Abstract
The competition between kinetic energy and Coulomb interactions in electronic systems leads to complex many-body ground states with competing orders. Here we present zinc oxide-based two-dimensional electron systems as a high-mobility system to study the low-temperature phases of strongly interacting electrons. An analysis of the electronic transport provides evidence for competing correlated metallic and insulating states with varying degrees of spin polarization. Some features bear quantitative resemblance to quantum Monte Carlo simulation results, including the transition point from the paramagnetic Fermi liquid to Wigner crystal and the absence of a Stoner transition. At very low temperatures, we resolve a non-monotonic spin polarizability of electrons across the phase transition, pointing towards a low spin phase of electrons, and a two-order-of-magnitude positive magnetoresistance that is challenging to understand within traditional metallic transport paradigms. This work establishes zinc oxide as a platform for studying strongly correlated electrons in two dimensions.
Highlights
The competition between kinetic energy and Coulomb interactions in electronic systems can lead to complex many-body ground states with competing superconducting, charge density wave, and magnetic orders
Our system features an unprecedented level of agreement with the state-of-the-art Quantum Monte Carlo phase diagram of the ideal jellium model, including a Wigner crystallization transition at a value of the interaction parameter rs ∼ 30 and the absence of a pure Stoner transition
In-plane field dependence of transport reveals a new low temperature state with partial spin polarization separating the spin unpolarized metal and the Wigner crystal, which we examine against possible theoretical scenarios such as an anti-ferromagnetic crystal, Coulomb induced micro-emulsions, and disorder driven puddle formation
Summary
MgxZn1−xO layer (x ≈ 0.001) of 500 nm thickness grown on a homoepitaxial ZnO layer upon single crystal (0001) Zn-polar ZnO substrates.[32,33] The heterostructure has an electron mobility approaching 106 cm2/Vs in the metallic regime. Ohmic contacts were formed by evaporating Ti (10nm) followed by Au (50nm) on the sample surface. The 3He cell is based upon the design used in other ultra-low temperature experiments on high mobility 2DES.[34] The mixing chamber temperature is measured using a calibrated cerous magnesium nitrate paramagnetic thermometer for T ≤ 100 mK, and a ruthenium oxide thermometer for T ≥ 100 mK. Each measurement wire is passed through a large surface area (A ≈1 m2) sintered silver heat exchanger to overcome the Kapitza interfacial resistance that suppresses heat exchange at low T. The differential resistance data is obtained by measuring V -I data using DL Instruments 1211 current and 1201 voltage preamplifiers at discrete steps in the (B, T, n) parameter space, followed by differentiation using data analysis software
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