Hydrogenic impurity states in zinc-blende InxGa1−xN/GaN in cylindrical quantum well wires

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende (ZB) In_xGa_1−x  N/GaN cylindrical quantum well wires (CQWWs) is investigated using variational procedures. Numerical results show that the ground-state donor binding energy _Eb$E_b$ is highly dependent on the impurity position and the CQWWs structure parameters. The donor binding energy for a shallow donor impurity located at the center of the CQWWs is the largest. As the impurity position changes from the center of the wire to its edge, the donor binding energy gets smaller. Also, we have found that In concentration is a very important value to tailor the system, since the binding energies close to binding energy maxima are strongly dependent on In content.     

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.     

Impact parameter dependence ofK-shell excitation probabilities in Ar — Ar-collisions

The impact parameter (b) dependence of theK-shell excitation probabilities (P _K (b)) in the Ar on Ar collision system at 4.5, 8, 12 and 20 MeV impact energy have been measured over a wideb range (from the kinematik peak to far outside the adiabatic peak) for different projectile charge states. In agreement with the data of Lutz et al. and Nolte et al. a very strong increase ofP _K (b) towards very smallb values (corresponding to scattering angles up to 40°) has been observed. From the measuredb and projectile charge state dependences it has been derived that for the systems investigated here the 2pπ-2pσ rotational coupling mechanism gives the dominant contribution to theK-shell excitation at very smallb (kinematic peak regime) as well as at very largeb (adiabatic peak regime). The analysis of the data indicates that in disagreement with the general assumption of ab independent 2pπ vacancy probability P_2pπ , the probability P_2pπ increases towards smallerb. At the high projectile energy the data confirm the results of Nolte et al. of a large dynamically produced probability P _2pπ ^d and of strong contributions of direct 2pσ and 1sσ excitation to the measuredP _K (b) at small impact parameters.     

Fine structures of indirect exciton absorption in phosphorus-doped silicon

A new indirect exciton absorption structure has been observed in phosphorus-doped silicon crystals by using a wavelength modulation method. The structure is interpreted in terms of two-electron transitions involving a free exciton and the valley-orbit states of a phosphorus donor, in which the donor is left in the 1s (A₁) singlet state.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

Primordial inflation and present-day cosmological constant from extra dimensions

A semiclassical gravitation model is outlined which makes use of the Casimir energy density of vacuum fluctuations in extra compactified dimensions to produce the present-day cosmological constant as ρ _Λ ∼M ⁸/M _P ⁴, where M _P is the Planck scale and M is the weak interaction scale. The model is based on (4+D)-dimensional gravity, with D=2 extra dimensions with radius b(t) curled up at the ADD length scale b ₀=M _P /M ²∼0.1 mm. Vacuum fluctuations in the compactified space perturb b ₀ very slightly, generating a small present-day cosmological constant.The radius of the compactified dimensions is predicted to be b ₀≈k ^1/40.09 mm (or equivalently M≈2.4 TeV/k ^1/8), where the Casimir energy density is k/b ⁴.Primordial inflation of our three-dimensional space occurs as in the cosmology of the ADD model as the inflaton b(t), which initially is on the order of 1/M∼10⁻¹⁷ cm, rolls down its potential to b ₀.     

Structure and energy of a line vortex in a three-dimensional ordered Josephson medium

Two possible equilibrium configurations of line vortices in a three-dimensional ordered Josephson medium for any value of structural factor b are considered: the center of the vortex coincides with the center of one of the cells and the center of the vortex is on one of the contacts. Infinite sets of equations describing these configurations are derived. The infinite set can be made finite if currents away from the center are neglected. The assumption b = 0 is shown to be valid if pinning parameter I is less than 0.25. For I > 0.25, the structures and energies of both configurations of line isolated vortices are calculated throughout the range of structural factor b. As structural factor b increases, phase jumps at the contacts, currents in the central part of the vortex, and the total energies of the vortices decrease in both configurations. This leads to a decrease in critical field H _c1. For all values of I and b, the energy of the vortex centered on the contact is higher than that of the vortex centered in the middle of the cell.     

