Spin-Blockade in Single and Double Quantum Dots in Magnetic Fields: a Correlation Effect

The total spin of correlated electrons in a quantum dot changes with magnetic field and this effect is generally linked to the change in the total angular momentum from one magic number to another, which can be understood in terms of an `electron molecule' picture for strong fields. Here we propose to exploit this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel at specific values of the magnetic field. The spin-blockade regions have been obtained by calculating both the ground and excited states. In double dots the spin-blockade condition is found to be less stringent than in single dots.Comment: 4pages, to be published in Phys. Rev. B (Rapid Communication

Currents in a many-particle parabolic quantum dot under a strong magnetic field

Currents in a few-electron parabolic quantum dot placed into a perpendicular magnetic field are considered. We show that traditional ways of investigating the Wigner crystallization by studying the charge density correlation function can be supplemented by the examination of the density-current correlator. However, care must be exercised when constructing the correct projection of the multi-dimensional wave function space. The interplay between the magnetic field and Euler-liquid-like behavior of the electron liquid gives rise to persistent and local currents in quantum dots. We demonstrate these phenomena by collating a quasi-classical theory valid in high magnetic fields and an exact numerical solution of the many-body problem.Comment: Uses RevTeX4, figures included in the tex

Possibilities to Improve the Therapy of the Patients with Ureteral Concretions

The aim of the given study was to improve the results of treatment of patients with concretions of the upper third of the ureter.Materials and methods. The clinical trial was carried out as a non-interventional open, controlled, in two groups of patients with baseline control. The inclusion criteria concerned the patients with concretions of the upper third of the ureter 0.7-0.9 mm in size, which had one session of an extracorporeal shock-wave lithotripsy (ESWL), and after the procedure the size of the concretions fragments was up to3 mm. The study involved 60 patients, all patients in the main group (n = 30) were treated using phytotherapy, within 1 month starting with basic therapy under hospital conditions and continuing with preventative treatment in an outpatient setting. The control group (n = 30) received baseline therapy (up to 10 days) at the stationary stage. Results and discussion. In patients of the main group, fragments of concretions came out significantly faster, namely: in the main group, the fragments came out in 21 patients (70.0 %), while in the control group in 15 patients (50.0 %) (p1- p2, p <0.05) to the 7th day, and in the main group, the fragments came out in one patient more than 14 days, and in the control group – in 5 patients, p <0.05. The "stone path" in the bottom third of the ureter was in 1 patient (3.3 %) of the main group and in 3 patients (10.0 %) in the control group (p1-p2, p <0.05). The "stone path" departure time in patients of the main group was 2 days, and in patients of the control group it was 4.1 days from the time of its formation (p1-p2, p <0.05). Bacteriuria was observed in the main group on the 10th day 6.6 % less relative to the control group.Conclusion. At comparing the obtained results in patients of both groups in 1 month it was noted that high efficacy of the treatment was registered in 6 (20 %) patients of the main group and in 3 patients (10 %) of the control group (p <0.05), moderate efficiency was registered in 23 (76.7 %) patients of the main group and in 22 (73.3 %) patients of the control group respectively, low efficacy was registered in 1 (3.33 %) patient in the main group and 5 (16.7 %) patients of the control group (p <0,05), which points to the effectiveness of Urolesan capsules use in complex therapy of patients with concretions of the one third of ureter

Effective interactions and large-scale diagonalization for quantum dots

The widely used large-scale diagonalization method using harmonic oscillator basis functions (an instance of the Rayleigh-Ritz method, also called a spectral method, configuration-interaction method, or ``exact diagonalization'' method) is systematically analyzed using results for the convergence of Hermite function series. We apply this theory to a Hamiltonian for a one-dimensional model of a quantum dot. The method is shown to converge slowly, and the non-smooth character of the interaction potential is identified as the main problem with the chosen basis, while on the other hand its important advantages are pointed out. An effective interaction obtained by a similarity transformation is proposed for improving the convergence of the diagonalization scheme, and numerical experiments are performed to demonstrate the improvement. Generalizations to more particles and dimensions are discussed.Comment: 7 figures, submitted to Physical Review B Single reference error fixe

Re-entrant pinning of Wigner molecules in a magnetic field due to a Coulomb impurity

