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Register a new userTopological properties of Mo$_{2}$C and W$_{2}$C superconductors
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The topological electronic properties of orthorhombic-phase Mo$_{2}$C and W$_{2}$C superconductors have been studied based on first-principles electronic structure calculations. Our studies show that both Mo$_{2}$C and W$_{2}$C are three-dimensional strong topological insulators defined on curved Fermi levels. The topological surface states on the (001) surface of Mo$_{2}$C right cross the Fermi level, while those of W$_{2}$C pass through the Fermi level with slight electron doping. These surface states hold helical spin textures and can be induced to become superconducting via a proximity effect, giving rise to an equivalent $p+ip$ type superconductivity. Our results show that Mo$_{2}$C and W$_{2}$C can provide a promising platform for exploring topological superconductivity and Majorana zero modes.
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A recent experiment reported two new non-centrosymmetric superconductors NbIr$_{2}$B$_{2}$ and TaIr$_{2}$B$_{2}$ with respective superconducting transition temperatures of 7.2 K and 5.2 K and further suggested their superconductivity to be unconventional [K. Gornicka textit{et al}., Adv. Funct. Mater. 2007960 (2020)]. Here, based on first-principles calculations and symmetry analysis, we propose that $T$Ir$_{2}$B$_{2}$ ($T$=Nb, Ta) are topological Weyl metals in the normal state. In the absence of spin-orbit coupling (SOC), we find that NbIr$_{2}$B$_{2}$ has 12 Weyl points, and TaIr$_{2}$B$_{2}$ has 4 Weyl points, i.e. the minimum number under time-reversal symmetry; meanwhile, both of them have a nodal net composed of three nodal lines. In the presence of SOC, a nodal loop on the mirror plane evolves into two hourglass Weyl rings, along with the Weyl points, which are dictated by the nonsymmorphic glide mirror symmetry. Besides the rings, NbIr$_{2}$B$_{2}$ and TaIr$_{2}$B$_{2}$ have 16 and 20 pairs of Weyl points, respectively. The surface Fermi arcs are explicitly demonstrated. On the (110) surface of TaIr$_{2}$B$_{2}$, we find extremely long surface Fermi arcs ($sim$0.6 ${text{AA}}^{-1}$) located 1.4 meV below the Fermi level, which should be readily probed in experiment. Combined with the intrinsic superconductivity and the nontrivial bulk Fermi surfaces, $T$Ir$_{2}$B$_{2}$ may thus provide a very promising platform to explore the three-dimensional topological superconductivity.
The effect of disorder on the superconducting transition temperature $T_{c}$ of cuprate superconductors is examined. Disorder is introduced into the cation sites in the plane adjacent to the CuO$_{2}$ planes of two single-layer systems, Bi$_{2.0}$Sr$_{1.6}$Ln$_{0.4}$CuO$_{6+delta}$ and La$_{1.85-y}$Nd$_{y}$Sr$_{0.15}$CuO$_{4}$. Disorder is controlled by changing rare earth (Ln) ions with different ionic radius in the former, and by varying the Nd content in the latter with the doped carrier density kept constant. We show that this type of disorder works as weak scatterers in contrast to the in-plane disorder produced by Zn, but remarkably reduces $T_{c}$ suggesting novel effects of disorder on high-$T_{c}$ superconductivity.
We report the growth and properties of epitaxial MgB2 thin films on (0001) Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films via RF magnetron sputtering, followed by a post-deposition anneal at 850C in magnesium vapor. X-ray diffraction and cross-sectional TEM reveal that the epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3 substrate and a 30 degree rotation in the ab-plane with respect to the substrate. The critical temperature was found to be 35 K and the anisotropy ratio, Hc2(parallel to the film) / Hc2(pendicular to the film), about 3 at 25K. The critical current densities at 4.2 K and 20 K (at 1 T perpendicular magnetic field) are 5x10E6 A/cm2 and 1x10E6 A/cm2, respectively. The controlled growth of epitaxial MgB2 thin films opens a new avenue in both understanding superconductivity in MgB2 and technological applications.
We carried out Raman measurements on neutron-irradiated and Al-doped MgB$_2$ samples. The irradiation-induced topological disorder causes an unexpected appearance of high frequency spectral structures, similar to those observed in lightly Al-doped samples. Our results show that disorder-induced violations of the selection rules are responsible for the modification of the Raman spectrum in both irradiated and Al-doped samples. Theoretical calculations of the phonon density of states support this hypothesis, and demonstrate that the high frequency structures arise mostly from contributions at ${bf q} ot=0$ of the E$_{2g}$ phonon mode.
This review introduces known candidates for bulk topological superconductors and categorizes them with time-reversal symmetry (TRS) and gap structures. Recent studies on two archetypal topological superconductors, TRS-broken Sr2RuO4 and TRS-preserved CuxBi2Se3, are described in some detail.
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