Kavli Affiliate: Leon Balents
| First 5 Authors: Leonie Woodland, Izabella Lovas, M. Telling, D. Prabhakaran, Leon Balents
| Summary:
We present experimental and theoretical evidence of novel bound state
formation in the low transverse field ordered phase of the
quasi-one-dimensional Ising-like material CoNb$_2$O$_6$. High resolution single
crystal inelastic neutron scattering measurements observe that small transverse
fields lead to a breakup of the spectrum into three parts, each evolving very
differently upon increasing field. We show that this can be naturally
understood starting from the excitations of the ordered phase of the transverse
field Ising model, domain wall quasiparticles (solitons). The transverse field
and a staggered off-diagonal exchange create one-soliton hopping terms with
opposite signs. This leads to a rich spectrum and a special field, when the
strengths of the off-diagonal exchange and transverse field match, at which
solitons become localized; the highest field investigated is very close to this
special regime. We solve this case analytically and find three two-soliton
continua, along with three novel bound states. We also present calculations
using exact diagonalization of a recently refined Hamiltonian model for
CoNb$_2$O$_6$ and using diagonalization of the two-soliton subspace, both of
which provide a quantitative agreement with the observed spectrum. The
theoretical two-soliton model qualitatively and quantitatively captures a
variety of non-trivial features in the observed spectrum, providing insight
into the underlying physics of bound state formation.
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