Singly quantized vortices have already been observed in many systems, including the superfluid helium, Bose-Einstein condensates of dilute atomic gases, and condensates of exciton-polaritons in the solid state. Two-dimensional superfluids carrying spin are expected to demonstrate a different type of elementary excitations referred to as half-quantum vortices, characterized by a p rotation of the phase and a p rotation of the polarization vector when circumventing the vortex core. We detect half-quantum vortices in an exciton-polariton condensate by means of polarization-resolved interferometry, real-space spectroscopy, and phase imaging. Half-quantum vortices coexist with single-quantum vortices in our sample.
Spin currents and spin textures are observed in a coherent gas of indirect excitons. Applied magnetic fields bend the spin current trajectories and transform patterns of linear polarization from helical to spiral and patterns of circular polarization from four-leaf to bell-like-with-inversion.Studies of electron spin currents in semiconductors led to the discoveries of the spin Hall effect [1][2][3][4][5][6], persistent spin helix [7], and spin drift, diffusion, and drag [8][9][10]. There is also considerable interest in developing semiconductor (opto)electronic devices based on spin currents. An important role in spin current phenomena is played by spin-orbit (SO) coupling. It is the origin of the spin Hall effect and persistent spin helix. It also creates spin structures with the spin vector perpendicular to the momentum of the electrons in metals [11] and topological insulators [12][13][14]. While phenomena caused by SO coupling are ubiquitous in fermionic systems, they have yet to be explored in bosonic matter. Available experimental data for bosons include the optical spin Hall effect in photonic systems [10,16,17] and spin patterns in atomic condensates [18,19]. Here, we report the observation of spin currents and associated rich variety of polarization patterns in a coherent gas of indirect excitons. Applied magnetic fields bend the spin current trajectories and transform patterns of linear polarization from helical to spiral and patterns of circular polarization from four-leaf to bell-like-with-inversion. We also present a theory of exciton transport with spin precession that reproduces the observed exciton polarization patterns and indicates trajectories of spin currents.Excitons -bound pairs of electrons and holes -form a model system to study spin currents of bosons [20]. SO coupling for an exciton originates from the combined Dresselhaus and Rashba effects for the electron and the hole [21][22][23]. An indirect exciton can be formed by an electron and a hole confined in separate quantum-well (QW) layers arXiv:1302.3852v1 [cond-mat.mes-hall]
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We observe anisotropy in the polarization flux generated in a GaAs/AlAs photonic cavity by optical illumination, equivalent to spin currents in strongly coupled microcavities. Polarization rotation of the scattered photons around the Rayleigh ring is due to the TE-TM splitting of the cavity mode. Resolving the circular polarization components of the transmission reveals a separation of the polarization flux in momentum space. These observations constitute the optical analogue of the spin Hall effect.
We report on observation of the spin transport of spatially indirect excitons in GaAs/AlGaAs coupled quantum wells (CQW). Exciton spin transport over substantial distances, up to several micrometers in the present work, is achieved due to orders of magnitude enhancement of the exciton spin relaxation time in CQW with respect to conventional quantum wells.Spin physics in semiconductors includes a number of interesting phenomena in electron transport, such as currentinduced spin orientation (the spin Hall effect), 1-3 spininduced contribution to the current, 4 spin injection, 5 and spin diffusion and drag. [6][7][8][9][10][11] Besides the fundamental spin physics, there is also considerable interest in developing semiconductor electronic devices based on the spin transport, which may offer advantages in dissipation, size, and speed over chargebased devices; see ref 12 and references therein.Optical methods have been used as a tool for precise injection, probe, and control of electron spin via photon polarization in semiconductors. A major role in the optical properties of semiconductors near the fundamental absorption edge is played by excitons. The spin dynamics of excitons in GaAs single quantum wells (QW) was extensively studied in the past; see refs 13-15 and references therein. It was found that the spin relaxation time of excitons in single QW is of the order of a few tens of picoseconds. Because of the short spin relaxation time, no spin transport of excitons was observed until this work.Here, we report on the observation of the spin transport of spatially indirect excitons in GaAs coupled quantum wells (CQW). The spin relaxation time of indirect excitons is orders of magnitude longer than one of regular direct excitons. In combination with a long lifetime of indirect excitons, this makes possible spin transport of indirect excitons over substantial distances.The spin dynamics of excitons can be probed by the polarization resolved spectroscopy. In GaAs QW structures, the σ + (σ -) polarized light propagating along the z axis creates a heavy hole exciton with the electron spin state s z ) -1/2(s z ) +1/2) and hole spin state m h ) +3/ 2(m h ) -3/2). In turn, heavy hole excitons with S z ) +1 (-1) emit σ + (σ -) polarized light. Excitons with S z ) (2 are optically inactive. The polarization of the exciton emission P ) (I + -I -)/(I + + I -) is determined by the recombination and spin relaxation processes. For an optically active heavy hole exciton, an electron or hole spin-flip transforms the exciton to an optically inactive state ( Figure 1a) causing no decay of emission polarization. The polarization of emission decays only when both the electron and hole flip their spins. This can occur in the two-step process due to the separate electron and hole spin flips and the singlestep process due to the exciton spin flip. [13][14][15] The rate equations describing these processes 14,15 yield for the case when the splitting between S z ) (1 and (2 states ∆ is smaller than k B T the polarization of the exc...
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