In order to find the mysterious transition between the metallic state and the insulating state of matter, researchers at Columbia University discovered the characteristic of quantum criticality in a unique material.
The most famous semiconductor-silicon-is known for blurring the line between metal and insulator. Sometimes it conducts electricity like copper, and sometimes it blocks electricity like a piece of wood. All modern electronic products take advantage of the split personality of silicon. When electrons flow through the transistors in the computer chip, a small piece of silicon amplifies the electrical signal in the metal state or stops the electrical signal in the insulated state.
This metal-insulator transformation sets basic limits for all our electronic products. In silicon chips, they occur at room temperature, where there are recognized principles of classical electricity and thermodynamics. But at ultra-low temperatures close to absolute zero, the classical law of conduction will fail, and quantum physics will take over. We don't know much about how materials in the quantum world change between a metallic state and an insulating state. Understanding the laws that apply here is one of the biggest mysteries and challenges in physics today.
Researchers are particularly interested in understanding the exact point at which the metal turns into an insulator (called the quantum critical point). In a recent study in the journal Nature, Colombian physicists Abhay Pasupathy and Cory Dean and their colleagues provided new insights into quantum criticality with the help of tungsten diselenide, which is a kind of The material is synthesized in an ultra-pure form in the research laboratory. The author James Hone is a materials scientist at Columbia University.
Tungsten is a metal and selenium is a non-metal. It can be combined in the laboratory to form tungsten diselenide, a silicon-like semiconductor that can exist as a stack of single atoms. Ten years ago, researchers at Columbia University figured out how to separate and stack monolithic tungsten diselenide. In this latest work, the team described how the behavior of the two layers changes when they are twisted into a so-called moiré pattern. Through twisting, tungsten diselenide transforms from a simple semiconductor to a material that can be a metal or an insulator, depending on the number of electrons in the sample.
The moiré pattern forms a hexagonal lattice; when an electron is placed at each point on the lattice—a state called “half full”—the electrons repel each other and refuse to move. This is a quantum effect that explains why the tungsten diselenide layer can unexpectedly act as an insulator. By adding or removing electrons from the gate electrode, Pasupathy and his colleagues can see how the conductive properties of the material change. "It's a very clean and beautiful way-we just change the voltage and measure it," he said.
When they exchanged electrons in and out, the team found a smooth, continuous transition between the metallic phase and the insulating phase. Generally, materials undergo sudden structural transformations when they change phases—think of ice melting into water, or water boiling into steam. These transformations are not uniform. Here, the twisted tungsten diselenide maintains its lattice shape as it gradually passes through the boundaries of the metal insulator.
At that threshold-the quantum critical point-the properties of the material will fluctuate in time and space. This kind of quantum critical point can appear in many different scenarios-some physicists believe that the structure of the universe itself is the result of quantum critical fluctuations after the Big Bang. Closer to the earth, quantum physicists have long been interested in using these quantum critical points as a way to understand superconductivity. In this work, the Columbia team was able to "see" these quantum fluctuations through their influence on the conductive properties of the sample.
Researchers say that at this strange stage, the material behaves in unexpected ways, and there is still a lot to understand. But as the quantum critical point of tungsten diselenide is now well characterized, they hope to further study the manipulation and understanding of quantum phase transitions.
Read more: Augusto Ghiotto, En-Min Shih, Giancarlo Pereira, Daniel Rhodes, Bumho Kim, Jiawei Zang, Andrew Millis, Kenji Watanabe, Taniguchi, James Hone, Lei Wang, Cory Dean and Abhay Pasupathy. Distortion of quantum criticality in transition metal dichalcogenides.