Insulator to metal transition by bandwidth reduction

In a classic Mott system, the transition from the insulating to the metallic state occurs when the lattice spacing is reduced such that the increase in bandwidth <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>W</a:mi> </a:math> overcomes the local Coulomb repulsion <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>U</b:mi> </b:math> . In this picture, both <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mi>W</c:mi> </c:math> and the ratio <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi>W</d:mi> <d:mo>/</d:mo> <d:mi>U</d:mi> </d:mrow> </d:math> become larger with decreasing the distance between correlated sites. Here we show that, in transition-metal compounds, there is another path to metallization. It is associated with an expansion (instead of a compression) of the lattice and thus with a in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mi>W</e:mi> </e:math> . In this scenario <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mi>W</f:mi> <f:mo>/</f:mo> <f:mi>U</f:mi> </f:mrow> </f:math> grows with increasing distance—the opposite of the classical Mott case. Such a surprising inversion can be obtained by decoupling the main factors controlling <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mi>W</g:mi> </g:math> and screening. This mechanism explains the electronic behavior of the <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msubsup> <h:mi>t</h:mi> <h:mrow> <h:mn>2</h:mn> <h:mi>g</h:mi> </h:mrow> <h:mn>2</h:mn> </h:msubsup> </h:math> triangular lattice series <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:mi>LiV</i:mi> <i:msub> <i:mi>X</i:mi> <i:mn>2</i:mn> </i:msub> </i:mrow> </i:math> ( <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:mi>X</j:mi> <j:mo>=</j:mo> <j:mi mathvariant="normal">O</j:mi> </j:mrow> </j:math> , S, and Se) with increasing anionic size.