Energy Spectrum of Electrons of Deep Impurity Centers in Wide-Bandgap Mesoscopic Semiconductors

JETP Letters - The energy spectrum of deep impurity centers in wide-bandgap semiconductors (Eg > 2 eV) of mesoscopic sizes R ⪢ λD, where λD is the de Broglie wavelength, at...     

New method to extract the neutron-electron scattering length

A novel method to extract the neutron-electron scattering length b _ne from the precise neutron scattering data measured for a noble gas at several different densities n is proposed. The main point of this method is dividing the experimental data into two parts: the first, nearly proportional to n, corresponding to diffraction on neighboring atoms and the second one, a small contribution of n, e scattering independent on n. The proposed technique is demonstrated using the structure factor S(q) for gaseous krypton.Received: 12 October 2004, Revised: 1 December 2004, Published online: 7 March 2005PACS: 28.20.-v, 61.12.-q, 14.20.Dh, 13.40.-f     

On a possibility for observing the neutron electric polarizability

A new method is proposed for setting a lower or upper limit a α _n * on the neutron electric polarizability α _an . It is based on the fact that the real part of the s-wave scattering amplitude changes sign near the s-wave neutron resonance at E=E*. The methods consist of the observation of the energy behavior of the forward-backward scattering asymmetry ω ₁ which experiences a jump at E=E*. If the jump is such that dω ₁/dE>0, then α _n >α _n *, while if dω ₁/dE<0, then α _n <α _n *, and if dω ₁/dE∼0, then α _n ∼α _n *. Seven even-even nuclei are found with α _n * from 0.5 to 3.1 in 10⁻³ fm³. Some details of a possible experiment with ¹⁸²W are described. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 171–174 (10 August 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Carrier energy spectrum and lifetime in quantum dots in electric field

The S-matrix formalism is used to perform analytical calculations of the spectrum of quasi-stationary states of charge carriers in a core-shell quantum dot. Analytical expressions are obtained for the second-order perturbative corrections to the position and half-width of a quasi-stationary energy level, and level shifts are calculated numerically for a core-shell quantum dot in the presence of an electrostatic field. The corrections to level half-width due to Stark effect are analyzed as functions of level energy and barrier thickness. It is shown that there exists a level position E _cr such that the correction δΓ to the level half-width changes sign. An analytical expression for the quadratic Stark shift in a dc-biased quantum well is found in semiclassical approximation. It is shown that the corresponding correction δΓ to half-width also changes sign as energy passes through E _cr. As an example, the Stark shift is calculated for a core-shell quantum dot in the electrostatic field of an adjacent protein molecule.     

On the neutron charge radius and the new experiments proposed for the precise (n,e)-scattering length measurement

Strict comprehensive treating the generalized Dirak equation for nucleon in external electro-magnetic field argues quite clear that there is no physical reason to bring into consideration so-called “Foldy-term” while obtaining the neutron mean square charge radius < r _n ² >, caused by an electric charge distribution inside a nucleon, from the experimental value of the (ne) — scattering length b _ne , that term being, as a matter of fact, fictitious one. Consequently, the representing of the experimental quantity b _ne as a sum of “Foldy length” b _F and “intrinsic” one b_I, even so splitting the total value of < r _n ² > in “Foldy” and “intrinsic” < r _in ² >, turn out of having no profound physical sense, being rather ambiguous in actual fact. The formal phenomenological relation, originated from the generalized Dirak equation for nucleon, of the quantities b _ne , < r _n ² >, and neutron anomalous magnetic moment μ is inquired. Concise treating < r _n ² > in the frame-work of up-to-date nucleon cloudy bag model (CBM) is presented, no “Foldy term” being emerged, and < r _n ² >, calculated according this approach, provides b _ne -value which is in agreement with experimental result within accuracy of about 10%. On the other hand, the experimental b _ne -value proves to be described phenomenologically through solely the neutron anomalous magnetic moment μ with the same accuracy ～ 10%. Then the necessity of obtaining b _ne -value with more reliable accuracy then in previous experiments becomes obvious, corrections have to be reduced to the level of the precise declared. For these aims, two new proposed experiments have been Monte-Carlo modelled. The first renders the measurement of the energy dependence of an elastic scattering cross-section on ⁸⁶ Kr, having the unique small capture cross-section. The second one is to measure the energy dependence of neutron scattering angle anisotropy for natural Xe.     

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.     

Neutron scattering in argon and <Emphasis Type="Italic">ne</Emphasis> interaction

As a preparation for the new experiment to measure the ne scattering length a_ne the total neutron cross section of gaseous argon has been obtained by the time-of-flight method at the Dubna booster IBR-30 in the energy range from ～5 eV to ～30 keV. A combined one-level analysis of the newly obtained and other known data on cross sections of Ar and ³⁶Ar has made it possible to improve some neutron parameters and calculate the scattering cross section σ_s and the scattering length a separately for ³⁶Ar and ⁴⁰Ar at any energy.