Effect of well thickness on the Rashba spin splitting and intersubband spin-orbit coupling in AlGaN/GaN/AlGaN quantum wells with two subbands

By projecting the characteristic equation into the subspace of the conduction band, the Rashba spin splitting coefficient for the first two subbands (α₁, α₂) and the intersubband spin-orbit coupling coefficient (η₁₂) in AlGaN/GaN quantum well structure are obtained. Then sizable α₁, α₂ and η₁₂ in QWs are calculated by solving the Schrödinger and Poisson equations self-consistently. We find that the internal electric field is crucial for considerable spin-orbit coupling effect in III-nitride QWs and the spin-orbit coupling coefficient can be greatly modulated by the well thickness. Compared with the Rashba coefficient, the intersubband spin-orbit coupling coefficient is basically of the same order of magnitude. The results show the great possibility of spin manipulation in low-dimensional semiconductors, and III-nitride QWs are candidates for the design of spintronic devices.     

Stability of bipolarons in the presence of a magnetic field

The stability of large Fröhlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω _c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α _c (above which the bipolaron is stable) decreases with increasing ω _c and (ii) the critical Coulomb repulsion strength U _c (below which the bipolaron is stable) increases with increasing ω _c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω _c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ? 1) we obtain for ω _c ? 1 that E _estim ? E _estim(ω _c = 0) + c(u)ω _c/α ⁴ with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω _c ? α ²? 1 we find in 3D E _estim ? ω _c - α ² A(u) ln²(ω _c/α ²), (also with an explicit analytical expression for A(u)) whereas in 2D E _estim ^2D ? ω _c - α√ω _cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.     

Can “Two-” and “One-Dimensional” Multielectron Atoms Exist?

The quasi-classical Thomas–Fermi method is applied to 2D and 1D multielectron atoms. In terms of this method, such atoms are shown not to exist because of the fact that the physical boundary conditions that are analogous to the 3D version of the theory, where boundary conditions are met, cannot be fulfilled. Our theoretical results can be experimentally tested. Atomic number Z_{1, 2max} (~10²?) is assumed to exist in terms of this method. At Z > Z_{1, 2max}, low-dimensional multielectron atoms cannot exist, in contrast to oneor two-electron atoms and, e.g., an experimentally detected Bose condensate of low-dimensional atoms with Z ~ 10 (Na).     

A continuous variant for Grünwald-Letnikov fractional derivatives

The names of Grünwald and Letnikov are associated with discrete convolutions of mesh h, multiplied by h^−α. When h tends to zero, the result tends to a Marchaud’s derivative (of the order of α) of the function to which the convolution is applied. The weights of such discrete convolutions form well-defined sequences, proportional to k^−α−1 near infinity, and all moments of integer order r<α are equal to zero, provided α is not an integer. We present a continuous variant of Grünwald-Letnikov formulas, with integrals instead of series. It involves a convolution kernel which mimics the above-mentioned features of Grünwald-Letnikov weights. A first application consists in computing the flux of particles spreading according to random walks with heavy-tailed jump distributions, possibly involving boundary conditions.     

Collisionless magnetohydrodynamic turbulence in two dimensions

In this paper we give a formulation of two-dimensional (2D) collisionless magnetohydrodynamic (MHD) turbulence that includes the effects of both electron inertia and electron pressure (or parallel electron compressibility) and is applicable to strongly magnetized collisionless plasmas. We place particular emphasis on the departures from the 2D classical MHD turbulence results produced by the collisionless MHD effects. We investigate the fractal/multi-fractal aspects of spatial intermittency. The fractal model for intermittent collisionless MHD turbulence appears to be able to describe the observed k⁻¹ spectrum in the solar wind. Multi-fractal scaling behaviors in the inertial range are first deduced, and are then extrapolated down to the dissipative microscales. We then consider a parabolic-profile model for the singularity spectrum f (α), as an explicit example of a multi-fractal scenario. These considerations provide considerable insights into the basic mechanisms underlying spatial intermittency in 2D fully developed collisionless MHD turbulence.     

Optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone

The optical and electroluminescent properties of 3,4,6-triphenyl-α-pyrone (α-pyrone), a new blue fluoresce dye, are investigated using films prepared by wet and dry process and organic light-emitting diodes (OLEDs) fabricated with an α-pyrone-emitting layer. The optical properties of α-pyrone are found to be affected by its crystallinity. In the fabrication of OLEDs, wet processing (spin coating) is shown to be more suitable for preparation of the α-pyrone layer than dry processing (thermal evaporation). The best device performance is obtained for a device prepared using poly (n-vinylcarbazole) as the dye host, and a bathocuproine/tris-(8-hydroxyquinoline)aluminum bilayer as a hole-blocking and carrier-injection layer. The maximum luminance of this device is 3000 cd/m² at a current density of 0.2 A/cm², with a current efficiency of 1.8 cd/A at 0.02 A/cm².     

Impact of initial lattice structures on networks generated by traces of random walks

We show that the platform stage of network evolution plays a principal role in the topology of resulting networks generated by short-cuts stimulated by the movements of a random walker, the mechanism of which tends to produce power-law degree distributions. To examine the numerical results, we have developed a statistical method which relates the power-law exponent γ to random properties of the subgraph developed in the platform stage. As a result, we find that an important exponent in the network evolution is α, which characterizes the size of the subgraph in the form V∼t^α, where V and t denote the number of vertices in the subgraph and the time variable, respectively. 2D lattices can impose specific limitations on the walker’s diffusion, which keeps the value of α within a moderate range and provides typical properties of complex networks. 1D and 3D cases correspond to different ends of the spectrum for α, with 2D cases in between. Especially for 2D square lattices, a discontinuous change of the network structure is observed, which varies according to whether γ is greater or less than 2. For 1D cases, we show that emergence of nearly complete subgraphs is guaranteed by α<1/2, although the transient power-law is permitted at low increase rates of edges. Additionally, the model exhibits a spontaneous emergence of highly clustered structures regardless of its initial structure.     

Gap solitons in a model of a superfluid fermion gas in optical lattices

We consider a dynamical model for a Fermi gas in the Bardeen-Cooper-Schrieffer (BCS) superfluid state, trapped in a combination of a 1D or 2D optical lattice (OL) and a tight parabolic potential, acting in the transverse direction(s). The model is based on an equation for the order parameter (wave function), which is derived from the energy density for the weakly coupled BCS superfluid. The equation includes a nonlinear self-repulsive term of power 7/3, which accounts for the Fermi pressure. Reducing the equation to the 1D or 2D form, we construct families of stable 1D and 2D gap solitons (GSs) by means of numerical simulations, which are guided by the variational approximation (VA). The GSs are, chiefly, compact objects trapped in a single cell of the OL potential. In the linear limit, the VA predicts almost exact positions of narrow Bloch bands that separate the semi-infinite and first finite gaps, as well as the first and second finite ones. Families of stable even and odd bound states of 1D GSs are constructed, too. We also demonstrate that the GS can be dragged without much distortion by an OL moving at a moderate velocity (, in physical units). The predicted GSs contain ∼10³-10⁴ and ∼10³ atoms per 1D and 2D settings, respectively.     

Approximate normalized wave functions for the finite-size Coulomb problem

Using perturbation methods developed previously for the finite-size energy shift, thes-state wave functions for a bound lepton in the Coulomb field of a nucleus with an arbitrary charge distribution are developed through order (Zα)². This technique allows a determination of the finite-size contribution to the normalization of the wave function up to order (Zα)², both for small separations of the lepton and nucleus and for the lepton outside the nucleus. General features of the wave function are discussea, including the transition of the Dirac problem to the (singular) point charge limit. A practical application is developed using these results.     

