The team of Wang Shiyong and Zhuang Xiaodong of Shanghai Jiaotong University realized the precise construction of molecular quantum magnets

On October 24, Nature Chemistry published the latest results
of the collaboration between Wang Shiyong of the School of Physics and Astronomy of Shanghai Jiao Tong University and Professor Zhuang Xiaodong of the Synthesis Center of Zhangjiang Institute for Advanced Study of the School of Chemistry and Chemical Engineering under the title of "Quantum nanomagnets in on-surface metal-free porphyrins".
In this study, a series of molecular quantum magnets were successfully constructed in metal-free porphyrin-free molecular chains by surface chemical synthesis method, and collective quantum excited states and fractionated end states were observed (Figure 1).

Figure 1 Precise construction of a quantum magnet with metal-free molecules

Quantum magnets have SU(2) rotational symmetry and exhibit collective quantum excitation behavior
due to significant quantum fluctuations.
Despite the numerous theoretical research works on quantum magnetism, the experimental research of individual quantum magnets still faces great challenges, which greatly hinders the development of
this frontier field.
For traditional transition metal magnets, spin-orbit coupling and crystal field splitting effect introduce significant magnetic properties, breaking the rotational symmetry of spin, so the quantum fluctuation phenomenon is not obvious
.
The search for high-quality quantum spin systems has always been a scientific problem
to be solved in this direction.
Earlier, the research team found that delocalized PI electromagnetism
could be introduced in nanographene.
The delocalized magnetism has SU(2) rotational symmetry, and its magnetic exchange strength and magnetic exchange direction can be precisely controlled (Nature Communications, 11, 6076 (2020); J.
Am.
Chem.
Soc.
, 142, 18532–18540 (2020); Phys.
Rev.
Lett.
124, 147206 (2020); Nano Lett.
20, 6859–6864 (2020); Nature Communications 13, 1705 (2022); CCS Chemistry 022, 202201895, (2022)).
In this work, the researchers further accurately constructed a one-dimensional quantum spin chain
in a metal-free porphyrin system.
Through ultra-high-resolution scanning tunneling differential spectroscopy, it is confirmed with certainty that there is an end fractional excitation effect, which is consistent with the prediction of Haldane's theory (see Figure 2).

Figure 2 Left: Chemical structure of molecular quantum magnets, Right: End fractional spin observed by spectroscopy

The researchers combined three different synthesis methods to efficiently realize the construction of molecular quantum magnets, and developed a new method
for efficient preparation of low-dimensional magnetic systems.
As shown in Figure 2, porphyrin molecular precursors are first synthesized in solution; Then, surface chemical synthesis was used to prepare non-magnetic molecular chains of different lengths.
Finally, by scanning the atom operation of the probe, any combination of quantum spin chains (Figure 3a-b)
can be constructed in it accurately and controllably.
The researchers used chemically bond-resolved non-contact atomic force microscopy imaging to trace the molecular chemical structure during probe manipulation, and visually detected the process of molecular quantum magnets from non-magnetic to S=1/2 spin chains and S=1 spin chains (3c-e).

The researchers used scanning tunneling spectroscopy and Heisenberg model calculations to deeply characterize and analyze
the constructed series of molecular quantum magnets.
The results show that there are two non-local spins inside each porphyrin unit in molecular quantum magnets, and there is a ferromagnetic exchange between them with an intensity of 20 meV.
The spin presence strength between two adjacent porphyrin units is about 3 meV antiferromagnetic exchange
.
Experiments show that the S=1/2 antiferromagnetic molecular spin chain has a limited excitation energy gap, which is in line with the Heisenberg antiferromagnetic S=1/2 model
.
The integer S=1 antiferromagnetic molecular self-selected chain has a fractionated end state, which confirms Haldane's prediction
with real space certainty.

Fig.
3 Synthetic path of molecular quantum magnet, preparation of quantum magnet shown by scanning tunneling microscope, and quantum spin chain structure shown by non-contact atomic force
microscopy.

Quantum spin systems have always been an important field in quantum-correlated many-body systems, which have a great role in promoting the understanding of important problems such as high-temperature superconductivity and quantum spin liquids, and have potential application prospects
in quantum information and quantum computing.
Finding new methods and developing new experimental methods in quantum-correlated many-body systems has always been an important problem
in physics research.
So far, how to prepare accurate and tunable low-dimensional quantum magnetic systems and how to detect single-root low-dimensional quantum magnetic systems in real space has always been a difficult point
in the direction of condensed matter experimental physics.
This research work cleverly uses interdisciplinary methods to achieve accurate quantum magnet preparation at the atomic level of structures and solve the difficult problems
of material preparation.
Researchers systematically study the behavior of quantum magnets with a single spin accuracy, and the relevant experimental results can be directly compared with quantum theoretical calculations, experimenting with the organic combination
of theory and experiment.
In addition, the method has good scalability and is expected to be further developed to realize the construction of arbitrary low-dimensional quantum magnetic systems and provide a new platform to study quantum magnetism
.

Yan Zhao, Can Li, doctoral students of the School of Physics and Astronomy of Shanghai Jiao Tong University, and Jiang Kaiyue, doctoral students of the School of Chemistry and Chemical Engineering, are the co-first authors of the work, and Shiyong Wang and Xiaodong Zhuang are the corresponding authors
.
This work is mainly supported by the National Key R&D Program, the National Natural Science Foundation of China, the Shanghai Municipal Natural Science Foundation of China and the Fok Yingdong Young Teachers Fund, and completed
in cooperation with the research group of Professor Qin Mingpu of Shanghai Jiaotong University and the team of Academician Jia Jinfeng of Shanghai Jiaotong University.

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