Study Reports Evidence of Stabilized Ferromagnetic Order by Optical Pumping

By Pritam RoyReviewed by Laura ThomsonOct 6 2022

In an article published in Science Advances, researchers found that local optical pumping could stabilize and control ferromagnetic order in monolayered electrostatically doped, transition metal dichalcogenide (TMD) semiconductors at zero magnetic fields.

As a spatially defined probe of charge-carrier spin polarisation, circular dichroism (CD) was employed in reflectivity from excitonic states. At electron densities of n_e ~ 10¹² cm^-2, a circularly polarised optical pump stabilized the long-range magnetic order with carrier polarisation surpassing 80% over an 8 mm ´ 5 mm extent. This optical pump violated the symmetry between oppositely polarised magnetic states.

The optical magnetism control with local optical pumps improved spin and optical technologies. It also offered a flexible tool for investigating related phases in two-dimensional electron gases. The authors reported that magnetic interactions enhanced the original pump-induced spin polarisation over an order of magnitude in time-resolved investigations using pulsed optical excitation.

Optical Pumping as a Potential Global Symmetry Breaking Mechanism

Two-dimensional (2D) electron gases (2DEGs) with interacting electrons can display various correlated phases, such as Mott insulators, Wigner crystals, magnetism, and charge density waves. TMDs are a quickly developing platform for manipulating and analyzing 2DEGs due to their advantageous material characteristics and tuning possibilities.

The Bohr radius for free electrons in TMDs is only marginally more significant than the lattice constant due to the large effective mass of ~ 0.44 m_e and decreased dielectric screening. The resulting Bohr radius yields r_s values above 20 at experimentally attainable densities of 10¹¹ to 10¹² cm^-2. As a result, Coulombic interactions have the potential for significantly higher energies than those related to phase-space filling.

A novel mechanism called optical pumping that could effectively break the symmetry among equivalent spin configurations in TMDs was proposed in this paper. According to recent research, circularly polarised light could produce spin imbalances with microsecond-long relaxation durations by pumping specific monolayers or heterostructures of TMDs.

The primary objective of this work was to demonstrate that by continuous pumping with circular light, resident electrons in the electron-doped region of tungsten diselenide (WSe₂) monolayers might be dynamically spin-polarized or valley-polarized. Due to quick spin-conserving intervalley scattering, photo-generated electrons stimulated by the circularly polarised pump in one valley preferentially relaxed to the opposite valley.

The asymmetry of the valley populations was increased by the intravalley coupling of conduction electrons by photogenerated holes, creating dark excitons. Since these processes occurred at times that were quicker than the spin relaxation rate, the resulting spin polarisation was sustained in the existence of the continuous pump.

A significant population of resident carriers was also spin-polarised due to the comparatively low free-charge carrier densities of n ~ 10¹² cm^-2. This relatively low density disrupted the symmetry between ground-state spin configuration and stabilized magnetic order in orientation with the pumped spins.

Proof-of-Concept Analysis and the Experimental Findings

Mechanical exfoliation was used to separate the monolayer WSe₂, hexagonal boron nitride (hBN), and few-layer graphene (FLG) flakes from commercial bulk crystal and deposited them onto silicon dioxide (SiO₂) chips. First, optical and atomic force microscopy were used to evaluate the flakes’ thickness and cleanliness.

The experiment investigated the effects of circularly polarised optical pumping on monolayers of Wse₂ enclosed in hBN to prove the magnetic order’s optically stable and non-local nature.

Various materials and doping regimes were studied using CD, which acted as a direct signature of electron spin or valley polarization. The doping level in the monolayer was significantly altered by applying gate voltage between the FLG contact and the top gate. This alteration resulted in the emergence of charged excitonic and neutral resonances in the reflection spectra.

The authors also studied the temporal dynamics of spin polarisation with various pump separations. A spin imbalance was created under the pump using a local, 5-ns pulsed laser with a range of 633 nm. It was observed that when the system developed over time, micrometer-length scales of mesoscopic spin polarisation appeared across a microsecond time scale. With increasing pump separation, the peak spin polarisation recorded showed no systematic change, consistent with the spatially uniform CD under continuous wave (CW) pumping.

The study's main finding was the optical creation of mesoscopic spin polarisation in TMDs monolayer. Long-range spin polarisation might be explained by the diffusion of resident carriers that were optically pumped. The spatial spread of an initial, preserved spin imbalance resulted in a rapid decrease in the maximal spin polarisation away from the pump region under a noninteracting spin diffusion scenario. According to the time-resolved measurements, a consistent maximum spin polarisation was seen micrometers away from the pulsed pump.

Significance of the Study

In this paper, the authors reported that the ferromagnetic order in TMDs was optically controlled. According to the findings, TMDs were 2D magnetic materials with different physical makeup and characteristics from the traditional 2D magnets. For instance, the magnetism in TMDs was derived from correlated itinerant electrons, and the magnetic configuration could be tuned entirely nonlocally using electronic gating and optical fields. Even at low submicrowatt power levels, the local optical pump stabilized the magnetic state, offering a finer spatial resolution for investigating and managing magnetic domain structure.

The findings demonstrated that optical pumps were a potent tool for comprehending and manipulating TMDs, constituting a model setting for studying correlated phenomena in 2DEGs. For instance, Wigner crystals and Mott insulators could be manipulated and probed using magnetic phases and their CD.

The findings would also hasten technological advancements using TMDs, which were already a premier material platform for studying next-generation spin, optoelectronics, and valley. The creation of nonreciprocal photonics and optoelectronics would be stimulated by the development of CD and optically reconfigurable magnetism in atomically thin semiconductors.

This work established a link between magnetic and optical control in TMDs that can be utilized to directly interface magnetic solid-state memories and integrated photonics.

Reference

Hao, K., Shreiner, R., Kindseth, A., High, Alexander A. (2022). Optically controllable magnetism in atomically thin semiconductors. Science Advances, 8(39). https://www.science.org/doi/10.1126/sciadv.abq7650

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