•Single crystal pyrrhotite (Fe,Ni)1-zS was grown via Bridgman–Stockbarger method.•The soluble Rh behaves like an ordinary component at directional solidification.•The refractory Ru forms RuS2 ...crystals, which then behave like suspended particles.•X-ray diffraction analysis reveal pyrrhotite, cubanite and RuS2 structural features.
We have studied the composition and structure of the products of initial crystallization stage of melt with the following composition (in mol. %): Fe 31.90, Cu 16.03, Ni 1.70, S 50.31, and by 0.03 Ru and Rh. Directional solidification of the sample was carried out in the region of primary crystallization of pyrrhotite solid solution (Poss) in the Fe-Cu-Ni-S system. Results of optical microscopy and SEM-EDS showed that the distribution of the main components and Rh in the sample agrees with the theory of directed crystallization. Refractory ruthenium sulfide RuS2 (structure type FeS2, pyrite), formed in the sulfide melt, plays the part of mechanical dopant. On cooling of the crystallized sample, Poss partly decomposes to form nonstoichiometric isocubanite Cu1.1Fe2S3 (Icb*). On further cooling, Icb* decomposes into stoichiometric isocubanite CuFe2S3 (Icb) and Cu3Fe4S7 phase. These phase reactions do not influence the distribution of Rh and Ru in the sample volume. Crystallographic parameters, features of modulation and spatial orientation of pyrrhotite and cubanite phases of sulfide matrix were determined.
•High quality Bi1.1Sb0.9Te2S crystals were grown by the Bridgman technique.•Optical parameters were determined from far infrared reflectivity spectra.•ARPES spectra revealed a wide direct bandgap ...topological insulator phase.•The Dirac point is positioned well away from the valence and conduction bands.
Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm−1) and the frequencies of two infrared phonons (177.7 cm−1 and 77.4 cm−1).
A growth station for growing paratellurite single crystals is presented. Modification of Czochralski furnace consists of nonuniform heating of melt by remaining low temperature gradients near the ...front of crystallization. It is supposed that local overheating of the lower parts of the melt breaks particle associates in the melt. This assumption is confirmed by the growth of bubble-free TeO2 crystals weighting up to 1.8kg.
•Local overheating in Czochralski furnace was created.•Bubble-free TeO2 crystals were grown.•Improvement of quality is a result of decomposition of melt clusters.
Noncentrosymmetric borates K7MIn2-xYbx(B5O10)3 (M = Ca, Sr, Ba; x=0…2) were synthesized by the solid state reaction and the crystals were successfully grown by the top seeded solution growth method ...using the K2O-B2O3-MF2 flux. According to Rietveld refinement, the crystal structure belongs to the noncentrosymmetric R32 space group. Also, the octahedrally coordinated In atoms are located at wide ranges ~8Å which may be promising for phosphor and laser applications. Samples with ytterbium show a characteristic emission band in the range of 950-1050nm related to the 2F5/2 → 2F7/2 transition of Yb3+ ions that is commonly used for laser generation. IR, Raman and absorption spectra were obtained for the samples, as well. The short cut edge of UV absorption, SHG intensity comparable with KDP and low concentration quenching of luminescence suggest that the K7MIn2-xYbx(B5O10)3 borates are promising self-frequency doubling materials.
•K7MIn2(B5O10)3 and K7MYb2(B5O10)3 (M = Ca, Sr, Ba) crystal structures were refined by Rietveld method.•K7MIn2-xYbx(B5O10)3 (M = Ca, Sr, Ba; x=0…2) solid-solutions was investigated by solid state syntheses.•Noncentrosymmetric crystals of K7MIn2-xYbx(B5O10)3 (M = Ca, Sr, Ba; x=0…2) were grown by TSSG from LiBO2-LiF flux.•IR, Raman and absorption spectra were obtained.•The second harmonic generation for grown crystals was investigated, deff value relative to KDP was calculated
Recently discovered intrinsic antiferromagnetic topological insulator MnBi2 Te4 presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at ...elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the MnBi2 Te4 Dirac cone. Here, using micro( μ )-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi2 Te4 samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the Néel temperature at about 10–16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi2 Te4 samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Furthermore, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi2 Te4 DP gap size.
The recent realization of quantum anomalous Hall effect and Majorana fermions observation enhance interest in magnetism investigation in topological insulators. In this work, the electronic and ...magnetic structure of the Gd-doped topological insulator Bi
1.09
Gd
0.06
Sb
0.85
Te
3
were systematically studied by means of angle-resolved photoemission spectroscopy, resonance photoemission spectroscopy (ResPES) and SQUID magnetometry. Resonant features related to the Gd density of states near the Fermi level are experimentally observed. Study of magnetic structure showed antiferromagnetic ordered bulk at low temperatures as well as presence of hysteresis loop at elevated temperatures. Finally, possible mechanism of magnetism and its relation to observed electronic features are discussed.
