Orbital order and spin-orbit coupling in BaVS3

The correlated 3d sulphide BaVS3 undergoes a sequence of three symmetry breaking transitions which are reflected in the temperature dependence of the magnetic susceptibility, and its anisotropy. We introduce a microscopic model based on the coexistence of wide band a(1g) and localized e(g) d-electrons, and give the classification of the order parameters under the double space group and time reversal symmetries. Allowing for the relativistic spin-orbit coupling, the d-shell multipoles acquire a mixed spin-orbital character. It follows that orbital ordering is accompanied by a change in the susceptibility anisotropyComment: 2 pages, submitted to the SCES05 conference, uses elsart41.cl

Töltés, spin és pálya szabadsági fokokkal rendelkező frusztrált elektronrendszerek egzotikus fázisai és gerjesztései = Exotic phases and excitation in frustrated electron systems with charge, spin and orbital degrees of freedom

Erősen frusztrált töltés- és spin-rendszereket vizsgáltunk piroklór rácson, melyekben az alapállapot makroszkopikusan elfajult. Valamint a másodlagos kölcsönhatások szerepét ezen elfajulás feloldásában. - Meghatároztuk egy korrelált elektronrendszer fázisdiagramját planáris piroklór rácson. Negyed-betöltésnél az alapállapot rezonáló szinglett kristály. Dópolás hatására szupravezetővé válik. - Megmutattuk, hogy a kvantum-dimer modellben gyémántrácson (ill. korrelált spinnélküli-fermion modellben piroklór rácson) résnélküli folyadék állapot jön létre véges paramétertartományban. - A króm-spinelekben észlelt feles mágnesezettségi plató megértéséhez vezethet a klasszikus bilineáris-bikvadratikus Heisenberg-modell tanulmányozása piroklór rácson. Megvizsgáltuk a plató stabilitásának feltételeit. Monte-Carlo szimuláció segítségével feltérképeztük a modell fázisdiagramját a hőmérséklet és mágneses tér függvényében, valamint felvázoltuk a mágnesezettség és szuceptibilitás hőmérsékletfüggését. Egymással versengő kölcsönhatások is vezethetnek frusztrációhoz: - Feltérképeztük a bilineáris-bikvadratikus egyes-spinű háromszögrács Heisenberg-modell fázisdiagramját mágneses térben, különös tekintettel a kvadrupoláris fázisokra és azok gerjesztéseire. - Megmutattuk, hogy a Dzjaloshinskij-Moriya kölcsönhatás lényegesen megváltoztatja a feles-spinű Heisenberg létrák fázisdiagramját mágneses térben. | We studied highly frustrated charge and spin pyrochlore systems, where the ground state is macroscopically degenerate, and the effects of secondary interactions in lifting this degeneracy: - We determined the phase diagram of a correlated electronic model on the planar pyrochlore lattice. At quarter filling the ground state is a resonating singlet-pair crystal. Upon doping, we found an extended window for superconducting pairing. - We have shown that the quantum dimer model on diamond lattice (i.e. correlated spinless fermion problem on the pyrochlore lattice) can support a gapless liquid state in a finite parameter range. - The classical bilinear-biquadratic Heisenberg model on pyrochlore lattice is a good starting point to understand the half-magnetization plateau in chromium spinels. We have deduced the criteria for a stable plateau. Monte-Carlo simulation revealed the finite-temperature phase diagram in magnetic field, and the temperature dependence of the magnetization and susceptibility. Frustration can also be introduced by competition between different interaction terms: -We have mapped out the phase diagram of the spin-one bilinear-biquadratic Heisenberg model on the triangular lattice in a magnetic field, with emphasis on the quadrupolar phases and their excitations. -We show that Dzyaloshinskii-Moriya interactions can substantially modify the phase diagram of spin-1/2 Heisenberg ladders in a magnetic field

Széles tiltottsávú félvezető rétegek növekedése, szintézise és mikroszkópiája = Growth, synthesis and microscopy of wide bandgap semiconducting layers

