A scholarly couple at Shanghai Jiao Tong University observe a superconducting st

Recently, Associate Professor Li Tingxin and Associate Professor Liu Xiaoxue from Shanghai Jiao Tong University, in collaboration with a team from Wuhan University, have created a high-quality Bernal-stacked bilayer graphene and a single-layer tungsten diselenide heterojunction sample, achieving an external vertical displacement electric field of 1.6V/nm.

Through this, they have observed the superconducting state for the first time at the electron-doped end of crystalline graphene, and revealed the differences between the superconducting states at the electron-doped end and the hole-doped end under a parallel magnetic field.

In addition, the research team has also observed a series of spontaneous symmetry-breaking states in the conduction band of graphene.

As the carrier doping and the vertical displacement electric field change, the Fermi surface structure of these symmetry-breaking states will also change. In response to these changes, the team has drawn a complete phase diagram.

Through this, the research team has developed the quality and control ability of bilayer graphene devices to a new level.Under the extreme low-temperature conditions of minus 270 degrees Celsius, the team explored new quantum states and quantum properties based on this system.

Advertisement

More importantly, under the condition of electron doping, the research group also observed the superconducting state, which is also the first time that the academic community has observed superconductivity in single-crystal graphene with electron doping.

The strength of the superconducting state at both the hole end and the electron end can be effectively adjusted by the external vertical displacement electric field.

In the experiment, the highest superconducting transition temperatures measured by the research group were about 450mK and 300mK, respectively.

At present, in the single-crystal graphene system, by using the method of electrostatic doping, the above temperatures are also the highest superconducting transition temperatures that can be observed at present.In the study, the research team also conducted a detailed comparison of the electron doping superconducting properties and hole doping superconducting properties in bilayer graphene.

The results were quite unexpected: under similar superconducting properties such as superconducting transition temperature and superconducting critical perpendicular magnetic field, the two types of superconducting states, hole doping superconductivity and electron doping superconductivity, exhibit completely different dependencies on the parallel magnetic field.

Specifically: the superconducting state of hole doping violates the Pauli paramagnetic limit; the superconducting state of electron doping always follows the Pauli paramagnetic limit.

Previously, it was believed that the Ising spin-orbit coupling interaction introduced through the proximity effect could be used to understand the enhancement effect of tungsten diselenide on the superconducting state of the graphene system.

The hole doping superconductivity exceeding the Pauli paramagnetic limit is the direct result of the Ising spin-orbit coupling interaction.However, the team found that: using the Fermi surface analysis method, although a significant Ising spin-orbit coupling interaction can also be observed in the conduction band, the superconductivity of electron-doped does not violate the Pauli paramagnetic limit.

This observation suggests that: the enhancement effect of tungsten diselenide on superconductivity in bilayer graphene may not come solely from the Ising spin-orbit coupling interaction introduced by the nearest-neighbor effect.

At the same time, most of the graphene superconducting systems found at present are metastable structures, such as twisted graphene systems and three-layer rhombohedral stacking graphene systems, and this metastable structure limits the development of related applications.

Bernal stacked bilayer graphene, on the other hand, is a graphene with a stable structure. In the long run, it is expected to be used to build new types of superconducting quantum devices, providing new possibilities for research such as quantum computing.

Recently, the relevant paper was published in Nature with the title "Tunable superconductivity in electron- and hole-doped Bernal bilayer graphene" [1].The doctoral student from Shanghai Jiao Tong University, Li Chushan, is the first author. Associate Professor Li Tingxin and Associate Professor Liu Xiaoxue from Shanghai Jiao Tong University, as well as Professor Wu Fengcheng from Wuhan University, serve as the co-corresponding authors.

Academic Power Couple in the Field of Graphene

It is reported that the research boom in graphene superconductivity can be traced back to a few years ago. In 2018, a team from the Massachusetts Institute of Technology in the United States reported a correlated insulating state and superconducting state in the magic-angle bilayer graphene moiré superlattice system with a flat band energy band structure.

Subsequently, similar novel electronic states were also observed in the magic-angle multilayer graphene moiré superlattice system.

Soon after, the field welcomed rapid progress. At the same time, research on graphene superconducting states gradually expanded to crystalline graphene systems without moiré superlattices.In 2021, a team from the University of California, Santa Barbara, observed superconductivity for the first time at the hole end of rhombohedral stacked trilayer graphene.

The transition temperature of its strongest superconducting state is about 100mK, which is also the first time that superconductivity has been observed in crystalline graphene through electrostatic doping.

In 2022, the team from the University of California, Santa Barbara, observed superconductivity at the hole-doped end of Berna stacked bilayer graphene.

However, the emergence of the superconducting state requires an additional small parallel magnetic field (about 0.15T). Moreover, its highest transition temperature is relatively low, only about 30mK afterwards.

In 2023, a team from the California Institute of Technology found that in the heterostructure composed of Bernal stacked graphene and a single-layer transition metal dichalcogenide compound, tungsten diselenide, the spin-orbit coupling interaction in graphene can be enhanced through the proximity effect of tungsten diselenide.At this point, the superconducting state of hole-doped Bernal-stacked graphene can be observed under a zero magnetic field. Moreover, its highest transition temperature is raised to approximately 300 mK.

