Ternary nickel hydrides: A platform for unconventional superconductivity and quantum magnetism

Hydrides are famous for the possibility of reaching high-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>T</a:mi><a:mi mathvariant="normal">c</a:mi></a:msub></a:math> superconductivity under high pressure within a conventional mechanism. Here we propose ternary nickel hydrides <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>M</c:mi><c:mi>Ni</c:mi><c:msub><c:mi mathvariant="normal">H</c:mi><c:mn>2</c:mn></c:msub></c:mrow></c:math> (<e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>M</e:mi><e:mo>=</e:mo><e:mi>Li</e:mi></e:mrow></e:math>, Na) as materials that mirror key aspects of unconventional superconducting cuprates and nickelates while presenting unique characteristics. Compared to Ni oxides, Ni-H bands are wider due to shorter bond lengths and show a smaller charge-transfer energy. This leads to a larger scale of magnetic interactions than for <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>LaNi</f:mi><f:msub><f:mi mathvariant="normal">O</f:mi><f:mn>2</f:mn></f:msub></f:mrow></f:math>, which previous works in cuprates suggest should lead to a larger <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>T</h:mi><h:mi mathvariant="normal">c</h:mi></h:msub></h:math>. The presence of an interstitial anionic electron orbital, which hybridizes with the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>d</j:mi><j:msup><j:mrow><j:mi>z</j:mi></j:mrow><j:mn>2</j:mn></j:msup></j:msub></j:math> band, induces self-doping in the Ni <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>d</k:mi><k:mrow><k:msup><k:mrow><k:mi>x</k:mi></k:mrow><k:mn>2</k:mn></k:msup><k:mo>−</k:mo><k:msup><k:mrow><k:mi>y</k:mi></k:mrow><k:mn>2</k:mn></k:msup></k:mrow></k:msub></k:math> band, enabling metallicity and superconductivity in stoichiometric forms. A <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mi>t</l:mi><l:mo>−</l:mo><l:mrow><l:mi>J</l:mi></l:mrow></l:math> model computation yields dominant <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:msub><m:mi>d</m:mi><m:mrow><m:msup><m:mrow><m:mi>x</m:mi></m:mrow><m:mn>2</m:mn></m:msup><m:mo>−</m:mo><m:msup><m:mrow><m:mi>y</m:mi></m:mrow><m:mn>2</m:mn></m:msup></m:mrow></m:msub></m:math> superconducting symmetry for all doping levels. This, combined with the improved thermodynamic stability over known <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mi mathvariant="normal">N</n:mi><n:msup><n:mrow><n:mi mathvariant="normal">i</n:mi></n:mrow><n:mrow><n:mn>1</n:mn><n:mo>+</n:mo></n:mrow></n:msup></n:mrow></n:math> oxides, positions <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:mi>M</q:mi><q:mi>Ni</q:mi><q:msub><q:mi mathvariant="normal">H</q:mi><q:mn>2</q:mn></q:msub></q:mrow></q:math> as compelling materials for unconventional high-<s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:msub><s:mi>T</s:mi><s:mi mathvariant="normal">c</s:mi></s:msub></s:math> superconductivity under standard conditions.