Anderson localization and the pseudogap in the energy spectrum of holes in PbTe: Tl under resonant scattering

This paper reports on a comprehensive study of the effect of additional doping with the Na acceptor impurity on the low-temperature resistivity of PbTe samples doped with Tl (2 at %), an impurity producing a band of resonant states within the valence-band spectrum. By additional doping with Na, we have shifted the Fermi level within the band of the resonant states of Tl in PbTe and varied the hole filling (k _h ) of the thallium impurity states. The larger part of the PbTe: (Tl,Na) samples transfers to the superconducting state with a critical temperature T _c = 0.4–2.3 K. The T _c (k _h ) relation obtained argues for the fact that, in the region of resonant states in PbTe: Tl, Anderson localization of holes and a pseudogap in the density of delocalized states are observed.     

Electrical conductivity and colour centres in Tl-doped KCl crystal

The electrical conductivity and optical absorption of potassium chloride crystals doped with different concentrations of thallium have been measured before and after X-irradiation. The optical absorption spectrum exhibits the characteristic Tl⁺ ion band at 247 nm. The extrinsic conductivity and the absorption coefficient at 247 nm increase with impurity addition upto a certain concentration. Further increase in impurities decreases them. Room temperature X-irradiation decreases the Tl⁺ ion band and produces the F band. F centre concentration is higher in lightly doped crystal compared to pure or heavily doped KCl. These results have been interpreted in terms of formation of interstitial potassium ions and positive ion vacancies in the Tl-doped KCl lattice due to large ionic radius of Tl⁺ ions. The impurity ions precipitate into TlCl phase when the doping is heavy.     

Effect of Coulomb correlations in a variable-valence impurity system and phonon drag on the thermoelectric constants in two-dimensional systems

The temperature behavior of the longitudinal Nernst-Ettingshausen coefficient in 2D systems is studied theoretically taking account of phonon drag and Coulomb correlations in a system of mixed-valence impurities at low temperatures. It is shown that the effect changes sign at the transition from entrainment to scattering by a correlated system of impurity centers. A sign change does not occur in the case of scattering by randomly distributed impurity centers. This temperature behavior of the Nernst-Ettingshausen coefficient is due to the radical rearrangement of the impurity system as a result of strong Coulomb correlations present in a system of impurities with mixed valence. As a result, the character of the scattering of charge carriers by the correlated system of charge centers changes substantially. Fiz. Tverd. Tela (St. Petersburg) 40, 553–556 (March 1998)     

Metastable states and physical characteristics of a subsystem of interstitial impurities

The behavior of mobile interstitial impurities in a crystal matrix has been simulated numerically. The contribution of the impurity subsystem to the heat capacity, and the effect of clusterization on the temperature dependence of the impurity diffusion coefficient have been studied. Fiz. Tverd. Tela (St. Petersburg) 40, 475–480 (March 1998)     

Impurity photovoltaic effect in silicon solar cells doped with two impurities

In this work, a numerical study has been carried out to investigate the impurity photovoltaic (IPV) effect for silicon solar cells doped with two impurities (indium and thallium). It is found that the conversion efficiency \(\eta \) of the IPV solar cell doped with two impurities can improve by 2.21 % absolute, which is greater than that of the IPV solar cell doped with indium ( \(\Delta \eta =1.63\,\%\) ), but less than that of the one doped with thallium ( \(\Delta \eta =2.69\,\%\) ). It is concluded that introducing two IPV impurities may not be a good selection for implementing the IPV effect since one impurity with poorer IPV effect can absorb some sub-bandgap photons while contributing fewer currents. The location of impurity energy level is critical to the IPV cell performance. For an acceptor-type IPV impurity, the optimized location of the IPV impurity energy level locates at 0.20–0.26 eV above the valence band edge. Our results may help to make better use of the IPV effect for improving solar cell efficiency.     

The influence of thermal processing on the relative mobility of edge and screw dislocations in NaCl crystals

The effect of thermal processing on the mobility of dislocations is investigated in NaCl crystals doped with impurities of various types—high-solubility impurities (Ca²⁺) and low-solubility impurities (Pb²⁺). The results obtained after aging and thermal processing indicate that the type of impurity and its state (e.g., level of aggregation) in the crystal have a strong effect on the relative mobility of edge and screw dislocations, and also on the parameters of double transverse slip. Fiz. Tverd. Tela (St. Petersburg) 41, 1041–1043 (June 1999)     

Comment on “preparation of amorphous films of tin by quench condensation with impurities”

A recent paper by Buck [1] reported on a detailed investigation of quenchcondensed tin films with additions of various impurities. It was shown that the highest temperature to which an amorphous lattice order persisted scaled with the concentration of valence electrons, thus indicating that the amorphous state would be dependent on the electronic structure. Here, we show that the same crystallization temperatures also scale with the melting point of the impurity element in agreement with our previous observations [2], i.e. crystallization can be understood as due to the onset of impurity diffusion. It is also shown that the latter explanation, but not the former, can account as well for data on lead and thallium based compounds, indicating the more universal nature of the scaling of the crystallization temperature with the impurity element melting point.     

