Electron drag in intermediate magnetic fields with low electron density

Electron drag between two two-dimensional electron systems has been measured in intermediate magnetic fields (/τ<ω_ck_BT) with a relatively low electron density. We explore, in this sample, the unusual increase of drag in intermediate magnetic fields which was well characterized by a nearly temperature independent B³ dependence. The anomalous behavior of electron drag observed in higher density samples is found to persist for low sample density.     

Two-electron absorption in a biased quantum well

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency ω≈Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency ω≈2Δ. This additional peak is entirely due to electron–electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U², where U is the applied bias.     

Comparison of quasilinear and WKB approximations

It is shown that the quasilinearization method (QLM) sums the WKB series. The method approaches solution of the Riccati equation (obtained by casting the Schrödinger equation in a nonlinear form) by approximating the nonlinear terms by a sequence of the linear ones, and is not based on the existence of a smallness parameter. Each pth QLM iterate is expressible in a closed integral form. Its expansion in powers of reproduces the structure of the WKB series generating an infinite number of the WKB terms. Coefficients of the first 2^p terms of the expansion are exact while coefficients of a similar number of the next terms are approximate. The quantization condition in any QLM iteration, including the first, leads to exact energies for many well known physical potentials such as the Coulomb, harmonic oscillator, Pöschl–Teller, Hulthen, Hyleraas, Morse, Eckart, etc.     

Quantum Hamilton mechanics: Hamilton equations of quantum motion, origin of quantum operators, and proof of quantization axiom

This paper gives a thorough investigation on formulating and solving quantum problems by extended analytical mechanics that extends canonical variables to complex domain. With this complex extension, we show that quantum mechanics becomes a part of analytical mechanics and hence can be treated integrally with classical mechanics. Complex canonical variables are governed by Hamilton equations of motion, which can be derived naturally from Schrödinger equation. Using complex canonical variables, a formal proof of the quantization axiom p → pˆ = −i, which is the kernel in constructing quantum-mechanical systems, becomes a one-line corollary of Hamilton mechanics. The derivation of quantum operators from Hamilton mechanics is coordinate independent and thus allows us to derive quantum operators directly under any coordinate system without transforming back to Cartesian coordinates. Besides deriving quantum operators, we also show that the various prominent quantum effects, such as quantization, tunneling, atomic shell structure, Aharonov–Bohm effect, and spin, all have the root in Hamilton mechanics and can be described entirely by Hamilton equations of motion.     

Single InAs/GaAs quantum dots: Photocurrent and cross-sectional AFM analysis

Photocurrent (PC) spectroscopy is employed to study the carrier escape from self-assembled InAs/GaAs quantum dots (QDs) embedded in a Schottky photodiode structure. As a function of the applied field, we detect a shift of the exciton ground-state transition due to the quantum-confined Stark effect (). The tunneling time, which is directly related to the observed photocurrent linewidth due to τ/(2Γ), changes by a factor of five in the photocurrent regime. The measured linewidth dependency on the electric field is modeled by a simple 1D WKB approximation for the tunneling process, which shows that the energetic position of the wetting layer is important for the measured tunneling time out of the dot. In addition to that we present cross-sectional atomic force measurements (AFM) of the investigated photodiode structure. The method needs a minimum of time and sample preparation (cleaving and etching) to obtain the dot density, dot distribution, and give an estimate of the dot dimensions. Etching only the cleaved surface of the sample opens up the opportunity to determine the properties of a buried dot layer before or even after device fabrication.     

Nuclear quadrupole moments of 5/2<Superscript>-</Superscript> and 9/2<Superscript>-</Superscript> states in <Superscript>169</Superscript>Ta

The nuclear spectroscopic quadrupole moments for the πh_9/25/2^-, 1/2^-[541] and the πh_11/29/2^-, 9/2^-[514] isomeric states in ¹⁶⁹Ta have been measured employing the time differential perturbed angular-distribution technique following the nuclear reaction ¹⁵⁹Tb(¹⁶O, 6nγ)¹⁶⁹Ta at beam energy 104 MeV. The ratio of the intrinsic quadrupole moments has been derived as 1.87(13) from the measured quadrupole precession frequencies of the corresponding states. The model-independent analysis of the equilibrium deformation indicates strong prolate- and oblate-driving nature of the 1/2^-[541] and 9/2^-[514] orbitals in ^169,171Ta isotopes, respectively.     

