Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

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.     

Polarized neutron study of TbNi2

Neutron diffraction experiments have been carried out on a TbNi₂ single crystal. Below the Curie temperature, 42 K, a magnetic contribution is observed only on nuclear scattering peaks. Therefore, the terbium atoms form a ferromagnetic structure. Polarized neutron measurements performed in the paramagnetic state, in an applied magnetic field of 57 kOe, reveal a non-uniform polarization of the conduction band. Within the experimental accuracy, no 3d magnetic moment is observed on nickel atoms. This result is consistent with the assumption of rare earth magnetic ordering occurring through the polarization of conduction electrons.     

Nuclear spin switch in semiconductor quantum dots

We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 T can be generated by varying the pumping intensity. The excitation power threshold for such a nuclear spin "switch" is found to depend on both the external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of the spin transfer from electrons to the nuclei of the dot.     

Optical orientation of nuclei in nitrogen alloys GaAsN at room temperature

The intensity and giant circular polarization of edge luminescence in a longitudinal magnetic field have been measured in nitrogen alloys GaAsN under circularly polarized pumping. It has been found that these dependences are shifted with respect to zero field by a value B _eff. The magnitude of the internal field B _eff increases with the pumping intensity and reaches saturation (≈250 G) at large excitation densities. The saturation of the B _eff field with growth of pumping indicates that this is a field of nuclei, polarized dynamically due to hyperfine interaction with optically oriented deep paramagnetic centers, rather than a field of exchange interaction created on the center by spin-polarized photo-excited conduction electrons. The short time of nuclear polarization by electrons (<15 μs), measured under modulation of circular polarization of the exciting light with high frequency, points to a small number of nuclei undergoing hyperfine interaction with an electron localized at a center.     

Investigation of ultrafast nuclear spin polarization induced by short laser pulses

We theoretically investigate the dynamics of nuclear spin induced by short laser pulses and show that ultrafast nuclear spin polarization can take place. Combined use of the hyperfine interaction together with the static electric field is the key for that. Specifically we apply the idea to unstable isotopes, (27)Mg and (37)Ca, with nuclear spin of 1/2 and 3/2, respectively, and show that 88% and 62% of nuclear spin polarization can be achieved within a few to tens of ns, which is 2-3 orders of magnitude shorter than the time needed for any known optical methods. Because of its ultrafast nature, our scheme would be very effective not only for stable nuclei but also unstable nuclei with a lifetime as short as mus.     

Optically detected electron spin polarization and Hanle effect in AlGaAs/GaAs heterostructures

The spin polarization of optically created conduction electrons in p-type AlGaAs/GaAs heterostructures was observed via the degree of circular polarization of the photoluminescence. Application of a magnetic field perpendicular to the propagation of the light allows one to determine the spin relaxation time T₁ and the electron lifetime τ in the conduction band. By tilting the magnetic field with respect to an estimate of the effective nuclear field acting on the electrons can be obtained.     

Frequency-selective laser optical pumping and spin exchange of cesium with129Xe and131Xe in a high magnetic field

We report the experimental results of frequency-selective laser optical pumping and spin exchange of Cs with¹²⁹Xe and¹³¹Xe in a high magnetic field of 11.74 T. Our results show that hyperpolarized¹²⁹Xe and¹³¹Xe nuclear magnetic resonance (NMR) signals exhibit alternating phases when the laser frequency for pumping the cesium atoms is changed, which is explained on the basis of the high-field optical pumping of Cs. We obtain about 3% polarization of the¹²⁹Xe. The electron-spin polarization of the Cs atoms has been measured to be about 22% with a simple NMR method.     

The role of spin-orbit coupling and hyperfine coupling in optical pumping ofF-centres

Optical pumping experiments onF-centres in potassium halides are described. The ground state polarization of the electronic spins achieved by optical pumping of isolatedF-centres is strongly dependent on the magnetic field and the wave-length of the pumping light. Experimentally it does not show the simple relationship to the magnetic circular dichroism (MCD) that has generally been assumed. A closer theoretical analysis shows indeed that the phenomena depend critically on the kind of spin mixing that prevails in the pumping cycle.If spin orbit coupling in the absorption band is the dominant spin mixing mechanism the sign of the pumping effect will be the same everywhere in the absorption band, if some other mechanism like spin mixing by radiationless transitions or hyperfine coupling to the surrounding nuclear spins prevails, the sign will follow the MCD. In strong magnetic fields experiments argue in favour of the first alternative, in low fields spin mixing by hyperfine coupling becomes important. On this basis the mechanism of optical pumping ofF-centres is discussed, and rate equations are given.     

Proposal for a nuclear gamma-ray laser of optical range

A possibility of the amplification of the 7.6 eV γ radiation by the stimulated γ emission of the ensemble of the (229m)Th isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of (1) the excitation of a large number of (229m)Th isomers by laser radiation, (2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field, (3) the emission or absorption of the optical photons by thorium nuclei in the crystal without recoil, and (4) the nuclear spin relaxation through the conduction electrons of the metallic covering.     

