Elementary excitations in S = 1/2 Heisenberg spin chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction

The magnetic-excitation spectrum of copper pyrimidine dinitrate, a material containing S = 1 2 antiferromagnetic chains with alternating g-tensor and the DzyaloshinskiiMoriya interaction, and exhibiting a field-induced spin gap, is probed using tunable-frequency electron spin resonance spectroscopy in magnetic fields up to 25 T. The data are interpreted in frame of the sine-Gordon quantum-field theoretical concept proposed recently by Oshikawa and Affleck. The field-induced gap is measured directly; signatures of soliton and three breather branches are identified. An isotropic S = 12 Heisenberg antiferromagnetic (AF) chain with uniform nearest-neighbor exchange coupling represents one of the most remarkable models of quantum magnetism. Its ground state is a spin singlet, and the dynamics are determined by a gapless two-particle continuum of spin-12 excitations, commonly referred to as spinons. A uniform external magnetic field causes a substantial rearrangement of the excitation spectrum, making the soft modes incommensurate [1], although the spinon continuum remains gapless. Since the S = 12 AF chain is critical, even small perturbations can considerably change fundamental properties of the system. One of the most prominent examples is the S = 12 AF chain perturbed by an alternating g-tensor and/or the Dzyaloshinskii-Moriya (DM) interaction. In the presence of such structural peculiarities, the application of a uniform external field H induces an effective transverse staggered field h ∝ H, which leads to the opening of an energy gap Δ ∝ H2/3. It has been shown [2,3] that the gapped phase can be effectively described by the quantum-field sine-Gordon theory. The excitation spectrum consists of solitons, antisolitons, and multiple soliton-antisoliton bound states called breathers. The availability of exact solutions for the sine-Gordon model, allows very precise theoretical descriptions of many observable properties and physical parameters of sine-Gordon magnets (including the field dependence of excitation energies [2-4] and response functions [5,6]), which makes such systems a particularly interesting target for experimentally probing elementary excitations. Here, we report on a detailed study of the low-temperature elementary excitation spectrum in Institute of Physics Publishing Journal of Physics: Conference Series 51 (2006) 39–42 doi:10.1088/1742-6596/51/1/006 Yamada Conference LX on Research in High Magnetic Fields 39 © 2006 Yamada Science Foundation and IOP Publishing Ltd copper pyrimidine dinitrate (hereafter Cu-PM), which has been recently identified as a S = 12 AF chain with a field-induced spin gap [7], and is probably the best realization of the quantum sineGordon spin-chain model known to date. Cu-PM, [PM-Cu(NO3)2(H2O)2]n (PM = pyrimidine) crystallizes in a monoclinic structure belonging to the space group C2/c with four formula units per unit cell. The lattice constants obtained from the single-crystal X-ray diffraction are a = 12.404 Å, b = 11.511 Å, c = 7.518 Å, and β = 115.0◦. The Cu coordination sphere is a distorted octahedron, built from an almost square N-O-N-O equatorial plane and two oxygens in the axial positions. The Cu ions form chains running parallel to the short ac diagonal. The local principal axis of each octahedron is tilted from the ac plane by ±29.4◦. Since this axis almost coincides with the principal axis of the g-tensor, the g-tensors for neighboring Cu ions are staggered. The exchange constant determined from susceptibility measurements [7] is J = 36± 0.5 K. The excitation spectrum was studied using a high-field submillimeter wave ESR spectrometer [8]. Backward Wave Oscillators were employed as tunable sources of radiation, quasicontinuously covering the frequency range of 150 to 700 GHz. The magnetic field was applied along the c′′ direction, which is characterized by the maximal value of the staggered magnetization for Cu-PM [7]. The experiments were performed both in the Faraday and the Voigt configurations. 0 2 4 6 8 10 12 14 16 18 20 22 24 0.2 0.4 0.6 0.8 1.0 1.2 C1