Microscopic model for the magnetization plateaus inNH4CuCl3

Abstract

A simple model consisting of three distinct dimer sublattices is proposed to describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization plateaus only at 1/4 and 3/4 of the saturation magnetization. The field dependence of the excitation modes observed by ESR measurements is also explained by the model. The model predicts that the magnetization plateaus should disappear under high pressure.PACS numbers: 75.30. Cr, NH 4 CuCl 3 , TlCuCl 3 , and KCuCl 3 are isostructual quantum spin systems consisting of two-leg ladders separated by NH + 4 , Tl + , and K + ions. The two-leg ladders are composed of Cu 2+ ions with spin S = 1/2 which interact antiferromagnetically (AF) through the Cl − ions. These compounds can be considered as coupled two-leg S = 1/2 Heisenberg AF spin ladders, and show various types of magnetization curves. Shiramura et al. found that NH 4 CuCl 3 has magnetization plateaus at 1/4 and 3/4 of the saturation moment, 1 while TlCuCl 3 and KCuCl 3 have no plateaus in their magnetization curves. 2 For NH 4 CuCl 3 , Kurniawan et al. performed ESR experiments which revealed that NH 4 CuCl 3 has a finite excitation gap in the plateau regions. 3 By studying specific heat, they found AF order already at zero field and suggested a phase diagram in finite fields containing three magnetically ordered phases. 4 The origin of magnetization plateaus has attracted much interest recently. For weakly interacting dimer systems, the ground state is a spin singlet liquid with only short-range correlations between spins, and the triplet excitations require a finite excitation energy. An excited gas of triplet magnons has two characteristic energies. 5 Exchange interactions cause the transfer of excited triplets between neighboring dimers leading to a form of kinetic energy. They also lead to a short range repulsion between triplets in addition to the hard core repulsion forbidding double occupancy of a dimer. The triplet excitations are split in an external magnetic field, and the energy of the lowest component can be driven through zero. When the kinetic energy is dominant, which is realized in TlCuCl 3 and KCuCl 3 , we have no plateaus in the magnetization curve. 2 In these case, antiferromagnetic order perpendicular to the field appears simultaneously with magnetization parallel to the field, creating field-induced magnetic order, 6 which can be interpreted as a condensation of the lowest lying magnon mode. 7,8,9,10,11,12,13,14,15 In contrast to these case, when the interaction energy dominates, magnetization plateaus can appear accompanied by a magnetic superlattice. 16 Such plateaus have been observed at a rational fraction of the saturation moment. 1,17 In SrCu 2 (BO 3 ) 2 , the frustrated form of the dimer lattice leads to a narrow bandwidth for triplet excitations and magnetization plateaus associated with a magnetic superlattice of localized triplets. 18,19 An unexpected feature of NH 4 CuCl 3 is the absence of a magnetization plateau at a value 1/2 of the saturation magnetization although this...

Coexistence of a nearly spin-singlet state and antiferromagnetic long-range order in quantum spin systemCu2CdB2O6

“…There is also an example of a magnet including both spinsinglet states and AFLRO which are almost decoupled. * Electronic address: HASE.Masashi@nims.go.jpIn NH 4 CuCl 3 , the spin system consists of three different S = 1/2 dimer subsystems [9]. States in two dimer subsystems are spin singlet, whereas AFLRO appears in another subsystem in low fields.…”

“…In NH 4 CuCl 3 , the spin system consists of three different S = 1/2 dimer subsystems [9]. States in two dimer subsystems are spin singlet, whereas AFLRO appears in another subsystem in low fields.…”

C63/65u- andC35/

“…Recently, Matsumoto [21] theoretically suggested that there are three distinct magnetic dimer sublattices with different exchange interactions in NH 4 CuCl 3 , in contrast to the isostructural KCuCl 3 and TlCuCl 3 , and reproduced well the magnetization plateaus and field dependence of the magnetic resonance frequencies observed in ESR measurements [22,23] by his model. Successive structural phase transitions which may indicate the creation of such distinct magnetic dimers have been observed at low temperatures by means of various kind of experiments [24,25,26,27,28] while the isostructural KCuCl 3 and TlCuCl 3 undergo no structural phase transitions at low temperature.…”

Magnon dispersion in field-induced magnetically ordered phases

“…These results imply that there are three weakly interacting distinct dimer sublattices, A, B, C, whose intradimer interactions ; respectively, to reproduce the magnetization plateaus at 1 4 and 3 4 of the saturated moment. The details will be published elsewhere [10]. As we can see in Fig.…”