Hydrogenic impurity states in zinc-blende GaN/AlN coupled quantum dots

Based on the effective-mass approximation, we have calculated the donor binding energy of a hydrogenic impurity in zinc-blende (ZB) GaN/AlN coupled quantum dots (QDs) using a variational method. Numerical results show that the donor binding energy is highly dependent on the impurity position and coupled QDs structural parameters. The donor binding energy is largest when the impurity is located at the center of quantum dot. When the impurity is located at the interdot barrier edge, the donor binding energy has a minimum value with increasing the interdot barrier width.     

Nitrogen vacancy and oxygen impurity in AlN: spintronic quantum dots

Point defects with non-zero spin are prototypical spintronic quantum dots. Here two anion-site defects in AlN are studied intensively in terms of their spin and related properties. The charged states of the nitrogen vacancy and substitutional oxygen impurity, in both ground and spin-flip states, are analyzed. The theoretical analysis includes optical absorption and emission, diffuse excited states, spin densities and local mode force constants. The relevance to spintronic quantum dots in semiconductors is discussed.     

The formation of heavy-fermion states in non-Fermi-liquid impurity systems

A mechanism for the occurrence of heavy-fermion states in non-Fermi-liquid (NFL) metals with f-shell impurities is proposed. The impurity with an unstable valence is suggested to have an energy spectrum consisting of a deep f-level and quasicontinuum states (narrow band) in resonance with the Fermi energy. Depending on the impurity concentration, the single-site NFL states are generated by the two-channel Kondo scattering for the low concentration (the Kondo regime) or by the screening interaction for a relatively high concentration (the X-ray-edge regime). It is shown that the NFL states are unstable against the scattering of the NFL excitations by electron states of the narrow band. This scattering generates additional narrow Fermi-liquid (FL) resonances at/near the Fermi level in the Kondo regime and in the X-ray-edge regime. The mixed-valence states are shown to be induced by new FL resonances. The mixed valence mechanism is local and is related to the instability of single-site NFL states. The FL resonances lead to the existence of additional energy scales and of pseudogaps near the Fermi level in the mixed-valence states. They also considerably narrow the region with a nearly integer valence.     

Nuclear orientation study of160Tb impurity in gadolinium single crystal

The low temperature nuclear orientation of¹⁶⁰Tb in gadolinium single crystal has been studied in the temperature range 5–60 mK and in the external magnetic field B_ext ≤9 T, applied both in the a- and c-axis directions. The obtained results indicate the noncollinearity between the Tb magnetic moments and the magnetization of the gadolinium matrix.     

Nuclear orientation study of160Tb impurity in yttrium single crystal

The low temperature nuclear orientation of¹⁶⁰Tb impurity in yttrium single crystal has been studied in the temperature range 5–60 mK and in the external magnetic field up to 9 T applied along a- or c-crystal axes. The experimental results are interpreted as a consequence of the interaction of Tb³⁺ ion with crystal field produced by the yttrium neighbours. A possible influence of technology used is also discussed.     

Spin-wave impurity states in metallic ferromagnets

It is shown that spin waves in dilute ferromagnetic transition metal alloys can be described in terms of effective matrix-matrix and impurity-matrix exchange integrals. Such a parametrization is exact within the random phase approximation for long-wavelength spin waves. The effective impurity-matrix exchange integral is determined in the tight-binding approximation in terms of the impurity potential and local density of states. The present theory is applied qualitatively to NiFe alloys.     

Heavy-fermion states in non-Fermi-liquid impurity metals

It is shown that a new type of instability of a non-Fermi-liquid state to the interband scattering of multiparticle excitations can dominate the formation of heavy-fermion states in non-Fermi-liquid metals doped with unstable-valence f impurities. A new mechanism is proposed for the formation of a small energy scale and pseudogaps near the Fermi level in a mixed-valent state.     

