15: Chapter 15 Materials Breakthrough: Successful Fabrication of Superconducting Rings

The morning light had not yet fully dispersed the mist between the shelterbelts surrounding the base when a black new energy SUV silently drove along the dedicated lane into the underground garage entrance.

The car door opened, and a middle-aged man stepped out. He was dressed in a dark blue work jacket, had a sturdy build, and his plain face revealed a sharp, capable demeanor.

He carried a worn black tool bag, his gaze habitually sweeping over the garage's surveillance cameras and lighting layout, as if assessing the security level of the place.

Shen Hong had arrived.

Lin Shen greeted him at the entrance of the dedicated elevator to the second underground floor.

After a brief exchange of pleasantries, Shen Hong cut straight to the point: "Engineer Lin, can I take a look at the 'sweet spot' data you mentioned on the phone? Also, the approximate parameters for the cryogenic and magnetic field conditions currently available at the base."

Lin Shen liked this get-to-the-point style. He took Shen Hong to his studio and brought up last night's simulation results for the bismuth selenide (Bi₂Se₃)-niobium nitride (NbN) heterostructure.

Shen Hong leaned toward the screen, adjusted his glasses, and his eyes immediately became focused and sharp.

He quickly browsed through the band diagrams, density of states distribution, and the curves of topological invariants changing with parameters, his fingers unconsciously gesturing in the air, as if simulating the distribution of electronic states.

"Two-dimensional topological insulator plus s-wave superconductor... The approach is classic, but the parameter selection is very tricky," Shen Hong commented in a low voice. "This interlayer coupling strength, what interval of atomic layers does it roughly correspond to? Has the interface lattice mismatch been considered in the simulation?"

"We considered a mismatch of about 4% and introduced the corresponding strain field. The simulation shows that this 'sweet spot' window has a certain tolerance for mismatch, but the optimal range is indeed very narrow, corresponding to a thickness fluctuation of about 1-2 unit cells," Lin Shen pointed to a line of parameter descriptions on the screen.

"In other words, the requirement for the growth precision of Molecular Beam Epitaxy (MBE) is extremely high, close to its limit," Shen Hong mused. "However, if the theoretical prediction is reliable, it is worth a try. The key to verification lies in the quantized conductance platform at extremely low temperatures (mK level), or more directly, the observation of a zero-bias conductance peak within the superconducting energy gap using a Scanning Tunneling Microscope (STM). For these measurements, a strong magnetic field is necessary to suppress superconductivity and Landau level quantization, and to distinguish between topological edge states and ordinary surface states."

He turned his head to look at Lin Shen: "For sample preparation, do you have any partner institutions? There aren't many in the country that can do high-quality MBE growth for such two-dimensional material heterojunctions."

"We are contacting Professor Wu Qingquan's team at the National Laboratory for Materials Science. They are the top in the country in the growth of topological insulators and transition metal dichalcogenide heterojunctions," Lin Shen replied. "But it requires a coordination letter from the Jiuzhang Special Project headquarters."

"I know Professor Wu's team; their level is fine," Shen Hong nodded. "But growing such heterojunctions will have a high failure rate. We need an alternative material system, or at least an alternative parameter range; we cannot bet everything on this one combination."

"This is exactly what we need you and Topologist to work on together," Lin Shen said. "Topologist is skilled in theoretical modeling and numerical searching. Once he is in place, he can, based on this 'sweet spot' feature we have initially discovered, reverse-engineer or search for other candidate systems that may have similar characteristics but slightly more relaxed material growth or experimental conditions. And you need to design the most effective and feasible experimental verification scheme based on these theoretical predictions and evaluate its specific requirements for extreme condition equipment."

"Understood," Shen Hong said straightforwardly. "Give me half a day to familiarize myself with the base's local servers and the relevant material growth and characterization equipment parameters in the 'Star Sea' library. In addition, I need a status report on the main strong magnetic field and extremely low-temperature comprehensive measurement platforms in China, as well as a guide to the machine time application process."

