Experimental study of two-dimensional quantum Wigner solid in zero magnetic field

Abstract

We report the first experimental observation of a characteristic nonlinear threshold behavior from dc dynamical response as an evidence for a Wigner crystallization in high-purity GaAs 2D hole systems in zero magnetic field. The system under increasing current drive exhibits voltage oscillations with negative differential resistance. They confirm the coexistence of a moving crystal along with striped edge states as observed for electrons on helium surfaces. However, the threshold is well below the typical classical levels due to a different pinning and depinning mechanism that is possibly related to a quantum process.In strongly correlated many electron systems, remarkable manifestations of quantum physics emerge in response to strong inter-particle Coulomb energy (E C ). The Fractional Quantum Hall (FQH) [1] state (with filling factors of odd and even denominators) and Mott insulators [2] are good examples. The most prominent interaction-driven effect is the Wigner crystallization (WC) [3] of electrons which is a solid phase of spatially separated charges settling in a form of a lattice. Such a fascinating quantum state of matter (with spin ordering) can be utilized for futuristic applications such as quantum electronics and spintronics. The classical version of the crystallization, with the Debye temperature Θ D < E C , has been demonstrated with 2D electrons on helium surfaces (EHS) [4,5]. On the other hand, the more desired quantum version with the Fermi energy E F ≪ E C ≪ Θ D has not been previously observed in 2D systems in zero magnetic field. This letter includes experimental evidence of a quantum WC obtained via an ultra-sensitive dc transport study.The nonlinear transport behaviors [6] in the absence of the significant effect of heating, as demonstrated in both WC in EHS and the charge density waves (CDW), are evidence for the sliding of spatially ordered charges under bias. Specifically, the threshold/switching behaviors accompanied by resistivity oscillations highlights a characteristic signature for the pinning and depinning of a classical WC [5]. However, in order to capture a very delicate quantum WC, much more stringent requirements have to be met with respect to both the high-resolution of dc-VI transport measurement and, especially, the high-purity of the systems. First, interaction effect must be dominant which means the interaction parameter r s = a/a B , a ratio of the Coulomb energy (E C = e 2 /(ǫ · 2a) and the Fermi energy (E F = (π 2 /m * )p), has to be at least 37 [7]. In a zero magnetic field, it can only be realized * email:jianhuang@wayne.edu with very dilute charge concentrations which is difficult to achieve in bulk materials. a = 1/ √ πp is half of the average charge spacing, a B = 2 ǫ/m * e 2 -Bohr radius, and p-charge density.Second, the dilute charge density means an enormous average carrier separation, ∝ n −1/2 , which, in the presence of a usual level of disorder, exceeds the singleparticle localization length ξ. Consequently, interaction effects are overshadowed by the single...