[Paper Review] Probing spin correlations using angle-resolved photoemission in a coupled metallic/Mott insulator system

Veronika Sunko, Federico Mazzola+17 more|arXiv (Cornell University)|Jan 1, 2020

Electronic and Structural Properties of Oxides41 references39 citations

TL;DR

This study demonstrates that angle-resolved photoemission spectroscopy (ARPES), typically non-magnetic, can detect spin correlations in the Kondo-coupled system PdCrO2 by probing an intertwined spin-charge excitation. The coupling between metallic Pd layers and Mott-insulating CrO2 layers with S=3/2 spins produces a spectral feature with Cr character and energy-independent intensity across ~1 eV, identified as a lower Hubbard band, revealing strong magnetic sensitivity in a non-magnetic probe.

ABSTRACT

A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of the spectrum of possibilities for the motion of electrons in a solid. Understanding their interaction lies at the heart of the correlated electron problem. In the magnetic oxide metal PdCrO2, nearly free and Mott-localized electrons exist in alternating layers, forming natural heterostructures. Using angle-resolved photoemission spectroscopy, quantitatively supported by a strong coupling analysis, we show that the coupling between these layers leads to an “intertwined” excitation that is a convolution of the charge spectrum of the metallic layer and the spin susceptibility of the Mott layer. Our findings establish PdCrO2 as a model system in which to probe Kondo lattice physics and also open new routes to use the a priori nonmagnetic probe of photoemission to gain insights into the spin susceptibility of correlated electron materials.

Motivation & Objective

• To investigate spin-charge entanglement in a natural heterostructure of metallic and Mott-insulating layers.
• To determine whether angle-resolved photoemission can detect spin correlations in correlated electron systems.
• To establish PdCrO2 as a model system for Kondo lattice physics and Mott-metal coupling.
• To explore the potential of ARPES as a probe of dynamical spin susceptibilities beyond static magnetic order.

Proposed method

• Performed soft X-ray ARPES at the Cr L2,3 edge resonance to enhance Cr-derived spectral weight.
• Compared on-resonance (581.7 eV) and off-resonance (578 eV) spectra to isolate Cr character.
• Used momentum distribution curves and energy distribution curves to extract spectral weight of metallic bands (IMB) and lower Hubbard band (ILHB).
• Correlated ILHB intensity with Cr L2,3 X-ray absorption spectrum to confirm Cr origin.
• Applied strong coupling analysis and DFT+DMFT calculations to interpret ARPES data.
• Analyzed spectral replicas in momentum space to identify Kondo coupling origin.

Experimental results

Research questions

• RQ1 Can angle-resolved photoemission detect spin correlations in a system with no intrinsic magnetic response?
• RQ2 What is the origin of the energy-independent, Cr-characterized spectral weight observed in ARPES near the Fermi level in PdCrO2?
• RQ3 How does the coupling between metallic and Mott-insulating layers in PdCrO2 lead to an intertwined spin-charge excitation?
• RQ4 To what extent can ARPES probe spin susceptibility in Mott insulators without relying on magnetic probes?
• RQ5 Can the observed spectral features be explained by Kondo coupling rather than conventional band folding?

Key findings

• The diffuse spectral weight at ~2 eV below EF, with Cr character and energy-independent intensity, is identified as the lower Hubbard band of a Mott insulator.
• The intensity of this feature (ILHB) matches the Cr L2,3 X-ray absorption spectrum, confirming its Cr origin.
• The metallic Pd-derived band shows no resonance enhancement, confirming its Pd character and negligible spectral weight change across the Cr edge.
• Spectral replicas of the metallic band, observed at the Fermi level and extending over 1 eV, are attributed to Kondo coupling rather than magnetic band folding.
• The observed spectral weight is not suppressed away from the magnetic zone boundary, contradicting standard band folding expectations.
• The results establish that ARPES can detect spin correlations in Mott insulators via Kondo coupling, enabling non-magnetic probing of spin susceptibility.