We propose an extended BCS–Hubbard model and investigate its ground state phase diagram in an external magnetic field.By mapping the model onto a model of spinless fermions coupled with conserving Z_(2) variables whi...We propose an extended BCS–Hubbard model and investigate its ground state phase diagram in an external magnetic field.By mapping the model onto a model of spinless fermions coupled with conserving Z_(2) variables which are mimicked by pseudospins,the model is shown to be exactly solvable along the symmetric lines for an arbitrary on-site Hubbard interaction on the bipartite lattice.In the zero field limit,the ground states exhibit an antiferromagnetic order of pseudospins.In the large field limit,on the other hand,the pseudospins are fully polarized ordered.With the increase of the applied field,a first-order phase transition occurs between these kinds of phases when the on-site Coulomb interaction is less than a critical value U(c).Above this critical U_(c),a novel intermediate phase emerges between the fully polarized and antiferromagnetic phases.The ground states in this phase are macroscopically degenerate,like in a spin ice,and the corresponding entropy scales linearly with the lattice size at zero temperature.展开更多
基金supported by the National Key Research and Development Program of China (Grant No. 2017YFA0302901)the National Natural Science Foundation of China (Grants Nos. 11888101 and 11874095)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000)the Youth Innovation Promotion Association CAS (Grants No. 2021004)
文摘We propose an extended BCS–Hubbard model and investigate its ground state phase diagram in an external magnetic field.By mapping the model onto a model of spinless fermions coupled with conserving Z_(2) variables which are mimicked by pseudospins,the model is shown to be exactly solvable along the symmetric lines for an arbitrary on-site Hubbard interaction on the bipartite lattice.In the zero field limit,the ground states exhibit an antiferromagnetic order of pseudospins.In the large field limit,on the other hand,the pseudospins are fully polarized ordered.With the increase of the applied field,a first-order phase transition occurs between these kinds of phases when the on-site Coulomb interaction is less than a critical value U(c).Above this critical U_(c),a novel intermediate phase emerges between the fully polarized and antiferromagnetic phases.The ground states in this phase are macroscopically degenerate,like in a spin ice,and the corresponding entropy scales linearly with the lattice size at zero temperature.
