Gang Mu

High- T c superconductivity in ultrathin Bi 2 Sr 2 CaCu 2 O 8+ x down to half-unit-cell thickness by protection with graphene

High-Tc superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi2Sr2CaCu2O(8+x) (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here, we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure. More interestingly, although the critical temperature remains essentially the same with reduced thickness of Bi2212, the slope of the normal state T-linear resistivity varies by a factor of 4-5, and the sheet resistance increases by three orders of magnitude, indicating a surprising decoupling of the normal state resistance and superconductivity. The developed technique is versatile, applicable to investigate other two-dimensional (2D) superconducting materials.

Enhanced Superconductivity in Restacked TaS2 Nanosheets

Since interface superconductivity was discovered at the interface between two insulating layers LaAlO3 and SrTiO3, such interface-induced superconducting systems have been a research hotspot in superconductivity. Here, we report homogeneous interfaces formed by stacking chemically exfoliated monolayer TaS2 nanosheets randomly. Enhanced superconductivity of Tc = 3 K is observed, compared with 0.8 K of parent 2H-TaS2. The measurement of heat capacity shows the increase of electronic specific-heat coefficient γ of restacked TaS2 nanosheets compared to parent 2H-TaS2 crystals. Density functional theory calculations indicate that increase and delocalization of electron states near the Fermi surface due to the homogeneous interfaces effects could account for the enhanced superconductivity.

Observation of superconductivity in 1T′-MoS2 nanosheets

Studies on the preparation and physical properties of phase-pure 1T′-MoS2 are still scarce although a 1T′ phase MX2 (M = Mo and W; X = Se and Te) has recently been reported to be a Weyl semimetal, a quantum spin Hall insulator, and a superconductor. Herein, we report a perfect single-layer 1T′-MoS2 structure based on edge-sharing octahedra. Pure 1T′-MoS2 nanosheets were successfully prepared using LiMoS2 single crystals. The arrangement of zig-zag Mo–Mo chains in the 1T′-MoS2 layer evolved from that of parent Mo–Mo diamond-like chains in LiMoS2 crystals after the removal of all lithium ions. A well-defined 1T′-type lattice structure was observed and characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Surprisingly, superconductivity with an onset transition temperature (Tc) of 4.6 K has been observed in the lithium-free 1T′-MoS2 nanosheets, which is not observed in semiconducting bulk or single-layer 2H-MoS2.

Synthesis, Structural, and Transport Properties of Cr-Doped BaTi2As2O

The interplay between unconventional superconductivity and the ordering of charge/spin density wave is one of the most vital issues in both condensed matter physics and material science. The Ti-based compound BaTi2As2O, which can be seen as the parent phase of superconducting BaTi2Sb2O, has a layered structure with a space group P4/mmm, similar to that of cuprate and iron-based superconductors. This material exhibits a charge density wave (CDW) ordering transition revealed by an anomaly at around 200 K in transport measurements. Here, we report the synthesis and systematical study of the physical properties in Cr-doped BaTi(2-x)Cr(x)As2O (x = 0-0.154) and demonstrate that the transition temperature of the CDW ordering is suppressed gradually by the doped Cr element. The magnetization measurements confirm the evolution of the CDW ordering transition. These observed behaviors are similar to that observed in iron-based superconductors, but no superconductivity emerges down to 2 K. In addition, the first-principles calculations are also carried out for well-understanding the nature of experimental observations.

Effect of Local Structure Distortion on Superconductivity in Mg- and F-Codoped LaOBiS2

La1-xMgxO1-2xF2xBiS2 (x = 0.1-0.35) were synthesized, and their superconductive properties were investigated. The superconducting transition temperature (Tc) increased below the codoping level (x ≤ 0.25). La1-xMgxOBiS2 (x = 0-0.2) and La1-xMgxO0.6F0.4BiS2 (x = 0.1-0.3) were further prepared to explore the effect of Mg(2+). We found that the introduction of Mg(2+) and F(-) leads to local structure distortion. Larger distortion is beneficial for superconductivity in LaOBiS2, which was further confirmed by the results in La1-xCaxO1-2xF2xBiS2 (x = 0.2, 0.3).

Growth and characterization of millimeter-sized single crystals of CaFeAsF

High-quality and sizable single crystals are crucial for studying the intrinsic properties of unconventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors. Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using the self-flux method. Owning to the availability of the high-quality single crystals, the structure and transport properties were investigated with a high reliability. The structure was refined by using the single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powder data. A clear anomaly associated with the structural transition was observed at 121 K from the resistivity, magnetoresistance, and magnetic susceptibility measurements. Another kink-feature at 110 K, most likely an indication of the antiferromagnetic transition, was also detected in the resistivity data. Our results supply a basis to propel the physical investigations on the 1111 phase of the Fe-based superconductors.

Strong anisotropy effect in an iron-based superconductor CaFe0.882Co0.118AsF

The anisotropy of the Fe-based superconductors is much smaller than that of the cuprates and the theoretical calculations. A credible understanding for this experimental fact is still lacking up to now. Here we experimentally study the magnetic-field-angle dependence of electronic resistivity in the superconducting phase of iron-based superconductor CaFe

Structure and properties of type-II clathrate Cs8Na16−xTlxGe136

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Type-I clathrates of K7.69(2)Cu2.94(6)Ge43.06(6) and Rb8Ag2.79(4)Ge43.21(4)

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Phase diagram and weak-link behavior in Nd-doped CaFe2As2

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