A. A. Pronin

Glassy dynamics under superhigh pressure.

Nearly all glass-forming liquids feature, along with the structural alpha-relaxation process, a faster secondary process (beta relaxation), whose nature belongs to the great mysteries of glass physics. However, for some of these liquids, no well-pronounced secondary relaxation is observed. A prominent example is the archetypical glass-forming liquid glycerol. In the present work, by performing dielectric spectroscopy under superhigh pressures up to 6 GPa, we show that in glycerol a significant secondary relaxation peak appears in the dielectric loss at P>3 GPa. We identify this beta relaxation to be of Johari-Goldstein type and discuss its relation to the excess wing. We provide evidence for a smooth but significant increase in glass-transition temperature and fragility on increasing pressure.

Interplay between the structure and properties of new metastable carbon phases obtained under high pressures from fullerite C60 and carbyne

A brief review of structural, electrotransport, optical, elastic, and mechanical properties of carbon phases synthesized under pressure by heating fullerite C60 and carbynoid materials is given. A large variety of carbon modifications with a variable bonding type, a variable mean coordination number, a variable molecular or atomic structural type, a variable characteristic dimensionality (from zero-to three-dimensional structures), a variable degree of covalence, etc., were prepared. Emphasis in the review is given to the elucidation of the interplay between the structural and topological characteristics of carbon phases and their key electronic and mechanical properties. A version of the kinetic phase diagram of fullerite C60 transformations on heating under pressure is also suggested. This version is modified with respect to the interpretations known in the literature.

New scenario for the decay of spin-Peierls state in CuGeO3: Fe. Onset of a quantum critical point

It is found experimentally that the introduction of 1% Fe in the CuGeO3 matrix gives rise to a strong disorder in the magnetic subsystem and leads to an appearance of the χ∼1/Tα (α ≈0.36) low-temperature asymptotic form for magnetic susceptibility over a wide temperature range 1.7≤ T≤30 K. A model attributing this anomaly to the suppression of spin-Peierls state as a result of the formation of the quantum critical point is proposed.

Nature of the low-temperature anomalies in the physical properties of the intermediate-valent compound SmB6

The transport properties (Hall coefficient, thermopower, and resistivity) of high-quality single-crystal samples of the classical mixed-valent compound SmB6 are investigated over a broad temperature range (1.6–300 K) in magnetic fields up to 45 T for the first time following the quasioptical measurements in the 0.6–4.5 meV frequency range [B. Gorshunov, N. Sluchanko, A. Volkov et al., submitted to Phys. Rev. B (1998)]. Measurements in the intrinsic conduction region permit determination of the gap width Eg≈20 meV and evaluation of the behavior of the mobility and concentration of light and heavy charge carriers, as well as the temperature dependence of the carrier relaxation time, in samarium hexaboride. The results of experimental investigations in the “impurity” conduction region (Eex≈3.5 meV) are discussed within the Kikoin-Mishchenko exciton-polaron model of charge fluctuations. Arguments supporting the formation of a metallic state with an electron-hole liquid in SmB6 at liquid-helium temperatures are presented.

Pressure-induced change in the relaxation dynamics of glycerol

Glycerol is one of the best studied and most widely used glass-forming liquids; however, its dynamic properties are still under discussion. The dielectric spectra of glycerol are studied in detail over wide ranges of temperatures and pressures up to 4.5 GPa. Starting from the pressures of 2–3 GPa, qualitative change in the dynamics of structural relaxation processes in glycerol has been revealed. It is accompanied by the appearance of secondary relaxation and a change in the asymptotic behavior of the pressure dependence of the fragility. The relation between the parameters for different relaxation mechanisms is analyzed.

Microwave EPR spectroscopy of cobalt-doped germanium cuprate

Resonance magnetoabsorption spectra of CuGeO3 single crystals containing 2% Co impurity have been studied in the frequency range 60–360 GHz in magnetic fields of up to 16 T and in the temperature interval 2–60 K with the magnetic field B aligned parallel to the a crystallographic axis. In addition to the Cu2+ chain resonance, a new absorption line (unobserved previously in doped CuGeO3 and deriving apparently from the Co2+ ions) was detected in EPR spectra. Quantitative analysis of the spectra suggests that the spin-Peierls transition occurs in about 10% Cu2+ chains, while the spin-Peierls state in the remaining 90% chains is completely destroyed by cobalt doping. The results obtained reveal considerable deviations from the universally accepted scenario of CuGeO3 doping and are discussed within alternative theoretical models, namely, the quantum critical behavior (based on the EPR theory for quasi-one-dimensional systems) and a three-dimensional antiferromagnet with the Néel temperature lowered by disorder.

Magnetoresistance of carbon nanomaterials

This paper contains an analysis of published experimental data on the magnetoresistance in the region of Mott hopping conduction in various carbon materials whose properties are determined by nanoscale inhomogeneities. The validity of the mechanism of shrinkage of a localized state in a magnetic field and the spin polarization mechanism involving the spin-dependent contribution of hopping over doubly occupied electronic states is considered. A simple analytic model is proposed for the spin polarization mechanism. An expression for magnetoresistance in a weak magnetic field is derived within the proposed model. It is demonstrated that only a combined effect of the spin-polarization mechanism and wave function shrinkage is capable of providing an adequate description of the magnetoresistance of carbon nanomaterials. Promising areas to search for objects with high magnetoresistance among the materials of this class are discussed.

Effect of BiFeO3 ceramics morphology on electrodynamic properties in the terahertz frequency range

The nature of the contribution (additional to the phonon one) to the permittivity of bismuth ferrite ceramics has been studied using submillimeter and Fourier transform infrared spectroscopy. It has been shown that its numerical value depends on morphological features of samples, and the nature is determined by the defectness and crystallite interfaces. The effect of structure-phase features of bismuth ferrite ceramics on antiferromagnetic resonance line frequencies is indicated.

Quantum critical behavior induced by Mn impurity in CuGeO3

Results of high frequency (60-315 GHz) studies of ESR in CuGeO3 single crystals containing 0.9% of Mn impurity are reported. Quantitative EPR line shape analysis allowed concluding that low temperature magnetic susceptibility for T<40 K diverges following power law with the critical exponent 0.81 and therefore manifests onset of a quantum critical (QC) regime. We argue that transition into Griffiths phase occurs at TG~40 K and disorder produced by Mn impurity in quantum spin chains of CuGeO3 may lead to co-existence of the QC regime and spin-Peierls dimerisation.

Electron transport in carbynes modified under high pressure

The studies of hopping conductivity in carbynes modified under high pressure are reviewed. Experimental data are presented on the dc and ac conductivities, thermopower, magnetoresistance, and Hall effect. The results obtained are discussed within the framework of a model that takes into account the substantially nonuniform distribution and clusterization of sp2 bonds in the carbyne sp matrix.