Scientists Reveal Hidden Structure of a Quantum Fluid

Using a technique called magneto-optical spectroscopy under cryogenic conditions, the researchers cooled the device down to a temperature near absolute zero and measured how the electron and hole components responded to small magnetic fields. Electrical gates above and below the device allow researchers to tune the density of these excitons.

The excitons performed collectively as predicted, but the researchers were surprised to find that the condensate signatures persisted up to about 2 Kelvin — still very cold but millions of times warmer than previous BEC demonstrations in ultracold atomic gases. 

“What is unusual here is that the excitons are not just short-lived particles created by light. They form an equilibrium quantum fluid in a device that we can tune electrically and magnetically,” explained Ruishi Qi, a co-first author on the paper and a former doctoral researcher in Berkeley Lab’s Materials Sciences Division.  

In the atomically thin semiconductors, electrons and holes carry not only charge and spin, but also a quantum property known as “valley,” which is tied to their motion — or degrees of freedom — inside the crystalline material. These spin–valley degrees of freedom give excitons multiple possible “flavors” or spin patterns, such as up-up, down-down, up-down, or down-up. In the current study, the Berkeley Lab-led team found that the BEC is not a simple one-flavor quantum state: It has two components, each with different flavors of internal spin–valley structure. This gives rise to multiple distinct condensate phases that can be switched by a magnetic field.

“The exciting part is that this is not just a simple condensate,” Qi said. “It has internal structures that we can control. By simply applying a small magnetic field, we can switch the same exciton fluid between different quantum states.”   

In future work, the researchers hope to demonstrate superfluid-based quantum devices and circuits building on the exciton BEC. 

Researchers from UC Berkeley; the University of Texas at Austin; and the National Institute for Materials Science, Japan, contributed to the study.  

This work was supported by the DOE Office of Science. 

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