A Radical Spin Ladder

The spin ladder model is an important platform bridging one-dimensional and two-dimensional magnetic systems, with its physical properties modulated by geometric configuration. As such, the S = 1/2 two-leg antiferromagnetic spin ladder has attracted attention due to its potential connection to unconventional superconductivity. Here, we combine on-surface synthesis with scanning probe microscopy to achieve atomic-precision construction and unit-by-unit manipulation of such ladders of varying lengths on Au(111). By integrating tip-induced dehydrogenation with differential conductance spectroscopy, we systematically characterize the spin coupling strengths along both the rung and leg directions, revealing antiferromagnetic couplings. We observe the evolution of spin excitation spectra as a function of ladder length and a parity effect: even-length ladders exhibit a larger low-energy excitation gap than odd-length ones. Results agree with density matrix renormalization group simulations. Further calculations show the gap remains finite in the thermodynamic limit. This work lays the foundation for the future design and realization of more complex artificial spin-lattices and quantum spin devices.