Yuanyuan Xiang

The electronic orders in Hubbard models on a kagome lattice at van Hove filling are of intense current interest and debate. We study this issue using the singular-mode functional renormalization group theory. We discover a rich variety of electronic instabilities under short-range interactions. With increasing on-site repulsion $U$, the system develops successively ferromagnetism, intra-unit-cell antiferromagnetism, and charge bond order. With nearest-neighbor Coulomb interaction $V$ alone ($U=0$), the system develops intra-unit-cell charge density wave order for small $V$, $s$-wave superconductivity for moderate $V$, and the charge density wave order appears again for even larger $V$. With both $U$ and $V$, we also find spin bond order and chiral ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$ superconductivity in some particular regimes of the phase diagram. We find that the $s$-wave superconductivity is a result of charge density wave fluctuations and the squared logarithmic divergence in the pairing susceptibility. On the other hand, the $d$-wave superconductivity follows from bond order fluctuations that avoid the matrix element effect. The phase diagram is vastly different from that in honeycomb lattices because of the geometrical frustration in the kagome lattice.

We study the electronic instabilities of near 1/4 electron doped graphene using the singular-mode functional renormalization group, with a self-adaptive $k$ mesh to improve the treatment of the van Hove singularities, and variational Monte Carlo method. At 1/4 doping the system is a chiral spin-density wave state exhibiting the anomalous quantized Hall effect. When the doping deviates from 1/4, the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$ Cooper pairing becomes the leading instability. Our results suggest that near 1/4 electron or hole doping (away from the neutral point) the graphene is either a Chern insulator or a topoligical superconductor.

Facing the challenging employment situation and the changing labor market, developing student entrepreneurial intention has attracted significant policy consideration in China. This study describes the background of entrepreneurship education in China’s higher education institutes and explores the influences of entrepreneurship education on student entrepreneurial intention. Using data from a survey on students in China, this study finds that students in different types of institutions and different major fields had a different level of engagement in entrepreneurship education. Further, the higher the level of entrepreneurship education the students received, the stronger their self-efficacy of entrepreneurial decision-making was, and the stronger their entrepreneurial intention was. Student entrepreneurial decision-making self-efficacy played a mediating role between entrepreneurship education and student entrepreneurial intention. We found that entrepreneurship education has a positive effect on entrepreneurial intention. Entrepreneurship education course-taking has a positive effect on entrepreneurial decision-making. Furthermore, the positive effect of self-efficacy of entrepreneurial decision-making on entrepreneurial intention was also confirmed. We also found that self-efficacy of entrepreneurial decision-making played the significant role of mediator between entrepreneurship education and entrepreneurial intention. The findings also showed a difference between the current China context and the western context that taking entrepreneurship-related classes had more considerable influences on student entrepreneurial intention than entrepreneurship-related practicum. We discuss the implications of the improvement of higher education in China and relevance to other contexts.

In several recent experiments the superconducting gap of a single-unit-cell-thick FeSe film on SrTiO${}_{3}$ substrate has been observed by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. The value of the superconducting gap is about nine times larger than that of the bulk FeSe under ambient pressure, suggesting a much higher pairing energy scale and ${T}_{c}$ than all other iron-based superconductors and thus calling for a better understanding of its superconducting mechanism. In this paper we study the effects of screening due to the SrTiO${}_{3}$ ferroelectric phonons on Cooper pairing in FeSe. We conclude that it can significantly enhance the energy scale of Cooper pairing and even change the pairing symmetry. Our results also raise some concerns on whether phonons can be completely ignored for bulk iron-based superconductors.