Scientists Measure Ferromagnetic Exchange Energy in Monolayer MoS2

Researchers have made a breakthrough discovery in the field of quantum physics, observing ferromagnetic ordering of mobile electrons in a two-dimensional system, specifically in monolayer MoS2. This phenomenon was previously thought to be impossible due to the Mermin-Wagner theorem. The team, led by unnamed researchers, used photoluminescence with quasiresonant excitation on gated monolayer MoS2 to study the behavior of electrons.

The experiment involved injecting electrons into the material using a gate electrode and measuring the energy separation between the spin-up and spin-down bands. The results showed that the exchange energy is larger than the Fermi energy, indicating ferromagnetic ordering.

This discovery has significant implications for the development of new technologies, including quantum computing and spin-based electronics. Companies such as IBM and Google are already working on developing these technologies, and this breakthrough could pave the way for further advancements.

The scientists have investigated the behavior of electrons in a two-dimensional electron gas (2DEG) system, specifically a MoS2 monolayer sandwiched between two hBN layers. They’ve used a technique called quasiresonant, quasilocal photoluminescence (PL) spectroscopy to study the spin properties of excitons (bound pairs of electrons and holes) in this system.

The key findings are:

1. Spin polarization: The researchers observed a high degree of spin polarization, indicating that the exchange energy (Σ) is larger than the Fermi energy. This means that the spins of the electrons are aligned, leading to a magnetic-like behavior.
2. Dichroism: They found a significant dichroism (difference in absorption or emission between left- and right-circularly polarized light) at low electron densities, which disappears abruptly at a certain density (n ≃ 3 × 10^12 cm^-2). This suggests a first-order phase transition to a paramagnetic state.
3. Bir-Aronov-Pikus mechanism: The team proposes that the increase in dichroism at small n is due to an electron-hole exchange process, known as the Bir-Aronov-Pikus mechanism. This process allows excitons to scatter between valleys (K and K˜) within their lifetime, reducing the dichroism.
4. Trions: They observed three trion species (T1, T2, and T3) in one polarization configuration and a fourth trion (T4) in another. The energy of T4 is close to that of T3, but they exhibit different linewidth dependencies on electron density.

The implications of this study are significant:

• It provides evidence for spin ordering and a first-order phase transition driven by subtle corrections to Fermi-liquid theory.
• The observed dichroism and trion behavior can be explained by the Bir-Aronov-Pikus mechanism, which is important for understanding spin-related phenomena in 2DEG systems.

This research has potential applications in the development of spin-based electronics and optoelectronic devices.