[Show abstract][Hide abstract] ABSTRACT: Spin quantum beat spectroscopy is employed to investigate the in-plane anisotropy of the spin dynamics in (001) GaAs/AlGaAs quantum wells induced by an external electric field. This technique allows the anisotropy of the spin relaxation rate Γs and the electron Landé g factor g* to be measured simultaneously. The measurements are compared to similar data from (001) GaAs/AlGaAs quantum wells with applied shear strain and asymmetric barrier growth. All of these operations act to reduce the symmetry compared to that of a symmetric (001) quantum well in an identical manner (D2d → C2v). However, by looking at the anisotropy of both Γs and g* simultaneously we show that the microscopic actions of these symmetry breaking operations are very different. The experiments attest that although symmetry arguments are a very useful tool to identify the allowed spin dependent properties of a material system, only a microscopic approach reveals if allowed anisotropies will manifest.
[Show abstract][Hide abstract] ABSTRACT: We show experimentally, using spin quantum beat spectroscopy, that strain applied to an undoped symmetric (001) GaAs/AlGaAs multiple quantum well causes an in-plane anisotropy of the spin-relaxation rate Γs, but leaves the electron Landé g factor isotropic. The spin-relaxation-rate anisotropy gives a direct measure of the bulk inversion asymmetry and the strain contributions to the conduction-band spin splitting. The comparison of the measured strain-splitting coefficient C3 for the quantum well with the value for bulk GaAs suggests a dependence on electron quantum confinement. The isotropic g factor implies a symmetric conduction electron wave function, whereas the anisotropic spin-relaxation rate requires a nonzero expectation value of the valence-band potential gradient on the conduction-band states. Therefore, the experiment suggests that strain generates an effective valence-band potential gradient, while the conduction-band potential remains symmetrical to a good approximation.
No preview · Article · Oct 2011 · Physical review. B, Condensed matter
[Show abstract][Hide abstract] ABSTRACT: We measure simultaneously the in-plane electron g factor and spin-relaxation rate in a series of undoped inversion-asymmetric (001)-oriented GaAs/AlGaAs quantum wells by spin-quantum beat spectroscopy. In combination the two quantities reveal the absolute values of both the Rashba and the Dresselhaus coefficients and prove that the Rashba coefficient can be negligibly small despite huge conduction-band potential gradients which break the inversion symmetry. The negligible Rashba coefficient is a consequence of the “isomorphism” of conduction- and valence-band potentials in quantum systems where the asymmetry is solely produced by alloy variations.
[Show abstract][Hide abstract] ABSTRACT: We report a comparison of conduction electron spin-splitting in III-V quantum wells caused by asymmetric band edges with that due to applied electric field. Measurements in GaAs/AlGaAs quantum wells and calculations on a range of heterostructures, both symmetric and asymmetric, lead to the conclusion that in a heterostructure with nearly “isomorphous” band edges (i.e., with conduction and valence band-edge potentials related by a constant factor, exemplified by GaAs/AlGaAs) spin splittings will be unmeasurably small even in a highly asymmetric structure. Application of an external electric field or the presence of a Hartree potential gradient in the system will generally break isomorphism and therefore produce a significant spin splitting.
[Show abstract][Hide abstract] ABSTRACT: We fabricate a highly efficient colour conversion light emitting diode
consisting of surface-patterned blue emitters and semiconductor
colloidal nanocrystal quantum dots (NQDs). Electrically injected
carriers in the blue emitter (donor) are efficiently transferred to the
NQDs (acceptor) via nonradiative energy transfer in addition to
conventional radiative energy transfer. The existence of nonradiative
energy transfer is verified by the simultaneous observation of increased
donor emission decay rate, the transient transfer of carriers at the
acceptor and a 2-fold enhancement of the NQD electroluminescence.
Full-text · Article · Feb 2010 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: We report a study of the nonlinear birefringence in undoped (110)-oriented GaAs/AlGaAs quantum wells using time-resolved pump-probe Kerr spectroscopy. Due to the optical anisotropy of the (110) quantum well plane, photoexcited carriers can give rise to a nonlinear birefringence and so cause probe polarization rotation independent of the pump polarization, i.e., independent of spin orientation. We develop a methodology for accurate determination of electron-spin lifetimes using the Kerr technique which takes account of this phenomenon and present room-temperature measurements of wavelength and power density dependence of the spin-relaxation rate.
