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.     

Structural phase transitions and piezoelectric anomalies in ordered Sc<Subscript>0·5</Subscript>Ga<Subscript>0·5</Subscript>N alloys

Local-density approximation calculations (LDA) within density functional theory (DFT) and Berry phase approach within modern theory of polarization are performed to predict the structural and piezoelectric properties of ordered Sc_0·5Ga_0·5N alloys under compressive and tensile in-plane strain. This alloy is found to exhibit a tremendous piezoelectric response, associated with a phase transition from nonpolar p6₃/mcc(D _6h) space group to a polar p6₃ mc(C _6v) structure, at fixed Ga and Sc compositions when continuously changing the experimental accessible parameters (i.e. the compressive and tensile strain). The mechanism of the effects behind such anomalous behaviour is revealed and explained.     

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.     

Trifluoromethylated [60]Fullerenes: Synthesis and Characterization

Abstract

The high temperature reaction of C₆₀ with silver(I) trifluoroacetate followed by 500°C sublimation and HPLC purification has led to the characterization of the trifluoromethyl‐[60]fullerenes 1,4‐C₆₀(CF₃)₂, C _s‐C₆₀(CF₃)₄, C ₁‐C₆₀(CF₃)₄, and C ₁‐C₆₀(CF₃)₆ by EI‐MS and ¹⁹F NMR. The compounds C ₁‐C₆₀(CF₃)₄ and C ₁‐C₆₀(CF₃)₆ were obtained with 90+% compositional purity. A sample of C₆₀(CF₃)₂ also contained ca. 15–20% of a C _s‐symmetry isomer of C₆₀(CF₃)₄. The structural assignments are based on calculations at the AM1 and DFT levels of theory.     

The New Method for the Synthesis of Fullerols Based on Radical Reaction

Abstract

The method for the synthesis of fullerols C₆₀(OH) _x was developed, allowing to obtain products with low number of hydroxy groups attached to fullerene molecule. At the first stage, product C₆₀(tert‐BuO) _x was obtained by the radical reaction of C₆₀ with tert‐butyl‐peroxide under heating or UV‐irradiation. At the second stage, fullerol C₆₀(OH) _x was synthesized after the elimination of tert‐butyl groups by acid treatment. Several samples C₆₀(OH) _x containing different numbers (x = 2–5) of hydroxy groups were obtained, the main product had x = 2. The structure of the fullerols was confirmed by UV‐ and Fourier‐transform infrared (FTIR)‐spectroscopy and MALDI‐TOF mass‐spectrometry.     

Mean polarizabilities and second and third virial coefficients of the gases C2H4, C2H6, and SF6

Mean polarizabilities ₀(₀, T) as well as second and third virial coefficients B(T) and C(T) of the equation of state of the gases C₂H₄, C₂H₆, and SF₆ have been determined from refractive index measurements with a Michelson interferometer for wavelength ₀=632.99 nm in the overall ranges 250 KT 340 K and 0p3 MPa of temperature T and pressure p. Some negative C(T) values at low temperatures have been obtained. The C(T) data could be fitted satisfactorily to the simple three-parameter function C(T)= a(T–T ₀) exp[ –b(T–T ₀)], with the maximum near the critical temperature T _c. A possible correlation between C(T) and the vapor pressure p _v(T) is discussed.     

