Dmitriy A. Chareev

Single crystal growth and characterization of tetragonal FeSe1−x superconductors

The plate-like single crystals of tetragonal (P4/nmm) FeSe1−x superconductors were grown using the KCl–AlCl3 flux technique which produced single crystalline tetragonal samples of about 4 × 4 × 0.1 mm3 dimensions. The energy dispersive X-ray spectroscopy established a ratio of Fe : Se = 1 : 0.96 ± 0.02. The resistivity and magnetization measurements revealed a sharp superconducting transition at Tc = 9.4 K. Multiple Andreev reflections spectroscopy pointed to the existence of two-gap superconductivity with the gap values ΔL = 2.4 ± 0.2 meV and ΔS = 0.75 ± 0.1 meV at 4.2 K.

Enhanced critical current density in the pressure-induced magnetic state of the high-temperature superconductor FeSe

We investigate the relation of the critical current density (Jc) and the remarkably increased superconducting transition temperature (Tc) for the FeSe single crystals under pressures up to 2.43 GPa, where the Tc is increased by ~8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in Tc, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of Tc. Unlike other high-Tc superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning Jc abruptly increases even though the increase of the zero-resistivity Tc is negligible, directly indicating that the flux pinning property compared to the Tc enhancement is a more crucial factor for an achievement of a large Jc. In addition, the sharp increase in Jc in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.

New superconductor LixFe1+δSe (x ≤ 0.07, Tc up to 44 K) by an electrochemical route

The superconducting transition temperature (Tc) of tetragonal Fe1+δSe was enhanced from 8.5 K to 44 K by chemical structure modification. While insertion of large alkaline cations like K or solvated lithium and iron cations in the interlayer space, the [Fe2Se2] interlayer separation increases significantly from 5.5 Å in native Fe1+δSe to >7 Å in KxFe1−ySe and to >9 Å in Li1−xFex(OH)Fe1−ySe, we report on an electrochemical route to modify the superconducting properties of Fe1+δSe. In contrast to conventional chemical (solution) techniques, the electrochemical approach allows to insert non-solvated Li+ into the Fe1+δSe structure which preserves the native arrangement of [Fe2Se2] layers and their small separation. The amount of intercalated lithium is extremely small (about 0.07 Li+ per f.u.), however, its incorporation results in the enhancement of Tc up to ∼44 K. The quantum-mechanical calculations show that Li occupies the octahedrally coordinated position, while the [Fe2Se2] layers remain basically unmodified. The obtained enhancement of the electronic density of states at the Fermi level clearly exceeds the effect expected on basis of rigid band behavior.

Thermodynamic study of monoclinic pyrrhotite in equilibrium with pyrite in the Ag-Fe-S system by solid-state electrochemical cell technique

Abstract An equilibrium mixture of monoclinic pyrrhotite and pyrite was synthesized in the eutectic AlCl3- KCl melt at 525 K. The reaction 7FeS2(cr) + 12Ag(cr) = 8Fe0.875S(cr) + 6Ag2S(cr) was studied by an electromotive force technique in an all-solid-state electrochemical cell with an Ag+-conductive solid electrolyte in Ar at atmospheric pressure: (-) Pt | Ag | AgI | Ag2S, Fe0.875S, FeS2 | Pt (+). In the 490-565 K temperature range a linear electromotive force vs. temperature trend was obtained from which the temperature dependence of the sulfur fugacity was determined for the monoclinic pyrrhotitepyrite equilibrium: logfS2(mpo+py) = 14.079 - 14406⋅T-1, (500 < T/K < 565). In addition, standard thermodynamic functions were calculated for monoclinic pyrrhotite Fe0.875S at 298 K and atmospheric pressure: ΔGf(mpo, 298.15 K) = -(136 200 ± 3000) J/mol, S°(mpo, 298.15 K) = (66.7 ± 1.3) J/(mol-K), ΔHf(mpo, 298.15 K) = -(157 400 ± 3000) J/mol. Gaseous sulfur, S2-ideal gas at 1 bar (105 Pa) pressure- was taken as a standard state for sulfur.

