We propose a lattice scale two-band generalized Hubbard model as a caricature of the electronic structure of twisted bilayer graphene. Various possible broken symmetry phases can arise, including a nematic phase (which is a form of orbital ferromagnet) and an orbital-triplet spin-singlet superconducting phase. Concerning the mechanism of superconductivity -we propose an analogy with superconductivity in alkali-doped C60 in which a violation of Hund's first rule plays a central role.arXiv:1804.03162v3 [cond-mat.supr-con]
We report a density-matrix renormalization group study of the t-J model with nearest (t1 & J1) and next-nearest (t2 & J2) interactions on a 4-leg cylinder with concentration δ = 1/8 of doped holes. We observe an astonishingly complex interplay between uniform d-wave superconductivity (SC) and strong spin and charge density wave ordering tendencies (SDW and CDW). Depending on parameters, the CDWs can be commensurate with period 4 or 8. By comparing the charge ordering vectors with 2kF , we rule out Fermi surface nesting-induced density wave order in our model. Magnetic frustration (i.e. J2/J1 ∼ 1/2) significantly quenches SDW correlations with little effect on the CDW. Typically, the SC order is strongly modulated at the CDW ordering vector and exhibits d-wave symmetry around the cylinder. There is no evidence of a near-degenerate tendency to pair-density wave (PDW) ordering, charge 4e SC, or orbital current order. arXiv:1610.04300v1 [cond-mat.str-el]
We show that at the level of BCS mean-field theory, the superconducting Tc is always increased in the presence of disorder, regardless of order parameter symmetry, disorder strength, and spatial dimension. This result reflects the physics of rare events -formally analogous to the problem of Lifshitz tails in disordered semiconductors -and arises from considerations of spatially inhomogeneous solutions of the gap equation. So long as the clean-limit superconducting coherence length, ξ0, is large compared to disorder correlation length, a, when fluctuations about mean-field theory are considered, the effects of such rare events are small (typically exponentially in [ξ0/a] d ); however, when this ratio is ∼ 1, these considerations are important. The linearized gap equation is solved numerically for various disorder ensembles to illustrate this general principle.
Recent thermal conductivity measurements on Sr2RuO4 [E. Hassinger et al., Phys. Rev. X 7, 011032 (2017)] were interpreted as favoring a pairing gap function with vertical line nodes while conflicting with chiral p-wave pairing. Motivated by this work we study the effects of deep superconducting gap minima on impurity induced quasiparticle thermal transport in chiral p-wave models of Sr2RuO4. Combining a self-consistent T-matrix analysis and self-consistent Bogoliubovde-Gennes calculations, we show that the dependence of the residual thermal conductivity on the normal state impurity scattering rate can be quite similar to the d-wave pairing state that was shown to fit the thermal conductivity measurements, provided the normal state impurity scattering rate is large compared with the deep gap minima. Consequently, thermal conductivity measurements on Sr2RuO4 can be reconciled with a chiral p-wave pairing state with deep gap minima. However, the data impose serious constraints on such models and these constraints are examined in the context of several different chiral p-wave models.arXiv:1810.00932v2 [cond-mat.supr-con]
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.