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Register a new user$R$RuB$_{2}$ ($R$=Y, Lu), topological superconductor candidates with hourglass-type Dirac ring
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Topological properties and topological superconductivity in real materials have attracted intensive experimental and theoretical attention recently. Based on symmetry analysis and first-principles electronic structure calculations, we predict that $R$RuB$_{2}$ ($R$=Y, Lu) are not only topological superconductor (TSC) candidates, but also own the hybrid hourglass-type Dirac ring which is protected by the nonsymmorphic space group symmetry. Due to the band inversion around the time-reversal invariant $Gamma$ point in the Brillouin zone,$R$RuB$_{2}$ also have Dirac topological surface states (TSSs). More importantly, their TSSs on the (010) surface are within the band gap of bulk and cross the Fermi level, which form single Fermi surfaces. Considering the fact that both YRuB$_{2}$ and LuRuB$_{2}$ are superconductors with respective superconducting transition temperatures ($T_c$) of 7.6 K and 10.2 K, the superconducting bulks will likely induce superconductivity in the TSSs via the proximity effect. The ternary borides $R$RuB$_{2}$ may thus provide a very promising platform for studying the properties of topological superconductivity and hourglass fermions in the future experiments.
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We use c-axis resistivity and magnetoresistance measurements to study the interplay between antiferromagnetic (AF) and superconducting (SC) ordering in underdoped RBa_2Cu_3O_{6+x} (R = Lu, Y) single crystals. Both orders are found to emerge from an anisotropic 3D metallic state, upon which antiferromagnetism opposes superconductivity by driving the doped holes towards localization. Despite the competition, the superconductivity sets in before the AF order is completely destroyed and coexists with latter in a certain range of hole doping. We find also that strong magnetic fields affect the AF-SC interplay by both suppressing the superconductivity and stabilizing the Neel order.
An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi2B2C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron- boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R=Dy, Ho, Er and Tm, while for R=Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R=Ho and near 6 meV in R=Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R=Y, Lu. At low temperature the SC gap in R=Ho exhibits a standard single-band BCS-like dependence, which vanishes above T_c^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.
The structure of the layered transition-metal Borides $A$B$_2$ ($A =$ Os, Ru) is built up by alternating $T$ and B layers with the B layers forming a puckered honeycomb. Here we report superconducting properties of RuB$_2$ with a $T_c approx 1.5$K using measurements of the magnetic susceptibility versus temperature $T$, magnetization $M$ versus magnetic field $H$, resistivity versus $T$, and heat capacity versus $T$ at various $H$. We observe a reduced heat capacity anomaly at $T_c$ given by $Delta C/gamma T_c approx 1.1$ suggesting multi-gap superconductivity. Strong support for this is obtained by the successful fitting of the electronic specific heat data to a two-gap model with gap values $Delta_1/k_BT_c approx 1.88$ and $Delta_2/k_BT_c approx 1.13$. Additionally, $M$ versus $H$ measurements reveal a behaviour consistent with Type-I superconductivity. This is confirmed by estimates of the Ginzburg-Landau parameter $kappa approx 0.1$--$0.66$. These results strongly suggest multi-gap Type-I superconductivity in RuB$_2$. We also calculate the band structure and obtain the Fermi surface for RuB$_2$. The Fermi surface consists of one quasi-two-dimensional sheet and two nested ellipsoidal sheets very similar to OsB$_2$. An additional small $4^{rm th}$ sheet is also found for RuB$_2$. RuB$_2$ could thus be a rare example of a multi-gap Type-I superconductor.
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