Pinning of magnetic-field induced Wigner molecules (WMs) confined in parabolic two-dimensional quantum dots by a charged defect is studied by an exact diagonalization approach. We found a re-entrant pinning of the WMs as function of the magnetic field, a magnetic field induced re-orientation of the WMs and a qualitatively different pinning behaviour in the presence of a positive and negative Coulomb impurity

Instabilities and distribution functions associated with photoexcited carriers in semiconductors

Some aspects of photoexcited carriers in semiconductors are investigated theoretically. Three distinct phenomena are studied namely, photoluminescence, oscillatory photoconductivity and trap modified Gunn domains. Distribution functions are calculated for photoexcited electrons in gallium arsenide and are discussed within the context of recent photoluminescence experiments. The calculations apply to conditions of continuous, monochromatic photoexcitation and lattice temperatures in the liquid helium range. A Boltzmann equation approach 1s used to take account of Injection of electrons into the conduction band, electron scattering and electron recombination. The effects of Inelastic scattering by acoustic phonons, longitudinal optical phonons and other electrons are considered. The equation is solved numerically using an Iterative technique. The steady state distribution function and its time evolution from an initial state in which there are no electrons in the band are both computed. Under conditions such that electron-electron scattering has a negligible effect, the steady state distribution function is found to have a non-Maxwellian form. It has a high energy plateau which has some fine structure close to the injection energy (or energies). The origin of the plateau and the fine structure are explained in terms of the phonon scattering events undergone by the photoexcited electrons. Two models for electron-electron scattering are studied. Within a large screening model, distribution functions are computed without recourse to further approximations to the electron-electron scattering rates. Distribution functions applicable to situations in which the screening is not large are computed using a Taylor series approximation to the electron-electron scattering rates. The computations are of a preliminary nature but it is found that at an electron density of ~ 3 x 10¹⁶ m-³ electron-electron scattering could cause heating of those electrons whose energy is greater than the injection energy. Electron-acceptor luminescence spectra for C doped GaAs, under photoexcitation conditions such that electron-electron scattering has a negligible effect, are derived from the computed distribution functions and are compared with the results of some recent photoluminescence experiments. Fair agreement is obtained. The theory of oscillatory photoconductivity is studied, with special reference to negative current and instability effects. The theory is used to investigate a one dimensional model for the space-time dependence of the electric field in a semiconductor under conditions such that oscillatory photoconductivity occurs. The semiconductor is taken to be in a resistive circuit. The appropriate equations are solved numerically and thus the stability of the negative current state is investigated. It is shown that this state could be stable with respect to space charge formation if the circuit resistance is sufficiently high. In addition a numerical study is made of the types of domain instability which could otherwise form

N electrons in a quantum dot: Two-point Pade approximants

We present analytic estimates for the energy levels of N electrons (N = 2 - 5) in a two-dimensional parabolic quantum dot. A magnetic field is applied perpendicularly to the confinement plane. The relevant scaled energy is shown to be a smooth function of the parameter \beta=(effective Rydberg/effective dot energy)^{1/6}. Two-point Pade approximants are obtained from the series expansions of the energy near the oscillator ($\beta\to 0$) and Wigner ($\beta\to\infty$) limits. The approximants are expected to work with an error not greater than 2.5% in the entire interval $0\le\beta < \infty$.Comment: 27 pages. LaTeX. 6 figures not include

Exact continuum theory of anti-Klein tunneling in bilayer graphene

Exact conditions for anti-Klein transmission zeros are found analytically with a 4-component continuum approach which includes trigonal warping. Anti-Klein tunneling occurs at oblique incidence on steps and barriers with soft and hard walls as well as in the known case of normal incidence on a hard step. The necessary energy and angle of incidence depend on the crystallographic orientation of the step or barrier. At normal incidence on an armchair step in unbiased bilayer graphene, anti-Klein tunneling occurs because both the continuum and the tight binding Hamiltonians are invariant under layer and site interchange. At oblique incidence, anti-Klein tunneling is valley-dependent even in the absence of trigonal warping. An experimental arrangement that functions both as a detector of anti-Klein tunneling and a valley polarizer is suggested. There are cases where anti-Klein tunneling occurs in the 4-component theory but not in the 2-component approximation.Comment: 16 pages, 10 figure