Impact of electron correlations on two-particle charge response in electron- and hole-doped cuprates

Estimating many-body effects that deviate from an independent particle approach has long been a key research interest in condensed matter physics. Layered cuprates are prototypical systems, where electron-electron interactions are found to strongly affect the dynamics of single-particle excitations. It is, however, still unclear how the electron correlations influence charge excitations, such as plasmons, which have been variously treated with either weak or strong correlation models. In this work, we demonstrate the hybridized nature of collective valence charge fluctuations leading to dispersing acoustic-like plasmons in hole-doped <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>La</a:mi><a:mrow><a:mn>1.84</a:mn></a:mrow></a:msub><a:msub><a:mi>Sr</a:mi><a:mrow><a:mn>0.16</a:mn></a:mrow></a:msub><a:msub><a:mi>CuO</a:mi><a:mn>4</a:mn></a:msub></a:mrow></a:math> and electron-doped <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mi>La</b:mi><b:mrow><b:mn>1.84</b:mn></b:mrow></b:msub><b:msub><b:mi>Ce</b:mi><b:mrow><b:mn>0.16</b:mn></b:mrow></b:msub><b:msub><b:mi>CuO</b:mi><b:mn>4</b:mn></b:msub></b:mrow></b:math> using the two-particle probe, resonant inelastic x-ray scattering. We then describe the plasmon dispersions in both systems, within both the weak-coupling mean-field random phase approximation (RPA) and strong-coupling <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>t</c:mi></c:mrow><c:mtext>−</c:mtext><c:mrow><c:mi>J</c:mi></c:mrow><c:mtext>−</c:mtext><c:mrow><c:mi>V</c:mi></c:mrow></c:math> model in a large-<d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>N</d:mi></d:math> scheme. The <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>t</e:mi></e:mrow><e:mtext>−</e:mtext><e:mrow><e:mi>J</e:mi></e:mrow><e:mtext>−</e:mtext><e:mrow><e:mi>V</e:mi></e:mrow></e:math> model, which includes the correlation effects implicitly, accurately describes the plasmon dispersions as resonant excitations outside the single-particle intraband continuum. In comparison, a quantitative description of the plasmon dispersion in the RPA approach is obtained only upon explicit consideration of renormalized electronic band parameters. Our comparative analysis shows that electron correlations significantly impact the low-energy plasmon excitations across the cuprate doping phase diagram, even at long wavelengths. Thus, complementary information on the evolution of electron correlations, influenced by the rich electronic phases in condensed matter systems, can be extracted through the study of two-particle charge response. Published by the American Physical Society 2024