Magnons and excitation continuum in XXZ triangular antiferromagnetic model: Application toBa3CoSb2O9

Abstract

We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model using series expansion and mean field Schwinger boson approaches. The single-magnon spectrum computed with series expansion exhibits rotonic minima at the middle points of the edges of the Brillouin zone, for all values of the anisotropy parameter in the range 0 ≤ J z /J ≤ 1. Based on the good agreement with series expansion for the single-magnon spectrum, we compute the full dynamical magnetic structure factor within the mean field Schwinger boson approach to investigate the relevance of the XXZ model for the description of the unusual spectrum found recently in Ba3CoSb2O9. In particular, we obtain an extended continuum above the spin wave excitations, which is further enhanced and brought closer to those observed in Ba3CoSb2O9 with the addition of a second neighbor exchange interaction approximately 15% of the nearest-neighbor value. Our results support the idea that excitation continuum with substantial spectral-weight are generically present in two-dimensional frustrated spin systems and fractionalization in terms of bosonic spinons presents an efficient way to describe them.

Static and Dynamical Properties of the Spin-1/2Equilateral Triangular-Lattice AntiferromagnetBa3CoSbMa1, Kamiya2, Hong3 et al. 2016Phys. Rev. Lett.142201454View full textAdd to dashboardCite

“…Meanwhile, we note that a Schwinger bosons mean field approach yields a spectrum with broadening due to deconfined spinons [14], bearing some resemblance to the observed results. After extending this approach to discuss spinon confinement as a more reliable description of a magnetically ordered state, it may offer a way of reconciling the discrepancy between theory and experiments.…”

“…Introduction.-The S = 1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) is the paradigmatic example of a two-dimensional (2D) frustrated quantum magnet [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The combination of frustration, strong quantum fluctuations and low dimensionality is anticipated to produce strong deviations from semiclassical theories.…”

Spin liquids in geometrically perfect triangular antiferromagnets

“…Already the first theoretical studies of spin dynamics exposed salient deviations from non-interacting magnon scenario of linear spin-wave theory. Not only the spin waves are renormalized, sometimes changing their shape to produce roton minima around the M -points [84][85][86], but also the spectrum is washed out into a broad continuum at energies ω > J 1 [86][87][88][89]. These findings were initially interpreted as signatures of spinon excitationsan idea inspired by the work on square-lattice Heisenberg antiferromagnets.…”

“…This fluctuating component can be represented by an RVB state and held responsible for various spectral features [90], including the broadening of spectral lines at high energies interpreted as a spinon continuum [91,92]. While a similar physics could be envisaged in the triangular case [85], and spinons may indeed account for a large part of the calculated spectral weight [88], non-linear spin-wave theory offered an alternative explanation in terms of interacting magnons. The roton minima can be reproduced by including the 1/S corrections [93,94] that are crucial in this case, given the nearly 60 % reduction of the ordered moment with respect to its classical value [95,96].…”

Ba8CoNb6O24: A spin-12triangular-lattice Heisenberg antiferromagnet in the two-dimensional limit

“…A recent inelastic neutron scattering (NS) study of Ba 3 CoSb 2 O 9 [24] showed that even in presence of sizable perturbations [20,[25][26][27] relative to the pure QTLHAF, dynamical features are not captured by spin-wave theory (SWT). This observation suggests that alternative theoretical approaches are not only needed to describe spin-liquid states, but also to account for qualitative properties of the excitation spectrum of ordered magnets near their QMP [28,29].…”