The effect of chemical pressure on the iron selenide superconductivity

Объектами исследования являются новые сверхпроводящие материалы на основе селенида железа. Цель работы – исследование влияния химического давления на величину критической температуры в халькогенидах железа и получение среди исследуемых соединений соединения с максимальной критической температурой¬. В процессе работы проведён синтез двух систем материалов: FeSe1-xSx (0.1 ≤ x ≤ 0.9) без избытка железа и Fe1.02Se1-xSx-zTez, где 0 ≤ z ≤ x, 0 ≤ x ≤ 1. Определены оптимальные условия синтеза, при которых содержание второй несверхпроводящей фазы в исследуемых материалах предельно мало. Обнаружено, что в системе FeSe1-xSx переход быстро подавляется с ростом концентрации серы, а проводимость материалов становится преимущественно полупроводниковой. Увеличение концентрации серы в диапазоне 0 ≤ x ≤ 0.2 приводит к незначительному росту критической температуры. В системе Fe1.02Se0.5Te0.5-xSx имеет место немонотонная зависимость температуры перехода от концентрации серы – резкое падение критической температуры при небольшом содержании серы и увеличение критической температуры с дальнейшим повышением содержания серы. В системе Fe1.02Se0.6Te0.4-xSx увеличение содержания серы приводит к увеличению температуры перехода на 5 К (х = 0.1) (по сравнению с Fe1.02Se0.6Te0.4) с дальнейшим постепенным снижением её до 12 К (х = 0.4). В системе Fe1.02Te0.5Se1-xSx наблюдается увеличение объёма элементарной ячейки с ростом содержания серы и, вместе с этим, монотонное уменьшение критической температуры. Степень внедрения – материалы пока не внедряются в производство, тем не менее, существует возможность изготовления проводов с внутренней частью из керамики на основе FeSe. Предположение о развитии – исследование сосуществования антиферромагнитного упорядочения и сверхпроводимости в одном материале при разных температурах.The objects under research are new superconducting materials based on the iron selenide. The main goal is to investigate the effect of chemical pressure on the critical temperature value and to create the compound with the maximal critical temperature. During the work two systems of the materials were synthesized: FeSe1-xSx (0.1 ≤ x ≤ 0.9) without iron excess and Fe1.02Se1-xSx-zTez with 0 ≤ z ≤ x and 0 ≤ x ≤ 1. The optimal synthesis conditions are determined when the content of the second non-superconducting phase is as little as possible. It was found that in FeSe1-xSx system the transition is quickly suppressed while sulfur content increases and the materials trend to be semiconductors. The sulfur content increase in 0 ≤ x ≤ 0.2 range leads to the insignificant increase of the critical temperature. In Fe1.02Se0.5Te0.5-xSx system non-monotonic critical temperature versus sulfur concentration dependence takes place: rapidly decrease of the critical temperature with low sulfur content and critical temperature increase with further increase of sulfur content. In Fe1.02Se0.6Te0.4-xSx system the increase of sulfur content to x = 0.1 leads to the 5K increase of Tc (according to Fe1.02Se0.6Te0.4) and the slowly decreases to the value of 12 K at x = 0.4. In Fe1.02Te0.5Se1-xSx system the increase of the unit cell volume is observed while the sulfur content increases; it correlates well with the decrease of the critical temperature. Application extent: the materials aren’t applied to the production yet; nevertheless there is an opportunity to produce the wires with ceramic core based on the FeSe. The development assumption: the investigation of coexistence of antiferromagnetism and superconductivity in the same material at different temperatures.Программа развития УрФУ на 2013 год (п.2.1.1.1

Localization of charge carriers in layered crystals MexTiSe 2 (Me = Cr, Mn, Cu) studied by the resonant photoemission

The probability of charge transfer in layered titanium diselenide between monolayers containing Cr, Mn, and Cu with different concentrations and host lattice TiSe2 is estimated according to the resonant photoemission data. For this purpose, the Raman-Auger contributions and narrow bands just below the Fermi energy were separated in the valence band spectra. These contributions provide the information about charge transfer. It is shown that the localization of the 3d electrons is typical for Cr and Cu atoms and strongly depends on theirs concentration. In MnxTiSe2, Mn 3d electrons are delocalized and the probability of the charge transfer is the greatest as compared with other compounds under investigation. © 2013 AIP Publishing LLC

Features of structural ordering and magnetic properties of the cobalt intercalates of titanium dichalcogenides