•Amorphous SnSe2 layer was crystallized by heating to 250 °C to initiate vdW epitaxy.•SnSe2 film screw dislocations density is ∼18 and ∼2 μm−2 for Si and Bi2Se3 substrates.•SnSe2 film has a hexagonal ...lattice structure (space group P3¯m1 (no. 164))•Decrease in Se:Sn flux ratio switches SnSe2 epitaxy to nucleation of SnSe 3D islands.
We have studied the structural and morphological features of SnSe2 films grown on Si(111) and Bi2Se3(0001) surfaces in an in situ reflection electron microscope. On both substrates, the SnSe2 growth started at 100 °C as an amorphous layer, and when thickness reached 1 nm, crystallized by raising the growth temperature to 250 °C without interruption of Sn and Se fluxes. The introduction of this growth-initiating stage has decreased the concentration of screw dislocations on films’ surfaces to ∼18 and ∼2 μm−2 for the Si(111) and Bi2Se3(0001) substrates, respectively. High-resolution transmission electron microscopy investigation has shown that the layered SnSe2 film has a hexagonal lattice structure corresponding to the space group P3¯m1 (no. 164) with lattice parameters a = 0.38 nm and c = 0.62 nm. Raman spectroscopy has shown vibrational modes corresponding to the 1T-SnSe2 phase. We have shown that the decrease in Se:Sn flux ratio switches growth mode from Frank—van der Merwe type SnSe2 epitaxy to Volmer—Weber type nucleation of SnSe 3D islands.
Density-functional perturbation theory and Raman spectroscopy are used in the study of vibrations in bulk V sub(2) VI sub(3) binary and ternary topological insulators as well as in ultrathin films ...composed of one and two quintuple layers. Full dispersion curves along the Brillouin zone symmetry directions are calculated and analyzed and the zone-center Raman active modes are compared to the experimental results. The calculations of the two quintuple layer films reveal modes that correspond to displacements of entire quintuple layer blocks. These modes have low frequencies and are Raman active. The phonon dispersion curves of one and two quintuple layer Bi sub(2) Se sub(3) films are compared with recent helium atom scattering experiments Phys. Rev. Lett. 107, 186102 (2011) (http://dx.doi.org/10.1103/PhysRevLett.107.186102).
Shikhzarli is a typical active mud volcano (MV) in Azerbaijan with extremely high percentages of fire eruptions reaching 74%. The strong eruption of Shikhzarli MV on 13 March 2011 (Shamakhi–Gobustan ...region, central Azerbaijan, at 40°29′12.5′′N; 49°02′03.4′′E) was accompanied by gas ignition with a straight-flow vertical gas flare. A large (60 m height) short-living (30 min) gas flare and small (1.5 m height) gas spouts with longer lifetimes (5 h) were simulated in
SigmaFlow
, and the model was used to evaluate methane flux and the amount of combusted methane. Geological and mineralogical data, field observations, as well as eye witness reports and photographs in mass media were used to choose the boundary conditions for the gas fire simulation. This study provides the first quantitative evaluation for methane emission during an MV catastrophic eruption. As simulation predicts, 14,000 m
3
of methane was consumed for the life time (30 min) of the main 60-m high flare, which is equivalent to nine tons of cold gas at ambient conditions; the volume of erupted methane for the 5-h lifetime of a small spout (1.5 m high) was 72.6 m
3
(45 kg) of cold CH
4
. The amount of methane emitted during a single strong flame eruption is commensurate with the annual output of permanently active individual MV vents or may be ten to hundred times as high.
Novel KCaR(BO3)2 (R = La, Pr and Nd) compounds have been obtained by solid-state synthesis. Specifically, single crystals of KCaNd(BO3)2 and KCaPr(BO3)2 were grown by using a top seeded solution ...growth method from the KBO2 flux. These crystal compounds belong to the orthorhombic system, Pbca space group. KCaLa(BO3)2 exhibits high transparency in the range of 200–900 nm. Pr and Nd compounds have a strong emission bands that are related to the well-known electron transition of Pr3+ (3P0 → 3H6 at 630 nm, 3P0 → 3F2 at 670 nm) and Nd3+ (4F3/2 → 4I9/2 at 875 nm, 4F3/2 → 4I11/2 at 1054 nm).
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•KCaR(BO3)2 (where R–La, Pr, Nd) crystals were obtained.•KCaNd(BO3)2 structure was solved by single crystal X-ray analysis.•Luminescent and absorption spectra for KCaR(BO3)2 (where R–Pr, Nd) were measured.
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