EuRu2As2: A New Ferromagnetic Metal with Collapsed ThCr2Si2-Type Structure

EuRu₂As₂ was characterized by structural, resistivity, magnetization and M?ssbauer measurements. It crystallizes in collapsed ThCr₂Si₂-type structure with c/a=2.58 and space group I4/mmm. The temperature and field dependences of magnetization indicate that the material is ferromagnetically ordered at T _C=17.3±0.5 K, where the electrical resistivity shows a kink. M?ssbauer spectroscopy studies determine that the Eu²⁺ moments align basically along the crystallographic c-axis, which is in contrast to the case in EuFe₂As₂ in which the Eu²⁺ spins lie in the basal planes.     

Crystal chemistry and magnetism in silicides LaFe2−x Rh x Si2 (ThCr2Si2-type)

A complete solid solution has been found for mixed crystals LaFe₂Si₂?LaRh₂Si₂ of the ThCr₂Si₂-type and a statistical distribution of iron-and rhodium atoms on the 4d sites of the space group I4/mmm. Crystal symmetry, atom order and precise atom parameters have been determined from a single crystal study of LaFe_1.6Rh_0.4Si₂ (R=0.015). Magnetic susceptibilities (1.8<T<1100K) are characterized by a temperature independent paramagnetism throughout the solid solution. The iron atoms prove to be in a nonmagnetic state in good agreement with Mössbauer data at 5K and at room temperature, which further confirms a small amount of iron on a second crystallographic site (4e). LaRh₂Si₂ is not superconducting above 1.5K.     

Low Temperature Heat Capacity of Layered Superconductors SrNi2Ge2 and SrPd2Ge2

Low-temperature heat capacity C(T) of the weakly electron-correlated SrNi₂Ge₂ 122-layer compound undergoes a superconducting transition with onset at 1.4 K and a bulk T _c =0.75 K, where heat-capacity jump ratio ΔC(T _c )/γT _c =0.88–1.05. A small average superconducting energy gap E _g (ave)=2.21 kT _c =0.14 meV is derived for this multi-gap superconductor. Similar results for isostructural SrPd₂Ge₂ include T _c (onset)=3.5 K, bulk T _c of 2.92 K, ΔC(T _c )/γT _c =0.70 and E _g (ave)=2.54 kT _c =0.64 meV. The higher T _c onset could be associated with stoichiometric 1:2:2 grains in the polycrystalline samples. In addition, deviations of E _g /kT _c from the BCS ratio of 3.5 suggest that, just like their iron-based counterpart, these 122-layer germanides may also exhibit an unconventional, fully-opened multi-gap s-wave superconductivity.     

Structural and Superconductivity Properties of BaFe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript>

Extraordinary magnetic behaviors, resistivity properties, and lattice parameters of the main sample BaFe₂As₂ and BaFe_2?x Pt _x As₂ in the variation of x from 0 to 0.4 with the step of 0.1 were investigated. The bulk materials have been prepared by the solid-state reaction method and sealed into a quartz tube. The crystal structure of all samples exhibited the ThCr₂Si₂-type crystal structure which is in harmony with earlier studies in the literature. The superconducting states with magnetization measurements have been detailed in the wide temperature range 5–170 K, up to a field of 20 Oe. Increasing Pt and decreasing Fe elements in the BaFe_2?x Pt _x As₂ compound deteriorated superconductivity. Using magnetization measurement data, we present the variation of superconducting critical temperature (T _c) correlating with a Pt dopant rate from x = 0 to x = 0.4. The dopant rate of x = 0.3 exhibited the limit rate for maximum T _c; deterioration of superconductivity was revealed with a dopant rate of more than x = 0.3. This should be explained by varying T _c related to a lattice shrinking and pressure effect (geometric factor).     

The influence of oxygen deficiency on structural and electronic properties of layered superconductor (Fe2As2)(Sr4V2O6?x)

By means of the first-principles FLAPW-GGA approach, we studied the influence of oxygen deficiency on the structural and electronic properties of the superconducting phase (Fe₂As₂)(Sr₄V₂O_6−x ). We find that the formation of vacancies in 2c sites of oxygen sublattice [when the initial tetragonal structure of the stoichiometric crystal (Fe₂As₂)(Sr₄V₂O₆) is kept] is energetically more preferable than in 4f sites, when orthorhombic distortions arise. The influence of oxygen vacancies on the structure of (Fe₂As₂) blocks is very insignificant; thus, using the known structural indicators, it is impossible to explain the observed drop of transition temperature T _C for oxygen-deficient systems by structural factors. On the other hand, the results of the band structure calculations allow us to assert that the vacancy-induced drop of T _C may be caused by the electronic factor, i.e., by the established decrease in N^tot(E _F) for the oxygen-deficient system.     

