Spin-Wave Spectrum of the Quantum Ferromagnet on the Pyrochlore LatticeLu2V2O7

Abstract

Neutron inelastic scattering has been used to probe the spin dynamics of the quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined spin waves are observed at all energies and wavevectors, allowing us to determine the parameters of the Hamiltonian of the system. The data are found to be in excellent overall agreement with a minimal model that includes a nearestneighbour Heisenberg exchange J = 8.22(2) meV and a Dzyaloshinskii-Moriya interaction (DMI) D = 1.5(1) meV. The large DMI term revealed by our study is broadly consistent with the model developed by Onose et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although our ratio of D/J = 0.18(1) is roughly half of their value and three times larger than calculated by ab initio methods [2]. PACS numbers:The highly frustrated nature of the pyrochlore lattice leads to a rich diversity of fascinating properties when the lattice sites are decorated with "classical" (large S) spins [3]. Arguably the most celebrated example is ferromagnetically coupled Ising spins which give rise to spinice [4,5] and the emergence of magnetic monopoles [6,7]. While many examples of classical pyrochlores exist, there are few examples of pyrochlores where the spins of the magnetic ions are explicitly in the quantum (S=1/2) limit. Quantum effects can, however, play a decisive role even in classical pyrochlores if their low-energy physics maps onto an effective spin 1/2 model [3]. In either case, quantum effects may produce a range of novel phenomena including the realisation of a three-dimensional quantum spin-liquid ground state, emergent electromagnetism supporting photon-like excitations, etc. [8][9][10][11]. Interest in itinerant pyrochlore magnets is also motivated by the various anomalous transport properties they exhibit [12].Lu 2 V 2 O 7 is a ferromagnetic, small-gap Mott insulator that crystallises in the pyrochlore structure and displays a number of exceptional properties. Fig. 1(a) shows the V 4+ (S=1/2) sites in the pyrochlore lattice, which form a three-dimensionally coordinated network of corner sharing tetrahedra. Bulk measurements have established that the spins order ferromagnetically at T C =70 K [1,13]. Measurements of the thermal conductivity in Lu 2 V 2 O 7 by Onose et al. have been interpreted in terms of a magnon Hall effect [1], based on the observation that the thermal conductivity has a distinctive dependence on applied magnetic fields for temperatures below T C . This highly unusual and previously unreported phenomenon was shown to be consistent with a model in which the Dzyaloshinskii-Moriya interaction (DMI) between nearest neighbour spins deflects magnon wavepackets propagating from the hot to the cold side of the sample [1].Further evidence of the exceptional properties of Lu 2 V 2 O 7 was provided by Zhou et al. [13] who discovered that it displays a large (50%) magneto-resistance around 70 K in an applied magnetic field. Additionally, polarised neutron diffraction found evidence for an orbital ordered ground state ...

Magnetic Hyperfine Fields in Lu $$_2$$ 2 V $$_2$$ 2 O $$_7$$ 7 : A Model Approach

“…The calculated value of the exchange interaction constant is J = 10.42 meV that is in a good agreement with the one obtained from the spin wave spectrum J = 8.22 meV [8]. As a result we have found that the energy of the ferromagnetic state is lower than the energy of the antiferromagnetic state.…”

Magnon nodal-line semimetals and drumhead surface states in anisotropic pyrochlore ferromagnets

“…Considering a direct mapping, e. g., by inelastic neutron scattering for the bulk magnons [38] and electron energy loss spectroscopy for the surface magnons [39], we propose to tune the anisotropy (imbalance of J N and J N ) by applying strain. As the diameter of the nodal line is sensitive to the ratio J N /J N , it will in-or decrease accordingly; such a behavior can be detected in experiments.…”

Tunable Magnon Weyl Points in Ferromagnetic Pyrochlores

“…Since the DM interaction determines the distance between the Weyl points and Γ in reciprocal space, a search for Weyl points can help to identify the exact ratio. The tunability of the Weyl points and magnon arcs can be exploited, for example, in inelastic neutron scattering experiments [24]: the shifts of the bulk band crossings upon variation of the external magnetic field can be traced. By probing a fixed line in reciprocal space through the origin, say q, one will see a band gap closing and reopening at one point on q upon evolution of the field's azimuth.…”

“…(1). For Lu 2 V 2 O 7 , the ratio D/J N of DM interaction to exchange interaction has been determined recently, with values of 0.32 [1], 0.18 [24], 0.07 [25], and 0.05 [26]. Since the DM interaction determines the distance between the Weyl points and Γ in reciprocal space, a search for Weyl points can help to identify the exact ratio.…”