164Ta,identification of a new isotope

The new isotope¹⁶⁴Ta (T _1/2=13.6(2)s) was produced in the¹²⁷I(⁴⁰Ca, 3n) reaction and transported to a measuring station by helium jet. Decay properties were observed withα, γ, andγ-γ spectroscopy. TheZ assignment of the new isotope was based ongg-X-ray coincidence measurements. The mass assignment was deduced from measured excitation functions. The twoγ rays assigned to the new activity most likely originate from the 4⁺→2⁺→0⁺ decay sequence in¹⁶⁴Hf. A smallα branch, ≦0.016(5)%, with anα decay energy ofE _α=4,625(15) keV was found and assigned to the decay of¹⁶⁴Ta. The deduced upper limit of theα width for this branch,W _α≦0.6(2), is in accordance with those known for neighbouring nuclei.     

A Gribov equation for the photon Green’s function

We present a derivation of the Gribov equation for the gluon/photon Green’s function D(q). Our derivation is based on the second derivative of the gauge-invariant quantity Trln?D(q), which we interpret as the gauge-boson ‘self-loop’. By considering the higher-order corrections to this quantity, we are able to obtain a Gribov equation which sums the logarithmically enhanced corrections. By solving this equation, we obtain the non-perturbative running coupling in both QCD and QED. In the case of QCD, α _S has a singularity in the space-like region corresponding to super-criticality, which is argued to be resolved in Gribov’s light-quark confinement scenario. For the QED coupling in the UV limit, we obtain a ∝ Q ² behavior for space-like Q ²=?q ². This implies the decoupling of the photon and an NJLVL-type effective theory in the UV limit.     

Green function theory of Curie and order—order transitions in ferromagnetic systems

Dipolar critical temperatures in ferromagnetic systems with isotropic bilinear and biquadratic exchange are investigated by means of the Green function technique. Expressions are found for both the familiar Curie temperature, T_c, and the less well known order-order transition temperature, T_o, at which, under appropriate conditions, the magnetic ordering undergoes a change between fully aligned and canted ferromagnetism. At T = 0, a fully aligned state has <s_i^z = s for spin s and all lattice sites i, while a canted state has 〈s_i^z〉<s. It is shown independently of the Green function analysis that the T = 0 ground state is fully aligned if α, the ratio of biquadratic to bilinear exchange integrals, obeys ?[2s(s?1)]^?1<α< [2s²?2s+1]^?1. The region below the lower limit is identified as the range in which canted ferromagnetism can occur and is a range that does not appear to have been considered previously via the Green function formalism.The temperature dependence of the magnetic ordering is investigated by means of the double-time temperature-dependent Green function formalism. A new decoupling scheme is derived and used to reduce higher order Green functions to lowest order. It is found that a canted state, occuring at low temperatures, undergoes a transition to a fully aligned state at a temperature T₀ and subsequently becomes disordered at temperature T_c. Transitions to paramagnetism are found to be second order for α<α_c and first order for α?α_c where α_c is a critical value that depends on the atomic spin and weakly on the lattice structure. A phase diagram is given to illustrate the results, and a comparison is made with the corresponding results found in mean field theory.     

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

α particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic¹⁶O+⁵⁸Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘ _α gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10^?21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT?3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.     

Mg substitution in CuCrO2 delafossite compounds

A detailed investigation of the series CuCr_1−xMg_xO₂ (x=0.0-0.05) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg²⁺ for Cr³⁺ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x∼0.02-0.03, indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr₂O₄ spinels as soon as x=0.005. The thermopower is discussed in the temperature-independent correlation function ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.     

Fat Fisher zeroes

We show that it is possible to determine the locus of Fisher zeroes in the thermodynamic limit for the Ising model on planar (“fat”) φ⁴ random graphs and their dual quadrangulations by matching up the real part of the high and low temperature branches of the expression for the free energy. The form of this expression for the free energy also means that series expansion results for the zeroes may be obtained with rather less effort than might appear necessary at first sight by simply reverting the series expansion of a function g(z) which appears in the solution and taking a logarithm.Unlike regular 2D lattices where numerous unphysical critical points exist with non-standard exponents, the Ising model on planar φ⁴ graphs displays only the physical transition at c=exp(−2β)=1/4 and a mirror transition at c=−1/4 both with KPZ/DDK exponents (α=−1, β=1/2, γ=2). The relation between the φ⁴ locus and that of the dual quadrangulations is akin to that between the (regular) triangular and honeycomb lattices since there is no self-duality.     

