Evgenii G. Maksimov

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

Ya. G. Ponomarev, S. A. Kuzmichev, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, N. Z. Timergaleev, A. V. Yarigin, M. V. Lomonosov Moscow State University, Department of Physics, 119899 Moscow, Russia, E. G. Maksimov, S. I. Krasnosvobodtsev and A. V. Varlashkin, P. N. Lebedev Physics Institute, RAS, Moscow, Russia, M. A. Hein, G. Müller, H. Piel, Bergische Universität Wuppertal, Fachbereich Physik, D-42097 Wuppertal, Germany, L. G. Sevastyanova, O. V. Kravchenko, K. P. Burdina, B. M. Bulychev, M. V. Lomonosov Moscow State University, Department of Chemistry, 119899 Moscow, Russia.

Investigation of a superconducting Mg1−xAlxB2 system by tunneling and microjunction (Andreev) spectroscopies

A detailed investigation of multiband superconductivity in the Mg 1−xAlxB2 system was carried out by the methods of tunneling and Andreev spectroscopy. Temperature dependences of the superconducting gaps and their variation upon an increase in the degree of disorder and the Al concentration were studied. It is shown that the experimentally observed dependences cannot be explained in detail within the framework of the presently available microscopic theories.

Two-band Bardeen-Cooper-Schrieffer superconducting state of the iron pnictide compound Ba(Fe 0 . 9 Co 0 . 1 ) 2 As 2

The conductivity and permittivity optical spectra of iron-pnictide Ba(Fe0.9Co0.1)2As2 film (Tc=20 K) are analyzed. In the superconducting state, at all temperatures up to Tc the temperature dependences of the magnetic field penetration depth and of the superconducting condensate density are well described within the generalized two-band BCS model with intraband and interband pairing interactions considered. It is shown that the smaller superconducting energy gap 2{\Delta} = 3.7 meV develops in the electronic subsystem while the larger gap 2{\Delta}>= 7 meV opens in the hole subsystem. The normal state parameters (plasma frequencies and scattering rates) of electron and hole conduction bands are determined. At all temperatures the obtained data are consistent with the results of electronic photoemission experiments on Ba(Fe1-x Cox)2As2.

Leggett’s mode in Mg1−x AlxB2

A detailed investigation of multiband superconductivity and Leggett’s mode in the Mg1−xAlxB2 (0 ≤ x ≤ 0.45) system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps Δσ and Δπ and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett’s mode energy ε0 upon the values of the gaps Δσ and Δπ has been derived.

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.

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.

Theoretical analysis of tunneling experiments in the MgB2 system

A detailed theoretical analysis of the experimental data obtained earlier in the studies of the tunneling spectra in the MgB2 two-band superconducting system has been performed. It is shown that most these data are well described in the generalized two-band Bardeen-Cooper-Schrieffer theory with the constants of the intraband electron-phonon interaction that reasonably coincide with the ab initio calculation results. It is shown that the existence of specific collective excitation in this system induced by oscillations of the relative phase of two superconducting condensates (the Leggett mode) indicates the overestimation of the constants of the interband electron-phonon interaction in the ab initio calculations. The dependences of the superconducting gaps and the Leggett mode frequency on the temperature and the disorder degree in the Mg1 − xAlxB2 system have been thoroughly studied.

Origin of low-energy excitations in charge-ordered manganites

The low-energy excitations in the charge-ordered phase of polycrystalline La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In the frequency range from 4 cm−1 to 700 cm−1 (energies from 0.4 meV to 90 meV) and at temperatures down to 5 K, we do not detect any feature that can be associated with the collective response of the spatially modulated charge continuum. In the antiferromagnetically ordered phase, broad absorption bands appear in the conductivity and permittivity spectra around 30 cm−1 and 100 cm−1 which are assigned to former acoustic phonons optically activated due to a fourfold superstructure in the crystal lattice. Our results indicate that characteristic energies of collective excitations of the charge-ordered phase in La0.25Ca0.75MnO3, if any, lie below 1 meV. At our lowest frequencies of only few wave numbers a strong relaxation is observed above 100 K connected to the formation of the charge-ordered state.

Investigation of Specific Features of the Lattice Dynamics and the Ferroelectric Transition in Perovskite Crystals

The specific features of diffuse X-ray scattering in BaTiO3, KNbO3, and PbTiP3 perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO3. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO3 and KNbO3 crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO3, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO3, KNbO3, and PbTiO3 are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.

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.