Observation of Spontaneous Ferromagnetism in a Two-Dimensional Electron System

Abstract

What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high-quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally-clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter $r_s$, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when $r_s$ surpasses 35, and then a phase with strongly non-linear current-voltage characteristics, suggestive of a pinned Wigner solid, when $r_s$ exceeds $\simeq 38$. However, our sample makes a transition to an insulating state at $r_s\simeq 27$, preceding the onset of the spontaneous ferromagnetism, implying that, besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.

References in corpus (9)

• Metal-insulator transition in two-dimensional electron systems
• Phase Diagram of the Low-Density Two-Dimensional Homogeneous Electron Gas
• Melting of a 2D Quantum Electron Solid in High Magnetic Field
• NMR profiling of quantum electron solids in high magnetic fields
• Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom
• Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems
• Contrast between spin and valley degrees of freedom
• Spin Susceptibility of Interacting Two-dimensional Electrons with Anisotropic Effective Mass
• Dependence of Effective Mass on Spin and Valley Degrees of Freedom

Cited by in corpus (7)

• Anisotropic, two-dimensional, disordered Wigner solid
• Interstitial-induced ferromagnetism in a two-dimensional Wigner crystal
• Robust Quantum Hall Ferromagnetism near a Gate-Tuned ν = 1 Landau Level Crossing
• Signatures of correlated defects in an ultra-clean Wigner crystal in the extreme quantum limit
• Thin films as practical quantum materials: a status quo and beyond
• Discovery of an insulating ferromagnetic phase of electrons in two dimensions
• Developing fractional quantum Hall states at $\nu$ = $\frac{1}{7}$ and $\frac{2}{11}$ in the presence of significant Landau level mixing