It is shown that MnTe-Bi2Te3 system is quasi-binary and in fact hosts three intermediate phases. Along with already known MnBi2Te4 phase, another two, MnBi4Te7 and MnBi6Te10 have been found to exist. ...All the phases melt incongruently in a very narrow temperature range of 577–590 °C via peritectic reactions. Directional crystal growth results in hetero-phase ingots due to the narrow compositional range and narrow primary crystallization fields. The crystal structure of each phase is a derivation of the prototype tetradymit-type layered structure and the phases constitute a new homologous series with the chemical formula (MnTe)·n(Bi2Te3). X-ray diffraction patterns and Raman spectroscopy of the sorted-out single phase samples show that different phases have different number of the seven (7)- and five (5)-layer blocks and their different stacking manner in the unit cell. In particular, MnBi2Te4 exhibits the -7-7-7-, MnBi4Te7 -5-7-5-7-, and MnBi6Te10 -5-5-7-5-5-7- sequence of the blocks. Thus, these structures are the first derivatives of Bi2Te3 structure to contain a transition metal cation Mn2+.
•Phase diagram of the MnTe-Bi2Te3 quasi-binary system was plotted.•Novel ternary compounds, MnBi4Te7 and MnBi6Te10 were found in the system along the known MnBi2Te4.•Single crystals of all the three compounds were grown by the Bridgman-Stockbarger method.•Crystal structure of the sorted-out compounds were elucidated from powder data.•All the three ternary compounds were characterized by Raman spectroscopy.
The aim is to examine the spin‐polarized electronic band structures, the density of states (DOS) as well as magnetism of CoxZn1−xSe and NixZn1−xSe diluted magnetic semiconductors (DMSs) in the ...ferromagnetic (FM) phase, and with 12.5% and 6.25% concentrations of impurity. The calculations are implemented by the recent ab initio norm‐conserving FHI pseudopotential method within the local spin density approximation and Hubbard U (LSDA + U) method. The analysis of the total DOS (TDOS) curves shows the half‐metallic FM character for Ni‐doped and (Co, Ni)‐co‐doped ZnSe with a half‐metallic bandgap of 1 ÷ 1.6 eV. The exchange splittings produced by dopant d states are determined: for the CoxZn1−xSe, it is obtained that the effective potential for the majority spin is more attractive than for the minority spin. The total magnetic moments of Co‐doped, Ni‐doped, and co‐doped ZnSe DMSs are found to be equal to 3, 4, and 5.0 μB, respectively. The Curie temperature of NixZn1−xSe is higher than room temperature and this is half‐metallic ferromagnetic (HMFM) material for spintronics application. Co‐doped ZnSe is an excellent choice for electronic devices owing to its below Curie temperature.
To promote suitable semiconductor materials for spintronics devices, this study aims to evaluate the magnetic properties of the ZnSe:(Co,Ni). The main contribution to magnetization comes mostly from the d‐states of impurity atoms. The presence of Zn vacancy in ZnSe:Co systems affects the magnetization, in which ≈2.0 μB increases the total magnetic moment of the supercell (≈5.0 μB).
By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence ...multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.
•The (100) and (001) surfaces of TlGaSe2 are accessed by spectroscopic ellipsometry.•Polarization-and temperature-dependent dielectric function spectra are obtained.•Critical points for inter-band ...optical transitions are retrieved.•Optical transitions are assigned according to the calculated band structure.•Critical points associated with Tl-orbitals show remarkable temperature behaviour.
The dielectric function spectra of TlGaSe2 crystal with quasi-two-dimensional layered structure were studied over the photon range 1.5–5.0eV in the temperature range 80–400K. The (100) and (001) surfaces were accessed by spectroscopic ellipsometry and the dielectric function was retrieved after conventional treatment of the ellipsometric data. Inter-band optical transitions associated with the obtained dielectric function were determined by using standard critical point analysis. Assignment of the transitions was done within the framework of the calculated electronic band structure. An abrupt temperature-induced change in energy of the retrieved critical points for inter-band optical transitions between the electronic states formed with participation of thallium orbitals was obtained at 104.5, 123.3 and 247.0K. The last temperatures agree well with the temperature points of the subsequent structural phase transitions in TlGaSe2.
The phase equilibria of the Sb2Te3+2BiI3↔Bi2Te3+2SbI3 reciprocal system have been investigated over the entire concentration range using differential thermal analysis (DTA), powder X-ray diffraction ...(PXRD), and scanning electron microscopy (SEM) techniques. Liquidus surface projection, solid-phase equilibrium diagram at 300 K and a number of isopleths sections of phase diagram were constructed based on experimental results and literature data. The primary crystallization areas of all the existing phases, their homogeneity ranges, as well as, types and coordinates of non- and monovariant equilibria were revealed. It is determined that the BiTeI−SbTeI section of this reciprocal system is non-quasi-binary due to the incongruent melting of one of the initial ternary compound. However, it is stable at subsolidus and forms wide areas of solid solutions based on the initial compounds. Additionally, dielectric functions and Raman spectra of Bi1−xSbxTeI (x=0; 0.05 and 0.1) phases were studied. The nature of the dependence of the dielectric function on the photon energy was identified based on the obtained ellipsometric data, while Raman spectra show that no structural changes were observed with an increasing concentration of antimony in the solid solution.
