The shifts from a superconductive state to a resistive state in two dimensions offer a valuable platform for exploring continuous quantum phase transitions and critical phenomena.
Physicists at Princeton have observed an abrupt change in quantum behavior while working with a three-atom-thin insulator that can be easily transformed into a superconductor. This discovery can enhance our understanding of quantum physics in solids and advance the study of quantum condensed matter physics and superconductivity in new directions.
The research focused on the sudden cessation or “death” of quantum mechanical fluctuations, revealing unique quantum behaviors and properties beyond established theories. The study sheds light on changes near absolute zero temperatures in a superconductor.
By directly looking at quantum fluctuations near the transition, researchers found clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field. After understanding this phenomenon, scientists think there is a real possibility for an exciting, new theory to emerge.
In the physical world, phase transitions occur when a material changes from one state to another, such as from a liquid to a gas or from a solid to a liquid. However, quantum phase transitions occur at extremely low temperatures, close to absolute zero, and involve continuously adjusting an external parameter without raising the temperature.
Scientists are keenly interested in understanding how quantum phase transitions manifest in superconductors. These materials conduct electricity with zero resistance, as they play a crucial role in advancing technologies like information processing and magnetic applications in healthcare and transportation.
Sanfeng Wu, assistant professor of physics at Princeton University, said, “How a superconducting phase can be changed to another phase is an intriguing area of study. And we have been interested in this problem in atomically thin, clean, and single crystalline materials for a while.”
Nai Phuan Ong, the Eugene Higgins Professor of Physics at Princeton University and an author of the paper, said, “This came about because, as you go to lower dimensions, fluctuations become so strong that they ‘kill’ any possibility of superconductivity.”
Scientists are actively investigating how two-dimensional superconductivity can be disrupted without raising the temperature. Quantum phase transitions, induced by quantum fluctuations at temperatures close to absolute zero, are exciting in this context.
In their research, physicists at Princeton utilized tungsten ditelluride (WTe2), initially in bulk form, and gradually exfoliated it to create a two-dimensional material. The material transformed into a strong insulator in its single-layer form, displaying unique quantum behaviors such as switching between insulating and superconducting phases.
To control this switching behavior, the researchers developed a device resembling an “on and off” switch.
After converting tungsten ditelluride into a two-dimensional material, the researchers subjected it to two critical conditions. First, they cooled the material to extremely low temperatures, approximately 50 milliKelvin (-273.10 degrees Celsius or -459.58 degrees Fahrenheit). At such low temperatures, quantum mechanical effects become dominant.
Tiancheng Song, a postdoctoral researcher in physics and the paper’s lead author, said, “Scientists converted the material from an insulator into a superconductor by introducing some extra electrons to the material. It did not take much voltage to achieve the superconducting state. Just a tiny amount of gate voltage can change the material from an insulator to a superconductor.”
“This is a remarkable effect.”
The researchers discovered that they could precisely manipulate superconductivity characteristics by modifying the electron density in the material using gate voltage. At a critical electron density, the proliferation of quantum vortices rapidly disrupts superconductivity, leading to the quantum phase transition.
To identify the presence of these quantum vortices, the researchers induced a slight temperature gradient on the sample, creating a situation where one side of the tungsten ditelluride was slightly warmer than the other.
Ong said, “Vortices seek the cooler edge. In the temperature gradient, all vortices in the sample drift to the cooler part, so you have created a river of vortices flowing from the warmer to the cooler part.”
The movement of vortices creates a measurable voltage signal in a superconductor. This is known as the Josephson effect, named after Nobel Prize-winning physicist Brian Josephson. According to Josephson’s theory, when a flow of vortices crosses a line drawn between two electrical contacts, it generates a weak transverse voltage. This voltage can be detected by a nanovolt meter.
Ong said, “We can verify that is the Josephson effect; if you reverse the magnetic field, the detected voltage reverses.”
Wu said, “This is a particular signature of a vortex current. The direct detection of these moving vortices gives us an experimental tool to measure quantum fluctuations in the sample, which is otherwise difficult to achieve.”
The researchers were surprised by two unexpected phenomena. Firstly, the vortices displayed remarkable robustness, persisting at higher temperatures and magnetic fields than anticipated. They survived at temperatures and fields well above the superconducting phase, even in the resistive phase of the material.
The vortex signal abruptly disappeared when the electron density was tuned just below the critical value, representing the quantum critical point (QCP). At zero temperature in a phase diagram, this point is where quantum fluctuations drive the phase transition.
Wu said, “We expected to see strong fluctuations persist below the critical electron density on the non-superconducting side, just like the strong fluctuations seen well above the BKT transition temperature. Yet, we found that the vortex signals ‘suddenly’ vanish when the critical electron density is crossed. And this was a shock. We can’t explain at all this observation — the ‘sudden death’ of the fluctuations.”
Ong added, “We’ve discovered a new type of quantum critical point, but we don’t understand it.”
In condensed matter physics, two established theories currently explain a superconductor’s phase transitions, the Ginzburg-Landau theory and the BKT theory. However, the researchers found that neither theory explains the observed phenomena.
Journal Reference:
- Tiancheng Song, Yanyu Jia, Guo Yu, Yue Tang, Pengjie Wang, Ratnadwip Singha et al. Unconventional Superconducting Quantum Criticality in Monolayer WTe2. Nature Physics DOI: 10.1038/s41567-023-02291-1