Quasiholes in a MgZnO/ZnO Heterojunction as Vacancions

Recombination of two-dimensional electrons of a low density in a MgZnO/ZnO heterojunction with localized valence-band holes is considered. It is suggested that quasiholes appearing in the process of photoluminescence of strongly interacting two-dimensional electrons should be considered as vacancion quasiparticles in a quantum Wigner crystal. Vacancions formed upon the removal of an electron from the crystal are delocalized owing to the tunneling effect. The vacancion energies E(k) form a band of width D that depends on the probability of vacancy tunneling. The width D corresponds to the width of the photoluminescence band of the two-dimensional electron system. The shape of the photoluminescence band of the Wigner crystal is obtained using the tight-binding approximation for the vacancion dispersion relation E(k) is compared with experimental results.

     

Shear modulus of polydomain, mono-domain and non-mesomorphic side-chain elastomers: Influence of the nematic order

We investigate the behavior of the complex shear modulus of a series of elastomers including mono-domain and poly-domain liquid crystal samples, and a non-mesomorphic sample. We find that the dynamics of the glass transition are strongly modified by the nematic order. This result explains why the truly elastic response of liquid crystal elastomers can only be observed in the isotropic phase at very high temperatures and at very low frequencies. Between the elastic regime and the glassy state, the elastomers have a visco-elastic regime, which is characterized by a Rouse-like behavior for mono-domain and poly-domain samples, and by a Zimm-like behavior for the non-mesomorphic sample. We also show that the mono-domain sample exhibits marked anisotropy of the shear-modulus G ^′. This anisotropy, which is observed for the first time, is a function of frequency and is inverted between low and high frequencies, due to relaxation effects of the orientational order. Received 28 January 2000 and Received in final form 16 October 2000     

Degenerate Electron Gas at Finite Temperatures. II

The degenerate high density electron gas is considered at finite temperatures using SOMMERFELD 's expansion. In particular the energy and the life time of one-particle excitations are investigated. For this purpose the real part of the self energy function is calculated up to the order e⁴. Using results derived in [4] the spectral function A(poM) = 2 JmG(poM) can be given explicitely. Furthermore an equation of state and a correct expression for the specific heat are given, the latter containing additional terms to the result of GELL -MANN .     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Pair exchange interaction in a crystalline nitroxide radical ESR study of dimers in the triplet state

The dimerization of the 9-aza-bicyclo (3,3,1) nonan-3-one-9-oxyl in the solid state is investigated by use of ESR spectroscopy. The ESR spectrum of a single crystal is characteristic of symmetric pairs of exchange-coupled radicals in a thermally accessible triplet state. The presence of well-resolved hyperfine structure is evidence for strongly localized excitations with a jumping rate lower than 10⁷ Hz.

The ESR spectrum is well described by the spin hamiltonian

At 35 GHz the observed splitting of the m _s=+ 1?0 transition has been found to be slightly different from that of the m _s=0?-1 one; this anomaly is explained by the mixing of the m _s electronic states.

The parameters and the principal directions of the zero-field splitting, spectroscopic and hyperfine tensors are determined and discussed. The principal directions of the dipolar tensor indicate a nearly equal spin density on the nitrogen and oxygen atoms; from the fine structure parameters D and E, determined to be (-0·0723 ± 0·0005) cm^-1 and (-0·0044±0·0003) cm^-1 at T=293 K respectively, it is suggested that the unpaired electron is partly delocalized on the molecule.

The singlet-triplet energy gap (J) and the zero-field splitting parameters are shown to be linearly temperature-dependent. These variations with temperature are attributed to the thermal expansion of the crystal lattice.     

Coulomb Crystals with Isotopic Impurities

Vibration modes and thermodynamic properties of a body‐centered cubic (bcc) Coulomb crystal with a small admixture of substitutional isotopic impurities are studied analytically applying the perturbation theory of disordered crystal spectra and the Lifshitz‐Krein trace formula. We calculate the density of phonon states of the perfect bcc Coulomb crystal and use it to compute the heat capacity of the crystal with impurities in a wide range of temperatures. It is shown that the ratio of an impurity contribution to the crystal heat capacity over the perfect crystal specific heat tends to a constant in the low‐temperature quantum regime and decays as T^–2 in the classic regime of high temperatures. It is also shown that even a small concentration of heavy impurities amplifies significantly the total crystal heat capacity. The results are compared with those obtained using the more conventional linear mixing theory. It is demonstrated that both methods give similar results at all tempera‐tures when the impurity mass is not too different from that of the base ions but a strong discrepancy is observed at low and intermediate temperatures when impurities are noticeably lighter or heavier. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatially-resolved spectroscopic diagnosing of aluminum wire array Z-pinch plasmas on QiangGuang-I facility

