Single Impurity Atom Research Articles

Nearest neighbour hopping conductors with negative magnetoresistance

Abstract DC resistance and magnetoresistance measurements are performed on insulating AlAl 2 O 3 samples which are prepared from conducting metal-insulator granular composite samples by subjecting them to high density current pulses. This treatment drastically reduces the room-temperature conductivity of the mixture, due to the explosion of a fraction of the metallic grains in percolation channels. Providing the pulse current density is high enough, the samples show the hopping conductivity. Two distinctly different nearest-neighbour hopping (NNH) types of samples may be obtained. In the first (M) type samples, conduction is dominated by hopping between adjacent metallic grains imbedded in a dielectric host medium. Negative orbital magnetoresistance (NMR), with a quasilinear field dependence, is observed at both 77 and 300 K. In the samples of the second (H) type (those which have been subjected to higher current pulses or to several sequential pulses of the same amplitude), conduction is found to be dominated by NNH between single impurity atoms. Positive magnetoresistance is now observed.

Luminescence Spectroscopy on Individual Nanostructures and Impurity Atoms Using Stm and Sem

AbstractIn the exploration of the nano-world of semiconductors there is a strong focus on low-dimensional structures and ultra-small devices. Two fundamental problems, which challenge progress in this field are: (i) large ensembles of nano-objects, like Quantum Dots (QDs), do not have identical geometrical shapes and electronic properties, and, (ii) the properties of a low-dimensional structure can be dominated by a few impurity atoms, whereas the properties of a macroscopic structure is determined by the quasi-continuous background of dopant impurities. To allow QDs and discrete impurities to be studied, novel experimental techniques are required. In this paper we describe how local luminescence has been excited from single QDs using electrons injected from a Scanning Electron Microscope (SEM), from the tip of a Scanning Tunneling Microscope (STM) or using highly focused photons for excitation. We present images of QDs as well as characteristic spectra of individual QDs. We finally show how the local character of the excitation enables us to excite and image individual impurities in low-dimensional structures, including the measurement of characteristic emission spectra from a single impurity atom in GaAs.

Self-consistent calculation of hyperfine fields and adiabatic potential of impurities in iron

Hyperfine fields of impurities of the atomic number Z=1–56 at the substitutional site and those of light impurities of Z=1–9 at the interstitial sites in ferromagnetic iron are calculated by the KKR method adapted to the system containing a single impurity atom. The potential of the impurity atom is determined self-consistently by use of the local spin density functional formalism. The results for nonmagnetic sp valence impurities agree with those of the previous nonself-consistent calculation by Katayama-Yoshida, Terakura and Kanamori except for a few cases, confirming their theory of the systematic variation of hyperfine fields. The calculation for magnetic impurities of transition elements is presented for the first time in this paper. The calculations mentioned so far assume that impurities are situated at the center of each site. For the purpose of discussing the stability of the impurity positions, the change of the adiabatic potential due to displacements from the center is calculated by carrying out similar self-consistent calculations for off-center impurity positions. It is concluded that positive muon and some light impurities including boron will be displaced from the center when trapped in a vacancy.

Positive muon spin depolarization in quenched aluminium dilute alloys

Aluminum dilute alloys (Al-0.047 at% Mg, Al-0.052 at% Cu and Al-0.051 at% Si) were quenched from 600‡ and 450‡C. The positive muon behavior in the quenched alloys has been studied. Three peaks (Peak I, II and III) are found. Peak I is due to the trapping and detrapping of positive muons at single impurity atoms. The muon spin depolarization rate at the trap in Peak I was found to be 0.36μsec−1. Peak II is connected to impurity clusters. Peak III is the trapping of muons at quenched-in vacancies. The depolarization rate at the trapping site was found to be 0.2μsec−1. The activation energy of the trapping rate was found to be 25 meV. As the quenched-in vacancies annealed out, the trapping rates decreased to near zero.

Geometrical significance of the orientation of defect-induced electric field gradients

One of the features of defect-induced axial symmetric electric field gradients(efg) is the orientation of their symmetry axis with respect to the crystal lattice. Especially in the case of simple trapped defects(mono-, di-vacancies, single interstitials or single impurity atoms) this orientation should follow the symmetry of the geometrical probe atom defect arrangement. This will be illustrated by PAC measurements at111In/111Cd for the case of trapped impurity atoms in Cu.