Spontaneous Stoichiometry Change in Single Crystals of Superconducting (Ba1−x K x )Fe2As2 Grown by a Rapid-Heating Sn-Flux Method

To prevent the loss of K in growing single crystals of Ba_1−x K _x Fe₂As₂ we developed a rapid-heating Sn-flux method. Large single crystals with the optimal superconducting transition temperature T _C≈38 K were obtained and their structural, chemical and superconducting properties were investigated. Additionally, the effect of post-growth annealing on these crystals at different temperatures was examined. Scanning electron microscopy microprobe studies on a crystal with the composition goal of Ba_0.25K_0.75Fe₂As₂ revealed a well defined separation of two phases with compositions that are suggestive of rational ratios of the K and Ba content.     

Pseudogap and Superconducting Energy Gap in Single Crystals of URu2Si2 by Point Contact Spectroscopy

We performed point contact spectroscopy measurements in the superconducting and normal state of a single crystal of the heavy fermion URu₂Si₂. The differential resistance as a function of bias voltage shows the features of the superconducting energy gap up to 1.37 K, above T _C we observe another feature that we identify as a pseudogap that persists up to 2 K. The superconducting gap does not fit a BCS behavior.     

Effect of Ag on the electrical transport and iso-thermal transformation of amorphous As2S3

The step-wise technique is used to follow the possible changes in d.c. transport of three different glasses of the system Ag-As₂S₃ isothermally annealed atT _g<T<T _m. The isothermal time-dependence of different characteristic electrical quantities (e.g.E _σ,σ_20°C, andσ ₀) has been drawn and correlated with the microstructural changes recorded for samples annealed at the same conditions. The effect of silver content (up to 25 at %) on the interplanar spacings (d _hkl) and relative intensities (I/I ₀) of the diffraction lines of As₂S₃ illustrates that the structure of the three-component system Ag-As₂S₃ can be represented as a solid solution of an element (e.g. S), binary phases (e.g. AsS, As₂S₃), and/or ternary phases (e.g. AgAs₂S₃, Ag₆As₂S₃), depending on the concentration of silver in the composition.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Structural, Electronic Properties and Fermi Surface of ThCr2Si2-Type Tetragonal KFe2S2, KFe2Se2, and KFe2Te2 Phases as Parent Systems of New Ternary Iron-Chalcogenide Superconductors

The first principles FLAPW–GGA method was used for the comparative study of the structural and electronic properties of three related tetragonal ThCr₂Si₂-type phases KFe₂ Ch ₂, where Ch are S, Se, and Te. The main trends in electronic bands, densities of states, and Fermi surfaces for AFe₂ Ch ₂ were analyzed in relation to their structural parameters. We found that anion replacements (S↔Se↔Te) produce no critical changes in the electronic structure of KFe₂ Ch ₂ phases. On the other hand, our analysis of structural and electronic parameters for hypothetical KFe₂Te₂ allows us to propose this system as a perspective parent phase for search of new iron-chalcogenide superconducting materials.     

A new family of superconducting silicides: The ThMxSi2−x compounds (M = Rh or Ir with 0 < x < 1) of α-ThSi2-type structure

New superconducting materials have been prepared by substitution of silicon by Rh or Ir atoms in the α-ThSi₂ disilicide. The ThM_xSi_2?x compounds crystallize in the tetragonal α-ThSi₂-type structure with a large homogeneity range: 0? x ? 0.96 for Rh and 0? x ? 1 for Ir. A sharp increase of the superconducting transition temperature T_cr is observed for x > 0.75 in both cases. Among the superconducting compounds with an α-ThSi₂-type structure, ThRh_0.96Si_1.04 and ThIrSi have the highest T_cr values with respectively 6.45 and 6.5 K about twice that of α-ThSi₂ (T_cr = 3.16 K). Superconductivity enhancement has been attributed to the formation of Rh-Rh or Ir-Ir metallic type bonding.     

Magnetic properties of new ternary silicides : Tb2Ir3Si5 and both polymorphic forms of TbIr2Si2

A new ternary silicide Tb₂Ir₃Si₅ of U₂Co₃Si₅-type structure has been prepared as well as two polymorphic forms of TbIr₂Si₂. Both modifications are tetragonal, the low-and high-temperature forms are isostructural with ThCr₂Si₂ and CaBe₂Ge₂ respectively. The three compounds order antiferromagnetically. A high Néel temperature is observed for the low-temperature modification (T_N = 80K, θ_p = 42K). Its magnetic structure is similar to that of TbRh₂Si₂ for which ferromagnetic layers perpendicualr to the c-axis are coupled antiferromagnetically (+?+?). Tb₂Ir₃Si₅ and the high-temperature modification of TbIr₂Si₂ have low T_N and θ_p values. The difference observed in the magnetic behavior of these compounds is likely due to the influence of the environment of the Tb³⁺ ion on the density of states at Fermi level.     

