M. Vladimirova

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)

Measurements of nuclear spin dynamics by spin-noise spectroscopy

We exploit the potential of the spin noise spectroscopy (SNS) for studies of nuclear spin dynamics in n-GaAs. The SNS experiments were performed on bulk n-type GaAs layers embedded into a high-finesse microcavity at negative detuning. In our experiments, nuclear spin polarisation initially prepared by optical pumping is monitored in real time via a shift of the peak position in the electron spin noise spectrum. We demonstrate that this shift is a direct measure of the Overhauser field acting on the electron spin. The dynamics of nuclear spin is shown to be strongly dependent on the electron concentration.

Ballistic spin transport in exciton gases

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

Nonlinear Optical Probe of Indirect Excitons

Studies of spatially indirect excitons (IX) in semiconductors have attracted considerable research efforts since early 1960’s, fueled by the prediction of the remarkable quantum properties. This resulted in recent demonstration of quantum coherent effects including spontaneous coherence [1–3], long-range spin currents and associated polarization textures [4, 5] of indirect excitons. An IX can be formed by an electron and a hole confined in separate coupled quantum wells (CQW). Application of the electric field across the CQWs bends the band structure so that the IX state became the ground state of the system [6, 7]. The spatial separation of electrons and holes within IX allows them to achieve long lifetimes, which may be orders of magnitude longer than the lifetimes of spatially direct excitons (DX). At the same time, the spatial separation of electrons and holes strongly reduces the oscillator strength of IXs,with respect to DXs. This determines the choice of the experimental methods for studies of these quasiparticles. The most frequently used optical methods are based on the emission (photoluminescence, PL) spectroscopy. The PL signal scales linearly with the emission rate in time-resolved experiments and is nearly independent on the emission rate in cw experiments for the samples with low nonradiative recombination. A set of the linear optics methods was employed for studies of IXs, including the imaging spectroscopy [8], the time-resolved imaging [9], the polarization-resolved imaging [4], and the first-order coherence measurements [1–3]. However, the powerful methods of nonlinear optics, which have been successfully applied for DXs in quantum wells (QW) [10], [11] remain unexplored in the studies of IXs. A nonlinear optical process, in its broadest definition, is a process in which the optical properties of the medium depend on the light field itself [12]. In the case of optical pumping in semiconductors, light-induced modifications of the optical properties of the medium can persist for a long time after the perturbing light is turned off. In this case, a pump-probe arrangement can be used, with pump and probe interactions separated in time [13]. This allows for time-resolved studies of optical and spin coherence in the medium. In semiconductor QWs resonant optical pumping of DX resonance with circularly polarized light, and subsequent detection of the pump-induced dispersive response is widely used to study exciton population and spin dynamics [14]. Experimentally, either modification of intensity (photoinduced reflectivity) or the rotation of the polarization plane of the linearly polarized probe pulse (photoinduced Kerr rotation) upon reflection from the sample are measured [15]. These signals are proportional to the square of the oscillator strength of the excitonic transition and have a pronounced resonant character [12]. Thus, because the oscillator strength of IX is orders of magnitude lower than for DX, it is impossible to simply transpose the ideas developed for nonlinear spectroscopy of DX to IX. In this paper, we show how IXs, despite their vanishing oscillator strengths, can induce measurable photoinduced reflectivity and Kerr rotation. Our proposal relies on two peculiar properties of the CQW structures. The first essential property is the spin conserving tunneling of electrons between the QWs [16]. It allows for substantial spin polarization of IX via optical orientation of DXs. This has been unambiguously demonstrated by polarization-resolved photoluminescence experiments [17]. Thus, optical pumping of IXs can de realized via the DX state. The second imporant effect is the spindependent coupling between DX and IX states. This coupling is quite strong in CQW, where each IX and each DX have either holes or electrons located in the same QW. This is why the presence of IX population in the structure alters DX resonance properties, mainly

Influence of magnetic quantum confined Stark effect on the spin lifetime of indirect excitons

We report on the unusual and counter-intuitive behaviour of spin lifetime of excitons in coupled semiconductor quantum wells (CQWs) in the presence of in-plane magnetic field. Instead of conventional acceleration of spin relaxation due to the Larmor precession of electron and hole spins we observe a strong increase of the spin relaxation time at low magnetic fields followed by saturation and decrease at higher fields. We argue that this non-monotonic spin relaxation dynamics is a fingerprint of the magnetic quantum confined Stark effect. In the presence of electric field along the CQW growth axis, an applied magnetic field efficiently suppresses the exciton spin coherence, due to inhomogeneous broadening of the

Spin waves in magnetic quantum wells with Coulomb interaction andsdexchange coupling

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Transition from spin-orbit to hyperfine interaction dominated spin relaxation in a cold fluid of dipolar excitons

-factor distribution.

Nuclear spin relaxation inn-GaAs: From insulating to metallic regime

We theoretically describe the spin excitation spectrum of a two dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work, we introduce equations that allow to consider the interplay between the Coulomb interaction of delocalized electrons and the

Spin temperature concept verified by optical magnetometry of nuclear spins

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Anomalously large spin susceptibility enhancement in n-doped CdMnTe quantum wells

exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found : mixed waves propagate below the single particle continuum, an anticrossing gap is open at a specific wavevector and the kinetic damping due to the electron motion strongly influences the coupling strength between electrons and impurities spins.