Compensation mechanism in Ni-diffused n-type GaP

Optical investigation of the diffusion profile of Ni acceptor in n-type Gap is presented. It is found that a single Ni impurity atom is responsible for compensation of 4 to 5 electrons from shallow donors. Generation of point defects, such as vacancies, by a dislocation climb mechanism is responsible for this large compensation.

Iron-related defects in silicon

Deep impurities in iron-doped silicon are investigated by means of capacitance transient spectroscopy, photocapacitance, photoconductivity and Hall-effect measurements. For interstitial iron (Fei), an energetic position of 0.38 eV above the valence band is established. Pairs of Fe with the acceptors B, Al, and Ga are found to produce deep levels in the lower half of the band gap. The experimentally observed trends in the activation energies of these pairs are in qualitative agreement with predictions of a model based on the electronegativities of the single impurity atoms. Higher order Fe-complexes are also found to produce electrically active centers, the most often found center has a energy level very similar to that of Fei.

On the Theory of a Heisenberg Impurity Ferromagnet

AbstractUsing the resolvent method a complete solution of the Wolfram‐Callaway problem for a Heisenberg ferromagnet with a single magnetic impurity atom is obtained. The spectrum of the system is determined and the exact wave functions of the continuous and discrete spectra of the system with a unit spin deviation are found. The theorem of completeness for eigenfunctions is proved. The treatment is performed with reference to the example of an infinite crystal with a simple cubic lattice.

Impurity Effects on Adatom Density of States

AbstractThe influence of both, impurity and surface states on the adatom electronic properties is studied using the Dyson equation approach. The surface Green function of a semi‐infinite crystal contaminated by a single impurity atom is calculated. It is found that the surface state energy depends on the location of the impurity atom. A similar calculation is performed for the case where an adatom interacts with the contaminated substrate and the energy Ea of an electron localized on the adatom is determined as a function of the impurity atom location and potential. The adatom density of states is also calculated to discuss the significant effect of the impurity near the surface.

Point defects and deep traps in III–V compounds

It is now generally agreed that almost all deep defects in as-grown III–V compounds, and in other semiconductors as well, are not simple, single, substitutional impurities or single vacancies or interstitial atoms. The deep defects observed at room temperature are complexes that may involve all of these. Complexes form from single defects and impurity atoms introduced during crystal growth and processing as the material cools to room temperature. The study of deep defects is therefore an inherently complex subject. In the author's opinion, it is no use to propose simple models with a minimal number of assumed defects and parameters which are adjusted so as to fit some limited set of data. Rather one ought to model the thermodynamic history of the material allowing all plausible defects in the calculations with the best theoretical estimates of their parameters to be adjusted only slightly to fit the widest possible range of data. This modeling must consider not only equilibrium defect concentrations, but also the kinetics of crystal growth, dissolution and oxidation as well as defect diffusion, trapping and complex formation both with and without the effects of photoionization, recombination and electromigration. We are almost to the point where such a model can be performed.

Infrared absorption and first-order Raman scattering due to single and paired impurity atoms in dilute Ge: Si alloys

Infrared absorption and first-order Raman scattering due to single and paired impurity atoms in dilute Ge: Si alloys

Photon antibunching in resonance fluorescence from impurity atoms in solids

The experimental conditions under which the intensity correlation function of resonance fluorescence of a single atom in a gas may be observed are briefly discussed. In general they can complicate the direct observation of photon antibunching. A direct observation should be possible in the case of a single impurity atom in a solid state matrix. The corresponding intensity correlation function is given.

Electronic states of a semi-infinite crystal with a surface impurity

The electronic states of a model Hamiltonian representing a single impurity atom adsorbed on the surface of a semi-infinite crystal are described in general terms. The model potential field is constructed by abruptly terminating the potential of the infinite crystal at the surface and by replacing the potential outside the crystal by a constant. The adsorbed atom is represented by a spherical potential of the muffin-tin type superimposed on the constant potential on the vacuum side of the metal–vacuum interface. A method for evaluating the eigenvalues and eigenfunctions of this model Hamiltonian is presented.

Electronic thermal conductivity of a superconducting indium-chromium alloy film

Measurements have been made of the electronic thermal conductivity of a quench-condensed film of an indium-chromium alloy. The results are compared with calculations based on Shiba's theory of superconductors containing magnetic impurities by using ${\ensuremath{\epsilon}}_{0}$ (the normalized position in the energy gap of the excited state associated with a single impurity atom) as an adjustable parameter. The data are in good agreement with the theory for a value ${\ensuremath{\epsilon}}_{0}=0.7$.

