125 research outputs found
THE EFFECT OF SUBSTITUTING METAL VAPOUR WITH METAL HALlDE VAPOUR IN A HOLLOW CATHODE LASER DISCHARGE
Diszubsztituált aminosavak enantioszelektĂv szintĂ©zise fázistranszfer katalizátorok segĂtsĂ©gĂ©vel: Enantioselective synthesis of disubstituted amino acids in the presence of phase transfer catalysts
The class of enantiopure α,α-disubstituted α-amino acids has gained considerable attention in the past decades since some of them have interesting properties as antibiotics. The synthesis of such materials has been a challenging task to which enantioselective phase transfer catalysis offers an effective solution. In our work the synthesis starts from the easily available Meldrum’s acid, that is converted into the key disubstituted malonate intermediates in an asymmetric alkylation step catalyzed by cinchona-crown ether derivatives. The aim was to obtain a new type of phase transfer catalyst – cinchona derivatives of aza-crown ethers – and compare the achieved enantiomeric excess values, while also optimizing the reaction conditions. The amino acids are gained by the selective hydrolysis of the key intermediates followed by a multistep reaction.
Kivonat
Az enantiomertiszta α,α-diszubsztituált α-aminosavak iránt számottevĹ‘ az Ă©rdeklĹ‘dĂ©s az utĂłbbi Ă©vtizedekben, mivel egyes kĂ©pviselĹ‘ik antibakteriális tulajdonságokkal bĂrnak. SzintĂ©zisĂĽk egy kihĂvást jelentĹ‘ feladat, amire az enantioszelektĂv fázistranszfer katalĂzis hatĂ©kony megoldást kĂnál. Munkánk során az előállĂtás a könnyen elĂ©rhetĹ‘ Meldrum-savbĂłl indul, amelybĹ‘l a kulcsfontosságĂş diszubsztituált malonát köztitermĂ©kek előállĂtása kĂĽlönbözĹ‘ cinkona-koronaĂ©ter származĂ©kok által katalizált, aszimmetrikus alkilezĂ©si reakciĂłban törtĂ©nik. A cĂ©l Ăşjfajta fázistranszfer katalizátorok – aza-koronaĂ©terek cinkona származĂ©kai – előállĂtása Ă©s összehasonlĂtása volt az elĂ©rhetĹ‘ enantioszelektivitás szempontjábĂłl, valamint a reakciĂłkörĂĽlmĂ©nyek is optimalizálásra kerĂĽltek. Az aminosavak előállĂtása a köztitermĂ©kekbĹ‘l szelektĂv hidrolĂzist követĹ‘ átalakĂtásokkal lehetsĂ©ges
Snake Trajectories in Ultraclean Graphene p-n Junctions
Snake states are trajectories of charge carriers curving back and forth along
an interface. There are two types of snake states, formed by either inverting
the magnetic field direction or the charge carrier type at an interface.
Whereas the former has been demonstrated in GaAs-AlGaAs heterostructures, the
latter has become conceivable only with the advance of ballistic graphene where
a gapless p-n interface governed by Klein tunneling can be formed. Such snake
states were hidden in previous experiments due to limited sample quality. Here
we report on magneto-conductance oscillations due to snake states in a
ballistic suspended graphene p-n-junction which occur already at a very small
magnetic field of 20mT. The visibility of 30% is enabled by Klein collimation.
Our finding is firmly supported by quantum transport simulations. We
demonstrate the high tunability of the device and operate it in different
magnetic field regimesComment: Accepted for publication in Nature Communication
Anyagok és rendszerek nagysűrűségű optikai adattároláshoz = Materials and systems for high density optical data storage
A pályázat eredeti cĂ©lkitűzĂ©sĂ©nek megfelelĹ‘en a nagysűrűsĂ©gű optikai adattárolásra alkalmas anyagok Ă©s rendszerek kutatása terĂ©n Ă©rtĂĽnk el fontos eredmĂ©nyeket. Az anyagok kutatásának terĂĽletĂ©n az Atomfizika TanszĂ©k kutatĂł csoportja elsĹ‘sorban a nemzetközi partnerek által kifejlesztett polimer minták optikai tárolásra valĂł alkalmazhatĂłságát vizsgálta. ModelleztĂĽk Ă©s mĂ©rtĂĽk az anyagok paramĂ©tereit Ă©s vizsgáltuk az anyagtulajdonságok hatását az optikai tárolĂł rendszerek működĂ©sĂ©re. A közreműködĹ‘ SZFKI optikai kristálymintákat kĂ©szĂtett Ă©s tesztelt, majd alkalmazta a mintákat optikai adattárolásra, vizsgálta a hologramok stabilitását Ă©s a kiolvasás hibáját. A rendszerek tĂ©májában a tanszĂ©ki csoport bekapcsolĂłdott kĂ©t eurĂłpai konzorcium munkájába fĹ‘leg a tĂ©rfogati adattárolĂłk modellezĂ©sĂ©nek terĂĽletĂ©n. A vastag anyagon valĂł diffrakciĂł számĂtására a tĂ©rfogati integrál megoldásának Ăşj mĂłdszerĂ©t javasoltuk, melynek alkalmazásával lehetĹ‘vĂ© válik a tĂ©rfogati holografikus rendszerek működĂ©sĂ©nek komplett modellezĂ©se, beleĂ©rtve a tárolĂłanyag viselkedĂ©sĂ©t is. Ăšj eredmĂ©nyeket Ă©rtĂĽnk el az optikai kártyára alapozott, hordozhatĂł, kompakt, polarizáciĂłs holografikus rendszerek kutatásában is. Az összesen 36 publikáciĂłbĂłl 11 referált folyĂłiratcikk, 3 könyvfejezet Ă©s nĂ©gy szabadalom. Az eredmĂ©nyekrĹ‘l összesen 16 rendezvĂ©nyen, illetve konferencián számoltunk be. A támogatás alatt három PhD ösztöndĂjasunk megszerezte a fokozatot Ă©s idĂ©n februárban beadásra kerĂĽlt egy MTA doktori