4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Non-isothermal crystallization of As2Se3 glass

The results of non-isothermal crystallization studies performed at different heating rates on batches of As₂Se₃ glasses prepared from melts at 400°C, 600°C and 800°C are reported. The peak temperature of crystallizationT _p, the enthalpy of crystallization ΔH _c and the activation energy for crystallizationE _c are independent of the melt temperature used in the preparation. Bulk nucleation with three-dimensional growth of crystals is indicated for As₂Se₃. The values of ΔH _c andE _c are found to be respectively 23·3 ± 0·9 cal/g and 36·5 ± 0·9 kcal/mol for As₂Se₃.     

Magnetic Pair Breaking in Superconducting SrPd2Ge2 Investigated by Scanning Tunnelling Spectroscopy

SrPd₂Ge₂ has been structurally identical with the 122-type iron pnictide compounds with iron completely replaced by Pd and pnictogen replaced by Ge yielding the superconducting transition below 3 K. Our previous works showed that in contrast to iron pnictides this system is electronically fully three-dimensional and revealed conventional superconductivity, even of type I. Here, by means of sub-Kelvin STM spectroscopy, we show that the dirty limit occurs in the sample driving it to type-II superconductivity. STM also proves that in magnetic field the sample undergoes the phase transition to the Abrikosov vortex-lattice state. The de Gennes–Maki theory of gapless superconductivity for dirty superconductors is successfully applied to model the tunnelling spectra of the superconducting density of states in magnetic field. We show that this theory is applicable in a wide range of magnetic fields starting from 60 % of the upper critical field H _c2.     

Structure,superconductivity and magnetism of new rare earth-rhodium silicides RE2Rh3Si5 of U2Co3Si5-type

New ternary silicides RE₂Rh₃Si₅ (RE = Y, La, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystalize in the U₂Co₃Si₅-type structure. Only the silicides containing the diamagnetic rare earths Y and La show a superconducting transition at T_Cr = 4.4 ± 0.2K and T_Cr = 2.7 ± 0.1K respectively, those with magnetic rare earths order antiferromagnetically at low temperature.     

Temperature Dependence of \sqrt{2} \times \sqrt{2} Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

We report the structural phase transition properties of the newly discovered K_0.8Fe_1.6Se₂ superconductor (T _c =31.8 K) using synchrotron single-crystal X-ray diffraction. The basic structure of the sample at room temperature is found to be tetragonal ThCr₂Si₂-type, modulated by a vacancy ordering induced superlattice structure together with a coexisting minority phase having a $\sqrt{2} \times \sqrt{2}$ ordering. At 520 K, the reflections corresponding to the $\sqrt{2} \times \sqrt{2}$ phase merge with the parent tetragonal phase. The superlattice peaks corresponding to the vacancy ordering disappear at 580 K, indicating an order-disorder phase transition at this temperature.     

Coexistence of Superconductivity and Spin Density Wave (SDW) in Ferropnictide Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

This work focuses on the theoretical investigation of the coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂. By developing a model Hamiltonian for the system and by using quantum field theory Green’s function formalism, we have obtained mathematical expressions for superconducting transition temperature (T _C), spin density wave transition temperature (T _sdw), superconductivity order parameter (Δ_Sc), and spin density wave order parameter (Δ_sdw). By employing the experimental and theoretical values of the parameters in the obtained expressions, phase diagrams of superconducting transition temperature (T _C) versus superconducting order parameter (Δ_Sc) and spin density wave transition temperature (T _sdw), versus spin density wave order parameter (Δ_sdw) have been plotted. By combining the two phase diagrams, we have demonstrated the possible coexistence of superconductivity and spin density wave (SDW) in ferropnictide Ba_1?x K _x Fe₂As₂.     

Refinement of the crystal structure of CeOs2Si2 (ThCr2Si2-type)

The crystal structure of CeOs₂Si₂ has been determined from single crystal X-ray counter data. CeOs₂Si₂ is tetragonal, I4/mmm, Z = 2; the cell parameters are a = 0.41617(5) and c = 0.98481(28) nm. The final reliability factor $\text{R}\text{=}\text{|Δ}\text{F}\text{|}\text{|}\text{F}{}_{\mathrm{o}}\text{|}$ is 0.027 for 129 observed reflections (|F_o|>26). CeOs₂Si₂ is isotypic with the ordered structure type of ThCr₂Si₂ (BaAl₄-derivative type).     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.