Comparison of the thermal conductivity of superconducting Zn-Mn alloys with Shiba's theory

Measurements by Sanchez of the thermal conductivity of Zn-Mn alloys are compared with calculations based on Shiba's theory of superconductors containing magnetic impurities, using ${\ensuremath{\epsilon}}_{0}$ (the normalized position within the energy gap of the excited state associated with a single impurity atom) as an adjustable parameter. The values of ${\ensuremath{\epsilon}}_{0}$ which best fit the data are in the range 0.7-1.0. However, all values of ${\ensuremath{\epsilon}}_{0}$ within the allowed range (0-1) fit the data to within the experimental uncertainty.

Nuclear magnetic resonance studies of electric field gradients in single crystal dilute alloys of aluminium

Dilute alloys of aluminium with gallium, silicon, germanium and magnesium, respectively, have been studied. The satellites to the main aluminium resonance have been used to obtain information about the electric field gradients due to a single impurity atom at neighbouring aluminium nuclei. Satellites due to nuclei which are as distant as fifth nearest neighbours to the impurity have been identified. For the magnesium alloy, the wipeout number for some satellites has been measured and has been found to be less than that for the main line.

A Calculation of the Electronic Structure of an Impurity Atom of Non-Transition Element in Transition Metals –The Nuclear Spin-Lattice Relaxation Time of an Impurity Atom (Al and Cu) in Pd–

Kohzuki and Asayama have shown that the nuclear spin-lattice relaxation time T 1 of Al in Pd 1- x Al x in the range of small x (<0.03) becomes about 10 times as long as that of pure Al and in the case of Pd 1- x Cu x , T 1 of Cu does not change appreciably in the whole range of x . These data are analysed in this paper by carrying out ab initio calculations of the electronic structure of a single impurity atom in Pd. It is shown that T 1 of Al nucleus is determined mainly by the contact interaction with conduction electrons. The increase of T 1 is a direct result of the reduction of the density of electrons with the Fermi energy at the impurity nucleus, ρ s (0, E F ), which arises from the fact that the Fermi level is situated at the valley between the bonding and antibonding states of the s states at the impurity site with the host d band. On the other hand, in the case of an impurity atom of Cu in Pd, the orbital relaxation, which can be neglected in pure Cu, compensates the decrease of the relaxation r...

Impurity-InducedgShift of Conduction Electrons

A simple theory of the $g$ shift of conduction electrons induced by nonmagnetic impurities embedded in a metal is described. A single impurity atom is placed at the center of a large sphere containing a free-electron gas. The major contribution to the $g$ shift is obtained by calculating the change in the orbital angular momentum due to the spin-orbit interaction. Quite large $g$ shifts obtain when the spin-orbit interaction with the impurities is large, when the scattering phase shifts are not too small, when one or more partial waves go through a narrow resonance, or when the phase shifts change rapidly with energy without becoming large, and when the alloys are concentrated. Application of the theory to very dilute lithium alloys has revealed that the $g$ shifts are too small to be observable by means of conduction-electron spin resonance. This was confirmed by performing spin-resonance experiments on dilute Li-Zn, Li-Ag, and Li-Cd alloys. The theory presented here provides the extension of the theory of the $g$ factor of atomic electrons to the case of electrons in the continuum states.

Annealing Study of Dilute Aluminum Alloys Electron Irradiated at Liquid-Nitrogen Temperature

Pure and dilute aluminum alloys (Mg, Zn, Ag, Ge, Au, and In) were irradiated at about 90 \ifmmode^\circ\else\textdegree\fi{}K by 2- or 3-MeV electrons. Isothermal annealing for 30 min at temperature intervals 10 \ifmmode^\circ\else\textdegree\fi{}C apart was carried out. The major results were the following: Stage-III annealing occurs by a second-order process with an activation energy ${E}_{\mathrm{III}}=0.60\ifmmode\pm\else\textpm\fi{}0.04$ eV for both pure and dilute aluminum alloys. Since this activation energy is the same as that found by quenching experiments, it is concluded that stage III in aluminum and dilute aluminum alloys occurs by the motion of vacancies. Two stages below stage III are dependent on the kind of impurity. A mechanism consisting of a breakup of interstituals from impurity traps (single-impurity atoms or impurity-atom clusters) can explain the experimental results. Reverse recovery of electrical resistivity was observed below and above stage III; this is due to Guinier-Preston-zone formation.