Scientists Reveal Hidden Structure of a Quantum Fluid

Bose-Einstein Condensates (BECs) are often described as a “fifth state of matter”: a quantum state in which many particles lose their individual identities and behave as one collective object. For more than 60 years, researchers have sought to create such condensates from excitons — electron-hole pairs — as a solid-state route to macroscopic quantum coherence, which is useful for quantum technologies. 

But now, a team led by Lawrence Berkeley National Laboratory (Berkeley Lab), including Allan MacDonald at The University of Texas at Austin, has observed a tunable BEC of excitons at (what materials scientists consider) a high temperature in an atomically thin semiconductor. The findings, published this summer in Nature, reveal not only that the excitons form a BEC, but also that the condensate has an internal structure that can be switched by a magnetic field.

The work enables a new platform for studying quantum fluids in solid materials. (A quantum fluid is an exotic state of matter in which gasses of electrons or other particles behave collectively like a fluid.) It also has implications for future quantum simulations, coherent optoelectronics in next-generation telecommunications and computing, and exciton-based devices enabling faster, more efficient computing. 

It has been difficult to realize BECs from excitons in controllable semiconductor devices because optically generated excitons have very short lifespans of around a billionth of a second, and BECs are normally attained with supercold gasses in a vacuum.

“While previous studies have shown that electrons and holes can bind into excitons, there wasn’t an easy way to determine whether those excitons formed a condensate, nor could they ascertain what kind of internal quantum order that condensate has. Our work provides a way to access that hidden structure directly,” said principal investigator Feng Wang. He is a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and a professor in UC Berkeley’s Department of Physics.  

This work was supported by the Department of Energy’s Office of Science. 

In many experiments, excitons are created by light and exist only as short-lived excited states. In the new study, Wang and team counter this transience by engineering a 2D semiconducting device with excitons in the ground state rather than an excited state. The researchers predicted that such a system would enable the excitons to reach equilibrium and persist as a BEC.

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. 

Adapted from a press release by Lawrence Berkeley National Laboratory.