Free and bound excitons in silver bromide under hydrostatic pressure

Abstract

Employing resonant Raman and luminescence spectroscopy, various exciton states and phonon modes are studied at low temperature in AgBr under hydrostatic pressure up to 0.7 GPa. The deformation potential for the indirect free exciton gap and mode Grüneisen parameters for various phonons are determined. Excitons bound to neutral donors and isoelectronic iodine are found to essentially derive from L-point valence band states.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Possibility of investigation the metastable donor states in ii-vi semiconductors under hydrostatic pressure

Abstract

The model of bistable donor centers was proposed for describing the metastable electron traps in cubic semiconductors. The detailed analysis of the model suggests that the bistable donors should be very common in II-VI semiconductors doped with VII group impurities. Under normal conditions the additional metastable states are degenerated with the conduction band. However, the analysis of the energy bands pressure coefficients for II-VI materials shows that under hydrostatic pressure the donor traps can change the electrical properties of the sample.     

Quantum charge density fluctuations and the phase transition in Ce

We have calculated the quantum quadrupolar interaction due to charge density fluctuations of localized 4f-electrons in Ce by taking into account the angular dependence, the degeneracy of the localized 4f -orbitals and the spin-orbit coupling. The calculated crystal field of 4 f electronic states is in good agreement with neutron diffraction measurements. We show that orientational ordering of quantum quadrupoles drives a phase transition at K which we assign with the transformation. In the phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory accounts for the first order character of the transition and for the cubic lattice contraction which accompanies the transition. The transition temperature increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the phase transition. Received 19 October 1998     

Wigner crystallization of electrons and holes in amorphous silicon nitride. Antiferromagnetic ordering of localized electrons and holes as a result of a resonance exchange interaction

The predicted quantitative relation between the density and trapping cross section of traps in Si₃N₄ and the Coulomb repulsion radius in the Wigner crystallization of carriers in localized states is observed experimentally. The absence of ESR for localized electrons and holes in Si₃N₄ is interpreted on the basis of a model of a resonance exchange interaction of electrons on account of tunneling via localized states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 489–494 (10 October 1996)     

Hydrostatic pressure investigations of resonant tunnelling through X-minimum-related states in a single barrier GaAs/AlAs/GaAs heterostructure

Abstract

We have investigated the resonant tunnelling of electrons through X-valley related states in a single AlAs barrier with Si donor & doping. Under high hydrostatic pressure all the peaks observed in the differential conductance-voltage (σ-V) characteristics shift with a pressure rate of -lSmeV/kbar, which is expected for X-minima related electronic states.     

High pressure X-ray diffraction studies on UAI2, UAI2, and UAI4 systems

Abstract

The UAI₂, UAI₂, AND UAI₄ compounds have been studied by high pressure X-ray diffraction up to a maximum pressure of ～ 35 GPa. The compressibility behaviour of UAI₂ has been found to be consistent with the itinerant 5f states, whereas that of UAI₂ and UAI₄, indicate a more localized nature. Further, UAI₂ has been found to undergo a structural transition at ～ 11 GPa and the structure of the high pressure phase has been identified to be of MgNi₂ type with space group P6₃/mmc. The structure of UAI₂ at ambient pressure is of MgCu₂ type with space group Fd3m. From the electronic considerations, for instance, free electrons per atom ratio e/a, it is anticipated that it may transform back to MgCu₂ type structure at still higher pressures. On similar considerations, it is expected that most of the AB₂ type Laves phase compounds of the ‘f’ electron systems may undergo the structural sequence: MgCu₂ – MgZn₂ (or MgNi₂) – MgCu₂ due to increased delocalization of their ‘f’ electron states.     

Ab initio modelling of band states in doped diamond

Presented in this study is an analysis of the electronic properties of doped diamond calculated using the Vienna ab initio simulation package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally into a 64-atom diamond supercell and include the single-electron acceptors boron and aluminium, the single-electron donors nitrogen and phosphorus and the double-electron donors oxygen and sulphur. Co-doping of diamond with sulphur and boron has also been briefly examined. The doped supercells have been relaxed, followed by calculation of electronic properties from the electronic density of states such as the indirect bandgap E _g, the valence bandwidth and an examination of the acceptor and donor states in the bandgap. It is anticipated that this study will provide a useful comparison of the third- and fourth-row donors and acceptors in diamond.     

