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 a 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, YCu₃(OD)₆Br₂[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 spectral measurements are in agreement with the low-temperature specific heat of the sample. Our results, thus, provide spectral evidence for a 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-neighbour Heisenberg model, suggesting the Dirac spinons have an unexpected origin.

凝聚态系统中具有狄拉克色散的涌现准粒子可以通过相对论电子的狄拉克方程来描述，类似于高能物理学中的狄拉克粒子。例如，具有狄拉克色散的电子已在石墨烯和拓扑绝缘体等电子系统中得到深入研究。然而，电荷并不是狄拉克费米子的先决条件，无电荷自由度的狄拉克费米子的出现已被理论上预测在狄拉克量子自旋液体中得以实现。这些准粒子带有 1/2 的自旋，但呈电中性，因此称为自旋子。在这里，我们证明了 Kagome 反铁磁体 YCu ₃ (OD) ₆ Br ₂ [Br _0.33 (OD) _0.67 ]的自旋激发是圆锥形的，内部有自旋连续体，这与两个狄拉克自旋子的卷积一致。通过光谱测量获得的具有自旋速度的狄拉克自旋液体模型的预测与样品的低温比热一致。因此，我们的结果为这种 Kagome 晶格反铁磁体中出现的狄拉克量子自旋液态提供了光谱证据。然而，锥形自旋激发的位置与最近邻海森堡模型计算的位置不同，这表明狄拉克自旋子有一个意想不到的起源。