Ă©rtekezĂ©s a tĂ©mában. | We achieved important results during the research of materials and systems for high density optical data storage. Concerning materials the research group of the department examined data storage capability of polymer layers developed by international partners. We modeled and experimentally tested characteristics of the materials and studied the effect of material properties on the operation of optical data storage systems. The contributor SZFKI prepared and tested optical crystal samples and applied them for optical data storage, studied stability of holograms and the bit-error-rate of hologram reconstruction. In the subject of systems the group of the department has become member of two European consortia with the task of modeling volume holographic systems. For the calculation of diffraction on thick material we proposed a new method that provides a useful tool for modeling complete volume holographic systems including the behavior of the storage material as well. We achieved new results also in the research of compact, portable polarization holographic systems using optical card form medium. From the 36 publications 11 are articles in referred journals, 3 of them are book chapters and four are filed patent applications. We presented our results on 16 conferences. During the project three PhD were defended and this year in February a new application was submitted to the Council of Doctors of HAS for the Doctor of Science degrees
Giant valley-isospin conductance oscillations in ballistic graphene
At high magnetic fields the conductance of graphene is governed by the
half-integer quantum Hall effect. By local electrostatic gating a \textit{p-n}
junction perpendicular to the graphene edges can be formed, along which quantum
Hall channels co-propagate. It has been predicted by Tworzid\l{}o and
co-workers that if only the lowest Landau level is filled on both sides of the
junction, the conductance is determined by the valley (isospin) polarization at
the edges and by the width of the flake. This effect remained hidden so far due
to scattering between the channels co-propagating along the \textit{p-n}
interface (equilibration). Here we investigate \textit{p-n} junctions in
encapsulated graphene with a movable \textit{p-n} interface with which we are
able to probe the edge-configuration of graphene flakes. We observe large
quantum conductance oscillations on the order of \si{e^2/h} which solely depend
on the \textit{p-n} junction position providing the first signature of
isospin-defined conductance. Our experiments are underlined by quantum
transport calculations.Comment: 5 pages, 4 figure
On the number of touching pairs in a set of planar curves
Given a set of planar curves (Jordan arcs), each pair of which meets --
either crosses or touches -- exactly once, we establish an upper bound on the
number of touchings. We show that such a curve family has touchings,
where is the number of faces in the curve arrangement that contains at
least one endpoint of one of the curves. Our method relies on finding special
subsets of curves called quasi-grids in curve families; this gives some
structural insight into curve families with a high number of touchings.Comment: 14 pages, 7 figure
Termikus effektusok vizsgálata akusztooptikai eszközökben = Investigation of thermal effects in acusto-optical devices
Kutatási projektĂĽnk során kĂĽlönbözĹ‘ tĂpusĂş Ă©s rendeltetĂ©sű akusztooptikai eszközökben kialakulĂł termikus jelensĂ©geket vizsgáltuk. Kidolgoztuk az akusztooptikai effektus termikus modelljĂ©t, amelyben az akusztikus nyaláb elnyelĹ‘dĂ©se, valamint az ultrahangkeltĹ‘ elektromos Ă©s akusztikus vesztesĂ©gei következtĂ©ben felszabadulĂł hĹ‘mennyisĂ©g-eloszlást, az ennek megfelelĹ‘ hĹ‘mĂ©rsĂ©klet-eloszlást szimuláltuk. A modellt akusztooptikai hatásfok-mĂ©rĂ©sekkel, valamint működĹ‘ cellán vĂ©gzett termovĂziĂłs mĂ©rĂ©sekkel fejlesztettĂĽk Ă©s ellenĹ‘riztĂĽk. A számolt Ă©s mĂ©rt hĹ‘mĂ©rsĂ©klet-eloszlás segĂtsĂ©gĂ©vel kiszámoltuk az akusztooptikai kristályok optikai Ă©s akusztooptikai paramĂ©tereinek termikus változását, Ă©s ezeket integráltuk a fĂ©ny hangoszlopon bekövetkezĹ‘ diffrakciĂłját Ă©s terjedĂ©sĂ©t leĂrĂł modellĂĽnkbe. MegállapĂtottuk, hogy az ĂĽzemi hĹ‘mĂ©rsĂ©kleten kialakulĂł termikus lencse jelentĹ‘sen befolyásolhatja a fĂ©nynyaláb terjedĂ©si irányát Ă©s intenzitás-eloszlását. Eljárásokat dolgoztunk ki az ultrahangkeltĹ‘ vesztesĂ©geink csökkentĂ©sĂ©re a technolĂłgia javĂtásával. Optimalizáltuk az akusztooptikai eszközök dobozolását hĹ‘technikai szempontbĂłl, Ă©s a nagyobb akusztikus teljesĂtmĂ©nyen működĹ‘ eszközöket kiegĂ©szĂtettĂĽk egy kĂ©tkörös aktĂv hĹ‘elvezetĹ‘ Ă©s hűtĹ‘elemmel. A bevezetett mĂłdszerekkel sikerĂĽlt az eszközök hĹ‘mĂ©rsĂ©kletnövekedĂ©sĂ©t nagyobb elektromos teljesĂtmĂ©ny esetĂ©n is korlátozni, valamint a