"It's already prepared," Lin Shen pulled up another file from the encrypted terminal. "This is the information initially compiled by the headquarters. The platforms are mainly concentrated in Hefei, the capital, and Shanghai. Competition for machine time is very fierce, especially for machine time combining high-quality strong magnetic fields (such as 45T hybrid magnets) and extremely low temperatures (dilution refrigerators, below 10mK). Our project needs to provide sufficient persuasive preliminary theoretical evidence to obtain priority."

Shen Hong took the materials and flipped through them quickly, his brows slightly furrowed: "It is indeed tight. Regular applications for the queue take at least half a year to a year. Unless... we take the emergency green channel of the 'Jiuzhang' special project, or cooperate with a team that is already conducting experiments in the relevant direction and hitch a ride for measurements."

"Both are possible, and we need to strive for them," Lin Shen affirmed. "The top priority is to perfect our theoretical predictions as soon as possible and form a report that can convince the expert review panel. This requires Topologist's theoretical depth and your experimental feasibility assessment."

As the two were discussing, the studio's access control prompted again. He Yun walked in, holding a tablet, her face showing a trace of fatigue, but her eyes bright.

"Engineer Lin, the auxiliary simulation you asked me to help run regarding the influence of stress regulation on the superconducting transition temperature (Tc) and energy gap of niobium nitride superconducting thin films on specific substrates has preliminary results," He Yun handed over the tablet. "The simulation shows that if we can induce moderate compressive stress in the niobium nitride thin film on a sapphire substrate by controlling the growth temperature and nitrogen partial pressure, its Tc can potentially be increased from the conventional ~15K to close to 18K, and the superconducting energy gap may also increase by about 10%. This might broaden the superconducting energy gap parameter window required in our theoretical model."

Lin Shen and Shen Hong were both invigorated. Although this was only an auxiliary simulation, it pointed to an important engineering detail: the specific preparation process of the material would directly affect its final performance, thereby having a subtle interaction with the assumptions of the theoretical model. If material parameters that better meet theoretical requirements could be actively regulated through process optimization, it would undoubtedly greatly increase the probability of experimental success.

"Very good!" Lin Shen praised. "This idea is very important. Theory guides material design, and process verification feeds back into theoretical correction. He Yun, continue to deepen this part of the work, especially the specific quantitative relationship model between stress, Tc, and energy gap, and strive to interface with the material growth team."

"Understood," He Yun nodded, then looked at Shen Hong. "Hello, Engineer Shen, I am He Yun, responsible for some of the simulation work. Regarding measurements under extreme conditions, I have a question. If we want to observe quantized conductance, besides ultra-low temperatures and strong magnetic fields, are there any special requirements for sample electrode preparation and measurement circuit noise suppression? My previous experience has been mainly in optical measurements, and I don't know much about ultra-low temperature electrical transport."

Shen Hong acknowledged He Yun's professional question: "Good question. The difficulty in ultra-low temperature electrical transport measurement really lies in the details. Electrodes need to use low thermal conductivity metals (such as gold or platinum) and be made using electron beam lithography or template methods to ensure good ohmic contact with the two-dimensional material. The measurement circuit requires multiple shielding and filtering, especially to suppress high-frequency noise and avoid interference from the Josephson effect or quasi-particle excitation. Usually, it is necessary to integrate a preamplifier inside the dilution refrigerator... I will design these in detail if it is decided to carry out the experiment."

The three discussed the details of theoretical simulation, material preparation, and experimental design for another hour, initially clarifying their respective work priorities for the next step. Lin Shen felt that with the addition of the experienced 'experimentalist' Shen Hong, the work that had originally been somewhat floating in the theoretical clouds began to fall rapidly to the ground, becoming concrete and actionable, while also seeing the obstacles ahead more clearly.

At noon, Lin Shen and Shen Hong had a simple lunch in the secure dining area of the base canteen. Shen Hong ate quickly and asked a few more questions about the base's internal small-scale low-temperature test platform (used for preliminary process screening) during the meal.

After the meal, Lin Shen took Shen Hong to the office and dormitory temporarily arranged for him. Walking in the quiet corridor, Shen Hong suddenly asked casually: "Engineer Lin, when I arrived, I felt that the monitoring and sensing density around the base was very high. Is there any special situation recently?"

