[Paper Review] Non-magnetic impurities as probes of insulating and doped Mott insulators in two dimensions

Subir Sachdev, Matthias Vojta|arXiv (Cornell University)|Sep 14, 2000

Physics of Superconductivity and Magnetism1 references3 citations

TL;DR

This paper proposes that non-magnetic impurities serve as sensitive probes of spinon confinement in two-dimensional Mott insulators and doped high-temperature superconductors. Using spin-charge separation and impurity susceptibility analysis, it demonstrates that a Curie-like magnetic response at low temperatures—evidenced by NMR and STM experiments—supports spinon confinement in the parent Mott insulator, distinguishing it from deconfined spin liquid states.

ABSTRACT

We characterize paramagnetic Mott insulators by their response to static, non-magnetic impurities. States with spinon deconfinement (or spin-charge separation) are distinguished from those with spinon confinement by distinct impurity susceptibilities and finite-size spectra. We discuss the evolution of physical properties upon doping to a d-wave superconductor, and argue that a number of recent experiments favor spinon confinement in the reference Mott insulating state.

Motivation & Objective

• To distinguish between paramagnetic Mott insulators with confined versus deconfined spinons using non-magnetic impurities as probes.
• To explain experimental observations in underdoped cuprates, particularly the persistence of local moments at non-magnetic impurities.
• To argue that the observed Curie-like susceptibility and spin resonance broadening point to spinon confinement in the parent insulating state.
• To connect the formation of local moments near impurities to quantum criticality and translational symmetry breaking in doped Mott systems.

Proposed method

• Analyzing the response of paramagnetic Mott insulators to static, non-magnetic impurities via impurity susceptibility and finite-size spectra.
• Using a spin-ladder model with anisotropic exchange couplings to realize a confined spinon phase with a gapped S=1 collective mode.
• Deriving the impurity susceptibility in the confined phase as χᵤ = (𝒜Δ)/(πcₓcᵧ) e^(−Δ/T), showing exponential suppression at low T.
• Comparing the evolution of physical properties upon doping: confined states exhibit a quantum critical point where local moments are Kondo-screened.
• Applying a Kondo-like Hamiltonian to describe the quantum phase transition between weak and strong doping regimes in the superconductor.
• Using theoretical models to reproduce STM tunneling current anomalies near Zn impurities in BSCCO, consistent with local moment formation.

Experimental results

Research questions

• RQ1 How does the response of a Mott insulator to non-magnetic impurities distinguish between confined and deconfined spinon states?
• RQ2 What experimental signatures in doped cuprates indicate that the parent Mott insulator has confined spinons rather than a deconfined spin liquid?
• RQ3 Why do NMR experiments show a Curie-like susceptibility for Li impurities in underdoped YBCO, even in the superconducting state?
• RQ4 How does the spin resonance mode in the superconductor relate to the S=1 excitation in the insulating state?
• RQ5 What is the role of translational symmetry breaking and bond-centered stripes in the insulating and lightly doped phases?

Key findings

• The impurity susceptibility in the confined spinon phase follows χᵤ = (𝒜Δ)/(πcₓcᵧ) e^(−Δ/T), showing exponential suppression at low temperature.
• A Curie-like susceptibility for Li impurities in underdoped YBCO, observed via NMR, provides direct evidence for local S=1/2 moments at T=0.
• The broadening of the spin resonance mode upon Zn doping is explained by unpaired moments near impurities, supporting spinon confinement.
• STM data on quasiparticle tunneling near Zn impurities in BSCCO are quantitatively reproduced by Kondo-like models with local moments.
• The quantum critical point separating weak and strong doping regimes is described by a Kondo Hamiltonian coupling a local S=1/2 moment to gapless Bogoliubov quasiparticles.
• Translational symmetry breaking via bond-centered stripes is expected in the confined insulator and may persist in lightly doped superconductors, with experimental support from photoemission data.