At sufficiently low temperatures, large assemblies of particles that are classified as bosons condense into a single quantum state. This remarkable phenomenon, known as Bose–Einstein condensation (BEC), can allow the particles to become a superfluid, whereby they flow without friction. Superfluidity has been seen in gaseous helium-4 and in ultracold atoms, but only at extremely low temperatures (a few kelvin). In the past few decades, there have been many attempts to achieve high-temperature BEC in semiconductors using electrically neutral composite particles called excitons, which are bound states of a negatively charged electron and a positively charged hole (electron vacancy). Writing in Nature, Wang et al.1 report compelling experimental evidence that charge-separated excitons in a pair of atomically thin semiconductors can exhibit BEC at temperatures as high as 100 K.

Read the paper: Evidence of high-temperature exciton condensation in two-dimensional atomic double layers

When an electron is excited from the ‘valence’ energy states of a semiconductor material to higher-energy conducting states, it leaves behind a hole. The electrostatic attraction between electrons and holes can bind them into excitons. Separately, electrons and holes are particles that are classified as fermions, which cannot form Bose–Einstein condensates. But because a bound state of two fermions is a boson, excitons can condense.

The effective masses of electrons and holes (the masses that these particles seem to have when responding to electrical forces) are much smaller than those of atoms. As a result, excitons can condense at much higher temperatures than can ultracold atoms. In addition, the energies needed to split excitons into electrons and holes are greater than the thermal energy of the excitons even at room temperature, so excitons can be stable at this temperature.

When electrons and holes are in the same spatial region of a semiconductor, there is a high chance that they will recombine (merge), releasing energy in the form of light through a process called electroluminescence. Such electron–hole recombination usually happens so quickly that BEC cannot occur, but recombination can be avoided by separating the electrons and holes into two adjacent semiconductor layers. However, despite expectations, experiments over the past few decades that have searched for high-temperature BEC of excitons have been largely unsuccessful.

In 2002, charge-separated excitons were produced by confining electrons and holes in two disconnected slabs (quantum wells) of the semiconductor gallium arsenide, separated by a slab of another semiconductor, aluminium gallium arsenide2. However, the electron–hole pairing interactions in such systems are weak because it is difficult to get the quantum wells close to each other and because the interactions are screened (weakened) by the presence of the other charges.

It was later recognized that, by replacing the quantum wells with atomically thin sheets of the semiconductor graphene (a two-dimensional form of carbon), the separation between the layers could be dramatically reduced, thereby greatly strengthening the electron–hole pairing interactions3. To decrease the detrimental effects of screening, researchers proposed alternative atomically thin semiconductor layers; namely, two sheets of bilayer graphene4 or of molybdenum disulfide5. For the bilayer graphene, strong experimental signatures of BEC were reported last year6, but at temperatures of only about 1 K.

Wang and colleagues studied charge-separated excitons in a device that comprises two semiconducting monolayers (one of molybdenum diselenide and the other of tungsten diselenide), separated by a few atomic layers of the 2D electrical insulator known as hexagonal boron nitride (Fig. 1). This material limits the tunnelling of charges between the two monolayers to suppress electron–hole recombination. The two ends of the device consist of electrodes called gates that are made from graphene. By combining voltages applied to the two gates and a voltage applied to the tungsten diselenide layer, the electron and hole densities in each monolayer can be tuned independently.

Figure 1 | Excitons in a semiconductor device. Wang et al.1 study monolayers of the semiconductors tungsten diselenide and molybdenum diselenide that are separated by sheets of the two-dimensional electrical insulator known as hexagonal boron nitride. The two ends of this device consist of electrodes called gates that are made from graphene (a 2D form of carbon). The device contains composite particles known as excitons, which are bound states of an electron vacancy (a hole) and an electron. A hole in one monolayer can tunnel to meet an electron in the other monolayer, and merge with it, releasing energy in the form of light. The authors demonstrate that, at temperatures up to about 100 kelvin, several features of this light depend only on the exciton density. This observation suggests that the excitons are in a single quantum state — a phenomenon known as Bose–Einstein condensation.

The authors detected a large electric current associated with tunnelling of charges between the layers. This current depends only on the exciton density, suggesting that the excitons coordinate strongly with each other. The electron–hole recombination induces strong electroluminescence, the intensity of which has a critical threshold that depends on the exciton density. This intensity also has a large enhancement that is strongly peaked at equal electron–hole densities. Such sensitivity of the electroluminescence enhancement to charge imbalance is consistent with exciton condensation.

These tunnelling and electroluminescence characteristics persist up to temperatures of about 100 K. Wang et al. therefore interpret this temperature as the transition temperature for BEC in this system, consistent with previous predictions5. Earlier this year, a theoretical investigation of superfluidity specifically for this molybdenum diselenide–tungsten diselenide system7 reported properties that quantitatively align with those in the current experiment, lending further credence to the authors’ conclusions.

In 2D systems, the transition temperature for BEC is generally higher than that for superfluidity. Above the superfluid transition temperature, there are disconnected regions of superfluidity that persist up to the BEC transition temperature. Both of these temperatures increase linearly with exciton density, at rates that depend on the electron and hole effective masses, but the BEC temperature increases much more quickly than does the superfluid temperature. For this reason, Wang and colleagues’ measurements, which are sensitive to BEC, reveal condensation at high exciton densities (of about 1012 per square centimetre) at temperatures up to 100 K.

Superfluidity could not be probed directly in the current experiment. But this is an exciting challenge for future work. Conclusive evidence for superfluidity could come from direct measurements of electric current flowing in opposite directions along the two layers, using independent electrical contacts for these layers.

The authors’ molybdenum diselenide–tungsten diselenide double layer is a straightforward semiconductor system. As a result, it is suitable for future condensate-based optoelectronics and ultrafast devices, and paves the way for the search for exciton-mediated high-temperature superconductivity.