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.     

Electronic structures and magnetic order of ordered-Fe-vacancy ternary iron selenides TlFe1.5Se2 and AFe1.5Se2 (A=K, Rb, or Cs)

By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe(1.5)Se(2) is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe(1.5)Se(2) (A=K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with the ground states. Thus, rich physical properties and phase diagrams are expected.     

Heavy-fermion superconductivity atT c =2K in the antiferromagnet UPd2Al3

UPd₂Al₃ is a new heavy-fermion superconductor with a recordT _c of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atT _N =14 K. Its Sommerfeld coefficient is reduced from _p =210mJ/K² mole in the paramagnetic to ₀=150mJ/K² mole in the antiferromagnetic phase.     

On the antiferromagnetism of CuO at 230K and its relevance for high Tc superconductivity

The magnetic susceptibility of CuO shows an anomaly at 230K, which, together with reported neutron and specific heat data, can be interpreted as an indication of antiferromagnetic order. This finding is discussed in the context of the recently discovered high T_c superconductivity of CuO-derived compounds.     

Probing the superconducting energy gap from infrared spectroscopy on a Ba0.6K0.4Fe2As2 single crystal with Tc=37 K

We performed optical spectroscopy measurement on a superconducting Ba0.6K0.4Fe2As2 single crystal with T{c}=37 K. Formation of the superconducting energy gaps in the far-infrared reflectance spectra below T{c} is clearly observed. A flat and close to unity reflectance is observed roughly below 150 cm;{-1} for Tor=T{c}. We determined the absolute value of the penetration depth at 10 K as lambda approximately 2000+/-80 A. A spectral weight analysis shows that the Ferrell-Glover-Tinkham sum rule is satisfied at low energy scale, less than 6Delta.     

X-ray investigations of the cubic to tetragonal phase transition in CsCaCl3 at Tc = 95 K

Some new crystallographic data on the cubic to tetragonal phase transition at T_c = 95 K in CsCaCl₃ allow us to infer the space group D¹⁸_4h for the low temperature phase connected with alternate rotations of the CaCl₆ along 〈0 0 1〉 directions. From the relative distortion (c-a)/a we estimate the rotation angle which is the order parameter of the transition, smaller than in other chloride perovskites as CsPbCl₃ and CsSrCl₃.     

Electronic structure, magnetism and superconductivity of layered iron compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe (Se, Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.     

Uniaxial magnetic anisotropy of rhombohedral CoCO3 crystals at T = 0 K

A method for calculating the contribution of exchange interaction to uniaxial anisotropy with the use of g’ factors has been worked out using CoCO₃ crystals as an example. The calculated contribution of dipole-dipole interactions to the anisotropy of CoCO₃ is 0.93 cm^?1. The sum of the contributions to the anisotropy constant of CoCO₃ with the inclusion of the dipole-dipole interactions is 36.1 cm^?1.     

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ～ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ～3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.     

The spin-spin correlation function in the two-dimensional Ising model in a magnetic field at T = Tc

The form-factor bootstrap approach is used to compute the exact contributions in the large-distance expansion of the correlation function 〈σ(x)σ(0)〉 of the two-dimensional Ising modeliin a magnetic field at T = T_c The matrix elements of the magnetization operator σ,(x) present a rich analytic structure induced by the (multi-)) scattering processes of the eight massive particles of the model. The spectral representation series has a fast rate of convergence and perfectly agrees with the numerical determination of the correlation function.     

晶胞结构与掺杂铁基砷化物超导电性的关系

通过研究掺杂FeAs-1111超导体系的超导电性与其晶胞结构之间的关系,发现两者之间呈现一定的规律性。为了能够定量的分析它们之间的关系,又研究了掺杂FeAs-1111超导体系晶胞体积与键参数之间的关系,发现两者之间也呈现较好的规律。     

Effect of 3d doping on the electronic structure of BaFe2As2

The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.     

The far infrared transmission of crystal quartz at 3 K

We present transmission measurements of Z-cut crystal quartz at 3 K in the wavenumber range 60–180 cm⁻¹. The results show that the absorption is extremely small and in fact on the average a factor of ten smaller than indicated by earlier measurements discussed in the literature. One consequence of our results is that crystal quartz is useful in the FIR as a substrate material for Fabry-Perot meshes used in cryogenic experiments.