MnAs dots grown on GaN(0001)-(1x1) surface

MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1$\cdot 10^{11}$ cm$^{-2}$ and $2.5\cdot 10^{11}$ cm$^{-2}$, respectively (as observed by AFM), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d - related contribution to the total density of states (DOS) distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both system behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intra-dot Curie temperatures substantially different. The intra-dot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.Comment: 20+ pages, 8 figure

Truthful Mechanisms for Matching and Clustering in an Ordinal World

We study truthful mechanisms for matching and related problems in a partial information setting, where the agents' true utilities are hidden, and the algorithm only has access to ordinal preference information. Our model is motivated by the fact that in many settings, agents cannot express the numerical values of their utility for different outcomes, but are still able to rank the outcomes in their order of preference. Specifically, we study problems where the ground truth exists in the form of a weighted graph of agent utilities, but the algorithm can only elicit the agents' private information in the form of a preference ordering for each agent induced by the underlying weights. Against this backdrop, we design truthful algorithms to approximate the true optimum solution with respect to the hidden weights. Our techniques yield universally truthful algorithms for a number of graph problems: a 1.76-approximation algorithm for Max-Weight Matching, 2-approximation algorithm for Max k-matching, a 6-approximation algorithm for Densest k-subgraph, and a 2-approximation algorithm for Max Traveling Salesman as long as the hidden weights constitute a metric. We also provide improved approximation algorithms for such problems when the agents are not able to lie about their preferences. Our results are the first non-trivial truthful approximation algorithms for these problems, and indicate that in many situations, we can design robust algorithms even when the agents may lie and only provide ordinal information instead of precise utilities.Comment: To appear in the Proceedings of WINE 201

Element specific characterization of heterogeneous magnetism in (Ga,Fe)N films

We employ x-ray spectroscopy to characterize the distribution and magnetism of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions adjacent to the samples surface. A sizable x-ray magnetic circular dichroism (XMCD) signal at the Fe L-edges in remanence and at moderate magnetic fields at 300 K links the high temperature ferromagnetism with the Fe(3d) states. The XMCD response at the N K-edge highlights that the N(2p) states carry considerable spin polarization. We conclude that FeN{\delta} nanocrystals, with \delta > 0.25, stabilize the ferromagnetic response of the films.Comment: 4 pages, 3 figures, 1 tabl

The impact of skull bone intensity on the quality of compressed CT neuro images

International audienceThe increasing use of technologies such as CT and MRI, along with a continuing improvement in their resolution, has contributed to the explosive growth of digital image data being generated. Medical communities around the world have recognized the need for efficient storage, transmission and display of medical images. For example, the Canadian Association of Radiologists (CAR) has recommended compression ratios for various modalities and anatomical regions to be employed by lossy JPEG and JPEG2000 compression in order to preserve diagnostic quality. Here we investigate the effects of the sharp skull edges present in CT neuro images on JPEG and JPEG2000 lossy compression. We conjecture that this atypical effect is caused by the sharp edges between the skull bone and the background regions as well as between the skull bone and the interior regions. These strong edges create large wavelet coefficients that consume an unnecessarily large number of bits in JPEG2000 compression because of its bitplane coding scheme, and thus result in reduced quality at the interior region, which contains most diagnostic information in the image. To validate the conjecture, we investigate a segmentation based compression algorithm based on simple thresholding and morphological operators. As expected, quality is improved in terms of PSNR as well as the structural similarity (SSIM) image quality measure, and its multiscale (MS-SSIM) and informationweighted (IW-SSIM) versions. This study not only supports our conjecture, but also provides a solution to improve the performance of JPEG and JPEG2000 compression for specific types of CT images

Spin effects in single electron tunneling

An important consequence of the discovery of giant magnetoresistance in metallic magnetic multilayers is a broad interest in spin dependent effects in electronic transport through magnetic nanostructures. An example of such systems are tunnel junctions -- single-barrier planar junctions or more complex ones. In this review we present and discuss recent theoretical results on electron and spin transport through ferromagnetic mesoscopic junctions including two or more barriers. Such systems are also called ferromagnetic single-electron transistors. We start from the situation when the central part of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle. Transport characteristics reveal then single-electron charging effects, including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations. Single-electron ferromagnetic transistors based on semiconductor quantum dots and large molecules (especially carbon nanotubes) are also considered. The main emphasis is placed on the spin effects due to spin-dependent tunnelling through the barriers, which gives rise to spin accumulation and tunnel magnetoresistance. Spin effects also occur in the current-voltage characteristics, (differential) conductance, shot noise, and others. Transport characteristics in the two limiting situations of weak and strong coupling are of particular interest. In the former case we distinguish between the sequential tunnelling and cotunneling regimes. In the strong coupling regime we concentrate on the Kondo phenomenon, which in the case of transport through quantum dots or molecules leads to an enhanced conductance and to a pronounced zero-bias Kondo peak in the differential conductance.Comment: topical review (36 figures, 65 pages), to be published in J. Phys.: Condens. Matte

Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The signature of coherent coupling between two quantum states is an anticrossing in their energies as one is swept through the other. In single semiconductor quantum dots containing an electron-hole pair the eigenstates form a two-level system that can be used to demonstrate quantum effects in the solid state, but in all previous work these states were independent. Here we describe a technique to control the energetic splitting of these states using a vertical electric field, facilitating the observation of coherent coupling between them. Near the minimum splitting the eigenstates rotate in the plane of the sample, being orientated at 45{\deg} when the splitting is smallest. Using this system we show direct control over the exciton states in one quantum dot, leading to the generation of entangled photon pairs