Copper ion induced reactions on110-108-106Cd,109-107Ag and110Pd. new rhenium,osmium and iridium isotopes

By⁶³Cu induced reactions on¹¹⁰Cd,¹⁰⁸Cd,¹⁰⁶Cd,¹⁰⁹Ag,¹⁰⁷Ag and¹¹⁰Pd targets, new isotopes were searched in the Ir, Os, Re region. Cross bombardments and excitation function measurements were used to identify newα emitting isotopes:¹⁷⁰Ir (E _α = 6.01 ±0.01 MeV?T _1/2=1.1±0.2s),¹⁶⁹Ir (E _α =6.11±0.01 MeV?T _1/2=0.4±0.1 s),¹⁶⁸Ir (E _α =6.22±0.02 MeV,¹⁶⁸Os (E _α =5.66±0.01 MeV?T _1/2=1.9±0.1 s),¹⁶⁷Os (E _α =5.84 ±0.01MeV ?T ^1/2=0.65±0.15s),¹⁶⁶Os (E _α =6.00±0.02 MeV?T _1/2=0.3 ±0.1 s),¹⁶⁵Os (E _α =6.20±0.02 MeV),¹⁶⁹Re (E _α =5.05±0.01 MeV),¹⁶⁸Re (5.26±0.01 MeV?T ^1/2=5.5 ±0.5 s). Theα-decay measurements are compared to theQ _α values obtained from different mass predictions.     

Conformal field theories near a boundary in general dimensions

The implications of restricted conformal invariance under conformal transformations preserving a plane boundary are discussed for general dimensions d. Calculations of the universal function of a conformal invariant ξ which appears in the two-point function of scalar operators in conformally invariant theories with a plane boundary are undertaken to first order in the ge = 4 − d expansion for the operator φ² in φ⁴ theory. The form for the associated functions of ξ for the two-point functions for the basic field φ^α and the auxiliary field λ in the N → ∞ limit of the O(N) nonlinear sigma model for any d in the range 2 < d < 4 are also rederived. These results are obtained by integrating the two-point functions over planes parallel to the boundary, defining a restricted two-point function which may be obtained more simply. Assuming conformal invariance this transformation can be inverted to recover the full two-point function. Consistency of the results is checked by considering the limit d → 4 and also by analysis of the operator product expansions for φ^αφ^β and λλ. Using this method the form of the two-point function for the energy-momentum tensor in the conformal O(N) model with a plane boundary is also found. General results for the sum of the contributions of all derivative operators appearing in the operator product expansion, and also in a corresponding boundary operator expansion, to the two-point functions are also derived making essential use of conformal invariance.     

Fundamental modes of new dispersive SH-waves in piezoelectromagnetic plate

Fundamental modes of new dispersive shear-horizontal (SH) acoustic waves propagating in the (6 mm) piezoelectromagnetic plate are studied. These SH-waves can propagate when the following boundary conditions are exploited for both the upper and lower surfaces of the plate: (1) when the surfaces are mechanically free, electrically and magnetically closed and (2) when the surfaces are mechanically free, electrically and magnetically open. The SH-waves depend on the electromagnetic wave speed $V_{\rm EM} =1/ {\sqrt {(\varepsilon\mu)}}$ and can only exist when the electromagnetic constant $\alpha \ne 0$ . The calculations (first evidence) were performed for the PZT-5H–Terfenol-D which is a composite with a large value of α. The limit cases of large values of α (α ²?=?0.5εμ, α ²?=?0.9εμ, and α ²?=?0.99εμ) are studied because they satisfy the limitation condition of α ²?<?εμ.     

A study of the reaction19F(α, n)22Na in the bombarding energy range 2·6 to 5·1 MeV

The total (α, n) reaction cross section for¹⁹F has been measured as a function of alpha energy in the energy range 2·6 to 5·1 MeV with a thin target. The excitation function exhibits a large number of resonances. The prominent amongst these for which theJ ^π values are known have been analysed to extract the partial widthsΓ ^α and Γ _n . Statistical analysis of the data in terms of strength function and average level spacing distribution has also been performed.