Dipolar excitons in a potential trap in a magnetic field
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ONIC PROPERTIES OF SOLID
Dipolar Excitons in a Potential Trap in a Magnetic Field A. V. Gorbunov* and V. B. Timofeev Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *email: [email protected] Received December 18, 2013
Abstract—The conditions for observing the Zeeman spin splitting compensation in an exciton Bose gas have been investigated. The magnetoluminescence of spatially indirect, dipolar excitons in a 25nmwide GaAs/AlGaAs quantum well upon their accumulation in a lateral electrostatic trap has been studied in the Faraday geometry. The critical magnetic field Bcr below which the spin (paramagnetic) splitting of the lumi nescence line for a heavyhole exciton at the trap center is almost completely compensated due to the exchange interaction in a dense Bose gas has been found to increase linearly with exciton concentration in qualitative agreement with the theory. Using a potential trap is fundamentally important. Incomplete com pensation is observed in a homogeneous photoexcitation spot for dipolar excitons: the splitting is consider ably smaller than that for a spatially direct exciton but differs noticeably from zero. The spin splitting com pensation effect is observed only under neutral charge balance conditions—there is no Zeeman splitting sup pression in a charged quantum well. DOI: 10.1134/S1063776114060119
1. INTRODUCTION Recently, we have found [1] that Zeeman splitting compensation is observed in a potential trap near the window in a Schottky gate upon the accumulation of 2D spatially indirect dipolar excitons (a 25nmwide GaAs/AlGaAs quantum well) at magnetic fields below some critical value, Bcr ≈ 2 T. The spin splitting sup pression effect was predicted theoretically for a ther modynamically equilibrium Bose condensate of 2D exciton polaritons in an optical microcavity at zero temperature [2]. This effect is related to the spinor nature of intracavity polaritons. They have the spin S = 1 with two allowed spin projections onto the struc ture growth axis, Sz = ±1, corresponding to two oppo site directions of the circular polarization, σ±. The exchange interaction in a spinor Bose condensate is assumed to be so arranged that the coparallel spins are repelled, while the antiparallel ones are attracted or repelled, but more weakly. As a result, in the absence of a magnetic field, the situation where the number of spins with the projections Sz = +1 and Sz = –1 is the same is energetically favorable. This corre sponds to a linear polarization of the emitted lumines cence light. A sharp increase in the degree of linear polarization is actually observed experimentally when the threshold of Bose condensation is exceeded in par ticle concentration (see, e.g., [3]). Obviously, all spins in a sufficiently strong perpendicular magnetic field (Faraday geometry) will be aligned with the field and will fill the lower Zeeman sublevel. However, in weak magnetic fields, as was shown in [2], no spin splitting (Zeeman effect) must be observed
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