S.A. Zvyagin, Spin dynamics of S = 1/2 Heisenberg chains with a staggered transverse field: electron spin resonance studies (Review Article), Low Temp. Phys. 38, (2012) [Fiz. Nizk. Temp. 38, 1032-1040, (2012)] DOI: https://doi.org/10.1063/1.4752094.