[Paper Review] Pseudogap metal and magnetization plateau from doping moiré Mott insulator

Yang Zhang, Liang Fu|arXiv (Cornell University)|Sep 12, 2022

Physics of Superconductivity and Magnetism4 citations

TL;DR

This paper proposes a novel pseudogap metal phase in doped moiré Mott insulators on the triangular lattice, driven by spin polarons—bound states of holes and spin flips—under intermediate magnetic fields. The phase exhibits a single-particle gap and a doping-dependent magnetization plateau at total spin $ S_p = N(1 - 3ar{ u})/2 $, distinguishing it from conventional Fermi liquids and fully polarized metals.

ABSTRACT

The problem of doping Mott insulators is of fundamental importance and long-standing interest in the study of strongly correlated electron systems. The advent of semiconductor based moiré materials opens a new ground for simulating the Hubbard model on the triangular lattice and exploring the rich phase diagram of doped Mott insulators as a function of doping and external magnetic field. Based on our recent identification of spin polaron quasiparticle in Mott insulator, in this work we predict a new metallic state emerges at small doping and intermediate field range, a pseudogap metal that exhibits a single-particle gap and a doping-dependent magnetization plateau.

Motivation & Objective

• To identify and characterize a novel metallic state in doped moiré Mott insulators that deviates from conventional Fermi liquid behavior.
• To understand the role of spin polaron quasiparticles—bound states of holes and spin flips—in stabilizing a metallic phase with a single-particle gap.
• To determine the conditions under which a magnetization plateau emerges as a function of doping and magnetic field.
• To establish a theoretical framework for distinguishing this pseudogap metal from fully polarized metals and antiferromagnetic states.

Proposed method

• Employing the infinite-U Hubbard model on the triangular lattice to describe strongly correlated electrons in moiré superlattices.
• Using density matrix renormalization group (DMRG) simulations to compute ground state energies and total spin $ S $ as a function of doping $ ar{ u} = 1 - n $ and Zeeman field $ h $.
• Defining the single-particle gap $ riangle E_{e,s} = E(N, S_N + s - s_0) - E_N $, where $ s_0 = 3/2 $ for spin polarons, to quantify the energy cost of adding an electron.
• Analyzing finite-size scaling of magnetization curves on two-leg ladders and three-leg cylinders to confirm the stability of the magnetization plateau.
• Comparing the Fermi surface structure and quantum oscillation periods between the spin polaron metal ($ 4a $) and fully polarized metal ($ 8a $) to distinguish the phases.
• Deriving the magnetization plateau condition $ S_p = N(1 - 3ar{ u})/2 $ from the spin polaron binding energy and doping-dependent spin content.

Experimental results

Research questions

• RQ1 Does a metallic state with a single-particle gap emerge in doped moiré Mott insulators at small hole doping and intermediate magnetic fields?
• RQ2 What is the microscopic origin of the magnetization plateau observed in the spin polaron metal phase?
• RQ3 How does the spin polaron quasiparticle structure differ from that of a conventional Fermi liquid or fully polarized metal?
• RQ4 Can the magnetization plateau be quantitatively linked to the doping level $ ar{ u} $, and what does this imply about the nature of the quasiparticles?
• RQ5 What experimental signatures—such as quantum oscillation frequency or Landau level degeneracy—can distinguish the pseudogap metal from other metallic phases?

Key findings

• A novel pseudogap metal phase emerges at small hole doping $ ar{ u} = 1 - n > 0 $ and intermediate magnetic fields, characterized by a single-particle gap and a doping-dependent magnetization plateau.
• The magnetization plateau is quantized as $ S_p = N(1 - 3ar{ u})/2 $, where $ N $ is the number of moiré unit cells, indicating a spin polaron origin with effective spin-3/2 quasiparticles.
• The single-particle gap arises from the spin mismatch between the added electron (spin-1/2) and the spin polaron (spin-3/2), with the gap energy $ riangle E_{e,s} $ equal to the spin gap.
• The Fermi surface in the spin polaron metal is a single pocket centered at $ m{k} = 0 $, leading to a quantum oscillation period of $ 4a $, in contrast to $ 8a $ in the fully polarized metal.
• The transition from the pseudogap metal to the fully polarized metal is signaled by a discontinuous change in Fermi surface volume and Landau level degeneracy.
• At zero magnetic field, small doping induces $ 120^ ext{circ} $ spin order consistent with antiferromagnetic correlations, confirming the absence of additional plateaus at low fields.