No preview · Article · Jan 2010 · Physical review. B, Condensed matter
[Show abstract][Hide abstract] ABSTRACT: For conduction electrons moving in quantum wells, a transverse electric field appears as an effective magnetic field causing spin relaxation via the Dyakonov‐Perel mechanism. For undoped samples, the strength of the effective magnetic field for a particular electric field and temperature is determined by the Rashba coefficient. By combining spin grating and relaxation
measurements, we have measured the Rashba coefficient for GaAs/AlGaAs quantum wells. There is good agreement with k.p theory at low temperature. We observe a deviation from the theoretical value at higher temperatures which could be a result of higher order terms in the Rashba spin‐orbit interaction not normally considered.
[Show abstract][Hide abstract] ABSTRACT: It is widely accepted that quantum wells with potential profiles lacking inversion symmetry will exhibit spin‐split conduction band states due to the Rashba spin‐orbit interaction. Here we measure
conduction band splitting for undoped asymmetric and symmetric GaAs/AlGaAs quantum wells in the absence of electric fields. Surprisingly the conduction band spin‐splitting measured in asymmetric quantum wells is no greater than that measured in symmetric quantum wells. This is a consequence of Ehrenfests theorem and the fact that the conduction and valence band edge profiles are related by a constant factor.
[Show abstract][Hide abstract] ABSTRACT: Asymmetric quantum well potentials are expected to produce a conduction band spin-splitting which contributes to Dyakonov–Perel
(Sov. Phys. Solid State 13:3023, 1971) spin relaxation. Much experimental work has focused on the effect of an electric field on spin dynamics (Karimov et al.,
in Phys. Rev. Lett. 91:246601, 2003) and little on asymmetry from alloy engineering. By combining time-resolved Kerr rotation measurements with transient spin
grating measurements in GaAs/AlGaAs quantum wells we have compared the conduction band spin-splitting resulting from asymmetric
alloy engineering with that from applied electric field. The latter is easily measurable, whilst the former is no greater
than that in symmetric wells. These results are consistent with an envelope function approximation model that considers the
potential profile in both the conduction and the valence bands (Winkler, in Springer Tracts in Modern Physics, vol. 191, 2003).
No preview · Article · Jan 2010 · Journal of Superconductivity and Novel Magnetism
[Show abstract][Hide abstract] ABSTRACT: A hybrid colloidal CdTe nanocrystal/bulk GaAs p-i-n heterostructure is demonstrated to have potential for highly efficient light harvesting photovoltaic devices. An array of rectangular channels is fabricated on the surface of the GaAs heterostructure penetrating through its active layer and subsequently filled with water soluble CdTe nanocrystals emitting in the near infrared. Photogenerated carriers in the highly absorbing colloidal nanocrystals are efficiently transferred by means of nonradiative energy transfer to the patterned heterostructure possessing high carrier mobility and converted to electrical current. A threefold enhancement of both photocurrent and monochromatic power conversion efficiency has been achieved.
No preview · Article · Jun 2009 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We fabricate a hybrid nanocrystal quantum-dot patterned p-i-n structure that utilizes nonradiative energy transfer from highly absorbing colloidal nanocrystal quantum dots to a patterned semiconductor slab to demonstrate a sixfold increase of the photocurrent conversion efficiency compared to the bare p-i-n semiconductor device.
No preview · Article · Mar 2009 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: To an electron moving in free space an electric field appears as a magnetic field which interacts with and can reorient the electron spin. In semiconductor quantum wells this spin-orbit interaction seems to offer the possibility of gate-voltage control in spintronic devices but, as the electrons are subject to both ion-core and macroscopic structural potentials, this over-simple picture has lead to intense debate. For example, an externally applied field acting on the envelope of the electron wavefunction determined by the macroscopic potential, underestimates the experimentally observed spin-orbit field by many orders of magnitude while the Ehrenfest theorem suggests that it should actually be zero. Here we challenge, both experimentally and theoretically, the widely held belief that any inversion asymmetry of the macroscopic potential, not only electric field, will produce a significant spin-orbit field for electrons. This conclusion has far-reaching consequences for the design of spintronic devices while illuminating important fundamental physics. Comment: 7 pages, 5 figs
[Show abstract][Hide abstract] ABSTRACT: Nonradiative energy transfer from a GaAs quantum well to a thin overlayer of an infrared organic semiconductor dye is unambiguously demonstrated. The dynamics of exciton transfer are studied in the time domain by using pump-probe spectroscopy at the donor site and fluorescence spectroscopy at the acceptor site. The effect is observed as simultaneous increase in the population decay rate at the donor and of the rise time of optical emission at the acceptor sites. The hybrid configuration under investigation provides an alternative nonradiative, noncontact pumping route to electrical carrier injection that overcomes the losses imposed by the associated low carrier mobility of organic emitters.
