Nonlinear electron transport in Si δ-doped GaAs

In this paper we report abnormal non-ohmic behaviour observed in Si δ-doped GaAs. We have performed measurements in Hall bar and Van der Pauw geometries in which electron transport phenomena can be studiekd by looking into the dependence of the device resistance R_xx on the applied current I or input power per electron, P. Transport of electrons in single Si δ-doped layers in GaAs, grown by MOCVD, in the linear and non-linear response regimes was studied. Our experimental results show that: i) R_xx dramatically decreases with increasingI (P) when sample current (input power) is larger than a critical value I_c (P_c); ii) I_c increases with electron density (Si δ-doping concentration); and iii) the decrease in R_xx with increasing I in the non-linear response regime can be observed both at zero and high magnetic fields over a wide temperature range. When R_xx is plotted as a function of the power loss rate per electron (P) the values of the critical power (P_c) for different samples are roughly the same. We suggest that redistribution of the ionised donors in the sample occurring when I >I_c takes the major responsibility for the observed abnormal phenomena.     

Asymptotic form of two-particle density matrix in atoms and molecules

For atoms and molecules with N electrons, the two-particle density matrix Γ_N(r′₁r′₂r₁r₂ is expressed in terms of the single-particle density matrix γN?1(r′₂r₂) for N?1 electrons for the sufficiently large r′₁ and r₁.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Polarization fluctuations in regions bounded by equi-potentials

It is shown that the integral of <{P(rτ), P(r')}>· n? as a function of r over a volume V_e bounded by an equi-potential is independent of r' and of V_e and is determined solely by [?(ω)?1] · n? provided the dielectric constant is local and that V_e is of macroscopic dimensions.     

North-South asymmetry of some solar parameters: A study based on the dominance of hemispheric sunspot activity during the solar cycles 18–24

Solar activity indices differ over the solar disk, and different activity parameters are not considered to be symmetric between the northern and southern solar hemispheres. In the present paper, the daily data of a set of solar parameters (solar radio flux F10.7, total solar irradiance TSI, plage area PA, coronal index CI, solar flare index SFI, and solar mean magnetic field ǀBǀ) as well as the daily hemispheric sunspot areas (SSAs) and sunspot numbers (SSNs) during a timeframe 1945–2017 (covering almost the last seven solar cycles, 18 ̶ 24) have been employed to investigate the north-south (N-S) asymmetry of the considered solar parameters based on the dominance of hemispheric distributions of SSAs and SSNs. The N-S asymmetry for each solar parameter has been investigated and the results revealed that it is a significant aspect through different years in the entire period. The grand average of each solar parameter for the northern and southern groups over each solar activity cycle has been calculated to investigate the statistical significance of N-S asymmetry of each solar parameter in each solar activity cycle. The solar cycles 19 and 24 displayed the dominance of the southern F10.7 and PA over the northern one. However, the solar cycle 23 showed the reverse. The grand average of CI displayed the southern preference in the solar cycle 19 while, the northern dominance of CI is revealed for the solar cycle 23. The grand averages of CI demonstrated nearly symmetric distribution in solar cycles 18, and 20–22. The N-S asymmetry of the grand averages of SFI exhibited a southern dominance during solar cycles 21 and 24. On the other hand, the northern preference is observed for the grand average of SFI through the solar cycle 23. The asymmetry of the grand averages of ǀBǀ obviously has the same dominance (sign) of hemispheric sunspot activity indices for the solar cycles 21–24. The periodic behavior of the N-S asymmetry of SSAs, SSNs, F10.7, and ǀBǀ has been investigated using Fast Fourier Transformation. Many mid- and long term periodicities have been detected. We present our results and discuss their possible explanations.     

