Quasi-localized States Research Articles

Superconductivity and transport properties of narrow-gap semiconductor Pb(Tl)Te film by HWE growth

The IV–VI semiconductor PbTe doped with a small amount of Tl shows superconductivity of T c max ∼ 1.4 K in spite of the small hole carrier densities of p∼10 20 cm −3. The superconducting generating mechanism has received much attention from the viewpoint of carrier resonance scattering with a Tl quasi-localized state. In order to investigate the superconductivity on well-defined film samples, we prepared high-quality Pb(Tl)Te films by the HWE method, and carried out the observation of superconductivity and electrical-transport properties. Furthermore, in the experiment with the coordinated system of Pb(Tl,Na)Te film, the results show for the first time the possible existence of two quasi-localized states of Tl causing strong scattering of carriers accompanied by reduction in Hall mobility μ, which play an important role in the superconductivity of this Pb(Tl)Te system.

Superconductivity in the PbTe-SnTe-GeTe system with In and Tl quasilocal impurity states

Superconducting (SC) materials based on AIVBVI solid solutions with deep impurity resonant states of group III elements (In, Tl) was investigated. It was shown that the necessary conditions for the appearance of SC with Tc>1 K are: i) the Fermi level is placed in an impurity band; ii) impurity states are situated near the additional valence band extremum with the high density of states (the Σ-extremum in materials based on PbTe∶Tl and the Σ- or Δ- extremum in materials based on SnTe∶In). On the other hand, in the case of (Sn1−zGez)1−xInxTe the changes of the band structure and the impurity band position may be caused by a structural phase transition observed by resistivity, Hall-effect, X-ray and magnetization measurements.

Low-frequency Raman scattering in glasses.

Low-frequency (10\char21{}80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) Raman spectra of glasses are considered theoretically. It is shown that taking into account the interaction of light both with phononlike and quasilocal vibrational states leads to the violation of the famous Shuker-Gammon formula for the scattering intensity. We find that this violation can explain the experimental results comparing Raman and inelastic-neutron-scattering spectra. Expressions allowing, in principle, the determination of the parameters of the interaction of light with both phononlike vibrations and quasilocal oscillations from experiments are derived.

Indium quasi-local level and superconductivity in GezSn1−zTe:In

Starting with the idea that indium creates a narrow impurity band of quasi-local resonant states in the valence band of the SnTeGeTe system, we have noticed that superconductivity with Tc>0.4 K in these materials requires not only partial electron occupation of the resonant states pinning the Fermi level, but also the following additional condition: on the energy scale the band of resonant impurity states should overlap additional valence band states with a high density of states. An analysis of a correlation between the experimental data of the resistivity, the Hall concentration and superconducting properties was made for solid solutions of GezSn1−zTe:In (0 ⩽ z ⩽ 0.4; In: 5 and 16 at.%). It was found that the superconducting parameters (Tc, Hc2(0)) have a bell-shaped function of the Ge content, revealing a maximum near the edge of the heavy-hole valence band.

Quantum versus classical control of tunneling.

Coherent tunneling can be suppressed or enhanced by exciting a vibrational mode coupled to the tunneling coordinate. The localization in a two-level system (TLS) driven by a periodic force is obtained as a semiclassical limit for such a mode. We derive and test a semiclassical quantization rule that relates the quasienergies of a classically driven TLS with the energy splittings in the model of a TLS coupled to a quantum oscillator. The dissipative stabilization of a quasilocalized state and the possibility of localization by a quasiperiodic force are analyzed from this perspective.

Resonant electronic states in disordered systems.

Long-lived quasilocalized (resonant) electronic states are predicted to lie above the mobility edge of a disordered system. A modification of the optimum fluctuation method is developed to estimate the density of such states. It is shown that the density of states tails exponentially with energy above the mobility edge and that this tail is almost symmetric to the tail of localized states with respect to the mobility edge. Possible manifestations of the predicted states are discussed.

States confined in the barriers of type-III HgTe/CdTe superlattices

We present a theoretical study of states quasi-localized in CdTe barriers of HgTe/CdTe superlattices. We show that the quasi-localization of both electrons and holes will lead to strong Coulomb interaction, and thus to the formation of excitons. It is further demonstrated that such quasi-localized states, including excitons, exhibit confinement effect similar to those of loclaized states in quantum wells.

Decay of the population of quasilocal states in disordered media.

En presence de diffusion elastique, la population des etats d'impurete dans la limite des temps longs est determinee pour le mouvement diffusif des electrons quittant les impuretes pour le continuum. En l'absence de processus de relaxation inelastique, on observe une lente decroissance en loi en puissance de la population

Asymptotic completeness in QED: (I). Quasilocal states

Projection operators onto the asymptotic scattering states are defined in the space of quasilocal states of QED in a Gupta-Bleuler formulation. They are obtained as weak limits for t → ± ∞ of expressions formed with interacting fields, in close analogy to the LSZ expressions known from field theories without infrared problems. It is shown that these limits exist in perturbative QED and are equal to the identity.

