Evidence for superposition of quantum states in the spin-1/2 coupled two-leg ladder antiferromagnet C₉H₁₈N₂CuBr₄

Quantum superposition is one of the fundamental principles of quantum mechanics that states that linear combinations of solutions to the Schrödinger equation are also solutions of the Schrödinger equation. To the best of our knowledge, observation of quantum superposition in the ground state of solid-state materials remains elusive. In this talk, we will highlight our studies of the spin-1/2 coupled two-leg ladder antiferromagnet C₉H₁₈N₂CuBr₄ (DLCB for short), based on the bulk and neutron scattering measurements. A Néel-ordered phase of DLCB below T_N=2.0 K was observed in the specific heat and neutron diffraction measurements [1]. However, the recent high-resolution neutron work [2] on the temperature dependence of spin dynamics across T_N indicates that the transverse magnetic excitations cannot be described by conventional spin wave theory associated with explicit symmetry breaking. We find that the spin gap in DLCB is not due to the spin anisotropy but to the separation between a singlet ground state and a triplet excited state. Since an antiferromagnetic spin-1/2 two-leg ladder system can be mapped onto the spin-1 chain, the notion of the Haldane gap is proposed to explain the opening of the spin gap in DLCB. Therefore, these point to the ground state of DLCB being a quantum superposition of a Néel-ordered phase and a symmetry-protected Haldane phase.