Zr55Al10Ni5Cu30非晶合金晶化相转变的价电子结构分析

基于EET理论,计算了F-Zr₂(Ni,Cu)和tI-Zr₂Cu及tI-Zr₂(Cu,Ni)相的价电子结构,用最强键键合力n₁、结构单元总成键能力F和单位成键能力F_v分析了F-Zr₂Ni亚稳相向tI-Zr₂Cu型稳定相的转变过程。研究发现:F-Zr₂Ni的n^{F-Zr₂Ni₁值比tI-Zr₂Cu的ntI-Zr₂Cu₁值大115.36%,FF-Zr₂Ni_v值比tI-Zr₂Cu的FtI-Zr₂Cu_v值大34.03%;F-Zr₂(Ni0.3,Cu0.7)的nF-Zr₂(Ni0.3,Cu0.7)₁值比tI-Zr₂Cu的ntI-Zr₂Cu₁值大0.36%,FF-Zr₂(Ni0.3,Cu0.7)_v值比tI-Zr₂Cu的FF-Zr₂Ni}_v值大1.25%; tI-Zr₂(Cu0.6,Ni0.4)的n^{tI-Zr₂(Cu0.6,Ni0.4)₁值比F-Zr₂(Ni0.3,Cu0.7)的nF-Zr₂(Ni0.3,Cu0.7)}₁值大12.95%, F^{tI-Zr₂(Cu0.6,Ni0.4)_v值比F-Zr₂(Ni0.3,Cu0.7)的FF-Zr₂(Ni0.3,Cu0.7)}_v值大14.41%; 从价电子结构角度看,F-Zr₂Ni不能直接转变为tI-Zr₂Cu,F-Zr₂(Ni,Cu)不易分解重构为tI-Zr₂Cu;F-Zr₂(Ni0.3,Cu0.7)可分解并转变为tI-Zr₂(Cu0.6,Ni0.4)。     

Sign reversal of the Hall effect in YBa2(Cu1?x)3O7?δ single crystals

We have measured the in-plane Hall effect and in-plane resistivity of cobalt-doped YBa₂Cu₃O_7– (YBCO) single-crystal samples. The concentration of cobalt ranged from 1 to 3%. As the applied field was varied, a sign reversal of the Hall resistivity was seen for 1 and 2% Co-doped samples, but not for the 3% sample. Our results are expressed in terms of conductivities, as recently recommended by the work of Dorseyet al. and Kopninet al. In the mixed state just below the superconducting transition temperatureT _c, the Hall conductivity _xy is the sum of two terms,C ₁/H andC ₂H, whereC ₁ andC ₂ are field-independent but temperature-dependent. As previously observed by Ginsberg and Manson for undoped YBCO, the coefficientC ₁ is approximately proportional to ² andC ₂ is approximately linear in, where = 1–(T/T _c ). The values ofC ₁ andC ₂ are presented for each cobalt concentration.     

Singlet‐Triplet Transition in the C60 2− Dianion

Abstract

The C₆₀ complexes with decamethylcobaltocene: (Cp*₂Co)₂C₆₀(C₆H₄Cl₂, C₆H₅CN)₂ (1) and [K · (18‐crown‐6)]₂ · C₆₀ · (DMF)₄ (2) have been obtained as single crystals by the diffusion method. The IR‐ and UV‐VIS‐NIR‐spectra justify the formation of the C₆₀ ^2? dianions in these salts. EPR measurements show that the low temperature signals of 1 in the 4–140 K range and 2 in the 4–60 K range have intensity corresponding only to 0.4% and 3.5% from total C₆₀. Because of this, most of the complexes are EPR silent, and, consequently, C₆₀ ^2? has a diamagnetic singlet (S = 0) state in these temperature ranges. The appearance of a broad EPR signal in the spectum of 1 above 140 K and 2 above ～60 K is assigned to a thermal population of a close lying excited triplet (S = 1) state. The singlet–triplet energy gap for C₆₀ ^2? in solid 1 and 2 was estimated to be 730 ± 10 and 300 ± 10 cm^?1.     

A new family of superconductors containing carbonate group

Three new oxycarbonate superconductors recently obtained in our laboratory are described. (1) Sr₂CuO₂(CO₃)_1–x (BO₃) _x (T _c = 35 K) was synthesized under ambient pressure, providing a new method for carrier doping on the layered copper oxycarbonate system. (2) Ca _n (Ca, Sr)₂Cu _n+1(CO₃)O _y (n = 1, 2, 3) was synthesized under high pressure. Superconductivity was achieved by substituting (BO₃)^3– for a part of (CO₃)^2–. (3) A new Hg system HgBa₂Sr₂Cu₂(CO₃)O₆₊ was synthesized.T _c obtained from the magnetic susceptibility is about 66 K.     