Wigner crystallization and a resonance exchange mechanism for electrons localized in an amorphous insulator with a high trap density

The possibility of Wigner crystallization of electrons in an amorphous insulator with a high trap density is discussed. A new exhange interaction mechanism is proposed, based on resonance tunneling of electrons between unfilled localized electronic states. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 483–488 (10 October 1996)     

High Pressure as a Tool for Characterizing Charge Carriers in a Conducting Polymer

A characteristic feature of conducting polymers is the existence of localized charge carriers. The localization process is closely related to the charge carrier-phonon interaction, which is a function of local molecular properties of the polymer chain on which the carriers are localized. Depending on this interaction in conducting polymers with a non-degenerate ground state singly charged polarons or doubly charged bipolarons may exist. It will be shown that high pressure is a useful tool for studying charge carrier properties by influencing the local molecular conformation. A transition between both types of charge carriers is observed in polypyrrole.     

On localization of f-electrons in high temperature phases of early actinides

Abstract

It has been suggested by Vohra and Holzapfel that the f-electrons in early actinide elements become localized at high temperatures close to melting. Their conclusion is based on the unusually large thermal expansions of Np and Pu and their observation of the high temperature β-Np structure under pressure in trivalent 3d Sc, which does not have any f-electrons. We show that these have alternative explanations. Further, the atomic volumes of the high temperature bcc phases of U, Np and Pu are considerably lower than the ones calculated from band theory assuming localized f-electrons. We conclude that the f-electrons continue to be delocalized at high temperatures in U, Np and Pu except Pa.     

Catalysis of the 〈b̄b〉 Condensate in the Composite Higgs Model

Increasing external pressure gives rise to s–d electron transfer in calcium that results in the localization of the charge density in the interstices of the crystal structure, i.e., the formation of an electride. The corresponding electronic states are partially filled and localized and, hence, electronic correlations could arise. We have carried out theoretical calculations for the high-pressure phases of Ca taking into account the Coulomb interactions between the electronic states centered on the interstitial site. The results of our calculations and proposed microscopic model showed that the structural phase transition under high pressure is due to an interplay of hybridization and correlation effects. Furthermore, it was found that the Coulomb repulsion can explain the experimentally observed anomalous increase in resistivity of the simple cubic phase of calcium under pressure.

     

Optical and thermal orientation of localized excitons in solid solutions under resonant excitation in a longitudinal magnetic field

A theoretical and experimental study of the effect of a longitudinal magnetic field on optical orientation and magneto-circular polarization of the luminescence of localized excitons in semiconducting solid solutions is reported. It is shown that recombination takes place through two types of emitting states differing substantially in the degree of anisotropy, g factor, and spin relaxation time. Estimates are made of the g factors, anisotropic and exchange splittings, lifetime, and spin relaxation time of localized states in a CdS_0.96Se_0.04/GaAs solidsolution epitaxial layer. Fiz. Tverd. Tela (St. Petersburg) 40, 900–902 (May 1998)     

Localized ferrons and the Mott transition via the ferromagnetic phase in doped antiferromagnetic semiconductors

The states of single-and double-charge donors in an antiferromagnetic crystal are investigated taking into account the formation of magnetized regions (localized ferrons) around them. Double-charge donors should be in a state of the (1s)(2s) type, which is energetically favored over the (1s)² state. In doped antiferromagnetic semiconductors, the usual Mott transition is impossible because the single-charge donors in them are ferrons. Instead, the donor electrons are delocalized through the transition of the crystal to the single-electron state. It is unlikely that a ferromagnetic-antiferromagnetic (FM-AFM) mixed state will occur in this case. In the case of double-charge donors, transition to an FM-AFM mixed state should occur.     

