Speaker: Dr. Zi-Yang Meng, Physics & Astronomy, Center for Computation & Technology, Louisiana State University 

Abstract:  Using large-scale dynamical cluster quantum Monte Carlo simulations, we study novel properties such as the metal-insulator transition, quantum critical point, non-Fermi liquid, unconventional superconductivity, etc, of Hubbard model on the square [1] and triangular [2] lattices.
In the square lattice case, the control parameters are hole doping, temperature T and next-nearneighbor hopping t'/t. We find that at t'/t>0, a line of finite temperature classical critical points converges to the quantum critical point at t'=0 and zero temperature; at t'/t<=0, we identify a line of Lifshitz transition points associated with a change of the Fermi surface topology. At the Lifshitz transition points, a van Hove singularity in the density of states crosses the Fermi level. Above the Lifshitz points at finite temperature, marginal Fermi liquid separates the pseudogap at underdope and Fermi liquid at overdope regions. We furthermore explore the evolution of the d-wave superconducting dome in t’/t<=0 region of the phase diagram [3].
In the triangular lattice case, we explore the Mott transition at half-filling as well as the superconducting instability in the hole-hoped side of the phase diagram [2]. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, in the hole-doped side, we find a doubly degenerate singlet pairing state at interaction strength close to the bare bandwidth. This superconducting state is mediated by antiferromagnetic spin fluctuations and has a chiral, d+id pairing symmetry. Our findings are relevant with the superconductivity in water-intercalated sodium cobaltates Na$_{x}$CoO$_{2} \cdoty$H$_{2}$O, as well as the superconducting phases of the organic compounds $\kappa$-(ET) $_{2}$X and Pd(dmit)$_{2}$.

References:
1. “Lifshitz Transition in the Two Dimensional Hubbard Model”, K. S. Chen, Z. Y. Meng, T. Pruschke, J. Moreno and M. Jarell, Phys. Rev. B 86, 165136 (2012).
2. “Unconventional superconductivity on the triangular lattice Hubbard model”, K. S. Chen, Z. Y. Meng, U. Yu, S. Yang, M. Jarrell and J. Moreno, Phys. Rev. B 88, 041103(R) (2013).
3. “Evolution of the superconductivity dome in the hole-doped square lattice Hubbard model”, Z. Y. Meng, K. S. Chen, S. Yang, T. Pruschke, M. Jarrell and J. Moreno, in preparation

About the Speaker: Dr. Zi-Yang Meng is a postdoctoral fellow in the Department of Physics and Astronomy, and the Center for Computation and Technology of Louisiana State University. He graduated from the University of Stuttgart, Germany in 2011, with a Ph.D. in theoretical condensed matter physics. His research interests are mainly in the numerical investigations on strongly correlated electronic systems, particularly using quantum Monte Carlo, cluster dynamical mean field theory and diagramatic techniques to study systems in the fields of quantum magnetism, graphene-related material, metal-insulator transition, unconventional superconductivity, disorder and localization, interacting topological state of matter, etc.

Date&Time: July 26, 2013 (Friday), 14:30 – 15:30
Location: 606 Conference Room