Lin Shen's heart sank; he did not expect Shen Hong's observation to be so keen. Considering Shen Hong's security clearance and the nature of his future work, he decided to reveal some information.

"Well, the headquarters reported that there have been some external abnormal signal activities recently, possibly targeting the newly launched special project. Everyone just needs to stay vigilant; internal security is very tight," Lin Shen answered cautiously.

Shen Hong nodded and didn't ask any more, just a flash of understanding and deeper focus in his eyes.

In the afternoon, Lin Shen continued to immerse himself in the simulation of 'tianhe-3', while also beginning to draft the framework for the first phase report of the 'Xuance Group'. He needed to clearly and persuasively present the basic principles of Topological Quantum Computing, the international status quo, the theoretical clues they discovered (bismuth selenide-niobium nitride system), potential experimental verification paths, required resources, and risk assessments.

In the evening, he received a message from Topologist via encrypted email, informing him that the handover work at his original unit had been basically completed, and he was expected to arrive at the base to report on the morning of the day after tomorrow. Attached to the email were some preliminary comments and questions from Topologist on the partial simulation data he had previously shared. The questions pointed directly to the core, showing a profound theoretical foundation.

The team's puzzle was falling into place piece by piece.

However, just as Lin Shen was preparing to end his day's work, Sun Qiming came to him again with a grave expression.

"Engineer Lin, the latest report from the security department," Sun Qiming's tone was more serious than last time. "This afternoon, a civilian communication base station about three kilometers southeast of the base detected a short, abnormal emission disguised as equipment debugging signals. The content of the signal has been preliminarily deciphered and contains targeted spectrum scanning instructions and positioning verification codes. Its encoding method is highly similar to the peripheral snooping signals detected before, but more precise, and the location of the emission source has moved."

Lin Shen's heart sank: "They... are getting closer?"

"It cannot be ruled out that they are conducting intelligence gathering before more precise triangulation or tentative infiltration," Sun Qiming nodded. "The headquarters has coordinated with relevant local departments to conduct a dragnet investigation of the surrounding area and has strengthened the alert level of all entrances and physical barriers of the base. At the same time, the families of all personnel involved in the 'Jiuzhang' and 'Chasing Light' core projects have been included in the key protection scope. Plainclothes guards have also been arranged for your parents."

A chill crept up Lin Shen's spine. The other party not only did not restrain themselves but was upgrading their actions! This had already exceeded the scope of conventional technological intelligence espionage and faintly revealed a dangerous aura of obtaining information at all costs.

"Is there... any possibility of a leak internally?" Lin Shen couldn't help asking.

"Currently, no abnormalities have been found in the background checks and real-time monitoring of all core personnel. But it cannot be ruled out that it is through technical means, such as remote eavesdropping, network penetration, or the turning of certain non-core personnel who have contact with peripheral information," Sun Qiming analyzed. "The headquarters requires that from today on, the local network of all core research areas will undergo physical isolation upgrades, and no wireless communication equipment without special encryption is allowed to be brought in. Work discussions within your group should also be conducted in designated secure conference rooms as much as possible to avoid leaving sensitive records in personal work areas."

This meant that the way of working would become more inconvenient and inefficient, but safety came first.

"I understand, I will notify team members to strictly comply," Lin Shen replied solemnly.

After Sun Qiming left, Lin Shen stood in front of the studio's window (although it was a screen simulating the scenery outside), looking at the calm forest night animation playing on it, but his mood was difficult to calm.

The prying eyes outside the laboratory were no longer just 'prying.' They were becoming more aggressive, like tentacles quietly tightening in the shadows.

And they, these people trying to light a torch at the forefront of science, not only had to fight against profound natural laws but also had to be on guard against the malice and greed of their own kind at all times.

He clenched his fists. The pressure was like a mountain, but retreat was absolutely impossible.

The first glimmer of theoretical light for topological quantum had already appeared, and there was an optimization direction for the preparation process of the superconducting ring. These were all precious sparks in the darkness.

Now, they must protect this faint spark in this increasingly dangerous environment and find ways to make it burn brighter until it illuminates the path to the new world.

No matter how the tentacles in the shadows dance, the exploration belonging to the light will never stop.