Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and 3.5 meV were identified

Pressure effects on electronic structure and magnetic properties of the FeSe(Te) superconductors

The magnetic susceptibility χ of the FeSe and FeTe compounds, which form the simplest FeSe(Te) system of the novel iron-based superconductors, is studied in the normal state under hydrostatic pressure. A substantial positive pressure effect on χ is detected at low temperatures for both compounds. At room temperature, this effect is found to be also strong, but it is negative for FeSe and positive for FeTe. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe and FeTe are close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and the height Z of chalcogen species from the Fe plane. The observed large positive pressure effects on χ in FeTe and FeSe at low temperatures are related to considerable sensitivity of the paramagnetism to the internal parameter Z. It is shown that available experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with the calculated behavior of the density of electronic states at the Fermi level.Магнитная восприимчивость χ соединений FeSe и FeTe, которые образуют простейшую систему FeSe(Te) новых сверхпроводников на основе железа, изучена в нормальном состоянии под действием гидростатического давления. Значительный положительный эффект влияния давления на χ обнаружен при низкой температуре для обоих соединений. При комнатной температуре этот эффект оказывается также сильным, но отрицательным − для FeSe и положительным − для FeTe. Расчеты из первых принципов зависимости электронной структуры и магнитной восприимчивости от давления показали, что FeSe и FeTe близки к магнитной неустойчивости с преобладанием обменно-усиленного спинового парамагнетизма. Вычисленная парамагнитная восприимчивость проявляет сильную зависимость от объема элементарной ячейки и высоты Z слоя халькогена над плоскостью железа. Наблюдаемые большие положительные эффекты давления на χ в FeTe и FeSe при низких температурах связаны со значительной чувствительностью парамагнетизма к внутреннему параметру Z. Показано, что имеющиеся экспериментальные данные о сильной и немонотонной зависимости температурного сверхпроводящего перехода от давления в FeSe качественно коррелируют с рассчитанным поведением плотности электронных состояний на уровне Ферми.Магнітну сприйнятливість χ сполук FeSe і FeTe, які утворюють найпростішу систему FeSe(Te) нових надпровідників на основі заліза, вивчено в нормальному стані під дією гідростатичного тиску. Значний позитивний ефект тиску на χ визначено при низьких температурах для обох сполук. При кімнатній температурі цей ефект виявляється також сильним, але негативним − для FeSe і позитивним − для FeTe. Розрахунки з перших принципів залежності електронної структури й магнітної сприйнятливості від тиску показали, що FeSe і FeTe близькі до магнітної нестійкості з переважанням обмінно-посиленого спінового парамагнетизму. Обчислена парамагнітна сприйнятливість проявляє сильну залежність від об’єму елементарної комірки й висоти Z шару халькогену над площиною заліза. Великі позитивні ефекти тиску на χ в FeTe і FeSe, які спостерігалися при низьких температурах, пов’язані зі значною чутливістю парамагнетизму до внутрішнього параметру Z. Показано, що наявні експериментальні дані про сильну й немонотонну залежність температурного надпровідного переходу від тиску в FeSe якісно корелюють із розрахованою поведінкою густини електронних станів на рівні Фермі

Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0)

Magnetization studies for FeSe₁–xTex (x≃0, 0.5, and 1.0) compounds were carried out in magnetic fields up to 50 kOe and in the temperature range 2–300 K. The superconducting transition was observed at Tc≃8 K and 13.6–14.2 K in FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, respectively. For the most samples, a nonlinear behavior of the magnetization curves in the normal state gives evidence of a commonly observed substantial presence of ferromagnetic impurities in the compounds under study. By taking these impurity effects into account, the intrinsic magnetic susceptibility χ of FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN≃70 K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe within the local spin density approximation

Determination of the lower critical field Hc1(T) in FeSe single crystals by magnetization measurements

In a recent work, Abdel-Hafiez et al. we have determined the temperature dependence of the lower critical field Hc1(T) of a FeSe single crystal under static magnetic fields H parallel to the crystallographic c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. In general, the first vortex penetration field may not reflect the true Hc1(T) due to the presence of surface barriers. In this work we show that magnetic hysteresis loops are very symmetric close to the critical temperature Tc = 9 K evidencing the absence of surface barriers and thus validating the previously reported determination of Hc1(T) and the main observations that the superconducting energy gap in FeSe is nodeless

The system Pd-Ag-S: Phase relations and mineral assemblages

© 2020 Mineralogical Society of Great Britain and Ireland. Phase equilibria in the system Pd-Ag-S were studied using the silica-glass tube method at 400°C and 550°C. In the system we synthesised three ternary phases: Coldwellite (Pd3Ag2S), kravtsovite (PdAg2S) and a new phase Pd13Ag3S4. At 400°C, coldwellite forms a stable association with vysotskite (PdS) and vasilite (Pd16S7); vysotskite and kravtsovite; phase Pd4S and a Ag-Pd alloy; it also coexists with a new phase Pd13Ag3S4. Kravtsovite is stable up to 507°C; the presence of kravtsovite in the mineral assemblage reflects its formation below this temperature. The occurrence of coldwellite, vysotskite and Ag2S together in equilibrium reflects the formation of this mineral assemblage above this temperature (507°C). Coldwellite is stable up at 940°. Mineral assemblages defined in this study can be expected in Cu-Ni-PGE mineral deposits, associated with mafic and ultramafic igneous rocks, in particular in mineralisations with known silver-palladium sulfides