Spherical and deformed configurations of18O in the model of K-harmonics with a potential well

From the point of view of the model ofK-harmonics an estimation of the relative positions of the spherical (K=K _min) and deformed (K=K _min+2) configurations of¹⁸O is carried out, which is useful for more exact calculations. In these configurations the structure (1s)⁴ (1p)¹² of the 16 nucleons (the hypothetical nucleus¹⁶O) is separated out as determining and their binding energy is calculated. Nuclear forces are taken in the schematic form of a rectangular potential well. At first it is derived from the realistic nucleon-nucleon rectangular well and results in overbound states. It is, therefore, derived directly from the experimental data (the binding energy and the root mean square radius of¹⁶O and¹⁸O respectively). Although the energies of the hypothetical nucleus¹⁶O are close together in both configurations of¹⁸O, they differ. In Appendix a very effective expression for the logarithmic derivative of the MacDonald function is given.The work has been begun in the Nuclear Research Institute of the Czechoslovak Academy of Sciences, e.     

Energy inversion of thef 7/2 andh 9/2 neutrons in yrast states of154Yb

The N=84 nucleus¹⁵⁴Yb was investigated through the¹⁰⁶Cd(⁵⁴Fe,2p) reaction. We found the 16⁺ yrast level in this nucleus to be formed by the (h_11/2 ² f_7/2h_9/2) configuration, in contrast to the lower-Z isotones where (h_11/2 ² f_7/2 ²)16⁺ is the yrast state. Manybody shell model calculations with empirical two-body interactions support our conclusion.Dedicated to Prof. Dr. O. Schult on the occasion of his 60^th birthday     

Nuclear Spin Orientation Created in Heavy Ion Collisions and the Sign of the Q Moment of 13B

The momentum dependences of the nuclear spin polarization P and alignment A of ¹³B(, T _1/2 = 17.36 ms) produced in the 100A MeV ¹⁵N + Be collisions have been measured by detecting β-ray asymmetry. Because both the P and A were significantly smaller than the prediction from a simple kinematical model, some relaxation mechanisms must be take into account. Comparing the signs of the observed alignment of ¹²B, the sign of the quadrupole coupling constant eqQ of ¹³B in TiO₂ was determined to be positive.     

Precise Nuclear Quadrupole Moments of 8B and 13B

The electric quadrupole coupling constants eqQ/h of ⁸B (, T _1/2 = 769 ms) and ¹³B (, T _1/2 = 17.4 ms) in single crystal TiO₂ have been precisely measured by the β-NQR technique. The ratios of these Q moments to Q(¹²B) were determined as ∣Q(⁸B)/Q(¹²B)∣ = 4.882(32) and ∣Q(¹³B)/Q(¹²B)∣ = 2.768(24).     

LDA and GGA investigations of some ground state properties of aluminium with the all electron MAPW method

Both in local-density approximation (LDA) and in generalised-gradient approximation (GGA) the electronic structure of Aluminium is evaluated by use of the modified augmented plane wave (MAPW) self-consistent scheme. The LDA based on the exchange correlation functional by Vosko, Wilk and Nusair gives the equilibrium lattice constant in good accord with its experimental value. The hole sheet of the Fermi surface, h₂, is well described by weakly distorted spheres with origin at and in the reciprocal lattice. Near and above the equilibrium lattice constant the electronic sheet, e₃, is found to be quite similar to the model originally proposed by Ashcroft. However, even moderate compressions induce a drastic variation.Received: 17 November 2003, Published online: 2 April 2004PACS: 71.18. + y Fermi surface: calculations and measurements; effective mass, g factor - 71.20.-b Electron density of states and band structure of crystalline solids     

Refractive scattering and reactions,comparison of two systems:16O+16O and20Ne+12C

