Using the breakthrough ESR-STM technique, researchers from the IBS Center for Quantum Nanoscience collaborated with a US-based team to construct quantum-mechanically coupled artificial spin chains and reveal their quantum many-body states, including the resonating valence bond state.

▲ Measuring quantum spin states of artificial atomic spin clusters using STM-ESR (left). Quantum states of coupled 2 spins, 'valence bond' (top right), and coupled 4 spins, 'resonating valence bond' (bottom right).

Much of the progress in nanotechnology has traditionally pursued a top-down approach, such as miniaturization in the semiconductor industry. However, this method faces a minimum size limit of a few atoms due to the emergence of dominant quantum effects at that scale. Due to this reason, molecular manufacturing of complex structures with atomic-level precision requires a paradigm shift in the research efforts towards a bottom-up approach. The scanning tunneling microscope (STM) offers an unprecedented spatial resolution in its ability to precisely manipulate individual atoms, which allows the researchers to delve into a bottom-up approach to engineer artificial structures at the atomic scale. Furthermore, an STM equipped with electron spin resonance (ESR-STM) provides a breakthrough in the field of nanoscience by allowing the precise quantum control of individual atoms’ spin states.

“Building artificial structures by assembling one atom at a time remains a technical marvel. This technology enables us to control the electronic spins of the atoms within a nanostructure, which opens up possibilities of future experiments that can further our understanding of quantum mechanics,” states Andreas Heinrich, Director of the IBS Center for Quantum Nanoscience (QNS).

In a study published in Nature Communications on February 15, 2021, researchers from QNS collaborated with IBM in the US to construct artificial quantum spin arrays on a surface using an STM. The team engineered the quantum states of the spin arrays using a tunable atomic-scale magnetic field emanating from the tip of the STM. Then the arrays’ quantum many-body states were measured at sub-atomic resolution using electron spin resonance.

These coupled spins featured strong quantum fluctuations due to antiferromagnetic exchange interactions between the neighboring atoms. Atom-selective ESR measurements on the spin arrays gave access to the properties of exotic quantum many-body states, such as a finite-size realization of a resonating valence bond state. “Atom-selective measurements of these spin arrays provide a versatile quantum-matter toolkit, which allows highly entangled quantum states to be generated and probed in detail,” said Kai Yang from IBM.

“Atomic-scale experimental modeling of coupled spin systems, enhanced by atom-selective ESR-STM measurements, opens a new avenue to explore nanoscale spin science,” said PHARK Soo-hyon from QNS. “This bridges the gap between the physics of individual atoms and macroscopic objects, and it may provide a microscopic understanding of exotic physical phenomena such as spin liquid and high-temperature superconductivity, as well as a platform to design well-defined atomic-scale spintronic devices.”

IBS Communications Team
William I. Suh