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Register a new userMultigap nodeless superconductivity in CsCa2Fe4As4F2 probed by heat transport
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Recently, a new family of iron-based superconductors called 12442 was discovered and the muon spin relaxation ($mu$SR) measurements on KCa$_2$Fe$_4$As$_4$F$_2$ and CsCa$_2$Fe$_4$As$_4$F$_2$ polycrystals, two members of the family, indicated that both have a nodal superconducting gap structure with $s+d$ pairing symmetry. Here we report the ultralow-temperature thermal conductivity measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals ($T_c$ = 29.3 K). A negligible residual linear term $kappa_0/T$ in zero field and the field dependence of $kappa_0/T$ suggest multiple nodeless superconducting gaps in CsCa$_2$Fe$_4$As$_4$F$_2$. This gap structure is similar to CaKFe$_4$As$_4$ and moderately doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, but contrasts to the nodal gap structure indicated by the $mu$SR measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ polycrystals.
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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.
We report a comprehensive study of the centrosymmetric Re$_3$B and noncentrosymmetric Re$_7$B$_3$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with $T_c$ = 5.1 K (Re$_3$B) and 3.3 K (Re$_7$B$_3$), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation/relaxation ($mu$SR). In both Re$_3$B and Re$_7$B$_3$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via tranverse-field $mu$SR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by temperature-dependent upper critical fields $H_mathrm{c2}(T)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $mu$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of both Re$_3$B and Re$_7$B$_3$. Our results suggest that a lack of inversion symmetry and the accompanying antisymmetric spin-orbit coupling effects are not essential for the occurrence of multigap SC in these rhenium-boron compounds.
We report a comprehensive study of the noncentrosymmetric superconductor Mo$_3$P. Its bulk superconductivity, with $T_c = 5.5$ K, was characterized via electrical resistivity, magnetization, and heat-capacity measurements, while its microscopic electronic properties were investigated by means of muon-spin rotation/relaxation ($mu$SR) and nuclear magnetic resonance (NMR) techniques. In the normal state, NMR relaxation data indicate an almost ideal metallic behavior, confirmed by band-structure calculations, which suggest a relatively high electron density of states, dominated by the Mo $4d$-orbitals. The low-temperature superfluid density, determined via transverse-field $mu$SR and electronic specific heat, suggest a fully-gapped superconducting state in Mo$_3$P, with $Delta_0= 0.83$ meV, the same as the BCS gap value in the weak-coupling case, and a zero-temperature magnetic penetration depth $lambda_0 = 126$ nm. The absence of spontaneous magnetic fields below the onset of superconductivity, as determined from zero-field $mu$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of Mo$_3$P and, hence, spin-singlet pairing.
We grew the single crystals of the SnAs-based van der Waals (vdW)-type superconductor NaSn$_2$As$_2$ and systematically measured its resistivity, specific heat, and ultralow-temperature thermal conductivity. The superconducting transition temperature $T_c$ = 1.60 K of our single crystal is 0.3 K higher than that previously reported. A weak but intrinsic anomaly situated at 193 K is observed in both resistivity and specific heat, which likely arises from a charge-density-wave (CDW) instability. Ultralow-temperature thermal conductivity measurements reveal a fully-gapped superconducting state with a negligible residual linear term in zero magnetic field, and the field dependence of $kappa_0 / T$ further suggests NaSn$_2$As$_2$ is an $s$-wave superconductor.
The recently discovered kagome superconductor CsV$_3$Sb$_5$ ($T_c simeq 2.5$ K) has been found to host charge order as well as a non-trivial band topology, encompassing multiple Dirac points and probable surface states. Such a complex and phenomenologically rich system is, therefore, an ideal playground for observing unusual electronic phases. Here, we report on microscopic studies of its anisotropic superconducting properties by means of transverse-field muon spin rotation ($mu$SR) experiments. The temperature dependences of the in-plane and out-of-plane components of the magnetic penetration depth $lambda_{ab}^{-2}(T)$ and $lambda_{c}^{-2}(T)$ indicate that the superconducting order parameter exhibits a two-gap ($s+s$)-wave symmetry, reflecting the multiple Fermi surfaces of CsV3Sb5. The multiband nature of its superconductivity is further validated by the different temperature dependences of the anisotropic magnetic penetration depth $gamma_lambda(T)$ and upper critical field $gamma_{rm B_{c2}}(T)$, both in close analogy with the well known two-gap superconductor MgB$_2$. Remarkably, the high value of the $T_c/lambda^{-2}(0)$ ratio in both field orientations strongly suggests the unconventional nature of superconductivity. The relaxation rates obtained from zero field $mu$SR experiments do not show noticeable change across the superconducting transition, indicating that superconductivity does not break time reversal symmetry.
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