Favorable phase transitions induced by spinful electron–electron interactions in two-dimensional semimetal with a quadratic band crossing point

We study the effects of marginally spinful electron–electron interactions on the low-energy instabilities and favorable phase transitions in a two-dimensional (2D) spin-$1/2$ semimetal that owns a quadratic band crossing point (QBCP) parabolically touched by the upper and lower bands. In the framework of a renormalization group procedure, all sorts of interactions are treated on the equal footing to derive the coupled energy-dependent evolutions of all interaction couplings that govern the low-energy properties. Deciphering the essential physical information of such flows, we at first find that the tendencies of interaction parameters fall into three categories including Limit case, Special case, and General case based on the initial conditions. In addition, the 2D QBCP system is attracted to several distinct kinds of fixed points (FPs) in the interaction-parameter space, namely ${\mathrm{FP}}_1^{+}$/${\mathrm{FP}}_2^{-}$, ${\mathrm{FP}}_1^{\pm }$/ ${\mathrm{FP}}_2^{\pm }$/${\mathrm{FP}}_3^{\pm }$, and ${\mathrm{FP}}_1^{\pm }/{\mathrm{FP}}_3^{\pm }/{\mathrm{FP}}_{41,42,43}^{\pm }$ with the subscripts characterizing the features of FPs for the Limit, Special, and General cases, respectively. Furthermore, as approaching these FPs, we demonstrate that the spinful fermion–fermion interactions can induce a number of favorable instabilities accompanied by certain phase transitions. Specifically, the quantum anomalous Hall (QAH), quantum spin Hall (QSH), and nematic (Nem.) site(bond) states are dominant for ${\mathrm{FP}}_1^{\pm }$, ${\mathrm{FP}}_2^{\pm }$, and ${\mathrm{FP}}_3^{\pm }$, respectively. Rather, QSH becomes anisotropic nearby ${\mathrm{FP}}_{41,42,43}^{\pm }$ with one component leading and the others subleading. Besides, Nem.site(bond), chiral superconductivity, and nematic-spin-nematic (NSN.) site(bond) are subleading candidates around these FPs. Our findings provide valuable insights for further research into the 2D QBCP and similar systems.