hĹ‘mĂ©rsĂ©klet-eloszlás frekvenciafĂĽggĂ©sĂ©t Ă©s gradiensĂ©t a kristályban csökkenteni. | We examined theoretically and practically thermal effects in different type acousto-optic devices designed for different purposes. We elaborated a numerical model of the thermal processes appearing in these devices. In this model we calculate the heat distribution arising from the absorption of acoustic waves and electric transducer losses and calculate the corresponding temperature distribution across the device. The development of the model has been supported by continuous measurement of the acousto-optic interaction efficiency and temperature distribution on the acousto-optic crystal surfaces. Starting from the measured and calculated temperature distribution in the crystal we simulated the thermal changes of its photoelastic and optical parameters. We included the results in our model, which simulates the propagation of the optical beams through the acousto-optic crystal, and found that the thermal lens effect considerably influences their propagation direction and transversal intensity distribution. Based on the thermal imaging measurements we improved our acousto-optic transducer technology to reduce its losses. We optimized the housing of the devices to effectively remove the heat from the crystal walls and elaborated an active cooling system capable to control device temperature during operation. These improvements helped to limit and stabilize temperature increase and temperature gradients even at higher input electric power levels
Gate-controlled conductance enhancement from quantum Hall channels along graphene p-n junctions
The formation of quantum Hall channels inside the bulk of graphene is studied using various contact and gate geometries. p-n junctions are created along the longitudinal direction of samples, and enhanced conductance is observed in the case of bipolar doping due to new conducting channels forming in the bulk, whose position, propagating direction and, in one geometry, coupling to electrodes are determined by the gate-controlled filling factor across the device. This effect could be exploited to probe the behavior and interaction of quantum Hall channels protected against uncontrolled scattering at the edges
Fabry-Pérot Resonances in a Graphene/hBN Moiré Superlattice
While Fabry-Perot (FP) resonances and Moire superlattices are intensively studied in graphene on hexagonal boron nitride (hBN), the two effects have not been discussed in their coexistence. Here we investigate the FP oscillations in a ballistic pnp-junctions in the presence and absence of a Moire superlattice. First, we address the effect of the smoothness of the confining potential on the visibility of the FP resonances and carefully map the evolution of the FP cavity size as a function of densities inside and outside the cavity in the absence of a superlattice, when the cavity is bound by regular pn-junctions. Using a sample with a Moire superlattice, we next show that an FP cavity can also be formed by interfaces that mimic a pn-junction but are defined through a satellite Dirac point due to the superlattice. We carefully analyze the FP resonances, which can provide insight into the band-reconstruction due to the superlattice
Electron quantum optics in graphene
In the last decade, graphene has become an exciting platform for electron optical experiments, in many aspects superior to conventional two-dimensional electron gases (2DEGs). A major advantage, besides the ultra-large mobilities, is the fine control over the electrostatics, which gives the possibility of realising gap-less and compact p-n interfaces with high precision. The latter host non-trivial states, e.g., snake states in moderate magnetic fields, and serve as building blocks of complex electron interferometers. Thanks to the Dirac spectrum and its non-trivial Berry phase, the internal (valley and sublattice) degrees of freedom, and the possibility to tailor the band structure using proximity effects, such interferometers open up a completely new playground based on novel device architectures. In this review, we introduce the theoretical background of graphene electron optics, fabrication methods used to realise electron-optical devices, and techniques for corresponding numerical simulations. Based on this, we give a comprehensive review of ballistic transport experiments and simple building blocks of electron optical devices both in single and bilayer graphene, highlighting the novel physics that is brought in compared to conventional 2DEGs. After describing the different magnetic field regimes in graphene p-n junctions and nanostructures, we conclude by discussing the state of the art in graphene-based Mach-Zender and Fabry-Perot interferometers
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