Quantum spin liquid in proximity to the Mott insulator.*

There are two routes to realizing the quantum spin liquid. The first is geometric frustration in frustrated lattices such as triangle, kagome and more recently Kitaev lattices, as first envisioned by Anderson. The second is to look in systems which are close to the insulating side of a Mott insulator. The organic system is a long standing example. Recently TaS₂ and TaSe2 have been added to the list. This talk discusses recent experiments and their theoretical interpretations. In monolayer TaSe2, the incommensurate modulation period of the star of David clusters have been interpreted as arising form nesting vector of an underlying spinon Fermi surface. Furthermore, resonance features have been seen by STS near surface magnetic ions  and have been interpreted as originating from Kondo screening by the spinon Fermi surface, which produces Kondo resonance peaks near the edge of the Hubbard bands.[1] In bulk TaS2, sharp resonance peaks have been seen by high resolution STS which shifts with applied magnetic field. [2] These resonances have been interpreted as spinon-chargon bound state whose binding energy is enhanced by the magnetic field. [3]  For systems near the Mott transition, an external magnetic field will be linearly coupled to the gauge magnetic field in a U(1) spin liquid. I will discuss proposed experiments to measure the magnetic fluctuations spectra produced near the surface by the fluctuating gauge magnetic fields using NV centers. [4] This method may be sensitive to the gauge field Meissner effect associated with spinon pairing. Finally I will mention some recent advances towards observing a Dirac spin liquid in kagome systems.

[1] Yi Chen et al, Nature Phys 18, 1335 (2022).

[2] C.J. Butler, Phys. Rev. B 107, L161107 (2023).

[3] Wenyu He and Patrick Lee, Phys. Rev. B 107, 195155(23).

[4] P. A. Lee and S. Morampudi, Phys. Rev. B 107, 195102, (2023).