Hyperfine structure of the 8p 2 P 3/2 and 8p 2 P 1/2 levels of115In

The hyperfine structure of the 8p ² P _3/2 and² P _1/2 levels of¹¹⁵In was measured using high-resolution laser spectroscopy on a collimated indium atomic beam. Step-wise excitations from the metastable 5² P _3/2 state via the shortlived 6s ² S _1/2 state were employed. For the magnetic-dipole and electric-quadrupole interaction constantsa andb, respectively, the following results were obtained:a(8p ² P _3/2)=16.3 (3) MHz,b(8p ² P _3/2)=11 (3) MHz anda(8p ² P _1/2)=44.0 (5) MHz. The magnetic hyperfine structure is shown to be strongly influenced by core polarization. For the electric-quadrupole moment of¹¹⁵In the valueQ=0.79 (20) barn is deduced.     

Evaluation of noncirrhotic hepatic parenchyma with and without significant portal vein stenosis using diffusion-weighted echo-planar MR on the basis of multiple-perfusion-components theory

To determine whether diffusion-weighted echo-planar MR images are sensitive to liver perfusion difference.Noncirrhotic livers of 71 patients (43 males, 28 females; age range, 22-87 years; mean, 61 years) without (n=51) and with (n=20) significant (>70%) portal vein stenosis (accompanying proximal hepatic arterial stenosis and/or biliary tract obstruction in 10) by tumors were examined with diffusion-weighted echo-planar sequences (modified for b factors of 1, 28, 66, 288 and 600 s/mm²). On the basis of multiple-perfusion-components theory, i.e., assuming logarithm of signal intensity for liver perfusion is linearly attenuated versus logarithm of a smaller b factor, we defined the slope of the line as the perfusion-related D′ value. The D′ values of these livers were calculated from images with b factors of 1, 28, and 66 s/mm². The livers' apparent diffusion coefficient values for diffusion (ADC_d values) were calculated from images with b factors of 288 and 600 s/mm².The livers with significant portal vein stenosis had statistically lower mean D′ values than the livers without portal vein stenosis (P<.001 on the Mann-Whitney U test). However, there was no significant difference in ADC_d values between these liver types (P>.05).The D′ value calculated from diffusion-weighted echo-planar sequences with plural smaller b factors may be sensitive to liver perfusion difference.     

X and K band ESR study of the Pb Interface centres in thermally oxidized p-type (001)Si wafers at low temperatures and influence of medium-dose As+ ion implantation

Electron spin resonance (ESR) experiments have been carried out at cryogenic temperatures (4.2 ? T ? 35 K) and room temperatures at 9.0 and 20.9 GHz on the P_b0 and P_b1 (commonly referred to as P_b) spin-active defects residing at the Si/SiO₂ interface. The ESR lineshapes were shown to display gaussian characteristics with inhomogeneous line broadenings amounting to 0.7 ± 0.1 and 0.2 ± 0.1 mT at K band for P_b0, and P_b1 respectively, whereas the oscillator strength of both signals followed the paramagnetic law (~ T^?1) down to 4.2 K within experimental error. In general the observed P_b spectrum appeared to have fewer peaks than in other observations, at most displaying two distinguishable lines. Mostly however, only one somewhat broad signal (of measured peak-to-peak linewidth >B_ptp ? 1.5 ± 0.15 mT and g = 2.0058 ± 0.0002 for $\text{\$}\text{?}$B 6 [001] was observed. By fully incorporating ΔB_ptp data for the first time in these observations and using computer simulations, it has been shown that the pervailing experimental spectrum always contains the signals from both kind of centers although mostly these are not separately discernable. Further, it emerged that the actual appearance of the experimental spectra is dirigated by the presence of a distribution of the SiIII unsaturated bond orientations around the ones normally prescribed by the Si crystallinity at the interface. It is found that for both centers this “angle” distribution predominantly occurs vertically with respect to the (001) interface plane. Ion implantation of 10¹⁴ As⁺ cm^?2 at 60 or 80 keV into the oxide layer of the Si/SiO₂ structure is shown to randomize the P_b dangling bond (DB) orientations (resulting in an isotropic g value behavior) but the effect of this is totally eliminated by subsequent annealing at 1000°C in N₂ ambient. It is argued that ESR has become a very sensitive means to study the “purity” of interfacial DB positions with respect to the Si single-crystal prescribed positions and to enable the display of collective fingerprints of the interface defects.     

