Spin dynamics of S=1/2 Heisenberg chains with a staggered transverse field: electron spin resonance studies

This article is a review of the recent achievements in experimental studies of magnetic excitations in the copper pyrimidine dinitrate [PM-Cu(NO3)(2)(H2O)(2)](n) (PM = pyrimidine), an S = 1/2 antiferromagnetic chain material with an alternating g-tensor and the Dzyaloshinskii-Moriya interaction, by means of high-field electron spin resonance (ESR). Due to this alternation, in the 1D critical regime this material exhibits a field-induced gap. The excitation spectrum is formed by solitons and their bound states, breathers, and can be effectively described using the sine-Gordon model. With increasing temperature, the soliton-breather regime can be suppressed, resulting in a substantial evolution of ESR parameters. These changes can be described using a new theoretical concept recently proposed for S = 1/2 AF chains with a staggered transverse field. High magnetic field induces a transition into the spin-polarized state with the excitation spectrum formed by magnons. Nonmonotonous behavior of the field-induced gap is observed in vicinity of the saturation field. Experimental data are compared with results of existing theoretical approaches, revealing excellent agreement with predictions. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752094]