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.     

Unitary transformations with spin angular momentum operators

Useful formulae for replacing an exponentiated spin scalar product by a finite sum of powers of this product, are presented in the context of Coulomb interactions between magnetic ions and conduction electrons in metals.     

Theory of magnetic impurities in metals

The theory of magnetic impurities in metals is considered starting from the strong-correlation or ionic limit as a zero-order approximation. An appropriate model Hamiltonian is derived which includes collective intra-ionic structure and new kinds of mixing interactions between local and conduction electrons. The relation of the present approach to the Friedel-Anderson theory is discussed in some detail.     

Temperature dependence of the electrical resistivity in amorphous metals

In amorphous metals the electrical resistivity increases linearly in the temperature range from 2 to 40 K. This result differs fundamentally from the nonlinear behaviour known for crystalline metals and it suggests the conduction electrons not to be scattered by the vibrations of the amorphous point lattice. The temperature dependent part of the resistivity in amorphous metals is explained with scattering of conduction electrons by fluctuations ofp-electrons.     

Electrical conductivity of multi-layered metallic thin films

Multi-layered metallic thin films have quite unique properties such as electrical conductivity because of their enhanced interactions at the interfaces between different metals. A theoretical solution has been derived for the distribution functions of the conduction electrons, through Boltzmann's equation and certain interface boundary conditions. The solution of the electrical conductivity for the multi-layered metallic thin films is given.     

Conduction electron polarization in ferromagnetic rare earth crystals

We show that the average spin per atom in ferromagnetic rare earth metals is enhanced by means of a net spin polarisation of conduction electrons, arising as a consequence of multiple scattering of the conduction electrons.     

Predominance of d electrons in the conduction bands of rare earths and intermetallic compounds

With a view to study the nature of the conduction electrons in rare earth metals and intermetallic compounds self consistent augmented-plane-wave calculations have been performed for DyZn. The results indicate that 72 per cent of conduction electrons inside the APW sphere of Dy have d character. The dependence of the nature of the conduction electrons on the exchange potential has also been studied.     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

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.     

Dissipative Quanturn Tunneling in Metals

The influence of a bath consisting of both conduction electrons and Debye phonons on quantum tunneling in metals is studied by the perturbation approach developed by the authors. We find that the renormalized tunneling parameter and the condition oflocalization transition are modified in comparison with the results of a bath consisting of conduction electrons.     

Über das Verhalten der 4f-Elektronen in einigen Metallen der Seltenen Erden unter der Einwirkung von Licht

The visible spectra of the rare-earth metals Pr, Nd, Sm, and Er were examined whether they contain absorption lines due to transitions within the 4f-shell. Optical measurements were made on vacuum evaporated transparent films at 90° K. No spectral lines were found. It was shown by complete oxydation of the metal films that eventual absorption lines of the metals must be at least 3.5 times weaker or broader than those of the oxydes. The missing of the lines is suggested to be caused by the conduction electrons: 1. As the crystalline field is largely screened by the conduction electrons the oscillator strength of the electric dipole radiation is rigorously reduced. 2. Lines may be broadened by exchange of excitation between 4f-electrons and conduction electrons.     

Anomalous thermoelectric power of rare earth metals and their compounds

We propose a new mechanism for an anomalous thermoelectric power (ATP) in the paramagnetic state of certain rare earth metals and their compounds, in which the ions possess a nonmagnetic ground state in a given crystal field. The ATP is found to be due to higher order inelastic scattering (second Born approximation) of the conduction electrons by the crystal field split rare earth ions. It has a peak at a temperatureΔ/3 ~Δ/2, whereΔ is the splitting energy between the ground state and the first excited state. Our main result is that the appearance of an ATP requires interactions between the conduction electrons and the ions of other than the simple isotropic exchange type. This implies that the ATP may serve as a valuable tool to detect more complicated types of thek-f interaction than the isotropic exchange.     

Origin of increase of damping in transition metals with rare-earth-metal impurities

The damping due to rare-earth-metal impurities in transition metals is discussed in the low concentration limit. It is shown that all established damping mechanisms based on spin-orbit and/or spin-spin interactions cannot explain experimental observations even qualitatively. We introduce a different relaxation channel due to the coupling of the orbital moments of the rare-earth-metal impurities and the conduction p electrons that leads to good agreement with experiment. Using an itinerant picture for the host ions, i.e., write their magnetization in terms of the electronic degrees of freedom, is key to the success of our model.     

Spin-flip scattering of conduction electrons by dislocations in a metal

The spin-flip scattering of conduction electrons by dislocations in metals with a strong spin-orbit coupling is considered. Calculations are performed in terms of the model spin-orbit potential describing the spin-flip scattering of conduction electrons. It is shown that deformation of the crystal lattice in a metal leads to a change in the structure factor. The core of a rectilinear edge dislocation is calculated by the molecular dynamics method. The results obtained are compared with the experimental data on conduction-electron spin resonance (CESR) in copper.     

Radiation-detected optical pumping in solids

Radiation-detected optical pumping in solids has been developed to investigate the structure of unstable nuclei. Appreciable nuclear polarization of implanted or doped unstable-nuclei in a host crystal is achieved with the optical pumping in solids. The nuclear polarization achieved is enhanced/reduced by applying a radio-frequency magnetic field together with the optical pumping, radiation-detected magnetic resonance being thus observed to get information on electromagnetic properties of unstable nuclei. Two schemes have been successfully developed for the optical pumping of unstable nuclei in solids. One is to directly pump the atoms by the excitation from the ground state to a broad absorption band in visible and UV regions which shows a large magnetic circular dichroism. This scheme is applicable to many rare-earth atoms in alkaline-earth fluoride host. The other scheme is to pump the electrons in the conduction band of direct-type semiconductor and thus indirectly polarize the nuclei in the host material via a hyperfine interaction between the nuclei and the polarized conduction electrons. This scheme can be especially applied to the III to VI families of atoms in direct-type semiconductors. Principle of the methods, on-line experimental system, and a few examples of the results obtained so far are presented and discussed.     

Elastic properties of itinerant electron ferromagnets

Starting from a standard electron-phonon interaction model we discuss how the lattice properties depend on the magnetization of conduction electrons in ferromagnetic metals. The simple and sensitive relation between the elastic properties and the electronic structures we obtain might be used to study the electronic structure of ferromagnetic metals.     

Electric field gradient produced by conduction electrons in rare earth metals

The electric field gradient produced by conduction electrons in the heavy rare earth metals has been calculated taking into account non-spherical effects due to the magnetic ordering in these metals. To our knowledge the values obtained are the first to agree with the sign of the experimental results.     

The “tilt” effect in the absorption of ultra-sound by metals in a low magnetic field

The “tilt” effect in the electron-phonon interaction has been discovered experimentally in metals placed in a low magnetic field, such that the cyclotron radius of conduction electrons is larger than the sound wavelength.     

Core line asymmetries in XPS-spectra of metals

The asymmetric lineshapes predicted to arise from the coupling of the core hole to the conduction electrons have been observed in X-ray photoemission from a number of 4d and 5d metals.