•The phase equilibria of the Sb2Te3 + 2BiI3↔Bi2Te3 + 2SbI3 reciprocal system were investigated and new cation-substituted solid solutions were prepared.•The crystal lattice parameters, dielectric function, and Raman characteristics of Bi1-xSbxTeI solid solutions were determined in the compositional range of 0–0.1.•The obtained Bi1-xSbxTeI solid solution phases were found to be Rashba-type degenerate semiconductors with well -defined free carrier plasma edge and band gap energy less than 0.45 eV.
In this paper, the authors report first‐principles calculations of the Seebeck coefficient in TlInSe2. Experimental measurements demonstrate that TlInSe2 is characterized by giant values of the ...Seebeck coefficient at the temperature region of 320–430 K. To get insight on significant enhancement of Seebeck coefficient under temperature changing the density functional theory (DFT) and the Boltzmann transport equation are applied to calculate the semiclassical transport coefficients for TlInSe2 and its constructed models. The molecular dynamic simulations of 2 × 2 × 2 and 1 × 1 × 8 superlattices of TlInSe2, the TlInSe2_118a, TlInSe2_118b, TlInSe2_222a, TlInSe2_222b structures are performed to study their behavior in both “bulk” and “chain – like” modes. It is found that the distortions of InSe4 tetrahedra and displacements of Tl atom lead to formation of Tl–Se covalent bonds. The “bulk” TlInSe2_222b compound possesses the maximum value of Seebeck coefficient and (ZT)max in comparison with another studied compounds. The hybridization of Se and Tl orbitals near the top of valence band and hybridization of the Se, Tl, and In orbitals in conduction band may lead to increasing of Seebeck coefficient in TlInSe2 at 425 K.
Crystal structure models of TlInSe2: On the left hand side it can be seen that the structure corresponds to the tetragonal structure with space group D4h18; and on the right‐hand side is the structural model of incommensurate phase.
Feasibility of many emergent phenomena that intrinsic magnetic topological insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic ...structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi2Te4)(Bi2Te3)m with m = 0, ⋯, 6. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m = 0) to MnBi4Te7 (m = 1) and MnBi6Te10 (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, respectively, realized in the m = 0, 1, 2 and m = 3 cases. For large m numbers a hitherto-unknown topologically nontrivial phase can be created, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi2Te4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi2Te4)(Bi2Te3)m allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.
The dielectric function spectra of TlGaS2 crystal with quasi‐two‐dimensional layered structure have been studied by means of spectroscopic elipsometry over the photon range 1.5–6.0 eV in the ...temperature range 100–400 K. The absolute values of the dielectric function show a sudden temperature‐induced change around 180 and 280 K. The energy of the inter‐band optical transitions retrieved from the dielectric function spectra display an abrupt temperature change around the same temperatures. Structural phase transitions are behind the observed change of electronic band structure of TlGaS2.
MoS2 is a two-dimensional layered transition metal dichalcogenide with unique electronic and optical properties. The fabrication of ultrathin MoS2 is vitally important, since interlayer interactions ...in its ultrathin varieties will become thickness-dependent, providing thickness-governed tunability and diverse applications of those properties. Unlike with a number of studies that have reported detailed information on direct bandgap emission from MoS2 monolayers, reliable experimental evidence for thickness-induced evolution or transformation of the indirect bandgap remains scarce. Here, the sulfurization of MoO3 thin films with nominal thicknesses of 30 nm, 5 nm and 3 nm was performed. All sulfurized samples were examined at room temperature with spectroscopic ellipsometry and photoluminescence spectroscopy to obtain information about their dielectric function and edge emission spectra. This investigation unveiled an indirect-to-indirect crossover between the transitions, associated with two different Λ and K valleys of the MoS2 conduction band, by thinning its thickness down to a few layers.
Tl LII and In K X‐ray absorption fine structure (XAFS) measurements were performed on a TlInSe2 thermoelectric material in the temperature range of 25–300 K including the incommensurate–commensurate ...phase transition temperature of about 135 K. Most of the bond lengths obtained from the present XAFS measurements are in good agreement with existing X‐ray diffraction data at room temperature, while only the Tl‐Tl correlation shows inconsistent values indicating the commensurate properties of the Tl chains expected from the thermodynamic properties. The present XAFS data clearly support positional fluctuations of the Tl atoms found in three‐dimensional atomic images reconstructed from X‐ray fluorescence holography.
The present XAFS measurement solved a difficult task in investigating the local structural information of the positional fluctuations on incommensuratecommensurate phase transition in a Tl‐based thermoelectric material, and a reasonable structural model for the one‐dimensional Tl chains is proposed to clarify the (a) incommensurate and (b) commensurate phases as shown in the figure.
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