Superconducting properties of the SnTe-PbTe system doped with indium

An effect of the replacement of tin by lead in solid solutions Pb zSn 1−zTe doped with In on the impurity band position and corresponding changes of the superconducting parameters, Tc and H c2, is studied. Indium doping yields a threshold rise of Tc by an order of magnitude. As Fermi energy enters an impurity band, Tc varies drastically: it is described by a bell-shaped function of the lead content z, reflecting the impurity band movement across the heavy-hole Σ-band edge. The results are discussed in theoretical model taking into account an interaction of the valence band states with quasilocal impurity states and an additional term in electron - electron attraction associated with a virtual shift of the impurity level due to phonons.

Hot electrons in semiconductors with long-lived resonance impurity states

The nonlinear carrier transport phenomena are considered in the semiconductor doped with the impurities which form the quasi-local electron states. The resonant scattering of carriers by these centres has two aspects, first, the scattering as such, i.e. the change of the carrier flight direction, and second, the long electron residence at the centre during the scattering which leads to the decrease of the free carrier concentration in the band. In this paper the influence of both these sides of the resonant scattering upon the electron transport is considered for the first time. The Boltzmann kinetic equation is not applicable to this case so the special approach has been developed. The possibility of carrier transfer to the resonant centres leads to appearance of the N-shaped parts of the current-voltage curves which is similar to the Gunn effect.

Formation of self-trapped holes under optical excitation of impurity centres in LiF crystals

By the semi-empirical version of the HF method, referred to as INDO, combined with the embedded molecular cluster model, the electronic structures of O 2−V a , Mg c O a and Mg + c V a V c -centres in LiF crystals were calculated. Their common feature is the existence of quasi-local states within the valence band, the optical transitions from which result in the formation of the self-trapped holes.

Bistability transition in the self-trapping indiscrete lattice

A theoretical study of electronic properties on MoS2 nanoribbon is made on focusing the calculation of zero bias transport in the presence of disorders. Disorders including intrinsic and extrinsic vacancies and also weak uniform scatter defects are considered. The calculations are based on the tight-binding Green's function formalism by including an iterative procedure. The Slater–Koster transformations are used to determine the parameters. This model reduces the numerical calculation time. The unsaturated atoms at the edge of armchair (zigzag) ribbon induce some mid-gap states with nearly high (low) localization, which act as scattering centers. The antiresonances of created quasi-localized states due to vacancy cause the conductance of the armchair nanoribbon to decrease. Finally, the zigzag ribbon provides the highest sensitivity as well as selectivity between the smaller energy range, in the presence of the single weak scatter with potential value of 2 eV at the edge of the ribbon.

Phonon damping in conductors with quasilocal electron states

Decrement of the electron damping of phonons has been calculated in weakly doped metals and degenerate semiconductors. Local and quasilocal electron states in the field of impurity atoms have been taken into account. It is shown that in the frequency dependence of the decrement there appears a maximum caused by transitions between local, quasilocal, and band electron states.

Fractal Behaviour in Disordered Chains: 2D Stochastic Maps

The wave equation in disordered chains is expressed in terms of linear 2D stochastic mapping. The phase portraits found numerically are topologically different for quasi-localized and quasi-extended states. The maps have complex structure controlled by the disorder and the energy.

Superconductivity in AIVBVI Semiconductor Compounds with Quasilocal Impurity States

Superconductivity in an impurity band is studied with reference to tin telluride doped with indium. A theoretical model is put forward, which relates a high critical temperature of the low carrier density IV-VI compounds doped with group-III elements to the interaction between the lattice and quasilocal impurity states.

Picosecond luminescence studies of the recombination kinetics in GaInAsP

The temperature dependence of the radiative and nonradiative recombination processes in GaInAsP is studied by picosecond luminescence spectroscopy. The time resolved spectra reveal the importance of transitions involving quasi localized states below the band gap. The Auger coefficient increases at temperature above 150K and amounts to 4.3 10 −29 cm 6/s at room temperature.

Magnetic field and temperature effects on the luminescence quenching in quasi-one-dimensional mixed crystals: Ba[Pt1−xNix(CN)4] ⋅ 4H2O

The optical emission of Ba[Pt(CN)4] ⋅ 4H2O single crystals (MCP) is quenched by doping with [Ni(CN)4]2− complexes. The quenching process drastically reduces the E⊥c-polarized emission (E=electric field vector, c=stack axis). We have studied the E⊥c-polarized emission of mixed crystals Ba[Pt1−xNix (CN)4] ⋅ 4H2O with 0.01≲x≲0.1 as a function of temperature (1.7 K≲T≲30 K) and of magnetic field strength (0≤H≤5 T). The experimental results are explained by assuming two types of [Pt(CN)4]2− species within a linear system of quasi-localized states.

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1?x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb1−x Sn x Te single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (H uql) for the metallic state of Pb1−x Sn x Te(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>H uql the equationE F = const is valid instead of the equationn = const which is usual for degenerate semiconductors (E F is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb1−x Sn x Te energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (∼105 sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.

Magnetic field effects on 1,4-dibromonaphthalene phosphorescence emission

Triplet—triplet annihilative processes in 1,4-dibromonaphthalene are strongly affected by strong magnetic fields. As a result the phosphorescence emission intensity from the quasi-local states close to the exciton origin is strongly increased. Magnetic field effects are also manifest in time-resolved experiments.