Full-text · Article · May 2008 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We design, fabricate and demonstrate a hybrid nanocrystal/patterned p-i-n device that utilises fluorescence energy transfer and exhibits a three-fold increase of the measured photocurrent compared to its bare p-i-n counterpart.
[Show abstract][Hide abstract] ABSTRACT: We tune the exchange interaction in nanocrystals by manipulating the electron-hole wavefunction overlap under external electric fields and simultaneously probe the electronic structure and rich transient dynamics under strong magnetic fields (8T).
[Show abstract][Hide abstract] ABSTRACT: Measurements of spin dynamics of electrons in a degenerate two dimensional electron gas, where the Dyakonov-Perel mechanism is dominant, have been used to investigate the electron scattering time (tp*) as a function of energy near the Fermi energy. Close to the Fermi energy the spin evolution is oscillatory, indicating a quasi-collision-free regime of spin dynamics. As the energy is increased a transition to exponential, collision-dominated, spin decay occurs. The frequency and the value of tp* are extracted using a Monte Carlo simulation method. At the Fermi energy tp* is very close to the ensemble momentum relaxation time (tp) obtained from the electron mobility. For higher energies tp* falls quadratically, consistent with theoretical expectations for the onset of electron-electron scattering which is inhibited by the Pauli principle at the Fermi energy.
[Show abstract][Hide abstract] ABSTRACT: We perform an all-optical spin-dynamic measurement of the Rashba spin-orbit interaction in (110)-oriented GaAs∕AlGaAs quantum wells under applied electric field. This crystallographic orientation allows us to isolate the Rashba from other contributions, giving precise values of the Rashba coefficient. At low temperature, we find good agreement between our measurements and the k⋅p theory. Unexpectedly, we observe a temperature dependence of the Rashba coefficient that may signify the importance of higher-order terms of the Rashba coupling.
No preview · Article · Mar 2008 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Optical pump-probe measurements of spin dynamics at temperatures down to 1.5K are described for a series of (001)-oriented GaAs/AlGaAs quantum well samples containing high mobility two-dimensional electron gases (2DEGs). For well widths ranging from 5to20nm and 2DEG sheet densities from 1.75×1011to3.5×1011cm-2 , the evolution of a small injected spin population is found to be a damped oscillation rather than an exponential relaxation, consistent with the quasi-collision-free regime of the Dyakonov-Perel spin dynamics. A Monte Carlo simulation method is used to extract the spin-orbit-induced electron spin precession frequency |Omega(kF)| and electron momentum scattering time taup* at the Fermi wave vector. The spin decay time passes through a minimum at a temperature corresponding to the transition from collision-free to collision-dominated regimes and taup* is found to be close to the ensemble momentum scattering time taup obtained from Hall measurements of electron mobility. The values of |Omega(kF)| give the Dresselhaus or bulk inversion asymmetry (BIA) coefficient of spin-orbit interaction as a function of electron confinement energy in the quantum wells and show, qualitatively, the behavior expected from k•p theory.
[Show abstract][Hide abstract] ABSTRACT: We perform an all-optical spin-dynamic measurement of the Rashba spin-orbit interaction in (110)-oriented GaAs/AlGaAs quantum wells. The crystallographic direction of quantum confinement allows us to disentangle the contributions to spin-orbit coupling from the structural inversion asymmetry (Rashba term) and the bulk inversion asymmetry. We observe an unexpected temperature dependence of the Rashba spin-orbit interaction strength that signifies the importance of the usually neglected higher-order terms of the Rashba coupling.