Synthesis,characterization and luminescence study of dimethyl[N-arylmethylenethiobenzahydrazonato]indium

Five dimethylindium complexes of type Me₂InL [L = N-(4-methoxy)benzylidenethiobenzahydrazonato (1), N-(3,4-dimethoxy)benzylidenethiobenzahydrazonato (2), N-(4-N,N-dimethylamino)benzylidenethiobenzahydrazonato (3), N-(2-naphthyl)methylene thiobenzahydrazonato (4) and N-(9-anthryl)methylenethiobenzahydrazonato (5)] have been synthesized by reaction of trimethylindium with appropriate N-arylmethylenethiobenzahydrozones. The complexes obtained have been characterized by elemental analysis, ¹H NMR, IR and mass spectroscopy. Compounds 1–5 emit blue colors at λ_max = 432–479 nm when irradiated by UV light. The electroluminescent (EL) properties of 1–5 were examined by fabricating EL devices using 1–5 as emitter, respectively. The EL bands are located in the green region (513–578 nm).     

Contribution of the inhomogeneous waves in angular-spectrum representations

Angular-spectrum representations of wave fields separate naturally into two parts, V_h(R,t) containing only homogeneous plane waves and V_i(R,t) containing only inhomogeneous plane waves. Some properties of V_i(R,t) are presented here. We conclude that, for some problems, V_i(R,t) has several unphysical properties, and that under certain specified conditions, V_i(R,t) can not be neglected compared to V_h(R,t), even far from the sources of the field.     

Increase of the paramagnetic-to-antiferromagnetic transition temperature of Cr2O3 by a magnetic field

The ordering temperature of antiferromagnetic Cr₂O₃ increases with magnetic field H, for H perpendicular to the easy axus c. This result disagree with mean-field theory, but is consistent with recent predictions. Results for H∥c (including the region near the bicritical point) are also presented.     

The inverse problem for a general N × N spectral equation

The spectral transform ?u/?x = {A(ζ)+B(x,ζ)} · u, where u is an n-element column vector and A(ζ) and B(x,ζ) are nxn matrices, is considered. A set of spectral data is given and the problem of reconcstructing B(x, ζ) from this spectral data is solved for a large class of cases. A special case of this spectral transform is used to solve the Boussinesq equation.     

Ultrasonic attenuation determination of Hc1 and Hc2 for ErRh4B4 at 1.5 K

The attenuation coefficient of longitudinal sound waves propagating in ErRh₄B₄ has been measured as a function of applied magnetic field where the propagation direction (q) of the sound waves was oriented either parallel or perpendicular to H_applied. For both orientations there is evidence of a type II-1 superconducting transition at H_c1 for T ≈ 1.5 K. In addition, when q ⊥ H an increase in attenuation is evident at H_c2, which does not appear when q 6 H, consistent with theories developed by Tachiki et al. utilizing supercurrent screening of the internal magnetic fields.     

Syntheses and structure characterization of inorganic/organic coordination polymers: Ag(dpa), Co(O3PH)(4,4′-bpy)(H2O), Zn(O3PH)(4,4′-bpy)0.5 and Mn[O2PH(C6H5)]2(4,4′-bpy) (dpa=2,2′-dipyridylamine; 4,4′-bpy=4,4′-bipyridine)

Four novel coordination polymers: Ag(dpa) I, Co(O₃PH)(4,4′-bpy)(H₂O) II, Zn(O₃PH)(4,4′-bpy)_0.5 III and Mn[O₂PH(C₆H₅)]₂(4,4′-bpy) IV (dpa=2,2′-dipyridylamine; 4,4′-bpy=4,4′-bipyridine), were synthesized by microwave heating and characterized by X-ray crystallography. I crystallizes in monoclinic space group P21/n with a=11.576(2) Å, b=5.585(2) Å, c=15.243(4) Å, β=109.00(2)°, V=931.8(3) ų. II crystallizes in monoclinic Cc space group with a=22.477(7) Å, b=5.280(1) Å, c=10.404(4) Å, β=96.08(3)°, V=1227.8(7) ų. III crystallizes in monoclinic P21/c space group with a=9.758(2) Å, b=7.449(3) Å, c=10.277(2) Å, β=100.02(2)°, V=735.6(4) ų. IV crystallizes in monoclinic space group P2/c with a=10.174(1) Å, b=11.817(3) Å, c=18.784(4) Å, β=102.14(1)°, V=2207.8(8) ų. I consists of linear metal–metal chains wrapped by dpa ligands. II and III consist of two-dimensional M^II(O₃PH) inorganic sheets cross-linked by 4,4′-bpy ligands, while IV is formed by Mn[O₂PH(C₆H₅)]₂ sheets cross-linked by 4,4′-bpy ligands. I exhibits two-step thermal decomposition at ~200 and ~250°C, resulting in the reduction of Ag⁺ to Ag metal. II loses its coordination water at ~100°C, leaving vacant coordination sites at Co²⁺ ions, while the original framework remains intact. The removal of 4,4′-bpy in II–IV occurs at elevated temperatures above 250, 200 and 400°C respectively.     

