High-field ESR in low-dimensional spin systems

Here, I present results of our recent tunable-frequency high-field Electron Spin Resonance (ESR) studies of two low-dimensional quantum spin systems. The first one, copper pyrimidine dinitrate, is an S=1/2 antiferromagnetic chain material with alternating g-tensor and the Dzyaloshinskii-Moriya interaction, which exhibits a field-induced gap Δ. Employing ESR technique, the gap was observed directly.1 Experimental data are sufficiently detailed to make an accurate comparison with predictions based on the sine-Gordon quantum-field theory. Signatures of three breather branches and a soliton are identified. In addition, the temperature and field dependences of ESR parameters in the perturbative spinon regime (T > Δ/k_B) are studied.2 Excellent agreement with theory is found. The second material, NiCl₂-4SC(NH₂)₂ (known as DTN) is a quantum S = 1 chain system with strong easy-plane anisotropy that is regarded as a new candidate for the field-induced Bose-Einstein condensation (BEC) of spin degrees of freedom. Employing ESR, we were able to accurately estimate parameters of the spin-Hamiltonian, to study the frequency-field dependence of two-magnon bound-state excitations 3 (predicted by theory and observed in DTN for the first time), and to investigate the magnetic excitation spectrum in DTN in the field-induced ordered phase.4 Supported in part by NHFMF (through NSF and DOE) and DFG.