StatusThe thesis was presented on the 23 December, 2019
Approved by NCAA on the 31 July, 2020
Abstract– 2.55 Mb / in romanian
– 2.41 Mb / in english
ThesisCZU 539.21:538.945 (043.2)
7.30 Mb /
The thesis ,,Structural, magnetic and electronics correlations in nonconventional superconductors FeTe1-xSe(S)x and RbFe2Se(S)2 ” is prezented by Mrs. Dorina Croitori for the Ph.D degree in Physical Sciences. The dissertation has been completed in Chisinau in 2019. The field of research ,,Materials sciences and technology”.
Structure of thesis: introduction, five chapters, general conclusions and recomendations, bibliografy by 154 tittles, 86 pages of text, 87 of figures, 18 tables. The obtained results are published in 18 papers and abstracts at scientific conferences.
Keywords: nonconventional superconductors, antiferromagnetism, phase separation.
Scope of work: preparation and investigation of structural, magnetic, and the rmodinamic properties of iron chalcogenides Fe1+yTe1-xSex, AFeX2 (A = Rb, K, Cs; X = S, Se), and Rb1-xFe2-ySe2-zSz with uni- and bidimensional structure. Analisis of the mecanisms of superconductivity, magnetic exchange, of structural and electronic correlations.
Research objectives: development of the technology of synthesis of polycrystals and growth of single crystals of Fe1+yTe1-xSex, AFeX2 (A = Rb, K, Cs; X = S, Se) and Rb1-xFe2-ySe2-zSz with variable composition and concentraton of anion substitution. Investigation of physical properties of compounds in function of their composition and substitution in a large range of temperature (1.8 K-700 K) and magnetic fields (up to 9 T). Study of sample stoichiometriy, structural and magnetic analysis, determination of superconductor parameters, analysis of mecanisms of conductivity and of thermodynamic properties having the main scope to evidentiate the structural, magnetic, and electronic correlations.
Novelty and scientific originality: The perfect single crystals of Fe1+xTe, Fe1+yTe1-xSex, AFeX2 (A = Rb, K, Cs; X = S, Se), and Rb1-xFe2-ySe2-zSz with record values of superconducting and magnetic properties have beed grown. The principal structural, magnetic, and superconducting parameters and their variation with substitution have been determined. The percolation treshold for the superconducting state in Rb1-xFe2-ySe2-zSz system have been established. The anisotropic character of the antiferromagnetic order which coexists with the superconducting state within the phase separated scenario was evidenced. The vacancy ordering of Fe ions and the character of the structural transitions into desordered and phase separated state was revealed. It was demonstrated that the suppression of superconducting state is caused by the reduction of the density of states at the Fermi level. For the first time, the phase diagram of Rb1-xFe2-ySe2-zSz system which describes physical ground states, structural, magnetic, and electronic correlations was established.
Obtained resuls that contribute to solution of important scientific problem: The technology of the growth of perfect single crystals Fe1+yTe1-xSex, AFeX2 (A = Rb, K, Cs; X = S, Se), and Rb1-xFe2-ySe2-zSz with variable composition and concentration of anion substitution with performant magnetic and superconducting parameters of prospective for application in advanced electronics and electrotechics was developed. The mechanisms that govern the change of the physical properties of materials under study in function of composition and substitution concentration were established.
Theoretic value: The experimental results obtained for the first time in the thesis constitue the fundamental base for development of theory of nonconventional superconductivity and can form the base for new theoretical approaches related to phenomenon of coexistence of superconducting and antiferromagnetic states and to effect of phase separation. Applied value: Advanced technical characteristics of the studied materials and their applied potential are of particular importance for design of novel electronics and electrotechnic devices.
Implementation of scientific results: The obtained results were presented at a number international and national științific conferences and published in national and international journals with impact factor.
Under consideration  :