MATERIAL SCIENCE Switching between superconductors and insulators

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The article is based on the latest news from Princeton University that states that physicists have witnessed the sudden death of quantum fluctuations in a quantum phase transition. Read the article to know more about the ground-breaking discovery.

What did physicists at Princeton University do?

Physicists from Princeton University have discovered an abrupt fall of “quantum fluctuations” when a superconductor transforms into an insulator. The researchers cooled down a crystalline material to a temperature near absolute zero but the rarest of the rare happened with gate voltage adjustment. The resulting material exhibited characteristics of an insulator and superconductor! The research can open the doors for room-temperature superconductivity.

The experiment defies the laws of physics

Change of structure

The experiment started with physicists slicing a bulk crystal of a monolayered semiconductor metal called Tungsten Ditelluride (WTe₂) into a 2D ultra-thin material. The transition from a 3D crystalline structure to a 2D thin layer helps understand quantum effects in insulators and superconductors.

Cooling down the material

As we all know superconductors exist at very low temperatures, the next step involved cooling down Tungsten Ditelluride to a temperature near absolute zero. The noted temperature was fifty milliKelvins or -273.10 degrees Celsius, low enough to understand quantum effects in detail.

Controlling gate voltage

The third step was to convert the insulating Tungsten Ditelluride into a superconductor by introducing extra electrons. A small gate voltage was enough to transition insulating Tungsten Ditelluride into superconducting material. Scientists observed that controlling this low-magnitude gate voltage could adjust electron density.

Quantum critical point

The critical electron density point is called the quantum critical point (QCP). Above QCP, quantum fluctuations start to appear and disturb the supercurrent.

Quantum fluctuations are defined as random changes in the thermodynamic states of a material at the threshold of the quantum transition. The temperature, pressure, magnetic field, gate voltage, and electron density are subject to change for measuring quantum fluctuations. In the case of superconductors, quantum fluctuations are not favorable to sustain the superconductive state. The rise in quantum fluctuations eliminates the superconductive character of superconductive material, making it electrically resistive.

Three-dimensional structures have better superconductive characteristics than two-dimensional materials. Lower dimensions have a high amount of quantum fluctuations. It means that one-dimensional thin layers or insulators are unstable for superconductive behavior. In the experiment, quantum fluctuations gave rise to quantum vortexes. A quantum vortex is a microscopic whirlpool-like structure that traps magnetic fields. The word “whirlpool” clearly indicates that supercurrent starts to reduce. Multiple quantum vortices emerged spontaneously in Tungsten Ditelluride to oppose the superconductive behavior.

Nernst experiment

Princeton Physicists implemented the “Nernst Experiment” to detect quantum vortices. The method involved slightly heating some portions of Tungsten Ditelluride while the other side remained at the same low temperature. When one side was slightly heated, quantum vortices moved toward the cooler side. The supercurrent flow between these two layers confirmed “Josephson Junction” in Tungsten Ditelluride. Josephson junction is a measure of the macroscopic behavior of superconductors.

Quantum fluctuations

Physicists lowered the electron density below the QCP through the reduction in doping concentration. Strangely, quantum vortices suddenly disappeared! Professor Sanfeng Wu from the research team said “What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field."

What does the “sudden death of quantum fluctuations” mean?

Quantum fluctuations favor insulators. Princeton University professors quote in simple words “Quantum fluctuations kill superconductivity”. The sudden death of quantum fluctuations for insulating states means that the material exists in an undefined state. It requires a new understanding of superconductivity. Scientists commented that the experiment allowed for the quantum transition of Tungsten Ditelluride from an insulator to a superconductor like an electronic switch. The OFF state represented an insulator and the ON state of the switch was a superconductor.

To summarize the experiment

• 1. Bulk monolayer crystal Tungsten Ditelluride was peeled into ultra-thin 2D material. The physical transformation converted Tungsten Ditelluride into a strong insulator.

• 2. Tungsten Ditelluride was cooled down to millikelvins or -273.10 degrees Celsius but it required more electrons to turn into a superconductor.

• 3. A small gate voltage introduced extra electrons to initiate the quantum phase transition. When electron density reached above the QCP, the insulator Tungsten Ditelluride turned into a strong superconductor.

• 4. Superconducting characteristics started to weaken due to the lattice imperfections and other factors. Quantum fluctuations abruptly grew to generate quantum vortices.

• 5. The temperature of Tungsten Ditelluride was slightly increased from one side. A new method called the “Nernst Experiment” confirmed the presence of Josephson’s effect.

• 6. Physicists lowered electron density below QCP. The material turned into an insulator but quantum fluctuations suddenly ended. Tungsten Ditelluride showed the characteristics of an insulator and superconductor together.

Key takeaways from the research

The material was supposed to become an insulator below QCP with a high amount of quantum fluctuations. However, quantum fluctuations suddenly disappeared. Heating the superconducting material is supposed to initiate quantum fluctuations that go on to end superconductivity and allow high resistance to take over the material. However, the Tungsten Ditelluride behaved like an insulator that conducts along the surface. The experiment gave rise to a new quantum state that hasn’t yet been discovered or understood. In conclusion, the electrical behavior of Tungsten Ditelluride can enable manufacturers to build room-temperature superconductors.

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