The mechanochemical reaction of C60 and Bi(NO3)3·5H2O

The mechanochemical reaction of C₆₀ and Bi(NO₃)₃·5H₂O was performed under ball-milling condition. The product was well characterized by spectroscopic methods such as IR, Raman, UV-vis, ¹³C NMR, as well as elemental analysis method. The spectroscopic investigation indicates that some hydroxyl and nitro groups are attached to the C₆₀ and no profound structural changes occur for the C₆₀ cage in the product. The elemental analysis gives an average composition of C₆₀(NO₂)₂(OH)₂₁ for the product. A possible mechanism is suggested for the reaction.     

Electronic and Magnetic Properties of Superconducting Sr4V2Fe2As2O6 Versus Sr4Sc2Fe2As2O6

First principles of FLAPW-GGA calculations have been performed with the purpose to understand the peculiarities of band structure and Fermi surface topology for recently discovered 37 K superconductor: Sr₄V₂Fe₂As₂O₆—in comparison with isostructural phase Sr₄Sc₂Fe₂As₂O₆. Our main finding is that the replacement of Sc with V leads to drastic transformation of electronic, magnetic and conductive properties of these materials: as against non-magnetic Sr₄Sc₂Fe₂As₂O₆ which is formed from non-magnetic conducting [Fe₂As₂] and insulating [Sr₄Sc₂O₆] blocks, Sr₄V₂Fe₂As₂O₆ consists of non-magnetic conducting [Fe₂As₂] blocks and [Sr₄V₂O₆] blocks which exhibit magnetic half-metallic properties.     

Inelastic neutron scattering results on pure and doped fullerenes

Via inelastic neutron scattering (INS) generalized phonon densities of states are obtained for C₆₀ and superconducting A₃C₆₀ (A=K, Rb). Changes in the intramolecular modes agree with and augment recent Raman (RA), infrared (IR), and INS measurements, i.e., in the vicinity of the high-frequency modes of C₆₀ [H _g (8),A _g (2),H _g (7)] we find a pronounced shift of about 10 meV toward smaller energy transfers upon intercalating with potassium. Separated from these vibrations by a gap are the intermolecular spectra which for the doped samples display well-defined optic modes at 11 and 14 meV due to the vibrations of Rb and K ions. Indications of a weak anomalous temperature dependence of low-energy phonons ( <3 meV) nearT _c are observed for K₃C₆₀.     

Microhardness studies on as-grown faces of NaClO3 and NaBrO3 crystals

Single crystals of NaClO₃ and NaBrO₃ are grown from their aqueous solutions at a constant temperature of 35°C by slow evaporation by using good quality seed crystals. Systematic microhardness studies are made on as-grown faces of these crystals at various loads. Typical cracks are observed at the corners of the impressions in NaClO₃ whereas in addition to the cracks at the corners microcracks also appeared in NaBrO₃ crystals around the impressions. The impressions formed in NaBrO₃ are not very clear as in NaClO₃, a possible mechanism for it is discussed. The work hardening index number(n) for both these crystals is around 1.6 suggesting that these are moderately harder samples. The hardness studies point out that NaBrO₃ is harder than NaClO₃ (ΔH ≈ 100 kg/mm ²,this could be due to strong inter ionic forces acting between Na-Br in NaBrO₃ crystals. Using Gilman’s empirical relation, hardness values are calculated from the values of elastic constants (C ₄₄) and are found to be close to the experimental results.     

Synthesis,Spectral Investigations,and Thermal Stability of [60]Fullerene Complexes with TMTSF: 2(C60) · 2(TMTSF) · (C6H6), C60 · TMTSF · 2(CS2), and 2(C60) · 2(TMTSF) · (C6H5Cl)

Abstract

Molecular complexes of [60]fullerene with tetramethyltetraselenafulvalene (TMTSF): C₆₀ · (TMTSF) · 2(CS₂) (1), 2(C₆₀) · (2(TMTSF) · (C₆H₆) (2), and 2(C₆₀) · (2(TMTSF) · (C₆H₅Cl) (3) have been synthesized and their thermal stability and IR spectra vs. light polarization and temperature have been performed.     