Additionally, compared to traditional conventional superconductors, the superconducting state exhibits different characteristics. For instance, research on the dependence on parallel magnetic fields shows that the superconducting state breaks the Pauli limit followed by conventional superconductors.

Related experiments also show that the superconducting state of Bernal-stacked bilayer graphene is highly dependent on the perpendicular electric field.

Furthermore, there are experiments showing that the highest vertical displacement electric field strength achievable in the Bernal-stacked bilayer graphene system is about 1V/nm.

However, under such electric field conditions, the superconducting state has not completely disappeared. Therefore, the dependence of the superconducting state on the vertical displacement electric field is an incomplete conclusion and still requires further experimental research.The mechanisms of superconductivity in molybdenum-graphene and crystalline graphene, as well as the relationship between the superconducting states of these two systems, have previously remained an unsolved mystery.

At the same time, the specific mechanisms by which transition metal chalcogenides enhance the superconducting properties of graphene systems also need to be revealed through further experimental and theoretical research.

In recent years, the superconducting state of crystalline graphene has been a direction of common interest for the couple Li Tingxin and Liu Xiaoxue.

In 2022, during a casual conversation, Liu Xiaoxue pointed out: Currently, the highest vertical displacement electric field strength that can be achieved in the experiment of double-layer graphene system superconductivity is about 1V/nm, which makes it impossible to fully characterize the changes of the superconducting state with the displacement electric field.

If a larger displacement electric field could be applied to the double-layer graphene system, it might be possible to study such a question: that is, how the superconducting state and the spontaneous symmetry breaking state evolve with the electric field.Li Tingxin mentioned: If some of the techniques for preparing two-dimensional transition metal chalcogenide devices are applied to Bernal-stacked bilayer graphene devices, it is expected to enhance the range of vertical displacement electric fields.

The two believe that this idea is very feasible, and preparing such devices is also very suitable for training new doctoral students.

So, they decided to undertake this topic and designated the new doctoral student, Li Chushan, to prepare the samples.

Under the guidance of Liu Xiaoxue and Li Tingxin, Li Chushan successfully prepared several high-quality devices at the beginning of 2023. Next, it is necessary to conduct ultra-low-temperature transport measurements for these devices.Experiments Across the Thousand Miles of Beijing and Shanghai

Due to the impact of equipment embargo, the research group was unable to purchase a dilution refrigerator in a short period of time, and naturally, they could not carry out transport measurements at ultra-low temperatures below 1.5K.

Therefore, they used a low-temperature magnetic field system with the lowest temperature of 1.5K to select samples. Although the superconducting state of the system cannot be observed below this temperature range, it is sufficient to test the range of vertical displacement electric fields that can be applied to the prepared bilayer graphene devices.

At the same time, under high electric fields, they can also preliminarily observe the spontaneous symmetry breaking state that appears in bilayer graphene, and judge the quality of the samples based on this.

After selecting high-quality samples through low-temperature measurements, they can then place them in a dilution refrigerator to measure the superconducting properties of the samples.Generally speaking, the lowest temperature that a dilution refrigerator can achieve is around 10 mK (lattice temperature).

To conduct these experiments, they applied for and obtained the right to use the relevant experimental apparatus from the Institute of Physics, Chinese Academy of Sciences.

This device is located in Huairou District, Beijing, where they can only apply for one to two weeks of machine time each time.

In total, the team made three round trips from Shanghai to Beijing to complete all the measurements.

At the same time, high-quality graphene samples are very fragile and can be easily damaged by static electricity, which also poses very high requirements for the multiple experiments between Beijing and Shanghai. It was not until September 2023 that they finally completed a cumulative two measurements with the dilution refrigerator.After a careful analysis of the data, Li Tingxin, Liu Xiaoxue, and Li Chushan, along with their theoretical collaborator Professor Wu Fengcheng from Wuhan University, reached a consensus on the new physical phenomenon they discovered through multiple in-depth discussions.

Subsequently, the team of Wu Fengcheng began theoretical calculations, while Li Tingxin and Liu Xiaoxue started writing the paper.

In response to the reviewers' comments, Li Chushan and several other graduate students from the team rushed from Shanghai to Beijing again in March 2024 to conduct the third measurement in the dilution refrigerator temperature range.

Ultimately, all three reviewers in the second round of review gave positive feedback, and the paper was officially accepted by Nature in May 2024.

In the future, the team will take a two-step approach:On the one hand, further in-depth research will continue on the properties of superconductivity in graphene systems.

For example, the impact of Coulomb screening on the strength of the superconducting state in crystalline graphene will be studied, as well as the influence of the two-dimensional semiconductor proximity effect on the superconducting state of rhombohedral stacked triple-layer graphene.

Through this, it is hoped to provide key experimental information for a comprehensive understanding of the superconductivity mechanism in graphene systems.

On the other hand, new types of superconducting quantum devices will be prepared based on graphene superconducting design schemes.

For example, by combining graphene superconductivity with other topological quantum states that emerge in graphene systems, new devices will be prepared based on the topological superconductivity of the graphene system.