Pulsed optically detected NMR of single GaAs/AlGaAs quantum wells

While nuclear magnetic resonance (NMR) is one of the most important experimental tools for the analysis of bulk materials, the low sensitivity of conventional NMR makes it unsuitable for the investigation of small structures. We introduce an experimental scheme that makes NMR spectra of single, nanometer-sized quantum wells possible with excellent sensitivity and selectivity while avoiding the spectral broadening associated with some alternative techniques. The scheme combines optical pumping and pulsed radiofrequency excitation of the nuclei with time-resolved detection of the free induction decay through the polarization of the photoluminescence.     

通过极化原子束的磁偏转实现激光同位素浓缩

本文描述一种通过极化原子束的磁偏转实现同位素和同质异能素浓缩的新方法。用具有不同频率和偏振的激光进行选择性光抽运,使束中两同位素的原子分别反向高度极化,然后使这些反向极化的原子在经过自旋选态磁铁后沿不同方向偏转,从而实现同位素浓缩。推算了此法的选择性和产量,与其他方法相比,讨论了其优缺点和可能的应用前景。提出了用锂或钾进行实验的方案。 关键词：     

Spin-polarization and x-ray magnetic circular dichroism in GaAs

The combination of angular spin momentum with electronics is a promising successor to charge-based electronics. The conduction bands in GaAs may become spin-polarized via optical spin pumping, doping with magnetic ions, or induction of a moment with an external magnetic field. We investigated the spin populations in GaAs with x-ray magnetic circular dichroism for each of these three cases. We find strong anti-symmetric lineshapes at the Ga L₃ edge indicating conduction band spin splitting, with differences in line width and amplitude depending on the source of spin polarization.     

Optical pumping and population transfer of nuclear-spin states of caesium atoms in high magnetic fields

Nuclear-spin states of gaseous-state Cs atoms in the ground state are optically manipulated using a Ti:sapphire laser in a magnetic field of 1.516T, in which optical coupling of the nuclear-spin states is achieved through hyperfine interactions between electrons and nuclei. The steady-state population distribution in the hyperfine Zeeman sublevels of the ground state is detected by using a tunable diode laser. Furthermore, the state population transfer among the hyperfine Zeeman sublevels, which results from the collision-induced modification \delta a(\bm S \cdot \bm I) of the hyperfine interaction of Cs in the ground state due to stochastic collisions between Cs atoms and buffer-gas molecules, is studied at different buffer-gas pressures. The experimental results show that high-field optical pumping and the small change \delta a(\bm S \cdot \bm I) of the hyperfine interaction can strongly cause the state population transfer and spin-state interchange among the hyperfine Zeeman sublevels. The calculated results maybe explain the steady-state population in hyperfine Zeeman sublevels in terms of rates of optical-pumping, electron-spin flip, nuclear spin flip, and electron-nuclear spin flip-flop transitions among the hyperfine Zeeman sublevels of the ground state of Cs atoms. This method may be applied to the nuclear-spin-based solid-state quantum computation.     

激光极化129Xe的生物磁共振成像研究进展

激光光抽运自旋交换方法能够极大地增强＾１２９Ｘｅ核自旋极化,其获得的非平衡极化度远远高于在相同磁场里玻尔兹曼平衡值。     

170 nm nuclear magnetic resonance imaging using magnetic resonance force microscopy

We demonstrate one-dimensional nuclear magnetic resonance imaging of the semiconductor GaAs with 170 nm slice separation and resolve two regions of reduced nuclear spin polarization density separated by only 500 nm. This was achieved by force detection of the magnetic resonance, magnetic resonance force microscopy (MRFM), in combination with optical pumping to increase the nuclear spin polarization. Optical pumping of the GaAs created spin polarization up to 12 times larger than the thermal nuclear spin polarization at 5K and 4T. The experiment was sensitive to sample volumes of 50 microm(3) containing approximately 4 x 10(11)71 Ga/Hz. These results demonstrate the ability of force-detected magnetic resonance to apply magnetic resonance imaging to semiconductor devices and other nanostructures.     

Spin-dependent electron dynamics and recombination in GaAs<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys at room temperature

Conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs_1−x N _x alloys with a small nitrogen content (x = 2.1, 2.7, and 3.4%) is studied both experimentally and theoretically. It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40–45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in the dynamical polarization of bound electrons. A nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers has been developed which describes the experimental dependencies, in particular, the electron spin quantum beats observed in a transverse magnetic field. The text was submitted by the authors in English.     

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100 mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.     

Field induced magnetic density in the paramagnetic compound LuCo2

The magnetization density induced in LuCo₂ by an applied magnetic field was measured by means of polarized neutron diffraction. The measurements were performed on a single crystal at 100 K in an applied field of 57.2 kOe. The observed density is localized on Co atoms with a form factor which is, within the experimental accuracy, similar to that of 3d electrons in Co metal. Weak additional magnetic amplitudes reveal a nonuniform polarization of the conduction band. Its mean value is opposite to the Co moment as in Co metal.     

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces.