Sen Yang

Excitons in electrostatic traps

We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively confine excitons in the electrostatic traps. We present calculations for various classes of electrostatic traps and experimental proof of principle for trapping of indirect excitons in electrostatic traps.

Coherence Length of Cold Exciton Gases in Coupled Quantum Wells

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.

Origin of the inner ring in photoluminescence patterns of quantum well excitons

In order to explain and model the inner ring in photoluminescence (PL) patterns of indirect excitons in GaAs/AlGaAs quantum wells (QWs), we develop a microscopic approach formulated in terms of coupled nonlinear equations for the diffusion, thermalization and optical decay of the particles. The origin of the inner ring is unambiguously identified: it is due to cooling of indirect excitons in their propagation from the excitation spot.

Spin Currents in a Coherent Exciton Gas

We report the observation of spin currents in a coherent gas of indirect excitons. The realized long-range spin currents originate from the formation of a coherent gas of bosonic pairs--a new mechanism to suppress the spin relaxation. The spin currents result in the appearance of a variety of polarization patterns, including helical patterns, four-leaf patterns, spiral patterns, bell patterns, and periodic patterns. We demonstrate control of the spin currents by a magnetic field. We also present a theory of coherent exciton spin transport that describes the observed exciton polarization patterns and indicates the trajectories of the spin currents.A.A. High, A.T. Hammack, J.R. Leonard, Sen Yang, L.V. Butov, T. Ostatnický, A.V. Kavokin, and A.C. Gossard Department of Physics, University of California at San Diego, La Jolla, CA 92093-0319, USA Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic School of Physics and Astronomy, University of Southampton, SO17 1BJ, Southampton, United Kingdom Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106-5050, USA (Dated: January 13, 2013)

Spin transport of excitons.

Spin transport of indirect excitons in GaAs/AlGaAs coupled quantum wells was observed by measuring the spatially resolved circular polarization of the exciton emission. The exciton spin transport originates from the long spin relaxation time and long lifetime of the indirect excitons.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.

Effect of spatial resolution on the estimates of the coherence length of excitons in quantum wells

We evaluate the effect of diffraction-limited resolution of the optical system on the estimates of the coherence length of two-dimensional excitons deduced from the interferometric study of the exciton emission. The results are applied for refining our earlier estimates of the coherence length of a cold gas of indirect excitons in coupled quantum wells [S. Yang et al., Phys. Rev. Lett. 97, 187402 (2006)]. We show that the apparent coherence length is well approximated by the quadratic sum of the actual exciton coherence length and the diffraction correction given by the conventional Abbe limit divided by

Exciton front propagation in photoexcited GaAs quantum wells

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Repulsive interaction in the macroscopically ordered exciton state in GaAs ∕ Al x Ga 1 − x As coupled quantum well structures

. In practice, accounting for diffraction is necessary only when the coherence length is smaller than about one wavelength. The earlier conclusions regarding the strong enhancement of the exciton coherence length at low temperatures remain intact.

Fluctuation and commensurability effect of exciton density wave

Sen Yang, L. V. Butov, L. S. Levitov, B. D. Simons, A. C. Gossard Department of Physics, University of California at San Diego, La Jolla, CA 92093-0319, USA Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, United Kingdom and Materials Department, University of California at Santa Barbara, Santa Barbara, CA 93106-5050, USA (Dated: August 20, 2009)

Spontaneous coherence, interaction and kinetics of macroscopically ordered exciton state

The macroscopically ordered exciton state (MOES) - a periodic array of beads with spatial order on a macroscopic length - appears in the external exciton rings at low temperatures below a few Kelvin. Here, we report on the experimental study of the interaction in the MOES. The exciton PL energy varies in concert with the intensity along the circumference of the ring, with the largest energy found in the brightest regions. This shows that the MOES is characterized by the repulsive interaction and is not driven by the attractive interaction.