Page 49 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
46-47
4 3 2 1 0
= 3,0,1
a) experiment
b) numerics
200
150
100
50
0
0 2 4 6 8 10 12 0 2 4 6 8 10 12 H (T) H (T)
Figure 2
a) Experimental intensity colour map showing the magnetic field dependence of the magnetic excitations in BaCo2V2O8 for a transverse field applied along the b-axis and for Q = (3, 0, 1).
b) Theoretical intensity colour map to be compared with the experimental one, obtained from iTEBD calculations of the S(Q, ω) neutron scattering dynamical structure factors. The intensity colour scale shown on the right is in arbitrary units.
The nature of this transition was further elucidated by combining inelastic neutron scattering experiments (on ThALES and IN12, using unpolarised and polarised neutrons, respectively) and numerical infinite Time Evolving Block Decimation (iTEBD) calculations [2]. We were able to show that for H = 0, the spin dynamics is governed by an assembly of spinons carrying a spin 1⁄2. Because of inter-chain couplings, they are tied up in bound states of three flavours (two transverse degenerate, polarised along a and b; and one longitudinal, polarised along c), forming a discrete excitation spectrum [3]. Above Hc, however,
the low-energy spectrum essentially consists of two well- defined branches corresponding to spin 1 transverse modes polarised along b and c. These excitations can
be understood as kinetic bound states formed by two adjacent spinons strongly attached by the staggered field and moving together along the chain. Figure 2 illustrates how the magnetic field operates this change. Importantly, the very good agreement between the experimental and numerical results (see figures 2a and 2b, respectively) fully validates our model.
The magnetic excitations characterising the phases on both sides of the transition are thus two different types
of solitonic topological objects in competition. Our experimental results can be further described in field theory by the so-called ‘double sine-Gordon model’, which is relevant well beyond the field of magnetism.
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