Conduction electrons localized by charged magneto-acceptors A2−^{2-} in GaAs/GaAlAs quantum wells

A variational theory is presented of A$^{1-}$ and A$^{2-}$ centers, i.e. of a negative acceptor ion localizing one and two conduction electrons, respectively, in a GaAs/GaAlAs quantum well in the presence of a magnetic field parallel to the growth direction. A combined effect of the well and magnetic field confines conduction electrons to the proximity of the ion, resulting in discrete repulsive energies above the corresponding Landau levels. The theory is motivated by our experimental magneto-transport results which indicate that, in a heterostructure doped in the GaAs well with Be acceptors, one observes a boil-off effect in which the conduction electrons in the crossed-field configuration are pushed by the Hall electric field from the delocalized Landau states to the localized acceptor states and cease to conduct. A detailed analysis of the transport data shows that, at high magnetic fields, there are almost no conducting electrons left in the sample. It is concluded that one negative acceptor ion localizes up to four conduction electrons.Comment: 8 pages, 5 figure

Reservoir model for two-dimensional electron gases in quantizing magnetic fields: a review

We collect and review works which treat two-dimensional electron gases in quantum wells (mostly GaAs/GaAlAs heterostructures) in the presence of quantizing magnetic fields as open systems in contact with outside reservoirs. If a reservoir is sufficiently large, it pins the Fermi level to a certain energy. As a result, in a varying external magnetic field, the thermodynamic equilibrium will force oscillations of the electron density in and out of the quantum well (QW). This leads to a number of physical phenomena in magneto-transport, interband and intraband magneto-optics, magnetization, magneto-plasma dispersion, etc. In particular, as first proposed by Baraff and Tsui, the density oscillations in and out of QW lead to plateaus in the Integer Quantum Hall Effect (IQHE) at values observed in experiments. The gathered evidence, especially from magneto-optical investigations, allows us to conclude that, indeed, in most GaAs/GaAlAs hetrostructures one deals with open systems in which the electron density in QWs oscillates as the magnetic field varies. Relation of the density oscillations to other factors, such as electron localization, and their combined influence on the quantum transport in 2D electron gases, is discussed. In particular, a validity of the classical formula for the Hall resistivity {\rho}xy = B/Nec is considered. It is concluded that the density oscillations are not sufficient to be regarded as the only source of plateaus in IQHE, although such claims have been sometimes made in the past and present. Still, our general conclusion is that the reservoir approach should be included in various descriptions of 2D electron gases in the present of a magnetic field. An attempt has been made to quote all the relevant literature on the subject.Comment: 17 pages, 23 figure

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

Scaling the mobility of health workers in an enlarged Europe : An open political-economy perspective

The final, definitive version of this paper has been published in European Urban and Regional Studies, Vol. 23 (4), October 2016, published by SAGE Publishing.The enlargement of the European Union in 2004 and 2007 and the marketisation of health care are increasing the mobility of workers and driving a scalar transformation of the sector across Europe. Drawing on questionnaires and interviews in 17 European Union countries, and focusing on two case study New Member States, we analyse inter- and intra-country drivers and impacts of health care labour mobility. The data are analysed from an open political-economy perspective underpinned by an understanding of scale as a socially constructed material entity mediated by national and supranational state institutions, and the collective agency of workers. We emphasise the contradictory and contested nature of rescaling health care and the complex micro-dynamics of mobility. Although absolute outward migration across borders is relatively small, the movement of health care specialists is having a disproportionate effect on sender countries and regions within them.Peer reviewedFinal Accepted Versio

