High-Field ESR in Spin Systems with Reduced Dimensionality

Quantum fluctuations in low-dimensional magnets give rise to a variety of exotic strongly correlated states, making those systems an extremely attractive ground for testing various theoretical concepts. In this presentation I will focus on high-frequency and high-field Electron Spin Resonance (ESR) studies of two low-dimensional quantum magnets: copper pyrimidine dinitrate (Cu-PM), a spin-1/2 antiferromagnetic chain system with alternating g-tensor and Dzyaloshinskii-Moriya interactions, and the spin-ladder system (C₅H₁₂N)₂CuBr₄ (known as BPCB). In Cu-PM, the observation of a minimum of the spin gap in the vicinity of the saturation field, H_sat = 48.5 T, is associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized magnon state. This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG calculations for spin-1/2 Heisenberg chain with a staggered transverse field. In BPCB, the zero-field gap in the excitation spectrum, Δ = 16.5 K, is observed directly. In addition, our studies provide straightforward evidence of a pronounced anisotropy in this compound (~ 5% of the rung interaction), which is in contrast to an isotropic spin-ladder model, employed for this system previously. It is argued that such an anisotropy in BPCB is determined by the substantial spin-orbit coupling, which appears to be very important for describing magnetic properties of BPCB. The talk will give also a brief introduction into the recent development of the high-field ESR program at the Dresden High Magnetic Field Laboratory.