Munkánk során a széles tiltott sávú félvezetők közül a SiC-dal, GaN-del ZnO-dal és kontaktusaikkal foglalkoztunk. Meghatároztuk a jellemző hibákat és a diszlokáció sűrűség értékeket. Sikerült magyarázatot adnunk a gyémánt és SiC illeszkedésére. Zafír hordozóra növesztett ZnO rétegeket vizsgálva megállapítottuk, hogy MgO puffer réteg képes akár teljesen MgOAl2O3 spinel réteggé alakulni, ami csökkenti a misfit-et. Különféle félvezető nanoszerkezeteket állítottunk elő, mint pl. 300 nm hosszú és 15 nm széles szálakból álló Al1-xInxN nanofűvet magnetronos porlasztással. A megfigyelt szerkezetet leírtuk (a koncentráció a nanofű szálainak szélességében változik), hozzá modelt alkottunk és kísérletileg is igazoltuk. Az epitaxiális SiC kristályokat eredményező CO-ban hőkezelt (SiO2-dal borított) Si szeleteket tovább oxidálva azokból többféle nanoszerkezetet állítottunk elő. Mind SiC-hoz, mind GaN-hez készítettünk fém kontaktus rétegeket, melyeket átfogóan vizsgáltunk. Megállapítottuk, hogy a p-tipusú SiC-hoz ohmos kontaktusként használt Ti3SiC2 a Si poláros oldalon alakul ki az Al/Ti kontaktusok hőkezelése során. | During the project wide bandgap semiconductors (like SiC, GaN, ZnO) and contacts to them have been investigated. The characteristic defects and dislocation density values have been determined. An explanation was given for the matching of diamond and SiC.Investigating ZnO layers grown on sapphire with a MgO buffer it was found that the MgO can transform to a MgOAl2O3 spinel, which decreases the misfit. Different semiconducting nanostructures have been prepared, like for example the magnetron sputtered, columnar Al1-xInxN, which has a morphology similar to the grass, therefore we named that as nanograss. The observed structure was described in details (the concentration is changed along the diameter of the columns), a model was created and experimentaly proved. Different nanostructures have been prepared from the SiC nanocrystals formed in CO annealing of oxide covered silicon wafers seprating them from the silicon by further oxidization. Metal contacts have been prepared both to SiC and GaN, which have been investigated in details. On the Al/Ti contacts (used as ohmic contacts to p-type SiC) we have concluded, that Ti3SiC2 phase (responsible for teh ohmic behaviour) is formed on the silicon polar side of SiC, only

The electronic structure and the phases of BaVS3

BaVS3 is a moderately correlated d-electron system with a rich phase diagram. To construct the corresponding minimal electronic model, one has to decide which d-states are occupied, and to which extent. The ARPES experiment presented here shows that the behavior of BaVS3 is governed by the coexistence of wide-band (A_1g) and narrow-band (twofold degenerate E) d-electrons. We sketch a lattice fermion model which may serve as a minimal model of BaVS3. This serves foremost for the understanding of the metal-insulator in pure BaVS3 and its absence in some related compounds. The nature of the low temperature magnetic order differs for several systems which may be described in terms of the same electron model. We describe several recent experiments which give information about magnetic order at high pressures. In particular, we discuss field-induced insulator-to-metal transition at slightly subcritical pressures, and an evidence for magnetic order in the high-pressure metallic phase. The phase diagram of Sr-doped BaVS3 is also discussed. The complexity of the phases of BaVS3 arises from the fact that it is simultaneously unstable against several kinds of instabilities.Comment: Presented at the International Conference on Magnetism 2006 (Kyoto), 6 pages, 9 figure

