Title: Topological quantum paramagnet in a quantum spin ladder,
Abstract: It has recently been found that bosonic excitations of ordered media, such as
phonons or spinons, can exhibit topologically nontrivial band structures. Of
particular interest are magnon and triplon excitations in quantum magnets, as
they can easily be manipulated by an applied field. Here we study triplon
excitations in an S=1/2 quantum spin ladder and show that they exhibit
nontrivial topology, even in the quantum-disordered paramagnetic phase. Our
analysis reveals that the paramagnetic phase actually consists of two separate
regions with topologically distinct triplon excitations. We demonstrate that
the topological transition between these two regions can be tuned by an
external magnetic field. The winding number that characterizes the topology of
the triplons is derived and evaluated. By the bulk-boundary correspondence, we
find that the non-zero winding number implies the presence of localized triplon
end states. Experimental signatures and possible physical realizations of the
topological paramagnetic phase are discussed.