Radiative recombination at deep impurity centres in AlN:O

The ionization energy of the deep donor states due to oxygen and of the acceptor levels due to compensating V_A1 vacancies were determined from the optical properties of AlN:O. The theoretical calculations together with the experimental data (the emission spectra, the excitation of luminescence spectra and EPR data) indicate that the deepest donor level belongs to the ion O^?.     

Density of states of impurity bands in semiconductors

The formalism of Matsubara-Toyozawa for impurity bands in semiconductors is extended by including the correlation effect on electron hopping matrix elements. The density of states of impurity bands for various impurity concentrations is computed numerically. The comparison with Matsubara-Toyozawa's result indicates that the impurity bandwidth is drastically reduced by electron correlation.     

Resonant impurity states in the d-density-wave phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.     

Long-lived excited impurity states in diamond-like semiconductors

The lifetime of charge carriers in the lowest excited states of some impurities of groups III and V in diamond, silicon, and germanium can be several (four to six) orders of magnitude longer that the lifetime of free carriers. Accumulation of carriers in these long-lived states may give rise to several new effects, such as hopping photoconductivity via long-lived excited states of impurities in dc and microwave electric fields, slow relaxation of induced absorption, and infrared absorption at energies lower than the impurity ionization energy. Zh. éksp. Teor. Fiz. 112, 221–236 (July 1997)     

Theory of an impurity state in a crystal

A review is given of the theory of the effects of single-, or low-concentration impurities on the properties of otherwise perfect crystals. The subjects considered include electron density, anomalies in the temperature dependence of the spontaneous magnetization of a ferromagnetic insulator and the spectrum of lattice vibrations. All the problems are considered from a single mathematical point of view in which the approximation of localized perturbations and Green function techniques are used. The conditions for the occurrence of localized and virtual states are discussed. The review concludes with a formulation of the general methods of the theory.     

Local and pseudolocal Jahn-Teller states of a crystal with impurity Γ8 term

The multimode Jahn-Teller effect for an impurity in the Γ₈ ground state of a double point group is considered. The vibronic coupling with the trigonal (t₂) crystal vibrations is taken into account. It is shown that the vibronic coupling leads to the appearance of local and pseudolocal electron-phonon states. The spectral and symmetry characteristics of these states are obtained.     

Hydrogenic impurity states in zinc-blende InGaN quantum dot

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in a zinc-blende (ZB) InGaN/GaN cylindrical quantum dot (QD) is investigated using a variational procedure. Numerical results show that the donor binding energy is highly dependent on impurity position and QD size. The donor binding energy E_b is largest when the impurity is located at the center of the QD. The donor binding energy is decreased when the dot height (radius) is increased.     

Localized and nonlocalized impurity states in amorphous germanium

The purpose of this study was to investigate which impurities form localized states in amorphous Ge and to study the concentration dependence of the effect. The low temperature resistivity of amorphous alloy films deposited at 77°K was well fitted by the relation $\text{ρ = ρ}{}_{\mathrm{<mtext>0\; </mtext><mtext>exp</mtext>}}\text{[(}\text{T}{}_0\text{T}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{]}$. The increase in the number of localized states resulting from an increase in the number of impurities is evidenced by a decrease in T₀ Impurities which lead to shallow energy levels in the crystalline form do not form localized states near the Fermi level while impurities with deep lying levels do.     

A new method for impurity states in semiconductors

The causes of inapplicability of the Luttinger method for the theoretical investigations of impurity states in small gap semiconductors are discussed. For this purpose a new method has been developed and applied to GaAs and InSp. It is shown that the impurity states spectrum in these semiconductors must be either of hydrogen-like or Dirac spectrum type.     

Pair effects in the impurity band density of states

Results of a simple two center approximation for impurity bands are presented. For low concentrations the density of states deviates strongly from the single site CPA and agrees well with numerical “exact” results.     

Problem of impurity states in narrow-gap lead telluride-based semiconductors

A review of recent results of experimental investigations devoted to studying unusual properties of impurity states in doped narrow-gap lead telluride-based semiconductors is presented. These results are analyzed in the framework of existing theoretical concepts.