Superconductivity at 28 K in Sodium Graphite Intercalation Compound under High Pressure

The discovery of 15.1 K superconductivity in first-stage <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>CaC</a:mi></a:mrow><a:mrow><a:mn>6</a:mn></a:mrow></a:msub></a:mrow></a:math> reignited interest in searching for high-temperature superconductors in graphite intercalation compounds (GICs). However, despite nearly two decades of intensive research, progress in exploring high-temperature superconductivity in GIC materials has remained stagnant. Currently, research on the superconductivity of GICs mainly focuses on first-stage GICs, while the superconductivity of high-stage GICs has been largely overlooked. Here, we report an experimental discovery of superconductivity with maximum critical temperature (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>T</c:mi></c:mrow><c:mrow><c:mi>c</c:mi></c:mrow></c:msub></c:mrow></c:math>) of approximately 28 K in sodium carbide at about 14 GPa evidenced by a sharp drop of resistance and a characteristic decrease of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>T</e:mi><e:mi>c</e:mi></e:msub></e:math> under magnetic fields up to 6 T. The upper critical field <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mrow><g:msub><g:mrow><g:mi>μ</g:mi></g:mrow><g:mrow><g:mn>0</g:mn></g:mrow></g:msub><g:msub><g:mrow><g:mi>H</g:mi></g:mrow><g:mrow><g:mi>c</g:mi><g:mn>2</g:mn></g:mrow></g:msub></g:mrow><g:mo stretchy="false">(</g:mo><g:mn>0</g:mn><g:mo stretchy="false">)</g:mo></g:mrow></g:math> was estimated to be 8.8 T based on fitting with the Ginzburg-Landau superconducting model while the coherent length is approximately <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mn>61.2</k:mn><k:mtext> </k:mtext><k:mtext> </k:mtext><k:mi>Å</k:mi></k:mrow></k:math>. X-ray diffraction measurements combined with crystal structure predictions identified the superconducting candidate material as the second-stage GIC <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:msub><m:mrow><m:mi>NaC</m:mi></m:mrow><m:mrow><m:mn>8</m:mn></m:mrow></m:msub></m:mrow></m:math>, in which sodium atoms occupy the interlayer spaces between AA-stacked bilayer graphene. The current findings not only achieve the long-sought high-<o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>T</o:mi><o:mi>c</o:mi></o:msub></o:math> GICs, but more importantly, open up a new avenue for exploring high-temperature superconductors in light-element compounds.