Bipolaron formation in the presence of magnetic impurities

The formation of bipolarons in the presence of magnetic impurities is studied theoretically. We use the extended Hubbard Su–Schrieffer–Heeger (SSH) model with Brazovskii–Kirova and Kondo interaction terms. Parameters are chosen suitably for cis-polyacetylene. A Quantum Monte Carlo (QMC) algorithm is used to study the equilibrium lattice structure and charge distribution as a function of doping level and Kondo exchange integral. The magnetic impurities can have a destructive effect on bipolaron stability, instead favouring the two polaron configuration. However by suitably adjusting the doping level, bipolarons can be stabilized over a wide range of impurity strength.     

Impurity pairs and excitation relaxation in doped silicon

The kinetics of photoconductivity is studied in silicon doped with B, Al, Ga, In, P, As, and Sb with concentrations of 10₁₆–10₁₈cm_?3 at 4.2 and 10.5 K placed in an 8-mm microwave electric field under pulsed impurity excitation. It is found that infrared absorption by impurity pairs and a slow component of photoresponse relaxation arise at close impurity concentrations. It is shown that this component is due to an increase in the polarization hopping conductivity in the presence of the optical charge exchange of impurity states—isolated impurities and impurity pairs and dipoles (pairs of the major and compensating impurities). The hopping transfer processes of ion charges in the course of relaxation are analyzed. It is shown that the main contribution to polarization photoconductivity comes from hopping transitions in impurity pairs at relatively small concentrations and from hopping with the participation of isolated ions at increased concentrations.     

Anharmonicity of lattice vibrations induced by charged nickel impurities in II–VI semiconductors

The lattice vibrations induced by nickel impurities with a negative charge relative to the lattice in ZnSe: Ni, ZnO: Ni, ZnS: Ni, and CdS: Ni semiconductors are investigated using very sensitive field-induced vibronic spectroscopy. This technique is based on the interaction of lattice vibrations with impurity excitons and the effect of an ac electric field on these excitons. The phonon replicas of the zero-phonon line (ZPL) of impurity excitons (including intense peaks of combination replicas up to the eighth order) in the experimental spectra of the system under investigation are observed for the first time. These spectra make it possible to analyze the interaction between different vibrations. The experimental results are interpreted in terms of model calculations of the vibrations in a lattice with a charged impurity center and vibrations in a monoatomic chain with a strong anharmonicity. It is demonstrated that charged impurity centers initiate new lattice vibrations, namely, extrinsic anharmonic modes with a considerable third-or fourth-order anharmonicity.     

Observation of a Kondo effect for a dilute alloy containing Sm impurities

Measurements of the depressions of the superconducting transition temperature T_c with Sm impurity concentration and the specific heat jump at T_c as a function of T_c, and the temperature dependences of the normal state specific heat and magnetic susceptibility are reported for the matrix impurity system (LaSm)Sn₃. The results constitute the first definitive evidence of a Kondo effect for a dilute alloy containing Sm impurities.     

Microwave magnetoresistance and electron spin resonance in Ge:Mn thin films and nanowires

Resonant and nonresonant absorption of microwave radiation is found to occur in germanium films implanted with manganese at concentrations of 2, 4, and 8 at %. Electron spin resonance is observed in two temperature ranges: (i) in the vicinity of the phase transition of Mn₅Ge₃ clusters to the ferromagnetic state at T = 295 K; and (ii) in the range of temperatures below 60 K, at which collective ordering of Mn spins in the crystal lattice and spin-wave resonance take place. The dependence of the nonresonant signal of the microwave magnetoresistance on the magnetic field exhibits a nonmonotonic behavior identical for the X and K microwave bands. An analysis of the field dependence of the microwave magnetoresistance makes it possible to separate two components of the derivative of the magnetoresistance: the quasi-linear Lorentzian component observed in strong fields and the negative exponential anisotropic component determined by spin-dependent scattering of charge carriers from magnetic impurities. The length of the phase relaxation of charge carriers is estimated to be 350 nm at T = 2 K and exceeds the thickness of the film (120 nm) and the sizes of clusters and precipitates (3–5 nm). In quasi-one-dimensional nanowires of the composition Ge:Mn at the same impurity concentrations, microwave magnetoresistance is absent. These facts suggest that conduction in thin films has a quasi-two-dimensional character and that the measured microwave magnetoresistance is associated with charge carriers in the crystal lattice rather than with impurity clusters.     

Pb2+ Defects in perovskite-like KMgF3 crystals

Abstract

The physical properties of Pb²⁺ impurities in the perovskite-like lattice of KMgF₃ are very similar to those of (ns)² ions in alkali halides. The impurity inhomogeneous distribution leads to high dopant levels in the bottom region of the crystals, with a segregation coefficient equal to 0. 020. The optical features of the 336 nm emission support its attribution to luminescence of Pb²⁺ ions from the relaxed excited B state.     

