Interaction between the exciton and nuclear spin systems in a self-organized ensemble of InP/InGaP size-quantized islands

Magnetic interaction between spin-polarized nuclei and optically oriented excitons in a self-organized ensemble of size-quantized InP islands in an InGaP matrix has been studied in a magnetic field in Faraday geometry. The effective magnetic fields generated by polarized nuclei at excitons have been measured. The strengths of these fields were found to be different for active and inactive excitons because of the difference between the excitonic g factors. The heavy-hole g factor has been determined. The active and inactive excitonic states were found to be coupled through cross-relaxation. Fiz. Tverd. Tela (St. Petersburg) 41, 2193–2199 (December 1999)     

Optical orientation of donor-bound excitons in nanosized InP/InGaP islands

Spin splitting of optically active and inactive excitons in nanosized n-InP/InGaP islands has been revealed. Optically inactive states become manifest in polarized-luminescence spectra as a result of excitons being bound to neutral donors (or of the formation of the trion, a negatively charged exciton) in InP islands. The exchange-splitting energies of the optically active and inactive states have been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1745–1752 (September 1998)     

Dynamic polarization of 7Li nuclei in solutions containing radical ions

Dynamic nuclear polarization parameters, obtained at 75 G, are reported for ⁷Li ions in collision with several radical anions and with one radical cation. All systems show large negative ⁷Li N.M.R. enhancements indicative of weak scalar relaxation. However, radical induced relaxation rates derived from ⁷Li T ₁ measurements suggest stronger complexing of lithium ions with radical anions than with the radical cation as would be expected from simple coulombic considerations. Translational modulation of the dipolar interaction best accounts for proton and radical cation dipolar relaxation rates while rotational modulation best accounts for the corresponding radical anion rates; this supports the interpretation above. A model for the lithium radical collision is proposed which implies that, for radical anions, scalar coupling is only apparently weak and that the low ⁷Li scalar relaxation rates observed result from scalar correlation times (τ_c = 10^-8-10^-9 s) longer than any yet observed by this technique. The model predicts that, for certain ranges of τ_c, increasing strength of complex formation should lead to smaller scalar relaxation rates and more negative enhancements, in contrast with the behaviour of fluorocarbons where the reverse was true. The predicted dependence of enhancement upon τ_c also suggests that ⁷Li enhancements should be extremely sensitive to variations in the chemical properties of the system.     

Perfect alignment of self-organized Ge islands on pre-grown Si stripe mesas

Self-organized Ge islands grown on patterned Si(001) substrates have been investigated. Selective epitaxial growth (SEG) of Si is carried out with gas-source molecular beam epitaxy to form Si stripe mesas followed by subsequent Ge island growth. Self-aligned Ge islands with regular spacing are formed on the <110>-oriented ridges of the Si mesas. The regular spacing is driven by the repulsive interaction between the neighbor islands through the substrates. A mono-modal distribution of the islands has been observed on the ridges of the Si mesas. The spatial confinement as well as the preferential nucleation is believed to be the mechanism of this alignment of the self-organized Ge islands. Received: 16 July 1999 / Accepted: 6 August 1999 / Published online: 24 March 2000     

Dynamical polarization of radioactive nuclei

In order to polarize radioactive nuclei implanted in a highly polarized protonic target, it is proposed to use methods for the dynamical orientation of nuclei. The angular distribution of photons that originate from the cascade beta-gamma decay of the ²²Na(3⁺) state in a strong magnetic field is calculated. It is shown that, if the populations of Zeeman magnetic sublevels obey the Boltzmann distribution, the angular distribution of emitted photons is independent of the sign of spin temperature; at the same time, the tensor polarization of quadrupole nuclei occurring in the intrinsic field of a crystal causes a strong dependence of the anisotropy on the sign of spin temperature. A rich potential of a dynamical orientation for studying the magnetic structure of rare nuclei and the dynamics of their spin-spin interactions in dielectric host materials is demonstrated. Physical and technological advantages and disadvantages of the method in the on-line regime of the implantation of heavy nuclei are discussed.     

Field-asymmetric transverse magnetoresistance in a nonmagnetic quantum-size structure

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layers — a transverse magnetoresistance which is asymmetric in the field (there can even be a channge in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 380–385 (10 September 1998)     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

冷原子系综中两正交光场偏振模高效率存储的实验研究

高效率光量子信息存储是可扩展光量子信息处理的一个重要工具.本文对一个冷原子系综中两正交光场偏振模的高效率存储进行了实验研究.通过在雪茄型冷原子系统上施加一个中等强度的磁场,消除了原子Zeeman子能级的简并性,从而使磁敏感自旋波从电磁感应透明系统中被移出,由此完成了两正交光场偏振模高效率、长寿命的量子存储.实验测量了两偏振模存储效率与存储时间以及实验重复频率的关系,结果表明,随着重复频率的增加,存储效率逐渐降低,在10 Hz时,测量得到两偏振模存储效率为30%,同时存储寿命达到3 ms.测量结果为偏振纠缠在冷原子系统中的存储提供了重要的实验基础.     

