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Maximum-Density Droplet and Charge Redistributions in Quantum Dots at High Magnetic Fields

T. H. Oosterkamp, J. W. Janssen, L. P. Kouwenhoven, D. G. Austing, T. Honda, S. Tarucha

1999
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Physical Review Letters
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We have measured electron transport through a vertical quantum dot containing a tunable number (between 0 and 40) of electrons. Over a region of the magnetic field the electrons are spin polarized and occupy successive angular momentum states. This is the maximum-density-droplet ( MDD) state. The stability region where the MDD is the ground state decreases for increasing electron number. The instability of the MDD and other transitions in this high B region are accompanied by a redistribution
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... a redistribution of charge which abruptly changes the area of the electron droplet. [S0031-9007(99)08873-0] PACS numbers: 73.20. Dx, 71.45.Gm, 73.40.Hm In this Letter we study quantum dots [1, 2] in the quantum Hall regime and exploit the fact that dots contain a tunable and well-defined number of electrons. In particular, we consider the spin-polarized, maximumdensity-droplet (MDD) state at high magnetic fields that corresponds to filling factor n 1 in a large two dimensional electron gas. The stability of this state is set by a balance of forces [3-6] acting on this finite electron system, namely, the inward force of the confining potential, the repulsive force of the direct Coulomb interaction between electrons, and a binding force due to the exchange interaction. In addition, Zeeman energy and correlation effects are important. As the magnetic field and the electron number are changed, the relative strengths of these forces are tuned, which induces transitions between the MDD and other states. Earlier these transitions were measured in the linear response regime [1, 7] . We report detailed measurements in linear and nonlinear transport showing that the transitions are accompanied by a significant redistribution of charge in this many-body system. Our vertical quantum dot is made from a double barrier resonant tunneling structure with an InGaAs well, AlGaAs barriers, and n-doped GaAs source and drain contacts [8] . The device is processed in the shape of a submicron circular pillar with a diameter of 0.54 mm, and a self-aligned gate surrounds it. We discuss data taken on one particular device, but comparable results have been obtained on several devices. A magnetic field, B, is applied perpendicular to the plane in which the electrons are confined. The energy spectrum of the quantum dot is derived from transport experiments at a temperature of 100 mK in the Coulomb blockade regime. A dc source-drain voltage, V SD , is applied and the current, I, is measured versus gate voltage, V g . The electron number, N, varies from about 40 at V g 0 to N 0 at the pinch-off voltage, V g ഠ 22 V. Figure 1 shows the Coulomb current peaks versus B for N 0 to 18. On increasing V g , current peaks are measured for every extra electron that enters the dot.

doi:10.1103/physrevlett.82.2931
fatcat:qddbzg6fybbbtl2vqpvvl557dy