Direct Measurement of the Spin Hamiltonian and Observation of Condensation of Magnons in the 2D Frustrated Quantum MagnetCs2CuCl4

Abstract

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.

Effects of an external magnetic field on the gaps and quantum corrections in an ordered Heisenberg antiferromagnet with Dzyaloshinskii-Moriya anisotropy

“…(5) and an explicit expression for C 1 = 1 + D tan ϕ from Eq. (17). Note that the order of δE is O(S) while δS and δM are O(1).…”

“…(17). The classical part of the per-spin uniform magnetization is simply given by the canting angle: M cl = S sin ϕ, see Fig.…”

“…The generic question is: does the DM interaction enhance or suppress quantum fluctuations? This is important for the systems close to the spin-liquid state, such as triangular lattice AF Cs 2 CuCl 4 where the DM term is significant, 17,18,19 and also for the frustrated spin systems where quantum effects may help to select the ground state. 20,21 In this work we study the excitation spectrum and various T = 0 static properties of the square lattice, spin-S Heisenberg model with the DM interaction.…”

Field-induced decay dynamics in square-lattice antiferromagnets

“…(6). The second contribution is due to the remaining fluctuating terms inĤ 3 described by three-magnon processes:…”

“…The strong-field regime is now reachable for a number of layered square-lattice materials with moderate strength of exchange coupling between spins. [2][3][4][5] In addition, new field-induced dynamical effects can be present in the antiferromagnets with other lattice geometries 6,7 as well as in the gapped quantum spin systems near the condensation field for triplet excitations. [8][9][10][11][12][13][14] The ground-state properties of the square-lattice antiferromagnet in a finite field conform with the semiclassical picture of spins gradually tilted towards the field direction, 15 see Fig.…”