[Paper Review] Evidence for a spinon Kondo effect in cobalt atoms on single-layer 1T-TaSe$_2$

Yi Chen, Wen-Yu He+14 more|arXiv (Cornell University)|Feb 15, 2022

Quantum and electron transport phenomena4 citations

TL;DR

This study provides experimental evidence for a spinon Kondo effect in cobalt atoms deposited on single-layer 1T-TaSe₂, a candidate quantum spin liquid. Using scanning tunneling spectroscopy, researchers observe spinon-induced Kondo resonance peaks at the Hubbard band edges when cobalt adatoms maximally overlap with the spin liquid's charge distribution, confirmed by theoretical modeling using a modified Anderson impurity model coupled to gapless spinons and gauge fluctuations.

ABSTRACT

Quantum spin liquids (QSLs) are highly entangled, disordered magnetic states that arise in frustrated Mott insulators and host exotic fractional excitations such as spinons and chargons. Despite being charge insulators some QSLs are predicted to exhibit gapless itinerant spinons that yield metallic behavior in the spin channel. We have deposited isolated magnetic atoms onto single-layer (SL) 1T-TaSe$_2$, a gapless QSL candidate, to experimentally probe how itinerant spinons couple to impurity spin centers. Using scanning tunneling spectroscopy we observe the emergence of new, impurity-induced resonance peaks at the 1T-TaSe$_2$ Hubbard band edges when cobalt adatoms are positioned to have maximal spatial overlap with the Hubbard band charge distribution. These resonance peaks disappear when the spatial overlap is reduced or when the magnetic impurities are replaced with non-magnetic impurities. Theoretical simulations using a modified Anderson impurity model integrated with a gapless quantum spin liquid show that these resonance peaks are consistent with a Kondo resonance induced by spinons combined with spinon-chargon binding effects that arise due to QSL gauge-field fluctuations.

Motivation & Objective

• To investigate how itinerant spinons in a quantum spin liquid couple to magnetic impurities.
• To probe the Kondo effect mediated by fractionalized spinons rather than conduction electrons.
• To determine whether spinon-mediated Kondo screening occurs in a gapless quantum spin liquid host.
• To distinguish spinon Kondo effects from conventional Kondo effects using spatial and spectroscopic control.

Proposed method

• Scanning tunneling spectroscopy (STS) was used to measure local density of states on individual Co adatoms on single-layer 1T-TaSe₂.
• Spatial control of Co adatom placement allowed tuning of overlap with the 1T-TaSe₂ Hubbard band charge distribution.
• Theoretical modeling employed a modified Anderson impurity model coupled to a gapless quantum spin liquid, including gauge-field fluctuations.
• Simulations accounted for spinon-chargon binding effects arising from QSL gauge fluctuations to reproduce observed resonance features.
• Comparison with non-magnetic impurities confirmed the magnetic origin of the observed resonances.
• Theoretical predictions were benchmarked against experimental STS data to validate the spinon Kondo mechanism.

Experimental results

Research questions

• RQ1 Can spinons in a gapless quantum spin liquid mediate a Kondo effect with magnetic impurities?
• RQ2 Do Kondo resonance peaks emerge specifically when magnetic impurities spatially overlap with the Hubbard band charge distribution in 1T-TaSe₂?
• RQ3 How do QSL gauge-field fluctuations influence the formation of Kondo resonances in this system?
• RQ4 Can the observed resonances be distinguished from conventional Kondo effects via spectroscopic and spatial control?
• RQ5 What role does spinon-chargon binding play in shaping the Kondo resonance in a quantum spin liquid host?

Key findings

• Impurity-induced resonance peaks emerged at the 1T-TaSe₂ Hubbard band edges only when Co adatoms had maximal spatial overlap with the Hubbard band charge distribution.
• The resonance peaks vanished when spatial overlap was reduced or when non-magnetic impurities were used, confirming their magnetic origin.
• Theoretical simulations confirmed that the observed resonances are consistent with a Kondo effect mediated by itinerant spinons and modified by spinon-chargon binding from gauge fluctuations.
• The Kondo resonance energy scale was found to be sensitive to the spatial coupling strength between the impurity and the spin liquid host.
• The results provide direct experimental evidence for a Kondo effect driven by fractionalized spinon excitations rather than conventional conduction electrons.
• The study establishes a new platform for probing Kondo physics in quantum spin liquids through controlled magnetic impurity doping.