Photoluminescence study of basal plane stacking faults in ZnO nanowires

We have investigated the photoluminescence (PL) of ZnO nanowires (NWs) containing a high density (~1×106 cm-1) of basal plane stacking faults (BSFs). It was observed that the BSFs result in a specific PL peak at ~3.329 eV along with a donor bound excitonic emission (DoX) peak at 5 K. The observed BSF-related emission is of excitonic type and possesses longer PL lifetime than DoX (~360 ps vs. ~70 ps). Via comparison of the microstructural and the PL properties of the ZnO NWs, it is shown that the observed BSF-related emission is due to the formation of crystal phase quantum wells (QWs). This is explained by the fact that BSF in wurtzite (WZ) ZnO is the thinnest segment of zinc blende (ZB) phase ZnO inserted in the WZ matrix, resulting in band alignment of type II due to the conduction and valence band offsets of ZB with respect to WZ ZnO. The mechanism of the BSF related PL is suggested to be an indirect exciton transitions due to the recombination of electrons confined in the ZB QWs to holes in the WZ barriers localized near the BSFs. © 2013 Elsevier B.V. All rights reserved

Continuous spark plasma synthesis of Au/Co binary nanoparticles with tunable properties

We present here a scalable and environmentally friendly gas phase technique employing atmospheric pressure electrical spark discharge plasmas for the production of Au/Co binaries, an effective catalyst system for the decomposition of hydrogen-rich compounds, such as ammonium borane. We demonstrate that Au/Co alloy nanoparticles can be produced via the spark plasma-based technique. The possibility of varying the morphology and phase structure via real time heat treatment of the generated aerosol to form Au/Co/CoO particles with continuous control over a wide particle compositional range (from 24 to 64 at.% [Co]/([Co] + [Au]) content) is also demonstrated. Since our spark-based approach is proven to be capable of providing reasonable particle yields, these results may contribute to the transition of lab-scale, nanocatalyst-based hydrogen storage systems to real world applications

Transformations in CrFeCoNiCu High Entropy Alloy Thin Films during In-Situ Annealing in TEM

In-situ TEM-heating study of the microstructural evolution of CrFeCoNiCu high entropy alloy (HEA) thin films was carried out and morphological and phase changes were recorded. Post annealing investigation of the samples was carried out by high resolution electron microscopy and EDS measurements. The film is structurally and morphologically stable single phase FCC HEA up to 400 °C. At 450 °C the formation of a BCC phase was observed, however, the morphology of the film remained unchanged. This type of transformation is attributed to diffusionless processes (martensitic or massive). From 550 °C fast morphological and structural changes occur, controlled by volume diffusion processes. Fast growing of a new intermetallic phase is observed which contains mainly Cr and has large unit cell due to chemical ordering of components in <100> direction. The surface of the films gets covered with a CrO-type layer, possibly contributing to corrosion resistance of these

Examination of the Genetic Connectedness of Various Hungarian Pig Populations

Authors analysed field test data (collected and owned by the National Institute for Agricultural Quality Control between 1994-1999) of various pig breeds (Belgian Landrace, Duroc, Pietrain), and constructions (Duroc × Belgian Landrace, Belgian Landrace × Hampshire, Pietrain × Hampshire, Pietrain × Duroc, the D and E-lines of the Ka-Hyb hybrid pig). Gilts are kept in groups up to 25 pigs while boars are raised in smaller groups up to 15 on an ad libitum feeding regime. In the field test three ultrasonic back fat measurements are taken from boars and gilts between 80 and 110 kg at the middle of the spinal chord (shoulder, mid-back, loin). Average back fat thickness is calculated as the average of these three measurements. Heritability of average back fat depth was estimated using the REML method. The estimates varied between 0.12-0.51 depending on the genotype. The existence of genetic ties between the herds of the various pig populations was measured through the use of boars across the herds. Concerning the Belgian Landrace breed and the E-line of the Ka-Hyb hybrid pig no genetic ties were found, which means that the boars of these populations were only used within the herds. On the other hand week connections (5-14%) were found for the other genotypes. Boars having been used in two or more herds showed more reliable breeding values (lower prediction error variance) than those animals, which had progeny in one herd only. Based on the results authors suggested the exclusive application of the BLUP method in the Hungarian pig evaluation