Triplons, magnons, and spinons in a single quantum spin system:SeCuO3

Abstract

Quantum magnets display a wide variety of collective excitations, including spin waves (magnons), coherent singlet-triplet excitations (triplons), and pairs of fractional spins (spinons). These modes differ radically in nature and properties, and in all conventional analyses any given material is interpreted in terms of only one type. We report inelastic neutron scattering measurements on the spin-1/2 antiferromagnet SeCuO 3 , which demonstrate that this compound exhibits all three primary types of spin excitation. Cu 1 sites form strongly bound dimers while Cu 2 sites form a network of spin chains, whose weak three-dimensional (3D) coupling induces antiferromagnetic order. We perform quantitative modeling to extract all of the relevant magnetic interactions and show that magnons of the Cu 2 system give a lower bound to the spinon continua, while the Cu 1 system hosts a band of high-energy triplons at the same time as frustrating the 3D network.

Coherent description of the magnetic properties of SeCuO3 versus temperature and magnetic field

“…In parallel, high-field magnetization measurements showed the existence of a half-step magnetization plateau at 45 T [8], which supposedly emerges from the polarization of weakly coupled Cu(2) spins, while Cu(1) dimers remain in the singlet state. This new magnetic model was supported a few years later by inelastic neutron scattering, used to determine magnetic exchange couplings and demonstrate the simultaneous presence of magnon, triplon and spinon excitations [11].…”

“…To represent the full magnetic Table I. Magnetic exchange couplings determined from DFT calculations (labelled J th ) and compared to experimental data when available (labelled J exp ) [11]. For each Jij interaction, the related Cu(i)-Cu(j) pair is specified between parenthesis.…”

“…smaller than 1.5 K (J 8 , J 13 , J 14 and J 15 ). The others are ferromagnetic (FM), with two couplings which are significantly larger than the others, J 2 = -38 K and J 5 = -20 K. The related experimental values were not directly deduced from fitting the neutron dispersion data, but by performing a perturbative analysis of a four-site model [11]. It leads the authors to estimate the square values of J 2 and J 5 (see equation 2 of ref.…”

“…It leads the authors to estimate the square values of J 2 and J 5 (see equation 2 of ref. [11]), and thus only their absolute values, i.e. 40 and 41 K, respectively.…”

“…In this work we are interested in the monoclinic phase of SeCuO 3 , where the magnetic Cu 2+ ions form structural spin-1/2 tetramers [6][7][8][9]. This particularity has motivated a special interest to study quantum effects in this system [6][7][8][9][10][11]. For instance, magnetic susceptibility and torque magnetometry measurements evidenced that this compound shows an AFM long-range ordering up to T N = 8 K, with an easy magnetization axis along the reciprocal [ 101] ([ 101] * ) direction [6,7].…”