Superconductivity at 22 K in Co-doped BaFe2As2 crystals

Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals below Tc=22 K, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast with cuprates, the BaFe2As2 system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for BaFe1.8Co0.2As2 indicate the interband scattering due to Co is weak.     

Superconductivity in Mg-Doped Layered Intermetallic Compound NbB2

We have performed low temperature resistivity p（T） and specific heat C（T） measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is -4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat γn=4.51 mJmol^-1K^2 and the Debye temperature θD=419 K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/γnTc = 2.84 exp（-1.21Tc/T）. This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.     

Superconductivity and magnetism in HoNi2B2C

The effect of magnetic ordering on superconductivity in HoNi₂B₂C is studied. It is concluded that the abrupt suppression of superconductivity in the region of incommensurate antiferromagnetic ordering is due to a modification of the wave functions of the conduction electrons by the long-range magnetic order. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 702–707 (10 May 1996)     

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at T{c}=38 K in (Ba1-xKx)Fe2As2 with x approximately 0.4. The parent compound BaFe2As2 crystallizes in the tetragonal ThCr2Si2-type structure, which consists of (FeAs);{delta-} iron arsenide layers separated by Ba2+ ions. BaFe2As2 is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs);{-} layers, which suppress the anomaly and induce superconductivity. The T{c} of 38 K in (Ba0.6K0.4)Fe2As2 is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure.     

Superconductivity in the Th-doped Y NiC2 compounds

We have observed superconductivity in the two-phase samples with nominal compositions (Y _1−xTh_x)NiC₂ (x=0.1,0.2,0.3,0.4,0.5). As determined by magnetic, electrical and heat capacity measurements, the superconductivity phase transition temperature T_c increases with Th concentration to . Powder X-ray diffraction data provide the evidence for bulk superconductivity in an orthorhombic CeNiC₂-type majority fraction of the sample volume while the minor impurity phase ThC₂ has no effect on the superconductivity. The variation of room temperature lattice parameters, a, b, c and v of these substitutional intermetallic samples indicates the systematic substitution of Y ³⁺ ions by Th⁴⁺ ions irrespective of the existence of the 2nd impurity phase.     

Superconductivity at Tc = 44 K in LixFe2Se2(NH3)y

We report a new approach to measuring the dielectric behavior of thin films by means of differential interdigitated electrode (IDE) cells coupled with a dual-channel impedance measurement setup. The differential IDE cell consists of two identical IDE??s on a common substrate. With one IDE loaded with sample and the other one empty, the complex permittivities of both capacitors are measured simultaneously by means of a dual-channel impedance measurement setup. The net dielectric response of the material under study is then obtained by the difference of the two permittivities, which corrects for the substrate contribution. The applicability of this approach is examined with bulk glycerol and a 600 nm indomethacin film and is evidenced by the results being consistent with those measured by conventional methods. The main advantages of this new approach include a simplified preparation technique for thin film samples and a straightforward correction for the substrate contribution by subtracting the empty IDE signal obtained at the same temperature and thermal history.     

Superconductivity at 55K in Iron-Based F-Doped Layered Quaternary Compound Sm[O1-xFx]FeAs

We report the superconductivity in iron-based oxyarsenide Sm[O_1-xF_x]FeAs, with the onset resistivity transition temperature at 55.0K and Meissner transition at 54.6K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides up to now.     

Superconductivity at 108 K in YBa2Cu4O8 at pressures up to 12 Gpa

Abstract

Resistive measurements in a cryogenic diamond anvil cell show that the T, of YBa₂Cu₄O₈ can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p < 5 GPa, dT_c/dp ～5 K/GPa. At higher pressures dT_c/dp decreases gradually and T_c saturates at ～108 K. This behavior can be reproduced by a phenomenological model where T, depends only on the number of holes in the CuO₂-planes.     

Superconductivity at 45 K in La2CuO4+δ oxidized by NaClO

An efficient and convenient oxidizing agent, sodium hypochlorite solution, has been used to chemically prepare the superconductor La₂CuO_4+δ at room temperature. The crystallographic data show an enhanced orthorhombic distortion and an increase of the unit-cell volume. The magnetic susceptibility properties reveal a sharp transition beginning at 45 K and the oxidized sample is a homogeneous and single-phase bulk superconductor. The iodometric titration measurements indicate that the oxidized sample has a higher excess-oxygen content of 0.11. Scanning electron microscope observations showed that no grain size difference was observed for the sample before and after oxidation. Received: 15 September 2000 / Accepted: 19 January 2001 / Published online: 3 April 2001     

Superconductivity in ThMo_2Si_2C with Mo_2C square net

We report the superconductivity of a new quaternary compound ThMo_2Si_2C, synthesized with the arc-melting technique. The compound crystallizes in a tetragonal CeCr_2Si_2C-type structure with cell parameters of a = 4.2296 ? and c = 5.3571 ?. An interlayer Si–Si covalent bonding is suggested by the atomic distance. The electrical resistivity and magnetic susceptibility measurements indicate a Pauli-paramagnetic metal with dominant electron-electron scattering in the normal-state. Bulk superconductivity at 2.2 K is demonstrated with a dimensionless specific-heat jump of ?C/γnT = 0.98. The superconducting parameters of the critical magnetic fields, coherence length, penetration depth, and superconducting energy gap are given.     

Superconductivity at 40 K in La1.8Sr0.2CuO4

High-T _c superconductivity withT _c onsets up to 42 K (midpoint: 37 K, zero resistance: 34 K) is observed in X-rays homogeneous single phase La_1.8Sr_0.2CuO₄. The quarternary compounds La_2–x Ba _x CuO₄ and La_2–x Sr _x CuO₄ (0x0.3 for Ba and 0x<1 for Sr, depending on the heating conditions) are mixed-valence oxygen defect oxides, characterized by the presence of Cu²⁺ and Cu³⁺ simultaneously. These oxides have a tetragonal symmetry (space group:I 4/mmm) similar to that of K₂NiF₄. We report the synthesis, characterization, and superconducting properties of various high-temperature superconducting La–Ba–Cu–O and La–Sr–Cu–O compounds. Through the substitution of Sr for Ba in these oxygen defect compounds an increasing superconducting transition temperature onset from 28 K to 35 K for La_1.8Sr_0.1Ba_0.1CuO₄ was observed. A positive initial pressure coefficient ofdTc/dp=290 (mK/kbar) has been found for La_1.8Sr_0.2CuO₄ with a magnetic susceptibility change consistent with the 100% diamagnetic expectation value.