We realized a potential energy gradient -a ramp -for indirect excitons using a shaped electrode at constant voltage. We studied transport of indirect excitons along the ramp and observed that the exciton transport distance increases with increasing density and temperature.An indirect exciton in a coupled quantum well structure (CQW) is a bound state of an electron and a hole in separate wells (Fig. 1a). The spatial separation allows one to control the overlap of electron and hole wavefunctions and engineer structures with lifetimes of indirect excitons orders of magnitude longer than those of direct excitons. Long lifetimes of the indirect excitons allow them to travel over large distances before recombination [1][2][3][4][5][6][7][8][9][10] . Furthermore, indirect excitons have a built-in dipole moment ed, where d is close to the distance between the quantum well (QW) centers that allows their energy to be controlled by voltage: an electric field F z perpendicular to the QW plane results in the exciton energy shift E = edF z 11 . These properties allow studying transport of indirect excitons in electrostatically created in-plane potential landscapes E(x, y) = edF z (x, y).Exciton transport was studied in various electrostatic potential landscapes including circuit devices 12-14 , traps 15 , lattices 16,17 , moving lattices -conveyerscreated by a set of ac voltages 18 , and narrow channels 14,19,20 .Several exciton transport phenomena have been observed, including the inner ring in emission patterns 4,6,8,10,21,22 , transistor effect for excitons [12][13][14] , localization-delocalization transition in random potentials 4,6,8 and in lattices 16,17 , and dynamical localization-delocalization transition in conveyers 18 . Exciton transport was also studied in potential energy gradients created by voltage gradients in electrodes 1,7 .In this work, we study exciton transport in a potential energy gradient -a ramp -created by a shaped electrode at constant voltage. We utilize the ability to control exciton energy by electrode shape 23 and design the shape of a top electrode on the sample surface so that a voltage applied to it creates a constant potential energy gradient for indirect excitons in the CQW. The excitonic ramp realizes directed transport of excitons as a diode realizes directed transport of electrons.The advantages of this shaped-electrode-method include the suppression of heating by electric currents in electrodes (such currents may appear in the case when the ramp potential is created by a voltage gradient in the top electrode) and the opportunity to engineer the exciton energy profile along the ramp by designing the electrode shape. We also measure exciton transport in a narrow channel formed by a voltage applied to an electrode stripe of constant width -a flat-energy channel 14,20 .The CQW structure is grown by molecular beam epitaxy. An n + -GaAs layer with n Si = 10 18 cm 3 serves as a homogeneous bottom electrode. A semitransparent top electrode is fabricated by depositing a 100 nm indium tin oxide...

We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions. V

We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions. V C 2016 AIP Publishing LLC. 1-12 Due to the spatial separation, the IXs also acquire a built-in dipole moment ed, where d is the approximate distance between the QW centers. The dipole moment can be explored to control the IX energy: an electric field F z applied perpendicular to the QW plane shifts the IX energy by E ¼ ÀedF z .13 These properties are advantageous for creating excitonic devices and studying the transport of IXs in electrostatic in-plane potential landscapes Eðx; yÞ ¼ ÀedF z ðx; yÞ.IX transport has been studied in various potential landscapes that were created by laterally modulated voltage V(x, y). These potential landscapes include ramps, 1,6,14 An alternative method for creating potential landscapes for IXs was proposed in Ref. 34, which is based on the lateral modulation of the electrode density rather than voltage. Divergence of the electric field around the periphery of an electrode reduces the normal component F z . This enables one to control F z , and thereby the potential energy landscape for IXs, by adjusting only the electrode density while keeping the entire device at a constant, uniform voltage. This method is beneficial compared to the voltage modulation method because it does not require an energy dissipating voltage gradient. One instance of this method based on varying the electrode width-the shaped electrode methodhas been used to create confining potentials for IXs in traps 18 and ramps. 14,24 In this work, we present an excitonic device based on the electrode density modulation in which a potential energy gradient is created by a perforated electrode at a constant

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