Using high‐pressure–high‐temperature treatment at P = 4–7.7 GPa; T = 1373–1473 K with subsequent quenching, a new metastable phase of bismuth selenide (m‐Bi2Se3) is synthesized and its crystal structure, electrical resistivity, and annealing at heating are investigated. Using X‐ray powder diffraction, and analogy with high‐pressure structures of the rare‐earth element sesquichalcogenides, the crystal structure of the new metastable phase of m‐Bi2Se3 is determined as an orthorhombic distorted cation‐deficient structure of the Th3P4 type with the Fdd2 space group and Z = 10.66. The unit‐cell dimensions are: a = 13.4660(7) Å, b = 12.7772(7) Å, c = 9.0896(5) Å. The density of m‐Bi2Se3 (7.47 g cm−3) is slightly less than the initial rhombohedral phase of Bi2Se3, but the coordination number (CN) = 8 is higher than CN = 6 of the initial phase. The period of stability under ambient conditions of the new m‐Bi2Se3 phase is only about 2 months. After 300 °C annealing, m‐Bi2Se3 completely returns to the initial layered structure, and differential scanning calorimetry confirms that the reverse transformation as well as the direct transition occur through an amorphous state. The new phase is a narrow bandgap semiconductor with the energy gap of about 80 meV.

No abstract

No abstract

No abstract

This paper presents the results of a study on the structural properties and dynamics of conductivity of thin (d ∼ 100 nm) films of germanium telluride depending on the phase states reversibly switched by nanosecond pulsed laser radiation with a «top hat» beam profile. It was determined that the threshold of laser radiation energy density at which the phase transition in GeTe thin films from the amorphous to crystalline state is in the range of E = 7.5 ÷ 47.6 mJ/cm2, and the threshold for the reverse transition from the crystalline to amorphous state starts from 47.6 mJ/cm2 and is observed up to 90 mJ/cm2 with no visible damage caused by the ablation. The full time of conductivity change associated with the phase transition between the amorphous and crystalline phases is τCA = 20.2 ns, while for the reverse crystalline to amorphous transition, the conductivity full change time it makes τAC = 52 ns.

No abstract

This paper presents the results of an experimental study, implementation, and numerical simulation of the transmissivity of a polymer waveguide covered by a GST225 thin film with various phase states. The paper considers an optical synapse prototype based on the interface between the waveguide and an optically controlled GST225 film. We demonstrate the fundamental possibility of controlling an optical signal in the telecommunication C-range as it passes through the synaptic interface via the action of an external laser on an optically active GST225 film. Experimentally, 40% single- and multi-level modulations of the optical signal intensity are achieved. The numerical simulation results are in line with the experimental data. Based on this principle, next-generation all-optical storage and computing devices that simulate the properties of biological synapses and neurons can be developed.

No abstract