74 research outputs found
Excitons in Electrostatic Traps
We consider in-plane electrostatic traps for indirect excitons in coupled
quantum wells, where the traps are formed by a laterally modulated gate
voltage. An intrinsic obstacle for exciton confinement in electrostatic traps
is an in-plane electric field that can lead to exciton dissociation. We propose
a design to suppress the in-plane electric field and, at the same time, to
effectively confine excitons in the electrostatic traps. We present
calculations for various classes of electrostatic traps and experimental proof
of principle for trapping of indirect excitons in electrostatic traps.Comment: 4 pages, 3 figure
Coherence Length of Cold Exciton Gases in Coupled Quantum Wells
A Mach-Zehnder interferometer with spatial and spectral resolution was used
to probe spontaneous coherence in cold exciton gases, which are implemented
experimentally in the ring of indirect excitons in coupled quantum wells. A
strong enhancement of the exciton coherence length is observed at temperatures
below a few Kelvin. The increase of the coherence length is correlated with the
macroscopic spatial ordering of excitons.Comment: 5 pages, 3 figure
Р-стереогенні діамандоїдні фосфіни
Despite diamondoid phosphines have found many synthetic applications and are even available commercially the chemistry of chiral diamondoid phosphines remains largely unexplored.Aim. To develop the convenient preparative method for the preparation of sterically-congested P-stereogenic secondary diamodoidyl phosphines as potential organocatalysts and ligands in the asymmetric synthesis.Results and discussion. A convenient method for the synthesis of P-stereogenic diamondoid phosphines with high yields through the phosphorylation of hydroxydiamondoids in trifluoroacetic acid followed by the reduction of the corresponding asymmetric chlorophosphonates has been proposed. The secondary phosphines obtained form stable complexes with borane that can be used to separate diamondoid phosphines into enantiomers.Experimental part. The experimental procedures for the preparation of 1- and 4-diamantyl-1-adamantyl- and phenylphosphines were developed; the structures of new compounds were confirmed by NMR and HRMS spectral data.Conclusions. A number of P-stereogenic mixed diamondoidylaryl phosphines and the secondary phosphines containing exclusively diamondoid substituents has been prepared. A degree of steric bulkiness is determined by the combination of diamondoid substituents around a phosphorus atom where 1-diamantyl derivatives are the most sterically-congested. The compounds obtained are potential ligands in asymmetric catalysis.Received: 31.03.2020Revised: 24.06.2020Accepted: 27.08.2020Незважаючи на те, що діамандоїдні фосфіни широко використовуються в органічному синтезі і навіть доступні комерційно, хімія хіральних діамандоїдних фосфінів залишається не дослідженою.Мета. Розробити зручний препаративний метод синтезу стереоускладнених Р-стереогенних вторинних діамандоїдних фосфінів, які можуть бути використані як ліганди в асиметричному синтезі, а також як органокаталізатори.Результати та їх обговорення. Запропоновано зручний метод синтезу P-стереогенних діамандоїдних фосфінів шляхом фосфорилювання гідроксипохідних діамандоїдів у трифтороцтовій кислоті з подальшим відновленням відповідних асиметричних хлорофосфонатів з високими виходами. Одержані таким чином фосфіни утворюють стійкі комплекси з бораном, які розглядаються як проміжні сполуки для подальшого розділення енантіомерів.Експериментальна частина. Був розроблений препаративний метод синтезу 1- і 4-діамантил-, 1-адамантил- і фенілфосфінів, структури яких підтверджено мас-спектрометричними і ЯМР-спектральними даними.Висновки. Одержано ряд Р-стереогенних змішаних діамандоїларилфосфінів та вторинних фосфінів, які містять виключно діамандоїдні замісники. Ступінь стеричного навантаження сполук визначається комбінацією діамандоїдних замісників навколо атома фосфору, де похідні 1-діамантилу найбільш стерично ускладнені. Одержані сполуки є потенційними лігандами в асиметричному каталізі.Received: 31.03.2020 Revised: 24.06.2020 Accepted: 27.08.202
Dark-bright magneto-exciton mixing induced by Coulomb interaction in strained quantum wells
Coupled magneto-exciton states between allowed (`bright') and forbidden
(`dark') transitions are found in absorption spectra of strained
InGaAs/GaAs quantum wells with increasing magnetic field up to
30 T. We found large (~ 10 meV) energy splittings in the mixed states. The
observed anticrossing behavior is independent of polarization, and sensitive
only to the parity of the quantum confined states. Detailed experimental and
theoretical investigations indicate that the excitonic Coulomb interaction
rather than valence band complexity is responsible for the splittings. In
addition, we determine the spin composition of the mixed states.Comment: 4 pages, 4 figure
Manifestation of exciton Bose condensation in induced two-phonon emission and Raman scattering
The unusual two-photon emission by Bose-condensed excitons caused by
simultaneous recombination of two excitons with opposite momenta leaving the
occupation numbers of excitonic states with momenta unchanged
(below coherent two-exciton recombination) is investigated. Raman scattering
accompanied by the analogous two-exciton recombination (or creation) is also
analyzed. The excess momentum equal to the change of the electromagnetic field
momentum in these processes can be transferred to phonons or impurities. The
processes under consideration take place if there is Bose condensation in the
interacting exciton system, and, therefore, can be used as a new method to
reveal exciton Bose condensation. If the frequency of the incident light
( is the frequency corresponding to the recombination
of an exciton with p=0), the coherent two-exciton recombination with the excess
momentum elastically transferred to impurities leads to the appearance of the
spectral line corresponding to the induced two-photon
emission. In this case the anti-Stokes line on frequency also
appears in the Raman spectrum. If , there are both Stokes and
anti-Stokes lines on frequencies in the Raman spectrum. The
induced two-photon emission is impossible in this case. The spectral lines
mentioned above have phonon replicas on frequencies corresponding to the transmission of the excess
momentum (partially or as a whole) to optical phonons of frequency
