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Register a new userDual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System
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Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3 heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson-Schrodinger subband model. In particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. Our study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.
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Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO$_{3}$ -SrTiO$_{3}$ interfaces[1] and measure a pronounced, unexpected response. Pressure of $sim$2 GPa reversibly doubles the 2D carrier density $n_{s}$ at 4 K. Along with the increase of $n_{s}$, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure.
We investigate plasmon resonances in graphene with periodic carrier density modulation. The period is 8 um, and each period consists of 1.7- and 6.3-um-wide ribbons with different density. Using terahertz spectroscopy, we show two plasmon modes with their electric field mostly localized in the 1.7- or 6.3-um-wide ribbon arrays. We also show that plasmons are excited only in one of the micro-ribbon arrays when the Fermi energy of the other micro-ribbon array is set close to the charge neutrality point. These results indicate that plasmons can be confined by the carrier density modulation.
Electric field effect in electronic double layer transistor (EDLT) configuration with ionic liquids as the dielectric materials is a powerful means of exploring various properties in different materials. Here we demonstrate the modulation of electrical transport properties and extremely high mobility of two-dimensional electron gas at LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface through ionic liquid-assisted electric field effect. By changing the gate voltages, the depletion of charge carrier and the resultant enhancement of electron mobility up to 19380 cm$^2$/Vs are realized, leading to quantum oscillations of the conductivity at the LAO/STO interface. The present results suggest that high-mobility oxide interfaces which exhibit quantum phenomena could be obtained by ionic liquid-assisted field effect.
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 $r_ssimeq 27$, 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.
Large fluctuations of conductivity with time are observed in a low-mobility two-dimensional electron system in silicon at low electron densities $n_s$ and temperatures. A dramatic increase of the noise power ($propto 1/f^{alpha}$) as $n_s$ is reduced below a certain density $n_g$, and a sharp jump of $alpha$ at $n_sapprox n_g$, are attributed to the freezing of the electron glass at $n_s = n_g$. The data strongly suggest that glassy dynamics persists in the metallic phase.
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