Huazhong University has made progress in the study of quantum control of strongly correlated electron systems

C114 News on August 9 (Yu Yu) According to news from Huazhong University of Science and Technology, recently, the low-dimensional physics and quantum materials laboratory team led by Professor Fu Yingshuang, School of Physics, Huazhong University of Science and Technology, has been conducting research on quantum control of strongly correlated Electronic systems. progress in.

It is understood that 1T-TaS2 has been widely concerned by researchers because of its possible existence of quantum spin liquid state and complex Electronic correlation phenomenon. Among them, the quantum spin liquid state can construct qubits.

It is generally believed in the field that the insulation of 1T-TaS2 comes from the Mott-Hubbard mechanism, but the latest research shows that the interlayer coupling can also lead to its insulation, so it is a common band insulator. A straightforward way to resolve the dispute is to study monolayer samples to exclude the effect of interlayer coupling.

In the research work, Fu Yingshuang’s team used molecular beam epitaxy to accurately prepare a single-layer 1T-NbSe2 film on a graphene substrate, and analyzed the single-layer 1T-NbSe2 film by scanning tunneling microscopy (STM) and its spectroscopic characterization technology. The surface morphology and electronic structure were systematically studied. Experiments show that most SDs exhibit “bright” central structures at 77 K, while a small number of SDs mainly concentrated on the film boundaries exhibit “dark” central structures. The difference in the electronic structure of these two SDs is reflected in a peak at 160 mV in the center of the bright SD. This peak disappears in the dark SD center.

Through the variable temperature experiment, it was found that when the temperature decreased to 4.4 K, all SDs turned into dark states, and the peak at 160 mV also disappeared; when the temperature increased to 40-50 K, the SDs suddenly changed from From the dark state to the bright state, a correspondingly strong 160 mV peak appears in the tunneling spectrum.

In addition, the research team also speculated that this 160 mV peak is the upper Hubbard band (UHB) by analogy analysis of 1T-TaS2. Among them, when the UHB moves up into the conduction band, it will cause the SD to change from bright to dark. This speculation is supported by first-principles calculations. Molecular dynamics calculations show that 1T-NbSe2 does have obvious structural distortions when the temperature rises to 50K, mainly manifested as the nearest neighbor Nb atom shrinking toward the center, while the nearest neighbor Se atom shrinks in-plane and expands out-of-plane. This structural change will lead to the enhanced hybridization between the d orbital of the central Nb atom and the p orbital of the Se atom, which increases the bandwidth of the d orbital and reduces the effective Coulomb interaction.

This work means that the Mott parameter of the correlated insulating system can be regulated by stress, and the Mott parameter determines whether the system can realize the quantum spin liquid state. Calculations show that only a small amount of stress is required to change the effective Coulomb strength of 1T-NbSe2 significantly. For 1T-TaS2, the effect of stress is much smaller. This work provides an opportunity to artificially control the quantum spin liquid state in such material systems.

The result was recently published in Nano Letters, an authoritative journal in the nano field, under the title “Charge Transfer Gap Tuning via Structural Distortion in Monolayer 1T-NbSe2”. Liu Zhenyu, a doctoral student at the School of Physics, and Qiao Shuang, a postdoctoral fellow at the Beijing Computational Science Research Center, are the co-first authors, and Fu Yingshuang is the corresponding author of the paper. Researcher Huang Bing of Beijing Computing Center, Professor Lv Jingtao of the School of Physics of our school, and Associate Professor Zhang Wenhao, a team member, participated in the related work.