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The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

The unconventional multigap superconductivity in elemental Pb were reported previously by the surface sensitive tunneling experiments as well as predicted by several theory works. To obtain bulk evidence for such multiple gap behavior, the thermodynamic critical-field ${B}_{\mathrm{c}}$ was measured along three different crystallographic directions ([100], [110], and [111]) in a high-quality Pb single crystal by means of muon spin rotation/relaxation. No difference in temperature evolution of ${B}_{\mathrm{c}}$ for all three directions was detected. The average reduced gap $\ensuremath{\alpha}=\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=2.312(3)$ ($\mathrm{\ensuremath{\Delta}}$ is the zero-temperature gap value, and ${T}_{\mathrm{c}}$ is the transition temperature) was further obtained by employing the phenomenological $\ensuremath{\alpha}$ model. Our results imply that the elemental Pb is an isotropic superconductor with a single energy gap.

Recently, the niobium (Nb) doped topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layers, has been shown to exhibit both superconductivity with ${T}_{c}\ensuremath{\simeq}3$ K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth ${\ensuremath{\lambda}}_{\mathrm{eff}}\left(T\right)$ in the layered topological superconductor candidate ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. The exponential temperature dependence of ${\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature ${T}_{c}=2.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the gap to ${T}_{c}$ ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.95(19)$. We also reveal that the ratio ${T}_{c}/{\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}$ is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field $\ensuremath{\mu}\mathrm{SR}$ experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.

We explore two methods for single-crystal growth of the theoretically proposed magnetic Weyl semimetals $R\mathrm{AlGe}$ ($R$ = Pr, Ce), which prove that a floating-zone technique, being both crucible- and flux-free, is crucial to obtain perfectly stoichiometric $R\mathrm{AlGe}$ crystals. In contrast, the crystals grown by a flux-growth technique tend to be Al-rich. We further present both structural and elemental analyses, along with bulk magnetization and electrical resistivity data on the crystals prepared by the floating-zone technique. Both systems with the intended 1:1:1 stoichiometry crystallize in the anticipated polar $I{4}_{1}md$ (No. 109) space group, although neither displays the theoretically expected ferromagnetic ground state. Instead PrAlGe displays a spin-glass-like transition below 16 K with an easy $c$ axis and CeAlGe has an easy-$ab$-plane antiferromagnetic order below 5 K. The grown crystals provide an ideal platform for microscopic studies of the magnetic field-tunable correlation physics involving magnetism and topological Weyl nodes.