Spectral evidence for Dirac spinons in a kagome lattice antiferromagnet

Abstract

Emergent quasiparticles with a Dirac dispersion in condensed matter systems can be described by the Dirac equation for relativistic electrons, in analogy with Dirac particles in high-energy physics. For example, electrons with a Dirac dispersion have been intensively studied in electronic systems such as graphene and topological insulators. However, charge is not a prerequisite for Dirac fermions, and the emergence of Dirac fermions without charge degree of freedom has been theoretically predicted to be realized in Dirac quantum spin liquids. These quasiparticles carry a spin of 1/2 but are charge-neutral, and so are called spinons. Here we show that the spin excitations of a kagome antiferromagnet, YCu${}_3$(OD)${}_6$Br${}_2$[Br${}_{0.33}$(OD)${}_{0.67}$], are conical with a spin continuum inside, which is consistent with the convolution of two Dirac spinons. The predictions of a Dirac spin liquid model with a spinon velocity obtained from the spectral measurements are in agreement with the low-temperature specific heat of the sample. Our results thus provide spectral evidence for the Dirac quantum spin liquid state emerging in this kagome lattice antiferromagnet. However, the locations of the conical spin excitations differ from those calculated by the nearest neighbor Heisenberg model, suggesting the Dirac spinons have an unexpected origin.

arXiv comment6 pages, 4 figures

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• Kitaev Quantum Spin Liquids
• Unconventional Magnetic Oscillations in Kagome Mott Insulators
• Magnetic ground states in kagome YCu${}_3$(OH)${}_6$[(Cl${}_x$Br${}_{1-x}$)${}_{3-y}$(OH)${}_y$]
• Thermodynamic evidence of fermionic behavior in the vicinity of one-ninth plateau in a kagome antiferromagnet
• Chemo-Structural Disorder in the kagomé spin $S$ = 1/2 systems ZnCu${}_3$(OH)${}_6$Cl${}_2$ and YCu${}_3$(OH)${}_6$Br${}_2$[Br${}_x$(OH)${}_{1-x}$]
• Cubic* criticality emerging from a quantum loop model on triangular lattice
• Nature of the 1/3 Magnetization Plateau in Spin-1/2 Kagome Antiferromagnets
• Magnetic excitations in the 1/3 plateau state in InCu${}_3$(OH)${}_6$Cl${}_3$
• Spin excitations arising from anisotropic Dirac spinons in YCu${}_3$(OD)${}_6$Br${}_2$[Br${}_{0.33}$(OD)${}_{0.67}$]
• Recent progress in quantum spin liquids, fractional magnetization plateaus, and unconventional superconductivity in kagome lattices
• Exerting chemical pressure on the kagome lattice as frustration control in the kapellasite family $A$Cu${}_3$(OH)${}_{6+x}$(Cl,Br)${}_{3-x}$
• Field-induced magnetic order in DyTa${}_7$O${}_{19}$ with two-dimensional pseudospin-$\frac{1}{2}$ triangular lattice
• Unnecessary quantum criticality in $SU(3)$ kagome magnets
• Scalable hybrid quantum Monte Carlo simulation of U(1) gauge field coupled to fermions on GPU
• Disorder-induced spin excitation continuum and spin-glass ground state in the inverse spinel CuGa${}_2$O${}_4$
• Disorder-induced spin-cluster magnetism in a doped kagome spin liquid candidate
• Emergence of a Fluctuating Ground State in Y-kapellasite under Pressure
• Universal collective Larmor-Silin mode emerging in magnetized correlated Dirac fermions
• Dynamical Spectral Function of the Kagome Quantum Spin Liquid
• Optimizing the dynamical preparation of quantum spin lakes on the ruby lattice
• An integrated theoretical and numerical approach to understand modern experiments on quantum magnetism
• Spin order and excitations in the collinear kagome antiferromagnet Y${}_{0.5}$Fe${}_3$Sn${}_3$