The charge state of pt in binary compounds and synthetic minerals determined by x-ray absorption spectroscopy and quantum chemical calculations

The binary synthetic compounds of Pt with chalcogens (O, S, Se, Te), pnictogens (As, Sb, Bi), and intermetallic compounds with Ga, In, and Sn of various stoichiometry were studied via X-ray absorption spectroscopy (XAS). The partial atomic charges of Pt in the compounds were computed using quantum chemical density functional theory (DFT) based methods: the Bader (QTAIM) method, and the density-derived electrostatic and chemical (DDEC6) approach. Strong positive correlations were established between the calculated partial atomic charges of Pt and the electronegativity (χ) of ligands. The partial charge of Pt in PtL2 compounds increases much sharply when the ligand electronegativity increases than the Pt partial charge in PtL compounds. The effect of the ligand-to-Pt atomic ratio on the calculated Pt partial charge depended on ligand electronegativity. The DDEC6 charge of Pt increases sharply with the growth of the number of ligands in PtSn (n = 1, 2; electronegativity χ(S) >> χ(Pt)), weakly depends on the phase composition in PtTen (n = 1, 2; χ(Te) is slightly lower than χ(Pt)), and decreases (becomes more negative) with increase of the ligand-to-Pt ratio in intermetallic compounds with electron donors (χ(L) χ(L)) was overcompensated by the gain of the hybridized s-p electron density, which was confirmed by Pt L1-edge spectra analysis. As a result, the total electron density at the Pt site followed the electronegativity rule, i.e., it increased with the growth of the number of the ligands-electron donors. The empirical correlations between the Pt partial atomic charges and parameters of XANES spectral features were used to identify the state of Pt in pyrite, and can be applied to determine the state of Pt in other ore minerals

Temperature dependence of tellurium fugacity for the kotulskite (PdTe)–merenskyite (PdTe<inf>2</inf>) equilibrium determined by the method of a solid-state galvanic cell

The thermodynamic properties of the kotulskite (PdTe)/merenskyite (PdTe2) equilibrium in the Ag-Pd-Te system were determined for the first time by the electromotive force (EMF) method. The thermodynamic properties were calculated from the temperature dependence of the EMF in a completely solid-state electrochemical cell with a common gas space:(-) C (graphite)| Ag | RbAg 4I 5| Ag 2Te , PdTe , PdTe 2| C (graphite)(+) , following a virtual chemical reaction: 2Ag + PdTe 2↔ Ag 2Te + PdTe. The measurements were carried out in the temperature range of 371–488 K at the atmospheric pressure of pure argon. As a result, the thermodynamic properties for the reaction PdTe + Te = PdTe2 at a pressure of 1 bar (105 Pa) were determined as: ΔrGo/J·mol−1 = − 27,639; ΔrS/J mol−1 K−1 = − 21.98; ΔrHo/J·mol−1 = − 34,176. Fugacity of gaseous tellurium (Te2) over the PdTe + 1/2Te2 ↔ PdTe2 equilibrium was also calculated: log f Te2 = (9.205 ± 0.072) − (11.44 ± 0.03)·(1000/T), (371.7 < T/K < 493.6)

Temperature dependence of the lower critical field Hc1(T) evidences nodeless superconductivity in FeSe

We investigate the temperature dependence of the lower critical fi eld Hc1(T) of a high-quality FeSe single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the first vortex penetration fi eld has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. A pronounced change of the Hc1(T) curvature is observed, which is attributed to multiband superconductivity. The London penetration depth Lambda_ab(T) calculated from the lower critical field does not follow an exponential behavior at low temperatures, as it would be expected for a fully gapped clean s-wave superconductor. Using either a two-band model with s-wave-like gaps of magnitudes Delta_1 = 0.41 +- 0.1meV and Delta_2 = 3.33+- 0.25meV or a single anisotropic s-wave order parameter, the temperature-dependence of the lower critical eld Hc1(T) can be well described. These observations clearly show that the superconducting energy gap in FeSe is nodeless

Thermodynamic Functions of PtS<inf>2</inf> in a Wide Temperature Range

© 2020, Pleiades Publishing, Ltd. Abstract—: The thermodynamic properties of crystalline platinum disulfide have been studied in the range from 5 to 875 K. The isobaric heat capacity of PtS2 has been determined by two methods: by adiabatic calorimetry from 5.32 to 344.96 K and by differential scanning calorimetry in the range 344.6–874.6 K. Using the experimental Cp(T) data, we evaluated the standard thermodynamic functions of platinum disulfide in a wide temperature range. The high-temperature Cp measurement results have been used to investigate regression models based on the Maier–Kelley and Khodakovsky equations for the temperature dependence of its isobaric heat capacity in the range from 298 to 875 K. The Debye characteristic temperature ΘD of PtS2 has been evaluated as a function of temperature