Study of the structure of a superconducting state of Co-doped BaFe2As2 multiband compounds

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe1 − xCox)2As2 compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ1 ≈ 15 cm−1, Δ2 ≈ 21 cm−1, and Δ3 ≈ 30–35 cm−1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe2As2 multiband compounds.