Hopping transport in molecularly doped polymers: Joint modelling of positional and energetic disorder

Hole transport in molecularly doped polymers (MDPs) is modelled as random walks in a bias field E over organic donors D embedded in a polymer matrix. Positional disorder for donor fraction p < 1 is represented by randomly placing donors at sites in a fcc lattice, while energetic disorder is given by a Gaussian distribution of site energies with width and spatial correlations in a sphere of radius R c . Random walks generated by Marcus or small polaron rates for steps between nearby donors yield the mobility w( E , T ). In addition to and R c , the rates depend on the parameters nand ufor the distance dependence and reorganization energy respectively. With tritolylamine (TTA) in polystyrene as the paradigm, a procedure is presented for fixing the interdependent parameters , u, nand R c that reproduce the field and temperature dependences of w( E , T ) over a wide range of p that includes dilute systems with different TTA packings enforced by saturated bonds. Positional disorder exceeds energetic disorder in dilute systems and yields constant w( E , T 0 ) near room temperature. Joint modelling of TTA and related systems accounts for the characteristic w( E , T ) of MDPs and substantially extends the picture of hopping between localized states, with nincreased by about 15% and reduced by about 25% from conventional analysis using the Gaussian disorder model. Similar parameter changes are expected in other MDPs based on the compensation temperature T 0 and on scaling TTA results.     

Kinetic effects of high pressure in congested and strained molecular systems. A useful application of high pressure in organic synthesis

Abstract

Steric congestion of reaction centers produces late transition states in organic reactions. Intrusion of steric hindrance induces a stronger pressure acceleration. Some relevant examples are given: Menshutkin, Wittig, Michael, Henry reactions, cycloadditions. The pressure parameter is, therefore, of high value in the synthesis of crowded molecules via sterically demanding reactions.     

Characteristics of the thermodynamics of the electronic subsystem in intercalated titanium dichalcogenides

Results of an electrochemical experiment are used to calculate the concentration dependences of the chemical potential and entropy of silver in Ag_xTiTe₂. It is shown that the electronic contribution to the thermodynamic functions must be taken into account in any explanation. The thermodynamic functions are modeled and the density of states in the conduction band is refined. It is confirmed that a band of localized states exists near the Fermi level, as was hypothesized previously to explain kinetic experiments. Various mechanisms are considered for the formation of these localized states. Fiz. Tverd. Tela (St. Petersburg) 40, 1187–1189 (July 1998)     

Impurity absorption of light involving resonant states of shallow donors in quantum wells

The energy spectrum of localized and resonant states of shallow donors in heterostructures GaAs/Al_xGa_1?xAs with quantum wells is calculated. The widths of the resonant states belonging to the second size quantization subband are determined. It is shown that the width of a resonance level is mainly determined by the interaction with optical phonons. The spectrum of impurity absorption of light due to electron transitions from the ground state of the donor to the resonant states belonging to the second size quantization subband is calculated.     

Polar Kerr rotation of the ferromagnet

Magneto-optical data on EuB ₆ , a ferromagnet with a Curie temperature T _C ∼ 15 K, are presented and discussed in detail. We have measured the polar Kerr rotation, covering a spectral range from the infrared up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields between 0 and 10 T. The Kerr rotation in high fields and at low temperatures is enormous. Our observations, which implicitly reflect the large magnetoresistive effects, are shown to discriminate between the spectroscopic response of localized and itinerant electronic states. Our data analysis is based on the phenomenological Lorentz-Drude model, following from the classical dispersion theory and appropriately extended to magneto-optical experiments. Received 30 January 2003 / Received in final form 21 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: degiorgi@solid.phys.ethz.ch     

Optical orientation of donor-bound excitons in nanosized InP/InGaP islands

Spin splitting of optically active and inactive excitons in nanosized n-InP/InGaP islands has been revealed. Optically inactive states become manifest in polarized-luminescence spectra as a result of excitons being bound to neutral donors (or of the formation of the trion, a negatively charged exciton) in InP islands. The exchange-splitting energies of the optically active and inactive states have been determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1745–1752 (September 1998)