33 research outputs found
Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N
The sign, magnitude, and range of the exchange couplings between pairs of Mn
ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have
been grown by metalorganic vapor phase epitaxy and characterized by
secondary-ion mass spectroscopy; high-resolution transmission electron
microscopy with capabilities allowing for chemical analysis, including the
annular dark-field mode and electron energy loss spectroscopy; high-resolution
and synchrotron x-ray diffraction; synchrotron extended x-ray absorption
fine-structure; synchrotron x-ray absorption near-edge structure; infra-red
optics and electron spin resonance. The results of high resolution magnetic
measurements and their quantitative interpretation have allowed to verify a
series of ab initio predictions on the possibility of ferromagnetism in dilute
magnetic insulators and to demonstrate that the interaction changes from
ferromagnetic to antiferromagnetic when the charge state of the Mn ions is
reduced from 3+ to 2+.Comment: 12 pages, 14 figures; This version contains the detailed
characterization of the crystal structure as well as of the Mn distribution
and charge stat
Conductance spectra of (Nb, Pb, In)/NbP -- superconductor/Weyl semimetal junctions
The possibility of inducing superconductivity in type-I Weyl semimetal
through coupling its surface to a superconductor was investigated. A single
crystal of NbP, grown by chemical vapor transport method, was carefully
characterized by XRD, EDX, SEM, ARPES techniques and by electron transport
measurements. The mobility spectrum of the carriers was determined. For the
studies of interface transmission, the (001) surface of the crystal was covered
by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC
current-voltage characteristics and AC differential conductance through the
interfaces as a function of the DC bias were investigated. When the metals
become superconducting, all three types of junctions show conductance increase,
pointing out the Andreev reflection as a prevalent contribution to the subgap
conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is
satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The
absolute value of the conductance is much smaller than that for the bulk
crystal, indicating that the transmission occurs through only a small part of
the contact area. An opposite situation occurs in In-NbP junction, where the
conductance at the peak reaches the bulk value indicating that almost whole
contact area is transmitting and, additionally, a superconducting proximity
phase is formed in the material. We interpret this as a result of indium
diffusion into NbP, where the metal atoms penetrate the surface barrier and
form very transparent superconductor-Weyl semimetal contact inside. However,
further diffusion occurring already at room temperature leads to degradation of
the effect, so it is observed only in the pristine structures. Despite of this,
our observation directly demonstrates possibility of inducing superconductivity
in a type-I Weyl semimetal.Comment: Accepted for Phys. Rev. B. 13 pages, 12 figures. Second version with
major revisions. The title was changed. One author R. Jakiela added. New
inset to Fig. 8(A). New fits in Fig. 8 (B) and Fig. 10 (B). Added figures 12
(C)-(E). Added Fig. 12 (F) with SIMS data. Rewritten chapters III-C-2 and
III-C-3. Reference no. 38 removed, 11 new references: 9, 21, 22, 40-44, 46-49
were adde
Biological properties of surface layers for ring of heart valve application
Oryginalna sztuczna komora wspomagania serca POLVAD opracowana w Polsce, została zastosowana dotychczas w leczeniu ponad 210 pacjentów. Najdłuższe wspomaganie serca za pomocą komory POLVAD trwało ponad rok. Dla protezy tej opracowywana jest innowacyjna zastawka dyskowa, z nisko profilowym pierścieniem wykonanym ze stopu tytanu. Dla zminimalizowania trombogenności pierścienia zastawki opracowano dyfuzyjne warstwy powierzchniowe: azotowaną typu TiN+Ti2N+αTi(N) i tlenoazotowaną typu TiO2+TiN+Ti2N+αTi(N), wytwarzane obróbką jarzeniową na potencjale plazmy. Trombogenność różnych kompozycji warstw została porównana w aspekcie aktywacji i adhezji płytek krwi do powierzchni biomateriału. Oceniono również wpływ metody sterylizacji biomateriału na intensywność adhezji trombocytów do jego powierzchni. Warstwy TiN oraz TiO2wykazały najniższą trombogenność, przy czym dla warstwy TiN korzystniejsza jest sterylizacja gazowa, podczas gdy dla warstwy TiO2- sterylizacja plazmowa.The original ventricular assist device POLVAD developed in Poland was used in over 210 patients so far. The longest POLVAD heart assistance excided one year. The innovative tilting disk valve with low profile ring made of titanium is developed for POLVAD. To minimize the valve ring thrombogenicity the diffusive surface layers were manufactured: nitriding TiN+Ti2N+αTi(N) and oxynitriding TiO2+TiN+Ti2N+αTi(N), in the glow discharge process on the plasma potential level. The thrombogenicity of different layers compositionwas compared regarding platelets activation and platelets adhesion to the material surface. The influence of material sterilization method on the platelets adhesion intensity was evaluated in addition. The nitriding TiN and oxynitriding TiO2layers have demonstrated the lowest thrombogenicity while the gas sterilization was the most profitable for nitriding layers – TiN and the plasma sterilization for oxynitriding layers – TiO2
The Fe-Mg interplay and the effect of deposition mode in (Ga,Fe)N doped with Mg
The effect of Mg codoping and its deposition mode on the Fe distribution in
(Ga,Fe)N layers grown by metalorganic vapor phase epitaxy is investigated. Both
homogeneously- and digitally-Mg codoped samples are considered and contrasted
to the case of (Ga,Fe)N layers obtained without any codoping by shallow
impurities. The structural analysis of the layers by high-resolution
transmission electron microscopy and by high-resolution- and synchrotron x-ray
diffraction gives evidence of the fact that in the case of homogenous-Mg
doping, Mg and Fe competitively occupy the Ga-substitutional cation sites,
reducing the efficiency of Fe incorporation. Accordingly, the character of the
magnetization is modified from ferromagnetic-like in the non-codoped films to
paramagnetic in the case of homogeneous Mg codoping. The findings are discussed
vis-`a-vis theoretical results obtained by ab initio computations, showing only
a weak effect of codoping on the pairing energy of two Fe cations in bulk GaN.
