[Paper Review] Metallic quantum criticality enabled by flat bands in a kagome lattice

Lei Chen, Fang Xie+5 more|arXiv (Cornell University)|Jul 18, 2023

Topological Materials and Phenomena8 citations

TL;DR

The paper demonstrates an orbital-selective Mott transition of molecular orbitals in a kagome lattice, yielding a metallic quantum critical point with strange-metal behavior via an extended dynamical mean-field theory of a two-orbital Hubbard/Anderson framework.

ABSTRACT

Strange metals arise in a variety of platforms for strongly correlated electrons, ranging from the cuprates, heavy fermions to flat band systems. Motivated by recent experiments in kagome metals, we study a Hubbard model on a kagome lattice whose noninteracting limit contains flat bands. A Kondo lattice description is constructed, in which the correlation effects are captured by symmetry preserving and exponentially localized molecular orbitals. These compact molecular orbitals represent the local degrees of freedom that emerge from topological flat bands. We identify a quantum critical point at which quasiparticles are lost and strange metallicity emerges. Our theoretical work opens up a new route for realizing beyond-Landau quantum criticality, as well as the associated strange metallicity and emergent quantum phases.

Motivation & Objective

• Motivate the study of strong correlations in flat-band kagome systems and their connection to strange metallicity.
• Develop an orbital-based description of flat-band kagome electrons through emergent molecular orbitals.
• Construct an Anderson/Kondo-lattice framework for the molecular orbitals to access nonperturbative quantum criticality.
• Identify and characterize a continuous selective Mott transition of molecular orbitals and its quantum critical point.

Proposed method

• Construct a two-orbital Hubbard model on the kagome lattice with SOC along z and spin- z rotational symmetry.
• Identify emergent molecular orbitals via a Wannierization procedure, yielding tight f and c orbital descriptions on a triangular lattice.
• Project to an effective Anderson lattice model with dominant on-site f-electron interactions and weak c-electron interactions.
• Apply extended dynamical mean-field theory (EDMFT) to solve the Bose-Fermi Anderson model and capture Kondo destruction and RKKY competition.
• Compute local and lattice spin susceptibilities to extract scaling forms and dynamical exponents at the quantum critical point.

Figure 1: Illustration of the lattice geometry, compact localized state, Wannier centers, and the qualitative phase diagram. a , Geometry of the kagome lattice with three sites per unit cell. The Wannier centers of the emergent Wannier orbitals are marked by the light-blue circles (centered at Wycko

Experimental results

Research questions

• RQ1 Can flat-band physics in a kagome lattice be encoded into an Anderson/Kondo lattice description via emergent molecular orbitals?
• RQ2 Does the orbital-selective Mott transition of these molecular orbitals yield a quantum critical point with strange-metal signatures?
• RQ3 What are the dynamical signatures (χ_loc, χ(q,ω)) and scaling properties at the orbital-selective Mott QCP?
• RQ4 How does the competition between Kondo screening and RKKY interactions drive dehybridization and a Fermi-surface jump across the QCP?

Key findings

• An orbital-selective Mott transition of the molecular orbitals is identified within an EDMFT treatment of the effective Anderson lattice model.
• The quantum critical point exhibits χ_loc(ω) with a logarithmic divergence and χ(q,ω) with ω/(kB T) scaling, indicating an interacting, beyond-Gaussian criticality.
• A continuous selective transition leads to a Kondo-destruction-type quantum critical point and a jump in Fermi surface across the QCP.
• The flat-band kagome structure provides a route to strange-metal behavior analogous to heavy-fermion quantum criticality, with potential implications for unconventional superconductivity.

Figure 2: Noninteracting bandstructure. a , The band structure of the two-orbital model in the original kagome lattice, with the parameter setting described in the Methods. b , The band structure for the middle two bands in the original Hamiltonian. The little group representations at high symmetry