Spatially-resolved crystal spectrometers with a high spectral resolution are developed to diagnose K-shell x-ray radiation from Z-pinch plasmas. These diagnostic apparatuses are successfully applied to aluminum wire array Z-pinch experiments on QiangGuang-I facility, a driver with a pulsed current up to about 1.5 MA in 80 ns. Time-integrated experimental results show that the K-shell x-ray emission lines of aluminum Z-pinch plasmas are dominated by line emissions from helium-like ionisation state. Bright spots that might have higher electron temperature or density are produced randomly in location and size along the z-axis during implosions. According to the experimental data, the electron temperature and the ion density are estimated to be between 250 eV and 310 eV, and between 7.0×10¹⁹cm^-3 and 4.0×10¹⁹ cm^-3 respectively, while the ion temperature is inferred to be about 10.2 keV, which is much higher than the electron temperature.     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

红宝石Cr3+:Al2O3的电子云延伸效应及压力对电子云延伸效应影响(英文)

本文以Cr³⁺自由离子的3d电子径向波函数为基础,对Cr³⁺:Al₂O₃中的电子云延伸效应进行了理论研究,引入了电子云延伸效应系数κ,得到了Cr³⁺:Al₂O₃中Cr³⁺离子的最优化3d电子径向波函数.并研究了压力对电子云延伸效应系数κ的影响.     

The interaction of two excitons in a benzene crystal

The paper deals with the investigation of pair interactions in a system of weakly interacting excitons in a benzene crystal as a function of the distance of molecules, while interactions due to phonons are neglected. It is demonstrated that this interaction has a repulsive character of the type [A/(R +a)]e ^–k (R+a), which determines the possibility of realizing the superfluid state of the exciton system provided that the density of excitons is sufficiently great.The author wishes to express her thanks to Prof. M. Trlifaj for suggesting the present theme and for useful discussions. Her thanks are also due to Miss A. Piútová for her help with the computations.     

Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca₂GeO₄: Cr⁴⁺ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca₂GeO₄: Cr⁴⁺ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr⁴⁺ ions substituted for Ge⁴⁺ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr⁴⁺ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr⁴⁺ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.     

Coupling effects in a photonic crystal microcavity with embedded semiconductor quantum dot

The present work investigates the effects of relevant parameters of InAs/GaAs quantum dot and photonic crystal slab-based microcavity on the QD–cavity coupling characteristics, in detail. We employ variational approach to find exciton state in QD and to find cavity modes we use the open source GME code. Calculations have performed in linear regime where excitons behave as bosons which correspond to the limit of low excitation. The dynamics of the system are studied using the first order correlation function (G⁽¹⁾(t,τ)). We will show how G⁽¹⁾ varies with time in both strong and weak coupling regimes. Our results indicate that the achieving of strong coupling regime is affected by the size of the quantum dot and how to engineer the photonic crystal microcavity to maximize the ratio of quality factor and mode volume.     

(e, 2e) triple differential cross section for ionization-excitation of helium

Simultaneous ionization and excitation of helium by electron impact is considered in an improved second Born approximation. The wave function of the low energy ejected electron is obtained in the field of residual He⁺ ion in 2s-state. The calculation has been done for the processe ⁻+He→e ⁻+He⁺(2s)+e ⁻ in the coplanar asymmetric geometry with Hartree-Fock wave function of Byron and Joachain for the helium ground state and the results are compared with the absolute experimental data of Dupreet al [J. Phys. B25, 259 (1992)] at ∼ 5.5 keV incident energy. Our results are found to increase the ratio of the recoil peak to binary peak intensity by about 30% over the first Born results and thus to bring it closer to the experimental data.     

Boron 10B—11B Isotope Substitution as a Probe of the Mechanism Responsible for the Record Thermionic Emission in LaB6 with the Jahn—Teller Instability1

We have tested the conduction band electrons of lanthanum hexaboride that is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. We performed infrared spectroscopic, DC (direct current) resistivity and Hall-effect studies of LaB₆ single crystals with various ¹⁰B and ¹¹B isotope contents. We find that only a small amount of conduction electrons behave as Drude-type charge carriers while about 70% of the electrons are involved in collective oscillations of electron density coupled to vibrations of both the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions loosely bound to the lattice. We suggest that exactly these non-equilibrium conduction electrons determine the extraordinary low work function of thermoemission in LaB₆.

     

Kinetics of flow stress in crystals with high intrinsic lattice friction

A relatively simple theoretical model, based on the concept of kink-pair mode of escape of screw dislocations trapped in Peierls valleys, has been developed to account for the observed temperature dependence of the critical resolved shear stress (CRSS), τ, and of the associated activation volume, v, in crystals with high intrinsic lattice friction at rather low temperatures. In this model, the CRSS varies with temperature T as τ^1/2?=?A–BT, and the associated activation volume v depends on temperature T as v ^?1?=?C–DT, where A, B, C and D are positive constants. Moreover, the activation volume v is found to be a function of τ such that vτ^1/2 is constant for a given slip system. Data analysis of the temperature dependence of the CRSS of W, α-Fe, Cr and V metal crystals shows excellent agreement between theory and experiment in both regime III (low temperature or high stress) and regime II (intermediate temperature/stress). However, the predicted temperature and stress dependence of the activation volume are borne out by experiment in regime II, but lack quantitative agreement in regime III. On the other hand, the CRSS of CdTe crystals at low temperatures (T?≤?200?K) is determined by the Peierls mechanism, whereas the weak temperature dependence of the CRSS above 200?K is probably governed by the breakaway of edge dislocation segments from arrays of pinning points due to localized defects in the crystal.     

