Tuning the carrier localization, magnetic and thermoelectric properties of ultrathin ( LaNiO3−δ)1 /( LaAlO3</mml:…

Using a combination of density functional theory calculations with an on-site Coulomb repulsion term (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>DFT</a:mi><a:mo>+</a:mo><a:mi>U</a:mi></a:mrow></a:math>) and Boltzmann transport theory within the constant relaxation time approximation, we explore the effect of oxygen vacancies on the electronic, magnetic, and thermoelectric properties in ultrathin <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mrow><b:mo>(</b:mo><b:msub><b:mi>LaNiO</b:mi><b:mrow><b:mn>3</b:mn><b:mo>−</b:mo><b:mi>δ</b:mi></b:mrow></b:msub><b:mo>)</b:mo></b:mrow><b:mn>1</b:mn></b:msub><b:mo>/</b:mo><b:msub><b:mrow><b:mo>(</b:mo><b:msub><b:mi>LaAlO</b:mi><b:mn>3</b:mn></b:msub><b:mo>)</b:mo></b:mrow><b:mn>1</b:mn></b:msub><b:mrow><b:mo>(</b:mo><b:mn>001</b:mn><b:mo>)</b:mo></b:mrow></b:mrow></b:math> superlattices (SLs). For the pristine SL (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>δ</c:mi><c:mo>=</c:mo><c:mn>0</c:mn></c:mrow></c:math>), an antiferromagnetic charge-disproportionated (AFM-CD) (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msup><d:mi>d</d:mi><d:mn>8</d:mn></d:msup><d:munder><d:mi>L</d:mi><d:mo>̲</d:mo></d:munder></d:mrow><d:mrow><d:msup><d:mrow/><d:mn>2</d:mn></d:msup><d:msub><d:mrow><d:mo>)</d:mo></d:mrow><d:mrow><d:mi>S</d:mi><d:mo>=</d:mo><d:mn>0</d:mn></d:mrow></d:msub><d:msub><d:mrow><d:mo>(</d:mo><d:msup><d:mi>d</d:mi><d:mn>8</d:mn></d:msup><d:mo>)</d:mo></d:mrow><d:mrow><d:mi>S</d:mi><d:mo>=</d:mo><d:mn>1</d:mn></d:mrow></d:msub></d:mrow></d:math> phase is stabilized, irrespective of strain. At <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mi>δ</e:mi><e:mo>=</e:mo><e:mn>0.125</e:mn></e:math> and 0.25, the localization of electrons released from the oxygen defects in the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>NiO</f:mi><f:mn>2</f:mn></f:msub></f:math> plane triggers a charge-disproportionation, leading to a ferrimagnetic insulator both at <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>a</g:mi><g:msub><g:mi>SrTiO</g:mi><g:mn>3</g:mn></g:msub></g:msub></g:math> (tensile strain) and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>a</h:mi><h:msub><h:mi>LaSrAlO</h:mi><h:mn>4</h:mn></h:msub></h:msub></h:math> (compressive strain). At <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mi>δ</i:mi><i:mo>=</i:mo><i:mn>0.5</i:mn></i:math>, an insulating phase emerges with alternating stripes of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msup><j:mrow><j:mi>Ni</j:mi></j:mrow><j:mrow><j:mn>2</j:mn><j:mo>+</j:mo></j:mrow></j:msup></j:math> (high-spin) and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msup><k:mrow><k:mi>Ni</k:mi></k:mrow><k:mrow><k:mn>2</k:mn><k:mo>+</k:mo></k:mrow></k:msup></k:math> (low-spin) and oxygen vacancies ordered along the [110] direction (S-AFM), irrespective of strain. This results in a robust <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mi>n</l:mi></l:math>-type in-plane power factor of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mn>24</m:mn><m:mspace width="4pt"/><m:mo>µ</m:mo><m:mi mathvariant="normal">W</m:mi></m:mrow><m:mo>/</m:mo><m:msup><m:mrow><m:mi mathvariant="normal">K</m:mi></m:mrow><m:mn>2</m:mn></m:msup></m:math> cm at <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:msub><q:mi>a</q:mi><q:mi>STO</q:mi></q:msub></q:math> and <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mn>14</r:mn><r:mspace width="4pt"/><r:mo>µ</r:mo><r:mi mathvariant="normal">W</r:mi></r:mrow><r:mo>/</r:mo><r:msup><r:mrow><r:mi mathvariant="normal">K</r:mi></r:mrow><r:mn>2</r:mn></r:msup></r:math> cm at <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"><v:msub><v:mi>a</v:mi><v:mi>LSAO</v:mi></v:msub></v:math> at 300 K (assuming relaxation time <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:mrow><w:mi>τ</w:mi><w:mo>=</w:mo><w:mn>4</w:mn></w:mrow></w:math> fs). Additionally, the pristine and <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"><x:mi>δ</x:mi></x:math> = 0.5 SLs are shown to be dynamically stable. This demonstrates the fine tunability of electronic, magnetic, and thermoelectric properties of ultrathin nickelate superlattices by oxygen vacancies.