Progress in the physical science has been achieved mainly along two directions in terms of the scale, represented by macroscopic (> 1 m) objects and atomic species (~ 1 Å). On the contrary, advance in the microscopic understanding on intermediate scale (~ 1 nm) objects, well-defined at an atomic scale, is rather slow in spite of continuous requests from the side of the top-down approach based nanotechnology, facing a size limit of a few atoms where the quantum effects become dominant. Scanning tunneling microscope (STM), with its unprecedented spatial resolution, allows a bottom-up approach to construct artificial structures at the real atomic scale. Furthermore, STM equipped with electron spin resonance (ESR) extends the field of nanoscience to the precision for quantum control of individual atomic spins. Using STM-ESR technique, in this works we constructed quantum mechanically coupled artificial spin chains composed of 3 or 4 spin-1/2 atoms and revealed their quantum many-body states, such as so called the resonating valence bond sate. This opens a new avenue to explore nanoscale spin science of a size, bridging physics of individual atoms and that of a macroscopic object, which may serve as microscopic understandings on exotic physical phenomena such as spin liquid and high temperature superconductivity and a platform to design well-defined atomic scale spintronic devices as well.
Designing and characterizing the many-body behaviors of quantum materials represents a prominent challenge for understanding strongly correlated physics and quantum information processing. We constructed artificial quantum magnets on a surface by using spin-1/2 atoms in a scanning tunneling microscope (STM). These coupled spins feature strong quantum fluctuations due to antiferromagnetic exchange interactions between neighboring atoms. To characterize the resulting collective magnetic states and their energy levels, we performed electron spin resonance on individual atoms within each quantum magnet. This gives atomic-scale access to properties of the exotic quantum many-body states, such as a finite-size realization of a resonating valence bond state. The tunable atomic-scale magnetic field from the STM tip allows us to further characterize and engineer the quantum states. These results open a new avenue to designing and exploring quantum magnets at the atomic scale for applications in spintronics and quantum simulations.