Two-dimensional superconductivity in a bulk superlattice van der Waals material <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ba</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mi>Nb</mml:mi><mml:mn>11</mml:mn></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>28</mml:mn></mml:msub></mml:mrow></mml:math>
Ke Ma, Shifeng Jin, Fanqi Meng, Qinghua Zhang, Ruijin Sun, Jun Deng, Long Chen, Lin Gu, Gang Li, Zhihua Zhang
Abstract
The observation of clean 2D superconductivity and bosonic Landau levels below 0.85 K in superlattice van der Waals material ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ have received much interest. Here we report the crystal structure, Hall resistivity, and 2D superconductivity of its sister compound, ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$. The atomic-resolution transmission electron microscopy images reveal an alternate stacking of $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ with a 3 \ifmmode\times\else\texttimes\fi{} 3 commensurate block layer, which is isostructural to ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{S}}_{28}$ and with a larger $\mathrm{Nb}{X}_{2}$ ($X=\mathrm{S}$, Se) interlayer distance (12.8 \AA{}). Transport property measurements demonstrate a twofold enhancement in superconducting temperature $({{T}_{\mathrm{c}}}^{0}=2.2\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, meanwhile the anisotropy in resistivity and carrier mobility in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ decrease drastically compared to its sulfur counterpart, driving the superconductor towards the dirty limit. Further measurements of angular-dependent upper critical field suggest the superconductivity in ${\mathrm{Ba}}_{6}{\mathrm{Nb}}_{11}{\mathrm{Se}}_{28}$ be Pauli-limit breaking (2 times the ${\ensuremath{\mu}}_{0}{H}_{P}$), and can be well described by the 2D Tinkham form. Our findings offer a valuable platform to investigate 2D superconductivity in bulk superlattice materials based on the transition-metal dichalcogenides.