Core polarization for l-forbidden M1 transitions in light nuclei

The experimental systematics of $\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?}\text{ld}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{1-}\text{forbidden}$ M1 transition rates in light nuclei with N, Z ? 20 are studied. Some surprising deviations from the expected behavior are qualitatively discussed in terms of an interplay between first- and second-order core polarization contributions. The B(M1) values in mirror pairs turn out to be close to each other within the experimental errors, the proton transition strengths being slightly larger. Isovector dominance is shown to account for this phenomenon.     

X-ray diffraction analysis of quantum-size ZnMgSSe/ZnSe/ZnMgSSe nanostructures

Quantum-size ZnMgSSe/ZnSe/ZnMgSSe nanostructures were studied by X-ray diffraction analysis. It was found that the ZnMgSSe solid solution decomposes into two phases with close lattice parameters. As the lattice-parameter mismatch between zinc selenide and barrier layers increases, the decomposition is accompanied by simultaneous broadening of peaks of each phase. It was found that almost pure ZnSe grows in the first stages of metastable barrier layer growth, thus thickening the well (ZnSe).     

Nuclear polarization in muonic atoms of deformed nuclei

The treatment of nuclear polarization correction in muonic atoms belonging to deformed nuclei is analyzed. The geometrical factors involved are expanded into a series of multipoles and the exact expansion coefficients are calculated. It is shown that, using reasonable assumptions about the nuclear spectrum, the nuclear polarization correction may be expressed as a shift of all hyperfine components plus a renormalization of the even multipole hyperfine interaction constants. All nuclear excited states contribute to the shift, but the ground-state rotational band gives an over-whelming contribution to the multipole moment renormalization. The effect of the ground-state band is analyzed in detail. The radial coefficients are calculated and an approximate formula, applicable over a broad range of atomic numbers and deformations, is obtained. By comparing our results with exact calculations we conclude that this part of the nuclear polarization correction may be calculated with accuracy better than 10 %.     

Production of nuclear polarization of unstable nuclei via polarization transfer reactions

To determine experimental conditions suitable for isotope analysis, we studied the plume dynamics of uranium. A uranium oxide sample was ablated by 2nd harmonic radiation from a Nd:YAG laser at a fluence of 0.5 J/cm². The temporal evolution of the ablation plume was investigated in 800 Pa helium environment. It was found that the observation at 3–5 μs after the ablation at the height of about 2.5 mm are most suited for obtaining higher sensitivity. Using the established conditions, we obtained the limit of detection of the isotope ratio (²³⁵U/²³⁸U) to be 0.01 %. In addition, the limit of detection of elemental abundance of uranium in uranium glass was also evaluated.     

Simulation of the indium profile in InGaAs/GaAs quantum-size heterostructures

A new approach to describe phenomena attendant on the growth of thin InGaAs epitaxial layers by hydride MOCVD in terms of which the boundary gas layer is considered as quasi-liquid is suggested. A numerical model for simulating the concentration profiles of the components in quantum-well heterostructures is developed. It is based on the assumption that a state close to thermodynamic equilibrium exists near the interface. The concentration profiles are simulated by jointly solving equations that describe heterogeneous equilibria and material balance at the interface. The indium profiles in InGaAs/GaAs quantum-size heterostructures are simulated at various parameters of the epitaxy process, such as temperature, initial component ratio in the gas phase, and boundary layer thickness. The results obtained agree well with the available experimental data.     

Tilted foil polarization of radioactive beam nuclei

Tilted foil polarization has up to now been mostly applied to nuclear reaction products recoiling out of a target traversed by a primary particle beam. Being a universal phenomenon it can be applied equally well to beams of particles, primary or secondary, radioactive or other. There are however some technical considerations arising from the nature of the beam particles. Radioactive beams are associated with ground state nuclei. They usually have low nuclear spin and as a consequence-as will be shown later-low polarization. Secondary beams are usually low in intensity and do not impose any constraints on the foils they traverse; unlike intense primary heavy ion beams which, if they traverse the foils, essentially limit the foil material to carbon. We review here briefly the tilted foil polarization process and then discuss an experiment with an isomer beam. Finally we review experiments with radioactive beams, past, present and planned for the future.     

Solid4He as an ensemble of superradiating nuclei

Summary We explore the possibility that the crystalline⁴He state is related to the onset of ?superradiance? (coherent oscillation in phase with particular classical modes of the electromagnetic field) for the ensemble of⁴He nuclei. Working at zero-temperature we find evidence for this phenomenon and compute the correct liquid-solid transition pressure and the correct energetics.