On the bifurcations occuring in the parameter space of the sixteen vertex model: I. Discrete bifurcations

The 16-dimensional parameter space of homogeneous sixteen vertex models is scanned for bifurcation points, i.e. points corresponding to models which possess extra symmetries not existing in nearby points. Equivalence classes of models having the same partition function are identified by means of a characteristic “normal” model, represented by a (4 x 4)-diagonal matrix N, and a pair of (2 x 2)-matrices A and B. In this paper the matrix N is assumed to be non-degenerate and the only bifurcations found are those associated with special types of matrices A and B, i.e. matrices whose decomposition in terms of Pauli-matrices corresponds to a vector a ≡ (a₁, a₂, a₃) or b ≡ (b₁, b₂, b₃) that is invariant with respect to one or more elements of the cubic symmetry group.The various bifurcation classes of models found include: a) the families of general- and “complementary” eight vertex models, b) discrete sets of doubly- and one-sided cyclic models and c) a number of secondary bifurcation classes within the eight-vertex families, among which is Baxters symmetric eight-vertex model.     

Surface polaritons on semi-infinite anisotropic media

We have investigated the nature of polaritons which propagate on the surface of semi-infinite anisotropic dielectric and magnetic media and find that the surface polaritons are regular surface waves [i.e., the penetration constant α (ω) is real] only for special orientations of the wave vector k and of the normal to the surface n relative to the principal axes. In the case of biaxial crystals, regular surface polaritons occur when k and/or n is along a principal axis of the dielectric tensor ε(ω) for dielectric media [or the magnetic permeability tensor μ(ω) for magnetic media]. When k and n are both directed along principal axes, the surface polaritons are TM with E in the saggital plane for dielectric media. One finds that surface polaritons which are coupled photon-virtual surface dipole excitation modes can exist on “surface active” anisotropic media in addition to the familiar coupled photon-real surface dipole excitation modes. When neither k nor n are along principal axes, the surface polaritons that occur are, with the exception of special orientations, “generalized” surface waves [i.e., α(ω) is complex]. The Poynting vectors of the propagating surface polaritons are always in the plane of the surface but not, in general, along k. These results, which hold for uniaxial crystals, are also applicable to gyromagnetic media and to gyrodielectric media. This theory is also applicable to surface polaritons which propagate along the interface between two media, one or both of which is anisotropic, and with one or the other serving as the “surface active” medium.     

Magnetic Resonance Imaging of Diffusion in the Presence of Background Gradients and Imaging of Background Gradients

The measurement of the self-diffusion coefficient D by an NMR technique that uses an applied gradient G_A can be corrupted by systems that have a background magnetic field gradient G₀ and also by imaging gradients G₁, when used in an imaging mode. In a nonimaging mode, the corrupting cross term G_A · G₀can be eliminated in the diffusion measurement by use of an alternating-pulse-field-gradient (APFG) sequence that allows an accurate and uncorrupted measurement of D. A Carr-Purcell echo train enables the measurement of the expectation value, 〈DG²₀〉; assuming D and G₀ to be uncorrelated will allow 〈G²₀〉 to be determined. An image of D or of 〈DG²₀〉 may be obtained without the corrupting G_A · G₀ and G_A · G_I terms by appending a standard imaging sequence to an APFG sequence or a Carr-Purcell sequence, respectively; assuming D and G₀ to be uncorrelated will allow (〈G²₀〉 to be determined within each pixel. Measurements of D and 〈G²₀〉 and their images are made in apple flesh in which minute air bubbles are shown to produce the large 〈G²₀〉. Their values in an 81 g Golden Delicious apple at a measuring frequency of 100 MHz were D = 1.42 × 10⁻⁵ cm²/s and [formula] = 8.9 G/cm.     