The nature of excitonic complexes in ZnTe

Uniaxial stress experiments were used to investigate the nature of the luminescence lines observed at low temperatures in ZnTe in the vicinity of the absorption edge. The single crystals used in this experiment were grown from solution of ZnTe in tellurium. Both “as-grown” crystals and crystals annealed in Zn vapour were investigated. The most intense line in “as-grown” crystals is attributed to an exciton bound to a neutral acceptor. The binding energy of the exciton in this center is 6 meV. After annealing a new center appears in the same spectral region. Stress experiments as well as the temperature dependence of the intensity of the luminescence indicate that this center is a complex consisting of an exciton and an ionized donor. Splitting of J = 1 (Γ₅) and J = 2 (Γ₃ + Γ₄) levels was found to be 1.2 meV.     

Transformation of As-Grown Phosphorus-Related Centers in HPHT Treated Synthetic Diamonds

This communication presents new data on phosphorus-containing centers in synthetic diamonds grown in the P–C system by high-pressure high-temperature (HTHP) method and annealed in the temperature range of 2,073–2,573 K. The electron paramagnetic resonance (EPR) study has shown that as-grown at 1,873 K diamonds contain single substitutional nitrogen (P1) and single substitutional phosphorus (MA1) centers. The main part of the spin density in the MA1 center locates on the carbon atom C₁ separated from phosphorus by one carbon atom. HPHT annealing (7 GPa, 2,073–2,273 K) results in aggregating substitutional nitrogen and phosphorus atoms. On the first step of annealing (2,073 K) of as-grown diamonds nitrogen–phosphorus NIRIM8 (NP1) centers are created. It is supposed that nitrogen and phosphorus atoms in this center are separated by two carbons. Further temperature increasing shifts the nitrogen atom toward phosphorus and creates two new nitrogen–phosphorus centers NP2 and NP3 with the supposed structures C₁–N–C–P and N–P–C₁, respectively. The main part of the spin density in MA1, NIRIM8 (NP1), NP2 and NP3 is located on the carbon atom C₁. Annealing these samples in the temperature range of 2,073–2,273 K has shown vanishing of NIRIM8 and increasing of NP2 and NP3 centers. HPHT annealing of diamonds at 2,573 K significantly changes the electron paramagnetic resonance (EPR) spectra: all previous nitrogen–phosphorus centers disappear and two new phosphorus centers NP4 and NP5 are created. Features of these centers are g ≈ 2.001 and high spin density located on the phosphorus atoms. The NP5 center is sensitive to X-ray irradiation and low-temperature annealing. The EPR spectra of both these centers are due to the hyperfine structure of one phosphorus atom. The structures of all phosphorus-containing centers are discussed.     

Photosynthetic electron transport in the cyanobacteriumSynechocystis sp. PCC 6803: High-Field W-band and X-band EPR study of electron flow through photosystem I

In this work, by using the respective advantages of W- and X-band electron paramagnetic resonance (EPR) spectroscopy techniques to investigate electron transport processes, we have studied the light-induced redox transients of the primary electron donor P₇₀₀ and the secondary acceptor A₁ in photosystem (PS) I complexes of intact cyanobacterial cellsSynechocystis sp. PCC 6803. We found that the kinetic behavior of the cation radical P₇₀₀ ^·+ generated by illumination with continuous light, and the EPR intensity of the radical pair P₇₀₀ ^·+A ₁ ^·? generated upon laser pulse illumination strongly depend on the illumination prehistory (either the sample was frozen in the dark or during illumination). Both these processes were sensitive to the presence of electron transport inhibitors which block electron flow between the two photosystems. In line with our X-band EPR data on the kinetics of light-induced redox transients of P₇₀₀, our high-field W-band EPR study of the radical-pair state P₇₀₀ ^·+A ₁ ^·? shows that photosynthetic electron flow through the PS I reaction center is controlled both on the donor and on the acceptor side of PS I.     