However, according to these computations, codoping reverses the sign of the
paring energy of Fe cations at the Ga-rich surface, substantiating the view
that the Fe aggregation occurs at the growth surface. In contrast to the
homogenous deposition mode, the digital one is found to remarkably promote the
aggregation of the magnetic ions. The Fe-rich nanocrystals formed in this way
are distributed non-uniformly, giving reason for the observed deviation from a
standard superparamagnetic behavior.Comment: 13 pages, 14 figure
Infrared Spectroscopy of Light Impurities in GaSb
In our work we study the doping behaviour of sulphur in Czochralski grown GaSb by means of the high resolution Fourier transform infrared spectroscopy and the secondary ion mass spectroscopy. We have revealed that the sulphur impurity forms an effective mass like donor state bound to the L-minimum of the conduction band. From the far infrared spectrum of this donor we derive the effective band masses of the L band minimum. We also observe local vibrational modes related to the arsenic and phosphorus isoelectronic impurities. From the nearest neighbour isotope splittings of these modes we conclude that the arsenic impurity occupies a tetrahedral substitutional site and the phosphorus impurity - a low symmetry lattice site
Cathodoluminescence Measurements at Liquid Helium Temperature of Poly- and Monocrystalline ZnO Films
Scanning electron microscopy, cathodoluminescence and secondary ion mass spectroscopy investigations are used to study an inter-link between structural quality, elements distribution and light emission properties of ZnO poly- and monocrystalline films grown by the atomic layer deposition. Cathodoluminescence and scanning electron microscopy investigations were performed at liquid helium temperature for four different types of ZnO films deposited on different substrates
Ti-Al-N MAX Phase a Candidate for Ohmic Contacts to n-GaN
Fabrication of a Ti₂AlN MAX phase for contact applications to GaN-based devices is reported. Sample characterisation was done by means of X-ray diffraction and secondary ion mass spectroscopy. Successful Ti₂AlN monocrystalline growth was observed on GaN and Al₂O₃ substrates by annealing sputter-deposited Ti, Al and TiN layers in Ar flow at 600°C. The phase was not seen to grow when the layers were deposited on Si (111) or when the first layer on the substrate was TiN. N-type GaN samples with Ti₂AlN layers showed ohmic behaviour with contact resistivities in the range 10¯⁴ Ωcm²
Ti-Al-N MAX Phase a Candidate for Ohmic Contacts to n-GaN
Fabrication of a Ti₂AlN MAX phase for contact applications to GaN-based devices is reported. Sample characterisation was done by means of X-ray diffraction and secondary ion mass spectroscopy. Successful Ti₂AlN monocrystalline growth was observed on GaN and Al₂O₃ substrates by annealing sputter-deposited Ti, Al and TiN layers in Ar flow at 600°C. The phase was not seen to grow when the layers were deposited on Si (111) or when the first layer on the substrate was TiN. N-type GaN samples with Ti₂AlN layers showed ohmic behaviour with contact resistivities in the range 10¯⁴ Ωcm²
Substrates Grown from the Vapor for ZnO Homoepitaxy
The novel method of preparation of epi-ready ZnO substrates is demonstrated. The substrates were made of unique ZnO crystals grown by chemical vapor transport method using hydrogen as the transport agent. The effect of low-level doping (Mn, Co, Cu, and V) on the structural quality of the crystals was investigated. Atomic layer deposition was used to verify usability of the substrates for homoepitaxy. The thermal annealing prior to the atomic layer deposition process and effect of thermal annealing of the epitaxial layers was studied. The X-ray diffraction and atomic force microscopy methods were applied to study the structural quality of the ZnO layers. Detection of the dopants in the substrates by secondary ion mass spectroscopy made possible the measurement of the thickness of the layers. The obtained root mean square roughness for both the substrates and layers ranged between 0.2 nm and 5 nm, and was dependent on the sample crystallographic orientation and sequence of polishing and annealing procedures. The optimal recipe for the epi-ready substrate preparation was formulated
Structural and Electrical Properties of SiC Grown by PVT Method in the Presence of the Cerium Vapor
The results of investigation of structural and electrical properties of bulk SiC crystals, which were grown by physical vapor transport method with different Ce impurity content added to the SiC source material, are presented. The gradual dosage of cerium from the SiC source and continuous presence of the cerium vapor over the SiC crystallization fronts during the crystal growth processes are confirmed. The cerium influences the overall concentration of structural defects. The increase of the concentration of both, donors and acceptors, and appearance of new shallow donors (15-32 meV) in 4H-SiC crystals are observed