Luminescence of exciton in GeS

The luminescence of one free and four bound excitons has been investigated in the layer compound GeS. Up to six phonon replicas of the bound excitons are observable. The energies of these phonons are in reasonable agreement with Raman- and IR reflectivity measurements. The luminescence is highly anisotropic and is only seen in the polarization E ∥ a which confirms that the transitions E ∥ b are forbidden. Measurements with E ∥ c were inconclusive because of the difficulties in the sample preparation. Time resolved measurements revealed decay times between 20 nsec and 1.5 μsec for the different bound excitons. One exciton has been identified as bound to a Si impurity. This is the first observation of intentional doping of GeS. Thermal dissociation energies for the bound excitons lie between 6.5 and 20 meV.     

Study of two-phonon excitations in superfluid 4He

We explain the second branch of excitations in superfluid ⁴He observed by Cowley and Woods, by accounting for two-phonon contributions to the dynamic structure function, S(k, ω). Our theory gives a good fit with the experimental data in the high energy region for several values of momentum transfer. It is observed that the contribution to S(k, ω) due to two-phonon excitations is of the order of k² as against its k⁴ dependence reported in earlier theories.     

Optical anisotropy of orthorhombic InS

The optical anisotropy of InS single crystals in the range of photon energy from 1.8 to 3.5 eV has been studied by absorption, electroreflectance and wavelength-derivative reflectance measurements. These systematic optical measurements for the polarizations, E//a and E//b, have revealed that the transition at the fundamental absorption edge of InS is allowed for E//b, and there exist three distinct doublet transitions having different selection rules in the photon energy region from 2 to 3.5 eV; Bo and B^'₀doublet allowed only for E//b, A₀ and A^'₀ allowed only for E//a, and E₁ and E^'₁ allowed for both polarizations. The observed results are discussed based on the anisotropic nature of two chemical bonds in InS, cation-cation and cation-anion.     

Comments on the use of loop reactors in sonochemical processes

For sonochemical processing on an industrial scale the traditional choice is either a batch or flow system. The former is straightforward in concept but it requires large scale powerful ultrasonic transducers capable of delivering high intensity ultrasound to large volumes of liquid. Unfortunately at the moment the cost and problems involved in building very large sonication devices for batch processes cannot justify the replacement of existing industrial processes. For this reason most sonochemists prefer some form of flow system where small quantities of reagents can be treated as they are pumped from a large vat of reagents through a smaller sonochemical reactor where high intensity ultrasound can be applied. In this short paper we draw attention to a problem which seems common in a number of papers dealing with such flow systems – a confusion between the terms continuous reactor and loop reactor. Further we emphasise the importance of calculating the actual amount of ultrasonic processing experienced by the reaction mixture within the sonication zone of a loop reactor during its operation. The parameters required for such a calculation are: ultrasonic processor volume (R_v in L), pump flow rate (F_r in L/min), stock solution volume in the reservoir (S_v in L) and the overall system operating time (S_o in min).     

Quasi-1 structure on q-Poincaré algebras

We use braided groups to introduce a theory of 1-structures on general inhomogeneous quantum groups, which we formulate as quasi-1 Hopf algebras. This allows the construction of the tensor product of unitary representations up to a quantum cocycle isomorphism, which is a novel feature of the inhomogeneous case. Examples include q-Poincaré quantum group enveloping algebras in R-matrix from appropriate to the previous q-Euclidean and q-Minkowski space-time algebras R₂₁x₁x₂ = x₂x₁R and R₂₁u₁Ru₂ = u₂R₂₁u₁R. We obtain unitarity of the fundamental differential representations. We further show that the Euclidean and Minkowski-Poincaré quantum groups are twisting equivalent by another quantum cocycle.     

Photoelektronenspektren und konformationsverhalten von hydroxylamin und methylhydroxylaminen

Gas-phase He(I) photoelectron spectra of hydroxylamine (1) and the methyl hydroxylamines (2–6) have been measured. From 1 a He(II) spectrum has also been obtained. To study the interactions of the n(N) and n(O) orbitals, MINDO/2 calculations have been performed for various conformations of 1–6. The findings are consistent with trans- forms being the most stable conformations of 1–6, but additional cis- conformers cannot be excluded.