Two-dimensional superconductivity with unconventional magnetotransport in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>PtPb</mml:mi> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> single crystals: A

This study presents a new nonsymmorphic symmetry-protected topological phase, as well as a two-dimensional (2D) surface topological superconductivity on ${\mathrm{PtPb}}_{4}$ surface. The bulk superconductivity of ${\mathrm{PtPb}}_{4}$ exhibited characteristic of a conventional moderately-coupled Bardeen--Cooper--Schrieffer nonideal type-II superconductor. Nevertheless, the superconductivity investigated via electrical transport exhibited anisotropic properties of 2D nature, verified through a Berezinsky-Kosterlitz-Thouless transition and 2D-like vortex dynamics in mixed-state transports. The obtained activation energy U(H), critical current density ${J}_{\mathrm{c}}(T,\phantom{\rule{0.16em}{0ex}}H)$, and scaling of pinning forces for ${F}_{\mathrm{p}}(H)$ for ${\mathrm{PtPb}}_{4}$ showed a relatively weak vortex pinning in ${\mathrm{PtPb}}_{4}$, accompanied by a crossover of the flux-line lattice from elastic relaxation to shear deformation. In the normal state, ${\mathrm{PtPb}}_{4}$ revealed nontrivial quantum interference transports with a crossover from weak-antilocalization- to weak-localization-dominated transverse magnetoresistance (TMR) in the low-field region, demonstrating the presence of topological surface states on ${\mathrm{PtPb}}_{4}$. Besides, the violation of Kohler's rule with a power-law exponent value of 1.4 also revealed a nontrivial MR characteristic of ${\mathrm{PtPb}}_{4}$. The pronounced exotic TMR characteristic confirmed that ${\mathrm{PtPb}}_{4}$ indeed hosts topological surface states, driven by a new nonsymmorphic symmetry-protected topological phase. All results are summarized in a constructed phase diagram of ${\mathrm{PtPb}}_{4}$ in the H-T plane, displaying different regimes of transport. The observed 2D superconductivity, as well as nontrivial quantum-interference transports, can be attributed to the dispersion of topological Dirac-like surface band in ${\mathrm{PtPb}}_{4}$. Our findings provide robust support for the emergence of a new nonsymmorphic symmetry-protected topological phase on the ${\mathrm{PtPb}}_{4}$ surface. Thus, ${\mathrm{PtPb}}_{4}$ is one of the few reported nonsymmorphic symmetry-protected topological superconductors, paving the way for the development of superconducting quantum computation.