A new molecular C60 complex with bis(methylenedithio)tetrathiafulvalene (BMDT-TTF): Synthesis, crystal structure, and properties

A new molecular C₆₀ complex of the composition (BMDT-TTF) · C₆₀ · 2CS₂ (I) with the bis(methylenedithio)tetrathiafulvalene (BMDT-TTF) organic donor is synthesized. The molecular and crystal structures of this complex are determined by x-ray diffraction. The (BMDT-TTF) · C₆₀ · 2CS₂ (I) compound crystallizes in a monoclinic crystal system. The main crystal data are as follows: a=13.550(5) Å, b=9.964(7) Å, c=17.125(8) Å, β=99.52(4)°, V=2280(2) ų, M=1229.45, and space group P2₁/m. Crystals of I have a layered structure: layers consisting of C₆₀ molecules alternate with layers composed of BMDT-TTF and CS₂ molecules. It is found that, in complex I, the donor and C₆₀ molecules are linked through the shortest contacts, which leads to a change in the molecular geometry of BMDT-TTF. The donor molecules in a crystal layer are characterized by the shortest S...S contacts. The IR data indicate the electroneutrality of the fullerene molecule. The electrical conductivity of (BMDT-TTF) · C₆₀ · 2CS₂ single crystals is measured using the four-point probe method at room temperature: σ_RT=2×10^?5 Ω^?1 cm^?1.     

Fine and hyperfine structure of the 22 P term of7Li; determination of the nuclear quadrupole moment

Microwave transitions between the Zeeman split doublet states 2² P _1/2 and 2² P _3/2 of⁷Li have been measured using the optical double resonance method. The fine structure separation was determined to bev _fs=10.053.184(58) MHz. The hyperfine structure was well resolved and led to considerably improved precision of the diagonal coupling constantsa _1/2=45.914(25) MHz,a _3/2=?3.055(14) MHz,b=?0.221(29) MHz. Furthermore, the contact, spin dipolar and orbital magnetic hyperfine constants are evaluated using additional results from level crossing experiments, which are more sensitive on the non-diagonal magnetic hyperfine constanta _3/2,1/2. The results are in good agreement with recent theoretical calculations. The nuclear quadrupole moment is derived fromb to beQ(⁷Li)=?41(6) mbarn.     

Photoluminescence and EPR of porous silicon formed on <Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript> and <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript> single crystals implanted with boron and phosphorus ions

The effect of combined doping by shallow donor and acceptor impurities on boosting the quantum yield of porous-silicon photoluminescence (PL) in the visible and near IR range was studied using phosphorus and boron ion implantation. Nonuniform doping of samples and subsequent oxidizing annealing were performed before and after porous silicon was formed on silicon single crystals strongly doped by arsenic or boron up to ≈10¹⁹ cm^?3. The concentration of known P_b centers of nonradiative recombination was controlled by electron paramagnetic resonance. It is shown that there is an optimal joined content of shallow donors and acceptors that provides a maximum PL intensity in the vicinity of the red part of the visible spectrum. According to estimates, the PL quantum yield in the transitional n ⁺⁺-p ⁺ or p ⁺⁺-n ⁺ layer of porous silicon increases by two orders of magnitude as compared to that in porous silicon formed on silicon not subjected to ion irradiation.     

