Electrical currents in a quantum spin Hall insulator are confined to the boundary of the system. The charge carriers behave as massless relativistic particles whose spin and momentum are coupled to each other. Although the helical character of those states is already established by experiments, there is an open question regarding how those edge states interact with each other when they are brought into close spatial proximity. We employ an inverted HgTe quantum well to guide edge channels from opposite sides of a device into a quasi-one-dimensional constriction. Our transport measurements show that, apart from the expected quantization in integer steps of 2e²/h, we find an additional plateau at e²/h. We combine band structure calculations and repulsive electron–electron interaction effects captured within the Tomonaga–Luttinger liquid model and Rashba spin–orbit coupling to explain our observation in terms of the opening of a spin gap. These results may have direct implications for the study of one-dimensional helical electron quantum optics, and for understanding Majorana and para fermions.

量子自旋霍尔绝缘体中的电流被限制在系统的边界。电荷载流子表现为无质量的相对论粒子，其自旋和动量相互耦合。尽管这些状态的螺旋特性已经通过实验确定，但是对于那些边缘状态在空间上紧密接近时如何相互影响仍存在一个悬而未决的问题。我们采用了倒置的HgTe量子阱来将边缘通道从设备的相对侧引导到准一维压缩区。我们的传输测量结果表明，除了预期的以2 e ² / h的整数步进行量化外，我们还在e ² / h处发现了一个额外的平稳期。。我们将能带结构计算与在Tomonaga-Luttinger液体模型和Rashba自旋-轨道耦合中捕获的排斥电子-电子相互作用效应结合起来，以自旋间隙的开口来解释我们的观察。这些结果可能对一维螺旋电子量子光学的研究，以及对马约拉那和对子费米子的理解有直接的意义。