Dimensionality Tuning of Heavy-Fermion States in Ultrathin <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>CeSi</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:m

Dimensionality tuning is an important method to modify the electronic states of quantum materials. However, the mechanism of such tuning in heavy fermion systems and its connection with transport properties remain largely unexplored. Here by combining molecular beam epitaxy, angle-resolved photoemission spectroscopy and transport measurements, we study the electronic states of the heavy-fermion compound <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>CeSi</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> as a function of film thickness. In three-dimensional thick films, our measurements reveal a dispersive Kondo peak at the Fermi level ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>E</c:mi> <c:mi>F</c:mi> </c:msub> </c:math> ) and satellite peaks originating from crystal electric field (CEF) excitations, characteristic of heavy-fermion systems. For two-dimensional ultrathin films, the CEF satellites are largely suppressed while the ground-state Kondo peak at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>E</e:mi> <e:mi>F</e:mi> </e:msub> </e:math> remains strong, although it develops at lower temperatures. Simultaneously, the maximum temperature <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>T</g:mi> <g:mi>max</g:mi> </g:msub> </g:math> of the magnetic resistivity, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:msub> <i:mrow> <i:mi>ρ</i:mi> </i:mrow> <i:mrow> <i:mi>m</i:mi> </i:mrow> </i:msub> <i:mo stretchy="false">(</i:mo> <i:mi>T</i:mi> <i:mo stretchy="false">)</i:mo> </i:mrow> </i:math> , changes from <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mo>≈</m:mo> <m:mn>100</m:mn> <m:mtext> </m:mtext> <m:mtext> </m:mtext> <m:mi mathvariant="normal">K</m:mi> </m:math> in thick films to <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mo>≈</p:mo> <p:mn>35</p:mn> <p:mtext> </p:mtext> <p:mtext> </p:mtext> <p:mi mathvariant="normal">K</p:mi> </p:math> in ultrathin films. This can be attributed to the dimensionality-driven reduction of CEF excitations during the Kondo process, in good agreement with spectroscopic results. Our work provides direct insight to understand the quantum confinement effects on strongly correlated <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mrow> <s:mn>4</s:mn> <s:mi>f</s:mi> </s:mrow> </s:math> -electron systems and opens up new opportunities to explore emergent phenomena in two-dimensional heavy-fermion materials.