Exact solution for a degenerate Anderson impurity in the limit embedded into a correlated host

We consider the one-dimensional t - J model, which consists of electrons with spin S on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the lattice sites and spin-exchange J between neighboring sites. The model is integrable at the supersymmetric point, J = t. Without spoiling the integrability we introduce an Anderson-like impurity of spin S (degenerate Anderson model in the limit), which interacts with the correlated conduction states of the host. The lattice model is defined by the scattering matrices via the Quantum Inverse Scattering Method. We discuss the general form of the interaction Hamiltonian between the impurity and the itinerant electrons on the lattice and explicitly construct it in the continuum limit. The discrete Bethe ansatz equations diagonalizing the host with impurity are derived, and the thermodynamic Bethe ansatz equations are obtained using the string hypothesis for arbitrary band filling as a function of temperature and external magnetic field. The properties of the impurity depend on one coupling parameter related to the Kondo exchange coupling. The impurity can localize up to one itinerant electron and has in general mixed valent properties. Groundstate properties of the impurity, such as the energy, valence, magnetic susceptibility and the specific heat coefficient, are discussed. In the integer valent limit the model reduces to a Coqblin-Schrieffer impurity. Received: 31 December 1997 / Accepted: 17 March 1998     

Energy gap and transition temperature of highly disordered and amorphous thallium- and indium-films

Thin and pure Tl-films and In-films with different impurities (Ag, Te, Ge, Sb) are condensed onto a cooled substrate at 4 K. Measurements of the energy gap by means of the tunnel effect and the transition temperature of the weak-coupling superconductor thallium are carried out as a function of the degree of disorder of the films. The ratioα=2Δ ₀/kT _c increases proportional to the reciprocal mean free path from 3.5 for the annealed film up to 3.8 for the highly disordered film. For In-films condensed by quenching with impurity additions, one finds a linear relation between energy gap and transition temperature. In-films with Sb-additive are obtained in an amorphous phase with a ratioα=2Δ ₀/kT _c of 4.4. The amorphous state of the In/Sb-films is confirmed by measurements of the electric conductivity and the Hall-effect.     

Thermal conductivity of quasi-one-dimensional antiferromagnetic spin-chain materials

We study heat transport in quasi-one-dimensional spin-chain systems by considering the model of one-dimensional bosonic spin excitations interacting with three-dimensional phonons and impurities in the limit of weak spin-lattice coupling and fast spin excitations. A combined effect of the phonon and impurity scatterings yields the following spin-boson thermal conductivity behavior: kappa(s) proportional to T2 at low, kappa(s) proportional to T-1 at intermediate, and kappa(s)= const at higher temperatures. Our results agree well with the existing experimental data for Sr2CuO3. We predict an unusual dependence on the impurity concentration for a number of observables and propose further experiments.     

Influence of impurities on the dynamics and synchronization of coupled map lattices

The influence of impurities and defects on the dynamics and synchronization of coupled map lattices (CML) is studied. In the context of CML we define impurities as sites in the lattice which have another local dynamics that from the whole lattice and defects as sites in the lattice without any dynamics. We show that synchronization and spatial intermittence are obtained as a function of the number of impurities present on a one-dimensional lattice. We also derive an analytical condition for a signal to “transpose” an impurity. For open flow models, we show that not only the presence of the impurity but also its position along the lattice and its local dynamics can be used to manipulate the lattice in order to obtain a regular or irregular motion. We also show how defects can be used to store information in a lattice.     

Phonon drag conductivity in the Fröhlich-Peierls phase in quasi-one-dimensional electron phonon systems with impurities

The effect of impurities on the static conductivity in the Fröhlich-Peierls phase is discussed by using the technique of the temperature Green function. The diagrams of the phonon drag type especially are shown to give the most important contribution to the static conductivity at low temperatures. The temperature dependence of this contribution is of the activation type with an activation energy equal to the pinning frequency of the phase mode of the charge density wave. The dependence of the preexponential factor on the impurity concentrations is also discussed.On leave of absence     

Striped morphologies induced by magnetic impurities in d-wave superconductors

We study striped morphologies induced by magnetic impurities in d-wave superconductors (DSCs) near optimal doping by self-consistently solving the Bogoliubov–de Gennes equations based on the t ? t′ ? U ? V model. For the single-impurity case, it is found that the stable ground state is a modulated checkerboard pattern. For the two-impurity case, the stripe-like structures in order parameters are induced due to the impurity-pinning effect. The modulations of DSC and charge orders share the same period of four lattice constants (4a), which is half the period of modulations in the coexisting spin order. Interestingly, when three or more impurities are inserted, the impurities could induce more complex striped morphologies due to quantum interference. Further experiments of magnetic impurity substitution in DSCs are expected to check these results.     

Impurity effect on surface states of Bi(111) ultrathin films

The surface impurity effect on the surface-state conductivity and weak antilocalization(WAL) effect has been investigated in epitaxial Bi(111) films by magnetotransport measurements at low temperatures. The surface-state conductivity is significantly reduced by the surface impurities of Cu, Fe, and Co. The magnetotransport data demonstrate that the observed WAL is robust against deposition of nonmagnetic impurities, but it is quenched by the deposition of magnetic impurities which break the time reversal symmetry. Our results help to shed light on the effect of surface impurities on the electron and spin transport properties of a 2D surface electron systems.