X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system

Abstract Minerals of the Fe-As-S system are the main components of Au ores in many hydrothermal deposits, including Carlin-type Au deposits, volcanogenic massive sulfide deposits, epithermal, mesothermal, sedimentary-hosted systems, and Archean Au lodes. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and often predominates. Knowledge of the chemical state of “invisible” Au (local atomic environment/structural position, electronic structure, and oxidation state) is crucial for understanding the conditions of ore formation and necessary for the physical-chemical modeling of hydrothermal Au mineralization. In addition, it will help to improve the technologies of ore processing and Au extraction. Here we report an investigation of the chemical state of “invisible” Au in synthetic analogs of natural minerals (As-free pyrite FeS2, arsenopyrite FeAsS, and löllingite FeAs2). The compounds were synthesized by means of hydrothermal (pyrite) and salt flux techniques (in each case) and studied by X-ray absorption fine structure (XAFS) spectroscopy in a high-energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The content of “invisible” Au in the synthesized löllingite (800 ± 300 ppm) was much higher than that in arsenopyrite (23 ± 14 ppm). The lowest Au content was observed in zonal pyrite crystals synthesized in a salt flux. High “invisible” Au contents were observed in hydrothermal pyrite (40–90 ppm), which implies that this mineral can efficiently scavenge Au even in As-free systems. The Au content of the hydrothermal pyrite is independent of sulfur fugacity and probably corresponds to the maximum Au solubility at the experimental P-T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in the structures of löllingite, arsenopyrite, and hydrothermal pyrite. The Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 Å (As), 0.13 Å (S) (arsenopyrite) relative to the Fe-ligand distance in pure compounds. Distortions of the atomic structures are localized around Au atoms and disappear at R > ∼4 Å. Chemically bound Au occurs only in hydrothermal pyrite, whereas pyrite synthesized without hydrothermal fluid contains only Au°. The heating (metamorphism) of hydrothermal pyrite results in the decomposition of chemically bound Au and formation of Au° nuggets, which coarsen with increasing temperature. Depending on the chemical composition of the host mineral, Au can play a role of either a cation or an anion: the Bader atomic partial charge of Au decreases in the order pyrite (+0.4 e) > arsenopyrite (0) > löllingite (−0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form a chemically bound refractory admixture in base metal sulfides/chalcogenides. The content of chemically bound noble metals can vary depending on the composition of the host mineral and ore history.

Single crystal growth, transport and scanning tunneling microscopy and spectroscopy of FeSe1−xSx

Single crystals of sulfur-substituted iron selenide, FeSe1−xSx, were grown within eutectics of molten halides, AlCl3/KCl, AlCl3/KCl/NaCl or AlCl3/KBr, under permanent temperature gradient. The innovative “ampoule in ampoule” design of a crystallization vessel allows obtaining mm-sized plate-like single crystals with a sulfur content up to x ∼ 0.19. The sharp anomalies in the physical properties indicate the superconducting and nematic phase transitions in FeSe0.96 at TC = 8.4 K and TN = 90 K, respectively. Scanning tunneling microscopy reveals the presence of dumbbell defects associated with Fe vacancies and dark defects at the chalcogen site associated with S within the FeSe1−xSx series of compounds. Scanning tunneling spectroscopy shows the presence of two different superconducting gaps at both hole and electron pockets of the Fermi surface for low S content levels. As a function of sulfur content, TC follows the conventional dome-shaped curve while TN decreases with x. The overall appearance of the T–x phase diagram of FeSe1−xSx suggests the importance of nematic fluctuations for the formation of the superconducting state in these compounds.

Pressure dependence of upper critical fields in FeSe single crystals

We investigate the pressure dependence of the upper critical fields ({\mu}

Magnetotransport properties of FeSe in fields up to 50 T

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Thermodynamic studies of pyrrhotite–pyrite equilibria in the Ag–Fe–S system by solid-state galvanic cell technique at 518–723 K and total pressure of 1 atm

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