Effect of the Lattice and Spin-Phonon Contributions on the Temperature Behavior of the Ground State Splitting of Gd<Superscript>3+</Superscript> in SrMoO<Subscript>4</Subscript>

The temperature behavior of the EPR spectra of the Gd³⁺ impurity center in single crystals of SrMoO₄ in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b ₂ ⁰ (T) = b₂(F) + b₂(L) and P ₂ ⁰ (T) = P₂(F) + P₂(L) (for Gd¹⁵⁷) describing the EPR spectrum and contributing to the Gd³⁺ ground state splitting ΔE is carried out. In terms of the Newman model, the values of b₂(L) and P₂(L) depending on the thermal expansion of the static lattice are estimated; the b₂(F) and P₂(F) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b ₂ ⁰ (T) and P ₂ ⁰ (T) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b₂(F) and P₂(F).     

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP₂) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (V_Zn, V_P, Ge_Zn), an S=1/2 EPR spectrum with an isotropic g=2.0123 and resolved hyperfine splitting from four equivalent I=1/2 neighbors is observed in electron-irradiated ZnGeP₂. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V_Zn⁻ and a proportional increase of the EPR signal assigned to the V_P⁰ under illumination (λ<1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V_Zn⁻ and V_P⁰ are in fact associated defects and the new defect is probably the isolated phosphorus vacancy V_Pi.     

High-field/high-frequency EPR/ENDOR — A powerful new tool in photosynthesis research

In photosynthesis research the elucidation of the spatial and electronic structures of the electron donor and acceptor ion radicals is very important for an understanding of the light-induced electron transfer process. Recent 3 mm (W-band, 95 GHz) high-field EPR and ENDOR studies on the primary donor cation radicals P^+. (bacteriochlorophyll dimer), the acceptor anion radicals Q^+. (quinones), and the charge-separated radical pair (P^+.Q^?.) in photosynthetic bacteria and biomimetic model systems are presented. From single crystals of, for instance,Rb. sphaeroides R-26 reaction centers both the hyperfine tensors of various protons and theg-tensor of P₈₆₅ ^+. have been determined and compared with calculated tensor values based on recent X-ray structure data. The results consistently reveal a breaking of the local C₂ symmetry of the electronic structure at the primary donor side of the reaction center. This is of particular interest since it might be relevant for the vectorial electron transfer along the protein complex. Among the quinone radical anions studied are frozen solutions of the electron acceptors of bacterial and plant reaction centers (ubiquinone and plastoquinone, respectively). The increased electron Zeeman interaction in high-field EPR leads to almost completely resolvedg-tensor components even in disordered samples. Theg-tensors and component linewidths are sensitive probes for specific anisotropic interactions with the environment. In the case of the transient correlated coupled radical pair P₈₆₅ ^+.-Q_A ^?. ofRb. sphaeroides (Fe replaced by Zn) the spin-polarized high-field EPR spectra allow an unambiguous determination of the relative orientation of theg-tensors of the donor and acceptor parts. Thereby high-precision structure information is obtained on the electron transfer pigments after light-induced charge separation.     

New hole configurations in X-irradiated KTiOPO4 Crystals

Electron paramagnetic resonance has been used to study the structure and thermostability of oxygen hole centers produced in KTiOPO₄ (KTP) crystals X-irradiated at 77 K. During the annealing of KTP crystals above 160 K the redistribution of charges took place. Four hole centers were observed at 40 K after the heating of the X-irradiated KTP crystal at different temperatures. The intensity of the hole center 1 decreased and new hole configurations (center 2 and center 3) appeared in the crystals. Theg-matrices were obtained from the angular dependencies of EPR spectra. The principalg-values for center 1 were 2.0040, 2.0209, and 2.0437. The principalg-values for centers 2 and 3 were 2.0053, 2.0204, 2.0431 and 2.0035, 2.0183, 2.0628, respectively. The transformation of Ti³⁺ spectra indicated that the trivalent titanium ions were involved into the recombination process. Subsequent annealing at temperatures above 220 K led to the formation of a new hole center 4 (g _a =2.0213,g _b =2.0236,g _c =2.0370).