Elastic, inelastic scattering as well as one-neutron transfer channels have been measured over a wide angular range for systems¹⁶O+¹⁶O at the incident energy of 350 MeV and²⁰Ne+¹²C at 390 MeV, respectively, using the Q3D magnetic spectrometer. In both cases differential cross sections have been measured down to about 50 nb/sr (or d/d _R10^–4) at large angles. For the¹⁶O+¹⁶O system refractive contributions are found at the level of these cross sections, whereas in the²⁰Ne+¹²C case a steeper decrease of the differential cross section with the angle is observed and the refractive contribution can not be determined. The elastic scattering data have been analyzed using standard Woods-Saxon potentials and potentials calculated in different versions of the double-folding model. Some properties of these potentials are tested in the calculations for inelastic scattering and one-neutron transfer within the DWBA. With the refractive pattern observed for the¹⁶O+¹⁶O system, the scattering and transfer data are found to be sensitive to the interaction potential at small internuclear distances down to about 2.5 fm.It should be acknowledged that part of the folding analysis reported here was done while one of the authors (D.T.K.) was staying at the Institute for Theoretical Physics, University of Tübingen. We also thank Prof. H. Clement and H. Abele for numerous discussions and contributions and Prof. G.R. Satchler for helpful comments on the use of the DWBA code PTOLEMY.     

Experimental results on40Ca(μ−,v μγ)40K* and16O(μ−,v μγ)16N**)

Photon spectra from radiative muon capture in doubly closed shell nuclei¹⁶O and⁴⁰Ca have been measured with a pairspectrometer. The⁴⁰Ca data agree with the most recent of the three previous experiments, for¹⁶O no other measurements exist. The comparison with theoretical calculations indicates that a careful treatment of the nuclear response function is required, ifg _P is to be determined from the data.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.     

Estimates of hypernuclear production via the exclusive (p,K+) reaction

We investigate the possibility of producing hypernuclei with proton beams via (p, K ⁺) reaction. We present differential cross sections calculations utilizing the distorted wave impulse approximation in momentum space. We consider the reactions¹²C(p,K ⁺) ¹³ and¹⁶O(p,K ⁺) ¹⁷ within the energy region 0.8 GeV1.2 GeV. We study both the case of formation of in S-state (¹²C) and P-state (¹⁶O). We take into account the contribution of both one-step and two-step processes when K pair is produced directly by incoming proton and intermediate pion, respectively. It is found that practically in all cases the two-step processes give significant contribution.Work supported by KFA Julich     

Weakly Non-Ergodic Statistical Physics

For weakly non ergodic systems, the probability density function of a time average observable is where is the value of the observable when the system is in state j=1,…L. p _j ^eq is the probability that a member of an ensemble of systems occupies state j in equilibrium. For a particle undergoing a fractional diffusion process in a binding force field, with thermal detailed balance conditions, p _j ^eq is Boltzmann’s canonical probability. Within the unbiased sub-diffusive continuous time random walk model, the exponent 0<α<1 is the anomalous diffusion exponent 〈x ²〉∼t ^α found for free boundary conditions. When α→1 ergodic statistical mechanics is recovered . We briefly discuss possible physical applications in single particle experiments.     

18O-selective IR MPD of perfluorodimethyl ether

The ¹⁸O-selective IR MPD of perfluorodimethyl ether (CF₃)₂O has been studied. The dissociation yields of (CF₃)₂ ¹⁸O (₁₈) and (CF₃)₂ ¹⁶O (₁₆) and the isotope selectivity (18/16) have been measured as functions of TEA CO₂-laser frequency, laser fluence and ether parent pressure. The (CF₃)₂O molecule has been found to provide highly efficient ¹⁸O separation. The MPD yield of the desired isotope ¹⁸O varies in the range 3–13%; the selectivity (18/16) achieves a value of 95 at the laser line 10P22, at moderate fluence Ø=4 J/cm² and at P _{(CF 3)2 O}=0.5 Torr.     

Photoproduction of η -mesons off nuclei for Eγ ? 2.2 GeV

The photoproduction of η -mesons off ¹²C , ⁴⁰Ca , ⁹³Nb , and ^nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S ₁₁(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η -mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η -mesons, e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross-section for η -mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU model show no unexplained in-medium modifications of the S ₁₁(1535) .     

Finite-size effects in a field-theoretic model with long-range exchange interaction

We present a systematic approach to the calculation of finite-size (FS) effects for anO(n) field-theoretic model with both short-range (SR) and long-range (LR) exchange interactions. The LR exchange interaction decays at large distances as 1/r ^d+2–2,0⁺,0⁺. Renormalization group calculations ind=d _u – are performed for a system with a fully finite (block) geometry under periodic boundary conditions. We calculate the FS shift of the critical temperature and the FS renormalized coupling constant of the model to one-loop order. The universal scaling variable is obtained and the FS scaling hypothesis is verified.