Field-induced gap in a quantum spin-12chain in a strong magnetic field

Abstract

Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field Hsat = 48.5 T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). 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.PACS numbers: 75.40. Gb, 75.10.Jm Introduction.-Due to recent progress in theory and the growing number of physical realizations, lowdimensional quantum magnets continue to receive a considerable amount of attention. They serve as model systems for investigating numerous fascinating phenomena in materials with cooperative ground states, in particular, induced by high magnetic fields. The way a magnetic field changes the ground-state properties and, correspondingly, the low-energy excitation spectrum of lowdimensional magnets is one of the fundamental aspects in quantum magnetism. For example, the zero-field ground state of an isotropic S = 1/2 Heisenberg antiferromagnetic (AF) chain with uniform nearest-neighbor exchange coupling is a spin singlet, and its spin dynamics is determined by a gapless two-particle continuum of fractional S = 1/2 excitations, called spinons. Application of an external magnetic field H leads to a pronounced rearrangement of the excitation spectrum, making the soft modes incommensurate [1, 2] but leaving the spinon continuum gapless. However, in a fully spin-polarized phase, H > H sat , the excitation spectrum is gapped and dominated by ordinary spin waves (magnons). The low-energy excitation spectrum of such an ideal isotropic Heisenberg S = 1/2 chain can be described in terms of an effective free massless boson theory [3][4][5].The presence of additional interactions such as Dzyaloshinskii-Moriya (DM) interaction [7] can significantly alter the physical properties of such spin systems (see for instance Ref.[6]), and, in particular, their highfield behavior. A Heisenberg spin-1/2 chain with exchange interaction J and DM interaction (or alternating g-tensor) in a field H can be mapped to a simple Heisenberg chain with a staggered transverse field h ∝ H [8-10], described by the effective spin Hamiltonian

Magnons in a two-dimensional transverse-field XXZ model

“…Such an effective model has successfully been applied to the quasi-one-dimensional quantum magnet copper benzoate [23][24][25] .…”

Boundary Resonances inS=1/2Antiferromagnetic Chains Under a Staggered Field

“…The boundary effects can account for the mysterious ''unknown modes'' found in two compounds KCuGaF 6 [15] and Cu-PM [16,17].…”

Thermodynamic properties of quantum sine-Gordon spin chain system KCuGaF6

“…21,22 In addition to copper benzoate, the following substances are known to be described by the effective model given by Eq. (1): Yb 4 As 3 , 23-25 PM·Cu(NO 3 ) 2 ·(H 2 O) 2 (PM, pyrimidine) [26][27][28][29][30] and CuCl 2 ·2[(CD 3 ) 2 SO]. [31][32][33] Because the elementary excitations and thermodynamic properties of quantum SG systems have been of great interest, numerous theoretical investigations have been published, [34][35][36][37][38][39][40][41][42][43][44][45] and the theoretical results have been used to analyze experimental results on the above-mentioned substances.…”

“…Moreover, new experiments have been proposed on the basis of theoretical results. 28,[46][47][48] KCuGaF 6 is an S = 1/2 AFHC system, which is represented by the effective model given by Eq. (1) in a magnetic field.…”