On exchange coupling and bonding in the Gd2@C80 and Gd2@C79N endohedral dimetallo-fullerenes

A series of computational experiments performed with various methods belonging to wave-function and density functional theories approaches the issue of bonding regime and exchange coupling in the title compounds. Gd₂@C₈₀ is computed with a very weak exchange coupling, the sign depending on the method, while Gd₂@C₇₉N has resulted with a strong coupling and ferromagnetic ground state, irrespective of the computational approach. The multi-configuration calculation and broken symmetry estimation are yielding closely coincident coupling constants, of about J ～ 400 cm^?1. No experimental estimation exists, but the ferromagnetic ground state of Gd₂@C₇₉N is confirmed from paramagnetic resonance data. The different behaviour is due to particularities of electron accommodation in the orbital scheme. The exchange effects localised on atom lead to preference for parallel alignment of the electrons placed in the 4f and 5d lanthanide shells, determining also a ferromagnetic inter-centre coupling. The structural insight is completed with a ligand field analysis of the density functional theory results in the context of frozen density embedding. The energy decomposition analysis of bonding effects is also discussed. Finally, with the help of home-made codes (named Xatom+Xsphere), a model for the atom encapsulated in a cage is designed, the exemplified numeric experiments showing relevance for the considered endohedral metallo-fullerene issues.     

A bonding evolution theory study on the catalytic Noyori hydrogenation reaction

ABSTRACT

The electronic rearrangements involved in Noyori hydrogenation reactions with double bonds (ethene and formaldehyde) are analysed using the bonding evolution theory. The study and analysis of the changes on the electron localisation function topology along a given reaction path reveals fluxes of electron density, allowing to unambiguously identify the main chemical events happening along the chemical reactions. This analysis shows that the first hydrogen transfer (with hydride character) occurs before the transition state (TS), while the second hydrogen transfer (with proton character) takes places after having reached the TS. The lower energy barrier found for formaldehyde over ethene is explained by two reasons. First, the hydride transfer is favoured for the C?=?O bond over C?=?C due to the electrophilic character of the carbon atom. Second, a negatively charged CH₃–X (X?=?CH₂, O) hidden intermediate is formed in the proximities of the TS region. The oxygen atom is able to stabilise this negatively charged species more effectively than the CH₂ group due to its higher electronegativity and the presence of V(O) lone pairs. The obtained analysis explains and rationalises catalyst chemoselectivity (C?=?O vs. C?=?C). Finally, a curly arrow representation diagram accounting for the electronic rearrangements is proposed on the basis of BET results.     

Transport through artificial single-molecule magnets: Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn+2 ion(S=5/2) are investigated by the non-equilibrium Green function method.We consider a minimal model where the Mn-hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with(2S+1) sublevels.In the sequential tunneling regime,the differential conductance exhibits(2S+1) possible peaks,corresponding to resonance tunneling via(2S+1) sublevels.At low temperature,Kondo physics dominates transport and(2S+1) Kondo peaks occur in the local density of states and conductance.These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.     

Two-dimensional density of states for electrons bound to impurities inside inversion layers at the semiconductor-insulator interface

The density of states for electrons bound to Na⁺ impurities inside the inversion layer at the semiconductor-insulator interface of an MOS structure is calculated as a function of impurity concentration. The impurity potential is considered unscreened and the electrical quantum limit is assumed. A simple one-electron Hamiltonian is used and the disorder is treated through a cluster calculation. It is shown that the impurity band has a considerable bandwidth for impurity concentrations in a range of the experimental regime (this result agrees with the experimental findings of Hartstein and Fowler), and that the upper Hubbard band stands well above the lower band at very low concentration (in rough agreement with recent calculation done by Phelps and Bajaj on D^? state).     

Amorphous metals and their superconductivity

Quite a number of metals and alloys can be forced into the amorphous state by quenching. In particular, condensation onto a substrate cooled to He-temperature is very effective. The superconductivity and related properties of these metals are considered in this article. The influence of the amorphous structure on the electron and the phonon system is discussed. In simple metals the electrons show a free electron behaviour. The transverse phonons are softened but maintainn the ω³-density of states at low frequencies. The electron-phonon interaction does not conserve momentum due to the loss of translational invariance in the amorphous metal and yields a large contribution to α²F(ω) at low frequencies. Therefore the amorphous superconductors are strong coupling. The strong coupling character alters the temperature dependence of several superconducting parameters such as critical fields etc. The characteristic superconducting properties and superconducting fluctuations are discussed.