The structure investigations of fog phase (BPIII) of liquid crystals

Summary The current understanding of the problems concerning the fog phase structure organization based upon experimental results of our group is presented. The correspondence between theoretical models and experimental results is discussed. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Dependences of HTS tape critical parameters on fluences under irradiation with heavy ions and high energy electrons

Experimental data on the study of modifications of two types of the YBCO(123)-based composite tape under irradiation with electrons with the energy of 23 MeV and ¹³²Xe²⁷⁺ (167 MeV), ⁸⁴Kr¹⁷⁺ (107 MeV), and ⁴⁰Ar⁸⁺ (48 MeV) ions in a wide range of irradiation doses are presented. It is shown that no changes in the HTS transition temperature and critical current occurred under electron irradiation up to the dose D ≈ 10⁻⁴ disp/a (displacements per atom). The threshold dose of irradiation with ¹³²Xe²⁷⁺ ions, at which superconductivity disappears, is determined. In comparison with the initial value, the critical current of ⁴⁰Ar⁸⁺ ions is found to increase by ≈18% in the zero magnetic field at T = 77 K. This effect can be explained by the pinning centers for Abrikosov vortices induced by relatively small ion irradiation doses. High doses of heavy ions lead to partial or complete amorphization of a superconductor, which results first in a decrease in the critical current and transition temperature and then in a total disappearance of superconductivity. X-ray diffraction is used to study the changes in the crystal structure of the YBCO(123) superconductor under ion irradiation.     

Optical evidence of quantum rotor orbital excitations in orthorhombic manganites

In magnetic compounds with Jahn–Teller (JT) ions (such as Mn³⁺ or Cu²⁺), the ordering of the electron or hole orbitals is associated with cooperative lattice distortions. There the role of JT effect, although widely recognized, is still elusive in the ground state properties. Here we discovered that, in these materials, there exist excitations whose energy spectrum is described in terms of the total angular momentum eigenstates and is quantized as in quantum rotors found in JT centers. We observed features originating from these excitations in the optical spectra of a model compound LaMnO₃ using ellipsometry technique. They appear clearly as narrow sidebands accompanying the electron transition between the JT split orbitals at neighboring Mn³⁺ ions, displaying anomalous temperature behavior around the Néel temperature T_N ≈ 140 K. We present these results together with new experimental data on photoluminescence found in LaMnO₃, which lend additional support to the ellipsometry implying the electronic-vibrational origin of the quantum rotor orbital excitations. We note that the discovered orbital excitations of quantum rotors may play an important role in many unusual properties observed in these materials upon doping, such as high-temperature superconductivity and colossal magnetoresistance.     

Properties of low-resistance joints between HTS tape conductors prepared by soldering

The electric resistance of contacts between superconducting tape conductors on the basis of the GdBa₂Cu₃O_{7 ? x} compound (2G) is investigated in an external magnetic field up to 5 T at T = 77 K. The mechanical tensile strength of both the initial conductors and the contacts is measured.     

Polarized luminescence in a cubic liquid crystal

In a liquid-crystalline cubic phase the kinetics of the luminescence induced by pulsed excitation of a luminescing dopant has been studied. The influence of the molecular reorientation on the decay kinetics of the polarized components of the light has been investigated. The correlation times of the molecular reorientation and the magnitude of the local order parameter have been determined.     

Surface ordering near the smectic-A-smectic-C transition in thin, free-standing, liquid-crystal films

Optical reflectivity measurements have been conducted near the smectic-A-smectic-C phase transition in free-standing films with thickness between two and eleven molecular layers. The temperature dependence of the reflectivity in thin film differs significantly from that in thick films. The optical thickness per layer increases in films with two to five layers as a result of cooling, in contrast with thick films. The average layer spacing was found to decrease with decreasing film thickness. Zh. éksp. Teor. Fiz. 111, 949–953 (March 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Spatial dispersion in chiral liquid crystals. Effects of higher orders

Abstract

Spatial dispersion effects in chiral liquid crystals are reviewed. New spatial dispersion phenomena are observed in the vicinity of the Bragg reflection wavelength–an anomaly of the refractive index and an optical anisotropy of the cubic blue phases. These effects can be explained by taking into account the spatial dispersion correction of the dielectric tensor of the medium of higher orders in the ratio of the light wave vector to the structural wave vector of chiral phases.     

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

Amorphous chiral smectic A phase of side chain copolymers

The chiral structure of the anomalous smectic phase recently observed in chiral copolymers has been studied by optical methods. The phase is amorphous, but shows a short range order, as a combination of smectic A and cholesteric structures. This phase is presumed to possess locally the TGB_A structure.