Electron Spin Coherence of Phosphorus Donors in Isotopically Purified <Superscript>29</Superscript>Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Kinetic Theory of Spin Coherence of Electrons in Semiconductors

We investigate the spin precession and dephasing of electrons in semiconductors, using the kinetic Bloch equations for a four-spin-band model. Various scatterings, such as carrier–carrier, carrier–phonon, and carrier–impurity scatterings are taken into account. Their contributions to the evolution of carrier distribution, optical coherence, and more importantly, spin coherence in undoped semiconductor quantum wells and in n-type bulk semiconductors are considered. We highlight the importance of the kinetic theory approach by comparing our results with those from Fermi's golden rule. Furthermore, a new spin dephasing mechanism is proposed, that is, spin dephasing due to spin conserving scatterings in the presence of inhomogeneous broadening.     

Pulse Electron Spin Resonance Studies of H and D Atoms in Impurity-Helium Solids

We have performed measurements of the two-pulse ESEEM (electron spin echo envelope modulation) spectra of H and D atoms within impurity-helium solids. The local environments of the atoms were determined from the modulation of the ESEEM signal by neighboring D₂ and HD molecules. We have measured changes in the atom environments due to coalescence of the nanoclusters within the impurity-helium solids and due to the tunneling exchange chemical reaction D+HD→H+D₂.      

Carrier-Density Dependence of Faraday Rotation and Spin Splitting in Cd1?xMnxTe

We employ spin-quantum-beat spectroscopy to investigate the carrier-density dependence of the spin-precession frequency and the magnitude of the Faraday rotation of Cd_1–x Mn _x Te samples at fixed magnetic field. We find an onset of saturation of the Faraday rotation at carrier densities as low as 4× 10¹⁶ cm^–3 and attribute it to electrons (not holes which dominate in other types of experiments). The spin splitting at fixed magnetic field remains density dependent down to 3 × 10¹⁵ cm^–3 (the lowest density accessible in our measurements) which suggests a direct influence of the carrier-density on the sp–d exchange not mediated by screening effects.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Hanle Effect Measurements of Spin Lifetime in Zn0.4Cd0.6Se Epilayers Grown on InP

We use the Hanle effect to study spin relaxation in Zn_xCd_1–xSe epilayers grown on lattice-matched InP substrates. We study three samples with a fixed composition (x = 0.4) and with varying levels of n-doping, as well as an undoped sample with x = 0.5. Our measurements show that the spin relaxation time changes non-monotonically as a function of carrier density, with a maximum transverse spin lifetime of ~10.5 ns at low temperatures for a sample doped near the metal–insulator transition.     

Zero-Magnetic-Field Spin Splitting of Conduction Subbands in InGaAs/InAlAs and GaAs/AlGaAs Quantum Wells

Zero-magnetic-field spin splitting in InGaAs/GaAs and GaAs/AlGaAs multiple quantum wells was investigated theoretically. The sp³s* empirical tight-binding method has been employed. It has been found that the splitting is much larger in InGaAs wells than that in GaAs wells. The origin of the splitting due to the structure inversion asymmetry was briefly discussed.     

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T ₁ ^–1) and the echo-decay rate (T ₂ ^–1) of ⁶³Cu were measured for the electron-doped infinite-layer superconductor Sr_0.93La_0.07CuO₂/T _c ^onset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-T _c material. Above T _c the ⁶³Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the ⁶³Cu Knight shift in the electron-doped CuO₂ layer can be understood as a compensation effect between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4p_z) and/or Cu(3d_z ²) orbitals.