Elastic properties of random L1<Subscript>2</Subscript>–Al<Subscript>3</Subscript>(Sc<Subscript>0.5</Subscript>TM<Subscript>0.5</Subscript>) alloys from first-principle SQSs calculations

Special quasi-random structures (SQSs) with 32 atoms have been generated to model appropriate supercell structure of pseudo-binary random L1₂–Al₃(Sc_0.5TM_0.5) (TM = Y, Ti, Zr, Hf, V, Nb and Ta) alloys. The optimized lattice parameters were in good agreement with the experimental data, and the obtained formation energies showed that all L1₂–Al₃(Sc_0.5TM_0.5) alloys were stable from energetic point of view. As the atomic radius of substitution elements TM in the same Period decreased, the values of C ₁₂ and C ₄₄ for L1₂–Al₃(Sc_0.5TM_0.5) alloys exhibited an overall tendency of increase, implying an enhanced Poisson effect and larger resistance to {100} 〈001〉 shear. The elastic isotropy of L1₂–Al₃(Sc_0.5TM_0.5) alloys was overall lowered and the ductility could be improved. The calculated electronic structure demonstrated that below the Fermi level the hybridization of transition-metal d states with Al p states was reduced with decreasing of atomic radius of substitution elements TM in the same Period, which uncovered underlying mechanism for stability and elastic properties of L1₂–Al₃(Sc_0.5TM_0.5) alloys.     

Molar Heat Capacity at Constant Volume of Trifluoromethane (R23) from the Triple-Point Temperature to 342 K at Pressures to 33 MPa

Molar heat capacities at constant volume (C _v) of trifluoromethane (R23) have been measured with an adiabatic calorimeter. Temperatures ranged from the triple point to 342 K, and pressures up to 33.5 MPa. Measurements were conducted on the liquid in equilibrium with its vapor and on compressed liquid and gaseous samples. The samples were of high purity, as verified by chemical analysis. Calorimetric quantities are reported for the two-phase (C ⁽²⁾ _v), saturated-liquid (C or C_x), and single-phase (C _v) molar heat capacities. The C ⁽²⁾ _v data were used to estimate vapor pressures for values less than 100 kPa by applying a thermodynamic relationship between the two-phase internal energy U ⁽²⁾ and the temperature derivatives of the vapor pressure. The triple-point temperature and the enthalpy of fusion were also measured. The principal sources of uncertainty are the temperature rise measurement and the change-of-volume work adjustment. The expanded relative uncertainty (with a coverage factor k=2 and thus a two-standard deviation estimate) is estimated to be 0.7% for C _v, 0.5% for C ⁽²⁾ _v, and 0.7% for C .     

Calculations of the water-dimer encapsulations into C84

The water-dimer formation and its encapsulation into D₂(22)-C₈₄ and D_2d(23)-C₈₄ fullerenes is evaluated. The water-dimer populations are computed using the potential-energy change from the G3 theory and anharmonic partition functions from the MP2/AUG-cc-pVQZ approach. The encapsulation energetics is treated at the M06-2X/6-31++G** level and it is found that the water-dimer storage in C₈₄ is attractive, yielding an energy gain of more than 10 kcal mol^?1. This substantial encapsulation energy together with the computed temperature increase of the water-dimer population in the saturated steam suggests that the (H₂O)₂@C₈₄ endohedrals could be produced in a high-temperature/high-pressure approach similarly to encapsulation of rare gases in fullerenes.     

C 1-stably ℒ p shadowable homoclinic classes

Let p be a hyperbolic saddle of diffeomorphism f on closed manifold M and H(p, f) be the homoclinic class associated with it. In this article, we introduce the notion of C ¹-stably ? _p shadowing and prove that if f is C ¹-stably ? _p shadowable on a homoclinic class H(p, f) then, H(p, f) has a dominated splitting. Moreover, we prove that if f is C ¹-stably Lipschitz ? _p shadowable on H(p, f) and H(p, f)-germ of f is expansive then the homoclinic class is hyperbolic.