Multiband Superconductivity of Heavy Electrons in aTlNi 2 Se 2
Hangdong WangChiheng DongQianhui MaoRajwali KhanXi ZhouChenxia LiBin ChenJinhu YangQi-Ping SuMinghu Fang
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We have made the first observation of superconductivity in TlNi2Se2 at T(C)=3.7 K, and it appears to involve heavy electrons with an effective mass m*=(14-20)m(b), as inferred from the normal-state electronic specific heat and the upper critical field, H(C2)(T). We found that the zero-field electronic specific-heat data, C(es)(T) (0.5 K≤T<3.7 K) in the superconducting state can be fitted with a two-gap BCS model, indicating that TlNi2Se2 seems to be a multiband superconductor, which is consistent with the band calculation for the isostructural KNi2S2. It is also found that the electronic specific-heat coefficient in the mixed state γN(H) exhibits a H(1/2) behavior, which is considered as a common feature of the d-wave superconductors. TlNi2Se2, as a d-electron system with heavy electron superconductivity, may be a bridge between cuprate- or iron-based and conventional heavy-fermion superconductors.Keywords:
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Strongly correlated material
Within the t-t'-J model, the physical properties of doped cuprates in the superconducting-state are discussed based on the kinetic energy driven superconducting mechanism. We show that the superconducting-state in cuprate superconductors is controlled by both superconducting gap parameter and single particle coherence, and then quantitatively reproduce some main features found in the experiments on cuprate superconductors, including the doping dependence of the superconducting gap parameter and superconducting transition temperature, the electron spectral function at [π,0] point, the charge asymmetry of superconductivity in the hole and electron doping, and the doping and energy dependence of the incommensurate magnetic scattering at both low and high energies and commensurate [π,π] resonance at intermediate energy. We also show that the incommensurate magnetic excitations at high energy have energies greater than the superconducting gap energy, and are present at the superconducting transition temperature.
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In order to investigate possible isostructural solid solutions of disubstituted N-phenylformamides and thioamides, we have studied the re-crystallization of pairs of compounds selected from 2,6-difluoro-N-phenylformamide (I), 2,6-dichloro-N-phenylformamide (II), 2,6-dimethyl-N-phenylformamide (III), 2,6-dichloro-N-phenylthioamide (IV), 2,6-dimethyl-N-phenylthioamide (V), 2,6-diisopropyl-N-phenylformamide (VI) and 2,6-diisopropyl-N-phenylthioamide (VII). For single-component 2,6-disubstituted-N-phenylformamides only the trans form occurs in the pure crystal, while for thioamides the cis form occurs, with only one exception. By forming solid solutions of pairs of these molecules the resulting structures all adopt similar N-H...O/S chains in the crystals. Solid solutions (1), (2) and (3), resulting from the mixing of (I) and (II), (II) and (III), and (IV) and (V), respectively, are all isostructural with each other (space group Pbca). Only co-crystal (1) is isostructural to both starting materials, while (2) is isostructural to only one of the starting pair, (II). Solid solution (3), which adopts the same Pbca structure as (1) and (2), is different to the monoclinic structures of both the reactants. Solid solution (4) is monoclinic, with similar hydrogen-bonded chains, and isostructural to the two components, resulting from the composition from the mixing of (VI) and (VII). Isostructural indices were used to quantify crystal-packing similarities and differences. Occupancy factors of the reactants in each co-crystal differ widely.
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No stars but stripes are known to occur in cuprate superconductors (see picture) and there has been an implicit belief that stripes play a seminal role in superconductivity. Based on recent experiments, it now appears that the relation between the stripes and superconductivity is by no means established. There is need for crucial experiments to understand this important aspect.
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Crystal structures of the guest free forms and some solvates of 1,3,5-triethyl-2,4,6-tris(4-halophenoxy)methylbenzenes (1X, X = I, Br, Cl, F) have been studied. The guest free forms of 1I, 1Br, and 1Cl are isostructural, but the crystal structure of 1F is different from the heavier halogen analogues. An entirely different crystal structure of the Me analogue, which is known to be isosteric to the corresponding bromo compound, shows the importance of the electronic factors of the halogens in this series of structures. 1I and 1Br form four types of architectures in their solvates depending upon the inter-halogen interaction geometries. All these solvates are two dimensionally isostructural to the guest free form. 1Cl forms three different types of frameworks in its solvates, and the ethylacetate solvates of the 1I, 1Br, and 1Cl are isostructural. 1F forms only one type of solvate which is isostructural to the corresponding 1Cl solvate.
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The widely held notion that high-temperature superconductivity originates in the cuprate-planes is proven to be faulty. In the cuprates such as YBa2Cu3O7, we argue that the superconductivity resides in the BaO layers. This superconductivity is s-wave, not d-wave, in the bulk. The trio of ruthenate compounds, doped Sr2YRuO6, GdSr2Cu2RuO8, and Gd2-zCe zSr2Cu2RuO10 all superconduct in their SrO layers, which is why they have almost the same ~49 K onset temperatures for superconductivity.
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Superconductivity in layered cuprates is induced by doping holes into a parent antiferromagnetic insulator. It is now recognized that another common emergent order involves charge stripes, and our understanding of the relationship between charge stripes and superconductivity has been evolving. Here we review studies of 214 cuprate families obtained by doping La$_2$CuO$_4$. Charge-stripe order tends to compete with bulk superconductivity; nevertheless, there is plentiful evidence that it coexists with two-dimensional superconductivity. This has been interpreted in terms of pair-density-wave superconductivity, and the perspective has shifted from competing to intertwined orders. In fact, a new picture of superconductivity based on pairing within charge stripes has been proposed, as we discuss.
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Copper oxide
Transition temperature
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Multiorbital systems such as the iron-based superconductors provide a new avenue to attack the long-standing problem of superconductivity in strongly correlated systems. In this work we study the superconductivity driven by a generic bosonic mechanism in a multiorbital model including the full dynamical electronic correlations induced by the Hubbard $U$ and the Hund's coupling. We show that superconductivity survives much more in a Hund's metal than in an ordinary correlated metal with the same degree of correlation. The redistribution of spectral weight characteristic of the Hund's metal reflects also in the enhancement of the orbital-selective character of the superconducting gaps, in agreement with experiments in iron-based superconductors.
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Redistribution
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