Growth, crystal structure and IR luminescence of KSrY1-xErx(BO3)2.

A series of novel KSrY1-xErx(BO3)2 (x = 0-1) phosphors that emit near-infrared radiation was synthesized using solid-state methods. Pure Y and Er crystals were grown using a KF flux via the top-seeded solution growth technique. In situ high-temperature single crystal X-ray diffraction, Raman spectroscopy and DFT calculations were used for characterization. Within the series, a polymorphic phase transition from space group P21/m to R3m was discovered between 550 and 600°C. The concentration dependence of the luminescence intensity was measured for the samples. A strong emission of Er3+ electron transition 4I13/2 → 4I15/2 was detected within the 1529-1549 nm range, with the maximum observed for the KSrY0.4Er0.6(BO3)2 composition.

Systematic rare Earth doping to adopt an R32 type huntite structure in NdSc3(BO3)4 crystals.

This study is focused on determining the type and quantity of REE impurities responsible for converting the structure of NdSc3(BO3)4 into an R32 polymorph. According to the single crystal X-ray diffraction of RxNdyScz(BO3)4 (R = Sm-Lu, x + y + z = 4) the samples probably contain several polymorphic modifications. However, the predominant structure has been defined as R32 for R = Eu, Er, Tm, and Yb and P3221 for R = Sm, Gd, Tb, Dy, and Ho. Another potential limitation to the future use of the crystals is a compositional zoning found in the crystals with significant substitution in the scandium position.

Evolution of Mn1-xGexBi2Te4 Electronic Structure under Variation of Ge Content.

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1-xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1-xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.

Electronic Structure of Pb Adsorbed Surfaces of Intrinsic Magnetic Topological Insulators.

Recently discovered intrinsic magnetic topological insulators (IMTIs) constitute a unique class of quantum materials that combine magnetism and nontrivial topology. One of the most promising applications of these materials is Majorana fermion creation; Majorana fermions are expected to arise when a superconductor is in contact with the surface of an IMTI. Here we study the adsorption of Pb ultrathin films on top of IMTIs of various stoichiometries. By means of XPS we figure out the formation of the Pb wetting layer coupled to the surface atoms for low coverages and overlayer growth on top upon further deposition. Investigation of the adsorbed surfaces by means of ARPES shows the Dirac cone survival, its shift in a binding energy, and the Pb electronic states appearance. The obtained results allow the Pb/IMTI interfaces to be constructed for the understanding of the proximity effect and provide an important step toward quantum device engineering based on IMTIs.

Synthesis and Growth of Rare Earth Borates NaSrR(BO3)2 (R = Ho-Lu, Y, Sc).

NaSrR(BO3)2 (R = Ho-Lu, Y, Sc) compounds were obtained for the first time. Their structures exhibit disordered positions of Sr2+ and Na+ atoms while RO6 polyhedra are connected through the BO3 groups. Large distances between R atoms and high transparency in the range of 250-900 nm make them promising for phosphor applications. A pathway to obtain single crystals was shown by growing NaSrY(BO3)2 and NaSrYb(BO3)2 by the top seeded solution growth method with Na2O-B2O3-NaF flux.

Study of an EuBO3-ScBO3 system and EuSc3(BO3)4, EuSc(BO3)2 orthoborates.

The EuBO3-ScBO3 system was investigated by solid state synthesis and DSC methods. In this system, a new EuSc(BO3)2 compound was found. It crystallizes in the R3̄ space group with unit cell parameters of a = 4.8939(1) Å and c = 16.2663(5) Å. Whereas another compound in the system EuSc3(BO3)4 possesses two modifications: a low-temperature α-C2/c (a = 7.687(1) Å, b = 9.810(2) Å, c = 12.021(2) Å, and ß = 105.379(4)°) and a high-temperature ß-R32 (a = 9.7473(1) Å and c = 7.9205(2) Å). The α-EuSc3(BO3)4 crystal was grown with LiBO2-LiF flux, and ß-EuSc3(BO3)4 was obtained by the solid state synthesis. All of the obtained crystals exhibited typical Eu3+ luminescence spectra with peaks at 589 nm, 596 nm, 615 nm, 657 nm and 689 nm, which corresponded to the 5D0 → 7FJ (J = 0, 1, 2 and 4) electron transitions. The strongest peak of luminescence was located at 615 nm and corresponded to the 5D0 → 7F0 transition.

Topologically Nontrivial Phase-Change Compound GeSb2Te4.

Chalcogenide phase-change materials show strikingly contrasting optical and electrical properties, which has led to their extensive implementation in various memory devices. By performing spin-, time-, and angle-resolved photoemission spectroscopy combined with the first-principles calculation, we report the experimental results that the crystalline phase of GeSb2Te4 is topologically nontrivial in the vicinity of the Dirac semimetal phase. The resulting linearly dispersive bulk Dirac-like bands that cross the Fermi level and are thus responsible for conductivity in the stable crystalline phase of GeSb2Te4 can be viewed as a 3D analogue of graphene. Our finding provides us with the possibility of realizing inertia-free Dirac currents in phase-change materials.

Shubnikov-de Haas oscillations in p and n-type topological insulator (Bi x Sb1-x )2Te3.

