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.

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• Signatures of bilayer Wigner crystals in a transition metal dichalcogenide heterostructure
• Observation of Wigner crystal of electrons in a monolayer semiconductor
• Direct observation of a magnetic field-induced Wigner crystal
• Competing correlated states around the zero field Wigner crystallization transition of electrons in two-dimensions
• Anomalous Hall Crystals in Rhombohedral Multilayer Graphene II: General Mechanism and a Minimal Model
• Sublattice structure and topology in spontaneously crystallized electronic states
• Observation of spontaneous valley polarization of itinerant electrons
• Anisotropic, two-dimensional, disordered Wigner solid
• Ground state phases of the two-dimension electron gas with a unified variational approach
• Interstitial-induced ferromagnetism in a two-dimensional Wigner crystal
• Nonlinear Dynamics, Avalanches and Noise for Driven Wigner Crystals
• Electronic transport, metal-insulator transition, and Wigner crystallization in transition metal dichalcogenide monolayers
• Robust Quantum Hall Ferromagnetism near a Gate-Tuned ν = 1 Landau Level Crossing
• Instability of the ferromagnetic quantum critical point in strongly interacting 2D and 3D electron gases with arbitrary spin-orbit splitting
• Dynamic Response of Wigner Crystals
• Spin-polarons and ferromagnetism in doped dilute Wigner-Mott insulators
• Nematic metal in a multi-valley electron gas: Variational Monte Carlo analysis and application to AlAs
• Pseudo-spin order of Wigner crystals in multi-valley electron gases
• 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
• Upper bound on the window of density occupied by microemulsion phases in two-dimensional electron systems
• Local noise spectroscopy of Wigner crystals in two-dimensional materials
• Discovery of an insulating ferromagnetic phase of electrons in two dimensions
• Ferromagnetic semimetal and charge-density wave phases of interacting electrons in a honeycomb moiré potential
• Chiral Wigner crystal phases induced by Berry curvature
• Spin effect on the low-temperature resistivity maximum in a strongly interacting 2D electron system
• Thermal melting of a quantum electron solid in the presence of strong disorder: Anderson versus Wigner
• Quantum melting of generalized electron crystal in twisted bilayer MoSe2
• Spin susceptibility in interacting two-dimensional semiconductors and bilayer systems at first order: Kohn anomalies and spin density wave ordering
• Critical gate distance for Wigner crystallization in the two-dimensional electron gas
• Continuous Wigner-Mott transition at $\nu =1/5$
• Microscopic Mechanism of Pair-, Charge- and Spin-Density-Wave Instabilities in Interacting D-Dimensional Fermi Liquids
• Developing fractional quantum Hall states at $\nu$ = $\frac{1}{7}$ and $\frac{2}{11}$ in the presence of significant Landau level mixing
• Electronic Crystal Phases in the Presence of Non-Uniform Berry Curvature and Tunable Berry Flux: The ${\lambda }_N$-Jellium model
• Defect liquids in a weakly imbalanced bilayer Wigner Crystal
• An unusual first order phase transition in a 2D superconductor
• Exchange Interactions of a Wigner Crystal in a Magnetic Field and Berry Curvature: Multi-Particle Tunneling through Complex Trajectories
• Quantum melting a Wigner crystal into Hall liquids
• Magnetic instability and spin-glass order beyond the Anderson-Mott transition in interacting power-law random banded matrix fermions