CoxTiS2 (x = 0.20, 0.30, 0.50, 0.75) compounds have a layered crystal structure with Ti atoms surrounded by the S octahedrons in the S-Ti-S layers and Co ions which could be surrounded both by the S octahedrons and tetrahedrons. The features of the crystal structure and magnetic properties of the polycrystalline CoxTiS2 samples were studied as a function of the cobalt concentration using X-ray diffraction and magnetization measurements. The disordered state of the Co atoms (x < 0.20) changes to the ordered one (x ≥ 0.20). The magnetic ordering arises at x = 0.20 with mostly antiferromagnetic interaction. For compound with x = 0.30 and 0.50 there is a weak ferromagnetic ordering. The ferromagnetic ordering is observed in Co0.75TiS2 below Tc = 375 K. © 2023, S.C. Virtual Company of Phisics S.R.L. All rights reserved.Russian Science Foundation, RSF: 22-72-00094The study was supported by the Russian Scientific Foundation (grant № 22-72-00094)

Observing of Ti, Cr and Mn 2p-3d resonance in valence band of titanium dichalcogenides

Ti, Mn, Cr 2p-3d resonant photoemission for compounds with substitution of Ti atoms of host lattice by the Cr atoms CrxTi1-xSe2 and for compounds with intercalation of Mn and Cr atoms MnxTiSe2, CrxTiTe2 is studied. It is shown that the resonant behaviour depends on the geometry of survey, on 3d metal atoms lattice positions and on the magnitude and localization of atomic magnetic moment. © Published under licence by IOP Publishing Ltd

Effects of ternary mixture of chalcogens on the Fel .02Se superconducting propertie

The present work was supported by the Ministry of Education and Science of Russia and by the Ural Branch of RAS (Project No 15-17-2-22)

Effect of the tellurium for selenium substitution on the phase composition and electrical resistivity of Fe7(Se1-xTex)8

The discovery of Fe(Se, Te)-type superconductors has attracted much attention due to their simplest crystal structure among the new families of iron-based layered compounds. However, the Fe(Se, Te) samples often exhibit an impurity hexagonal phase of the NiAs-type together with the main superconducting phase having a tetragonal PbO-type phase [1]. The aim of the present work is to study how the coexistence of the tetragonal and hexagonal phases in the Fe7(Se,Te)8 samples influences their superconducting properties bearing in mind the limited solubility of tellurium in Fe7(Se,Te)8.The present work was supported by RFBR (projects No 16-02-00480 and 16-03-00733)

Impact of 3dmetal oxides on the structure and properties of Fe(Se,Te)-type superconductors

The present work was supported by RFBR (projects No 16-02-00480 and 16-03-00733) the Ministry of Education and Science of Russia (project No 1362)

Resonant photoemission at the absorption edge of Mn and Ti and electronic structure of 1T-Mn0.2_{0.2}TiSe2_2

Resonant valence-band X-ray photoelectron spectra (ResPES) excited near 2p$_{3/2}$ core level energies, 2p X-ray photoelectron spectra (XPS) and L$_{3,2}$ X-ray absorption spectra (XAS) of Ti and Mn in single crystal of 1T-Mn$_2$TiSe$_2$ were studied for the first time. The ionic-covalent character of bonds formed by Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with results of atomic multiplet calculations of Ti and Mn L$_{3,2}$ XAS it is found that Ti atoms are in ionic state of 4+ and Mn atoms are in the state of 2+. In ResPES of Mn$_{0.2}$TiSe$_2$ excited near Ti 2p$_{3/2}$ and Mn 2p$_{3/2}$ absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to band structure calculations E$(\textbf{k})$ the Ti 3d states are localized in the vicinity of $\mathsf{\Gamma}$ point and the Mn 3d states are localized along the direction K-$\mathsf{\Gamma}$-M in the Brillouin zone of the crystal.Comment: 18 pages (preprint), 9 figure

The electronic structure formation of CuxTiSe2 in a wide range (0.04 < x < 0.8) of copper concentration

An experimental study of the electronic structure of copper intercalated titanium dichalcogenides in a wide range of copper concentrations (x = 0.05 - 0.6) using the x-rays photoemission spectroscopy, resonant photoemission and x-rays absorption spectroscopy has been performed. Negative energy shifts of the Ti 2p and Se 3d core levels spectra and a corresponding decrease of the photon energy in Ti 2p absorption spectra with increasing concentration of copper have been found. Such sign-anomalous shifts may be explained by the shielding effect of the corresponding atomic shells as a result of the dynamic charge transfer during the formation of a covalent chemical bond between the copper atoms and the TiSe2 matrix.Comment: 12 pages, 6 figure