We show Shubnikov-de Haas (SdH) oscillations in topological insulator (Bi x Sb1-x )2Te3 flakes whose carrier types are p-type (x = 0.29, 0.34) and n-type (x = 0.42). The physical properties such as the Berry phase, carrier mobility, and scattering time significantly changed by tuning the Fermi-level position with the concentration x. The analyses of SdH oscillations by Landau-level fan diagram, Lifshitz-Kosevich theory, and Dingle-plot in the p-type samples with x = 0.29 and 0.34 showed the Berry phase of zero and a relatively low mobility (2000-6000 cm2 V-1 s-1). This is due to the dominant bulk component in transport. On the other hand, the mobility in the n-type sample with x = 0.42 reached a very large value ~17 000 cm2 V-1 s-1 and the Berry phase of near π, whereas the SdH oscillations were neither purely two- nor three-dimensional. These suggest that the transport channel has a surface-bulk coupling state which makes the carrier scattering lesser and enhances the mobility and has a character between two- and three-dimension.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons at ambient and high pressures.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons (PAHs) was found at 500-773 K and ambient and high (3.5 GPa) pressures. The most intensive oligomerization at 1 bar and 3.5 GPa occurs at 740-823 K. PAH carbonization at high pressure is the final stage of oligomerization and occurs as a result of sequential oligomerization and polymerization of the starting material, caused by overlapping of π-orbitals, a decrease of intermolecular distances, and finally the dehydrogenation and polycondensation of benzene rings. Being important for building blocks of life, PAHs and their oligomers can be formed in the interior of the terrestrial planets with radii less than 2270 km.

Growth and Optical Properties of LixNa1-xBa12(BO3)7F4 Fluoride Borates with "Antizeolite" Structure.

Studied LixNa1-xBa12(BO3)7F4 (P42bc) solid solution belongs to the new class of "antizeolite" borates with [Ba12(BO3)6]6+ cation pattern, which contains channels filled by anionic clusters. Optical-quality crystals were grown from the compositions with different sodium-lithium ratio. The results of Rietveld refinement based on powder data demonstrate linear increase of parameter a and unit cell volume with Na/(Na + Li) ratio in cation site. Parameter c is less sensitive to the changes in stoichiometry, which is consistent with channel topology of LixNa1-xBa12(BO3)7F4 structure. Distinctive feature of LixNa1-xBa12(BO3)7F4 crystals is their deep purple color, which is due to both hole-type and electron-type centers. Crystals are characterized by linear dichroism effect.

Dual nature of magnetic dopants and competing trends in topological insulators.

Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials.

Spin-texture inversion in the giant Rashba semiconductor BiTeI.

Semiconductors with strong spin-orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin-orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.

Plasma-Wave Terahertz Detection Mediated by Topological Insulators Surface States.

Topological insulators (TIs) represent a novel quantum state of matter, characterized by edge or surface-states, showing up on the topological character of the bulk wave functions. Allowing electrons to move along their surface, but not through their inside, they emerged as an intriguing material platform for the exploration of exotic physical phenomena, somehow resembling the graphene Dirac-cone physics, as well as for exciting applications in optoelectronics, spintronics, nanoscience, low-power electronics, and quantum computing. Investigation of topological surface states (TSS) is conventionally hindered by the fact that in most of experimental conditions the TSS properties are mixed up with those of bulk-states. Here, we activate, probe, and exploit the collective electronic excitation of TSS in the Dirac cone. By engineering Bi2Te(3-x)Sex stoichiometry, and by gating the surface of nanoscale field-effect-transistors, exploiting thin flakes of Bi2Te2.2Se0.8 or Bi2Se3, we provide the first demonstration of room-temperature terahertz (THz) detection mediated by overdamped plasma-wave oscillations on the "activated" TSS of a Bi2Te2.2Se0.8 flake. The reported detection performances allow a realistic exploitation of TSS for large-area, fast imaging, promising superb impacts on THz photonics.

Dispersion properties of sulfur doped gallium selenide crystals studied by THz TDS.

High optical quality nonlinear crystals of solid solution GaSe(1-x)S(x), x=0, 0.05, 0.11, 0.22, 0.29, 0.44, 1 were grown by modified Bridgman method with heat field rotation. Ordinary and extraordinary wave dispersion was studied in detail as a function of sulfur content by terahertz time-domain spectroscopy (THz TDS) in the 0.3-4 THz range using cleaved and processed (cut and polished) crystals. Suitable dispersion equations for different parts of the entire transparency range were derived, utilizing comparative analyses of the measured data, the available published data, and approximations in the form of Sellmeier equations. A criterion was proposed for selecting measurement results of adequate quality, based on the etalon patterns in the transmission spectrum.

Signatures of Dirac fermion-mediated magnetic order.

The spin-momentum locking of topological states offers an ideal platform to explore novel magnetoelectric effects. These intimately depend on the ability to manipulate the spin texture in a controlled way. Here we combine scanning tunnelling microscopy with single-atom deposition to map the evolution of topological states under the influence of different magnetic perturbations. We obtain signatures of Dirac fermion-mediated magnetic order for extremely dilute adatom concentrations. This striking observation is found to critically depend on the single adatoms' magnetic anisotropy and the position of the Fermi level. Our findings open new perspectives in spin engineering topological states at the atomic scale and pave the way to explore novel spin-related topological phenomena with promising potential for applications.