Enhanced lifetimes of spin chains coupled to chiral edge states
We consider spin relaxation of finite-size spin chains exchanged coupled with a one-dimensional (1D) electron gas at the edge of a quantum spin Hall (QSH) insulator. Spin lifetimes can be enhanced due to two independent mechanisms. First, the suppression of spin-flip forward scattering inherent in the spin momentum locking of the QSH edges. Second, the reduction of spin-flip backward scattering due to destructive interference of the quasiparticle exchange, modulated by k F d, where d is the inter-spin distance and k F is the Fermi wavenumber of the electron gas. We show that the spin lifetime of the S = 1/2 ground state of odd-numbered chains of antiferromagnetically coupled S = 1/2 spins can be increased more than 4 orders of magnitude by properly tuning the product k F d and the spin size N, in strong contrast with the 1D case. Possible physical realizations together with some potential issues are also discussed. ; JFR acknowledges financial support by FCT- The Portuguese Foundation for Science and Technology, under the project 'PTDC/FIS-NAN/4662/2014' (016656), MINECO-Spain (Grant No. MAT2016-78625-C2) and Generalitat Valenciana Prometeo2017/139. FD acknowledges financial support from Spanish Government through grants MAT2015-66888-C3-2-R, Basque Government, grant IT986-16 and Canary Islands program Viera y Clavijo (Ref. 2017/0000231). FD acknowledges the hospitality of the Departamento de Física Aplicada of the Universidad de Alicante.