( is an integer number).Comment: 21 pages, 2 Postscript figures. Submitted to Phys. Rev. B (1998
Time evolution of condensed state of interacting bosons with reduced number fluctuation in a leaky box
We study the time evolution of the Bose-Einstein condensate of interacting
bosons confined in a leaky box, when its number fluctuation is initially (t=0)
suppressed. We take account of quantum fluctuations of all modes, including k =
0. We identify a ``natural coordinate'' b_0 of the interacting bosons, by which
many physical properties can be simply described. Using b_0, we successfully
define the cosine and sine operators for interacting many bosons. The
wavefunction, which we call the ``number state of interacting bosons'' (NSIB),
of the ground state that has a definite number N of interacting bosons can be
represented simply as a number state of b_0. We evaluate the time evolution of
the reduced density operator \rho(t) of the bosons in the box with a finite
leakage flux J, in the early time stage for which Jt << N. It is shown that
\rho(t) evolves from a single NSIB at t = 0, into a classical mixture of NSIBs
of various values of N at t > 0. We define a new state called the
``number-phase squeezed state of interacting bosons'' (NPIB). It is shown that
\rho(t) for t>0 can be rewritten as the phase-randomized mixture (PRM) of
NPIBs. It is also shown that the off-diagonal long-range order (ODLRO) and the
order parameter defined by it do not distinguish the NSIB and NPIB. On the
other hand, the other order parameter \Psi, defined as the expectation value of
the boson operator, has different values among these states. For each element
of the PRM of NPIBs, we show that \Psi evolves from zero to a finite value very
quickly. Namely, after the leakage of only two or three bosons, each element
acquires a full, stable and definite (non-fluctuating) value of \Psi.Comment: 25 pages including 3 figures. To appear in Phys. Rev. A (1999). The
title is changed to stress the time evolution. Sections II, III and IV of the
previous manuscript have been combined into one section. The introduction and
summary of the previous manuscript have been combined into the Introduction
and Summary. The names and abbreviations of quantum states are changed to
stress that they are for interacting many bosons. More references are cite
Photoluminescence investigations of 2D hole Landau levels in p-type single Al_{x}Ga_{1-x}As/GaAs heterostructures
We study the energy structure of two-dimensional holes in p-type single
Al_{1-x}Ga_{x}As/GaAs heterojunctions under a perpendicular magnetic field.
Photoluminescence measurments with low densities of excitation power reveal
rich spectra containing both free and bound-carrier transitions. The
experimental results are compared with energies of valence-subband Landau
levels calculated using a new numerical procedure and a good agreement is
achieved. Additional lines observed in the energy range of free-carrier
recombinations are attributed to excitonic transitions. We also consider the
role of many-body effects in photoluminescence spectra.Comment: 13 pages, 10 figures, accepted to Physical Review
Collective coherence in planar semiconductor microcavities
Semiconductor microcavities, in which strong coupling of excitons to confined
photon modes leads to the formation of exciton-polariton modes, have
increasingly become a focus for the study of spontaneous coherence, lasing, and
condensation in solid state systems. This review discusses the significant
experimental progress to date, the phenomena associated with coherence which
have been observed, and also discusses in some detail the different theoretical
models that have been used to study such systems. We consider both the case of
non-resonant pumping, in which coherence may spontaneously arise, and the
related topics of resonant pumping, and the optical parametric oscillator.Comment: 46 pages, 12 figure
Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire
Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of
considerable technological importance but remains a challenge due to the
amphoteric behavior of the dopant atoms. Here we show that placing a narrow
GaAs quantum well in the AlAs shell effectively getters residual carbon
acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical
studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined
emission. Theoretical calculations of NW electronic structure confirm quantum
confinement of carriers at the core/shell interface due to the presence of
ionized carbon acceptors in the 1~nm GaAs layer in the shell.
Micro-photoluminescence in high magnetic field shows a clear signature of
avoided crossings of the Landau level emission line with the Landau
level TO phonon replica. The coupling is caused by the resonant hole-phonon
interaction, which points to a large 2D hole density in the structure.Comment: just published in Nano Letters
(http://pubs.acs.org/doi/full/10.1021/nl500818k
European registry on the management of helicobacter pylori infection (HP-EUREG protocol): The first results of Russian centers
Aim: To assess the clinical practice of diagnosis and treatment in patients with Helicobacter pylori infection and to compare this practice with the international guidelines in the European Registry on the management of Helicobacter pylori infection, Hp-EuReg protocol), a multicenter prospective observational study initiated by the European Helicobacter and Microbiota Study Group. Materials and methods: The data of 813 patients infected with H. pylori and entered in the Hp-EuReg register by the Russian centers in 2013-2015 were analyzed. Results: The most common methods for the primary diagnosis of H. pylori infection are histology (40.3%), rapid urease test (35.7%), and serology (17.2%). The duration of H. pylori eradication therapy was 7, 10, and 14 days in 18.0, 49.3, and 25.1%, respectively. To monitor the effectiveness of treatment, the investigators used a histological examination (34%), a urea breath test (27.3%), H. pylori stool antigen (22.8%), and a rapid urease test (16.3%). A serological test was carried out in 2.5% of the cases. No monitoring was done in 13.5% of the patients. The average eradication efficiency was 82.6%. If the therapy was ineffective, 80% of physicians did not intend to prescribe a new cycle of treatment. Conclusion: Significant differences were found between clinical practice and the current guidelines
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