Stereocomplexation of polylactides containing ionic liquid end-groups

Linear and cyclic oligomers of epichlorohydrin were synthesized and converted by reaction with N-methylimidazole into linear or cyclic derivatives fitted with imidazolium ionic liquid groups. These oligomers containing hydroxyl groups were used as macroinitiators of cationic polymerization of L- and D-lactide. Two types of products were obtained: linear polylactides (PLA's) containing a few ionic liquid groups at the chain end and star PLA's containing a few ionic liquid groups at the core. The enantiomeric forms of PLA, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were used for stereocomplexation studies following earlier observation that if PLLA and PDLA containing one imidazolium ionic liquid group at the chain end are mixed in 1,4-dioxane solution, stereocomplex precipitates spontaneously in form of uniform microspheres. In a present contribution it is shown that if more than one ionic liquid group is introduced at the chain ends of PLLA and PDLA, morphology of precipitated stereocomplex is similar to that observed for PLA's fitted with one ionic liquid group but microspheres are slightly less regular. When few ionic liquid groups are present at the core of star PLLA no specific morphology is observed for precipitating stereocomplex with linear PDLA fitted with C4H9O- group. Only when end-groups able to interacting with each other are introduced, specific morphology appears. With ionic liquid end-groups in linear component, stereocomplex precipitates in form of irregular granules while with strongly interacting ureidopirymidine end-groups, microspheres, although not very regular, are formed. These results indicate that well defined morphology (regular spherical particles) is observed when interacting groups are present at the freely moving chain ends but not, when the same groups are embedded and immobilized at the core of star polymers and through it probably are hidden inside polymer random coil.Otrzymano liniowe i cykliczne oligomery epichlorohydryny, następnie grupy chlorometylowe przekształcono – w reakcji z N-metyloimidazolem – w imidazoliowe, analogiczne do grup występujących w cieczach jonowych. Oligomery zawierające grupy hydroksylowe zastosowano jako makroinicjatory kationowej polimeryzacji L- i D-laktydu, w wyniku której uzyskano dwa rodzaje produktów: liniowe polimery laktydu (PLA) z udziałem kilku grup cieczy jonowej na jednym zkońców łańcucha oraz gwiaździste polimery laktydu zawierające kilka grup cieczy jonowej w centrum gwiaździstej makrocząsteczki. Enancjomeryczne formy PLA: poli(L-laktyd) (PLLA) i poli(D-laktyd) (PDLA) zastosowano do wytworzenia stereokompleksów. Wykazano, że jeśli na końcu łańcucha znajduje się więcej niż jedna grupa cieczy jonowej, morfologia wytrącającego się stereokompleksu jest zbliżona do morfologii obserwowanej w wypadku obecności tylko jednej grupy cieczy jonowej, ale mikrosfery są mniej jednorodne. Wyniki wskazują, że regularne mikrosfery powstają jeśli grupy zdolne do wzajemnych oddziaływań znajdują się na końcach liniowych makrocząsteczek, nie powstają natomiast wtedy, gdy grupy te znajdują się w centrum gwiaździstej makrocząsteczki i są otoczone jej ramionami

Stereokompleksowanie polilaktydów zawierających ciecze jonowe jako grupy końcowe

Linear and cyclic oligomers of epichlorohydrin were synthesized and converted by reaction with N-methylimidazole into linear or cyclic derivatives fitted with imidazolium ionic liquid groups. These oligomers containing hydroxyl groups were used as macroinitiators of cationic polymerization of L- and D-lactide. Two types of products were obtained: linear polylactides (PLA's) containing a few ionic liquid groups at the chain end and star PLA's containing a few ionic liquid groups at the core. The enantiomeric forms of PLA, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were used for stereocomplexation studies following earlier observation that if PLLA and PDLA containing one imidazolium ionic liquid group at the chain end are mixed in 1,4-dioxane solution, stereocomplex precipitates spontaneously in form of uniform microspheres. In a present contribution it is shown that if more than one ionic liquid group is introduced at the chain ends of PLLA and PDLA, morphology of precipitated stereocomplex is similar to that observed for PLA's fitted with one ionic liquid group but microspheres are slightly less regular. When few ionic liquid groups are present at the core of star PLLA no specific morphology is observed for precipitating stereocomplex with linear PDLA fitted with C4H9O- group. Only when end-groups able to interacting with each other are introduced, specific morphology appears. With ionic liquid end-groups in linear component, stereocomplex precipitates in form of irregular granules while with strongly interacting ureidopirymidine end-groups, microspheres, although not very regular, are formed. These results indicate that well defined morphology (regular spherical particles) is observed when interacting groups are present at the freely moving chain ends but not, when the same groups are embedded and immobilized at the core of star polymers and through it probably are hidden inside polymer random coil.Otrzymano liniowe i cykliczne oligomery epichlorohydryny, następnie grupy chlorometylowe przekształcono – w reakcji z N-metyloimidazolem – w imidazoliowe, analogiczne do grup występujących w cieczach jonowych. Oligomery zawierające grupy hydroksylowe zastosowano jako makroinicjatory kationowej polimeryzacji L- i D-laktydu, w wyniku której uzyskano dwa rodzaje produktów: liniowe polimery laktydu (PLA) z udziałem kilku grup cieczy jonowej na jednym zkońców łańcucha oraz gwiaździste polimery laktydu zawierające kilka grup cieczy jonowej w centrum gwiaździstej makrocząsteczki. Enancjomeryczne formy PLA: poli(L-laktyd) (PLLA) i poli(D-laktyd) (PDLA) zastosowano do wytworzenia stereokompleksów. Wykazano, że jeśli na końcu łańcucha znajduje się więcej niż jedna grupa cieczy jonowej, morfologia wytrącającego się stereokompleksu jest zbliżona do morfologii obserwowanej w wypadku obecności tylko jednej grupy cieczy jonowej, ale mikrosfery są mniej jednorodne. Wyniki wskazują, że regularne mikrosfery powstają jeśli grupy zdolne do wzajemnych oddziaływań znajdują się na końcach liniowych makrocząsteczek, nie powstają natomiast wtedy, gdy grupy te znajdują się w centrum gwiaździstej makrocząsteczki i są otoczone jej ramionami