Psychologiczna ocena pacjentów z tachyarytmiami nadkomorowymi leczonych ablacją przeznaczyniową

Wstęp: U chorych z częstoskurczami nadkomorowymi (SVT) po udanej ablacji przeznaczyniowej przy uzyciu prądu o wysokiej częstotliwości (RFA) wykazywano istotną poprawę jakości życia. Cel pracy: Ocena wpływu RFA na obraz samego siebie i poziom lęku pacjentów z SVT. Materiał i metody: Do badania włączono 52 pacjentów (w tym 25 kobiet), w średnim wieku 43 lata (18&#8211;75 lat), poddanych ablacji SVT. Wskazaniami do RFA były: zespół Wolffa-Parkinsona- White&#8217;a (WPW) - u 28 chorych, częstoskurcz nawrotny w obrębie łącza przedsionkowokomorowego (AVNRT) - u 20 pacjentów i migotanie przedsionków (AF) - u 4 osób. Oceniano wpůyw RFA na obraz samego siebie i poziom lęku u pacjentów poddanych temu zabiegowi. Badania wykonano przed zabiegiem ablacji oraz 3 miesiące po RFA. Do badania poziomu lęku użyto inwentarza stanu i cechy lęku (STAI). Do oceny obrazu samego siebie wykorzystano test przymiotnikowy Gougha i Heilbruna (ACL), a do badania nastroju - krótką skalę oceny nastroju (SMCS). Wyniki analizowano, używając testu t-Studenta, wartoąśi p Ł 0,05 uznano za znamienne. Wyniki: Wstępne wyniki RFA były dobre u wszystkich pacjentów. Po 3 miesiącach obserwacji po zabiegu zanotowano nawrót SVT u 3 pacjentów. Jedynym istotnym powikłaniem RFA było wystąpienie bloku przedsionkowo-komorowego III° w 1. dobie po ablacji AVNRT. U tego chorego wszczepiono układ stymulujŕcy serce typu VDD. W 3. miesiącu po zabiegu zanotowano istotną redukcj poziomu lęku, mierzonego za pomocą kwestionariusza STAI (lęk jako: stan, p < 0,002; cecha, p < 0,02). Niektóre aspekty obrazu siebie ("heteroseksualność" i "feminizm") uległy istotnym zmianom (p < 0,03; p < 0,002). Pogorszenie obrazu samego siebie oraz podwyższenie poziomu lęku obserwowano u 3 pacjentów z nawrotem SVT w 3. miesiącu po RFA. Wnioski: Mimo krótkiego okresu obserwacji u pacjentów po ablacji SVT zanotowano istotną redukcję poziomu lęku oraz pozytywne zmiany w obrazie samego siebie

Psychologiczna ocena pacjentów z tachyarytmiami nadkomorowymi leczonych ablacją przeznaczyniową

Wstęp: U chorych z częstoskurczami nadkomorowymi (SVT) po udanej ablacji przeznaczyniowej przy uzyciu prądu o wysokiej częstotliwości (RFA) wykazywano istotną poprawę jakości życia. Cel pracy: Ocena wpływu RFA na obraz samego siebie i poziom lęku pacjentów z SVT. Materiał i metody: Do badania włączono 52 pacjentów (w tym 25 kobiet), w średnim wieku 43 lata (18&#8211;75 lat), poddanych ablacji SVT. Wskazaniami do RFA były: zespół Wolffa-Parkinsona- White&#8217;a (WPW) - u 28 chorych, częstoskurcz nawrotny w obrębie łącza przedsionkowokomorowego (AVNRT) - u 20 pacjentów i migotanie przedsionków (AF) - u 4 osób. Oceniano wpůyw RFA na obraz samego siebie i poziom lęku u pacjentów poddanych temu zabiegowi. Badania wykonano przed zabiegiem ablacji oraz 3 miesiące po RFA. Do badania poziomu lęku użyto inwentarza stanu i cechy lęku (STAI). Do oceny obrazu samego siebie wykorzystano test przymiotnikowy Gougha i Heilbruna (ACL), a do badania nastroju - krótką skalę oceny nastroju (SMCS). Wyniki analizowano, używając testu t-Studenta, wartoąśi p Ł 0,05 uznano za znamienne. Wyniki: Wstępne wyniki RFA były dobre u wszystkich pacjentów. Po 3 miesiącach obserwacji po zabiegu zanotowano nawrót SVT u 3 pacjentów. Jedynym istotnym powikłaniem RFA było wystąpienie bloku przedsionkowo-komorowego III° w 1. dobie po ablacji AVNRT. U tego chorego wszczepiono układ stymulujŕcy serce typu VDD. W 3. miesiącu po zabiegu zanotowano istotną redukcj poziomu lęku, mierzonego za pomocą kwestionariusza STAI (lęk jako: stan, p < 0,002; cecha, p < 0,02). Niektóre aspekty obrazu siebie ("heteroseksualność" i "feminizm") uległy istotnym zmianom (p < 0,03; p < 0,002). Pogorszenie obrazu samego siebie oraz podwyższenie poziomu lęku obserwowano u 3 pacjentów z nawrotem SVT w 3. miesiącu po RFA. Wnioski: Mimo krótkiego okresu obserwacji u pacjentów po ablacji SVT zanotowano istotną redukcję poziomu lęku oraz pozytywne zmiany w obrazie samego siebie