Columbia University: With a Little Twist, Researchers Delve Into a Quantum Physics Puzzle

In search of the mysterious transition between metallic and insulating states of matter, Columbia researchers find signatures of quantum criticality in a unique material.

The best known semiconductor—silicon—famously blurs the line between metal and insulator. Sometimes it conducts electricity, like copper, and other times it stops electrical currents, like a block of wood. All modern electronics take advantage of silicon’s split personality. As electrons flow through the transistors in a computer chip, a tiny piece of silicon switches from amplifying the electrical signal in a metallic state or stopping it in its insulating state.

Such metal-insulator transitions set the fundamental limits on all of our electronics. In silicon chips, they occur at room temperature, where well-established principles of classical electricity and thermodynamics rule. But at ultracold temperatures near absolute zero, the classical laws of conduction break down and quantum physics takes over. Little is yet known about how materials transition between metallic and insulating states in the quantum world; understanding the laws that apply here is one of the great mysteries—and challenges—in physics today.

Researchers are particularly interested in understanding the exact point at which a metal transitions to an insulator—known as a quantum critical point. In a recent study in Nature, Columbia physicists Abhay Pasupathy and Cory Dean, and their colleagues, provide new insights into quantum criticality with the help of tungsten diselenide, a material that’s been synthesized to an ultra-pure form in the lab of study co-author James Hone, a materials scientist at Columbia.

Tungsten, a metal, and selenium, a nonmetal, can be combined in the lab to form tungsten diselenide, a silicon-like semiconductor that can exist as stacked sheets of single atoms. A decade ago, Columbia researchers figured out how to isolate and stack single sheets of tungsten diselenide. In this latest work, the team describes how its behavior changes when two layers are twisted into what’s called a moiré pattern. With a twist, tungsten diselenide switches from a simple semiconductor to a material that can be either a metal or an insulator, depending on the number of electrons in the sample.

Moiré patterns form a hexagonal lattice; when a single electron is placed on each point on the lattice—a state known as “half filled”—those electrons repel each other and refuse to move. This is a quantum effect that explains why layers of tungsten diselenide can, unexpectedly, act as an insulator. By adding or removing electrons via gate electrodes, Pasupathy and his colleagues could see how the material’s conducting properties changed. “This is a very clean and beautiful way of doing it—we just change a voltage and measure,” he said.

As they swapped electrons in and out, the team found a smooth, continuous transition between metallic and insulating phases. Often, materials undergo a sudden structural shift as they change phases—think of ice melting to water, or water boiling to steam, where transitions occur unevenly. Here, the twisted tungsten diselenide kept its latticed shape as it gradually crossed the metal-insulator divide.

At that threshold—the quantum critical point—the material’s properties fluctuate in space and time. Such quantum critical points can occur in many different scenarios—the structure of the universe itself is thought by some physicists to be the result of quantum critical fluctuations following the Big Bang. Closer to Earth, quantum physicists have long been interested in these quantum critical points as a way to understand superconductivity. In this work, the Columbia team was able to “see” these quantum fluctuations via their influence on the conducting properties of the sample.

There is still much to learn about this strange phase where materials behave in unexpected ways, the researchers said. But with the quantum critical points of tungsten diselenide now well characterized, they hope to dig deeper into manipulating and understanding quantum phase transitions.

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