Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

Preparation and properties of spray-deposited ZnIn2Se4 nanocrystalline thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been deposited onto amorphous and fluorine doped tin oxide (FTO)-coated glass substrates using a spray pyrolysis technique. Aqueous solution containing precursors of Zn, In, and Se has been used to obtain good quality deposits at different substrate temperatures. The preparative parameters such as substrate temperature and concentration of precursors solution have been optimized by photoelectrochemical technique and are found to be 325 °C and 0.025 M, respectively. The X-ray diffraction patterns show that the films are nanocrystalline with rhombohedral crystal structure having lattice parameter a=4.05 Å. The scanning electron microscopy (SEM) studies reveal the compact morphology with large number of single crystals on the surface. From optical absorption data the indirect band gap energy of ZnIn₂Se₄ thin film is found to be 1.41 eV.     

The ESR study of single crystal spinel ZnCr2Se4 diluted with Sb and V

The single crystal of Sb³⁺ and V³⁺ doped zinc chromium selenide spinel ZnCr₂Se₄ were prepared by a chemical transport method and characterized by ESR spectroscopy in order to examine the effect of nonmagnetic antimony and magnetic vanadium on properties of the system. For antimony admixtures the Neel temperature is very similar to that of the parent spinel ZnCr₂Se₄ (22 K). However, upon incorporating vanadium ions, the T_N temperature decreases down to 17.5 K, determined for the maximum vanadium content (x=0.06). The temperature dependence of the ESR linewidth over paramagnetic region is interpreted by an occurrence of spin-phonon interaction. The strong broadening linewidth together with its strong temperature dependence for vanadium doped ZnCr₂Se₄ is explained by the complex paramagnetic relaxation model.     

Laser synthesis of thin layers of In4Se3, In4Te3 and modification of their structure and characteristics

Trends of structural modifications and phase composition occurring in In₄Se₃ thin films and In₄Se₃-In₄Te₃ epitaxial heterojunctions under laser irradiations have been investigated. Dynamics of the layer structure modification, depending on laser modes, i.e. pulse duration τ = 2-4 ms, irradiation intensity I₀ = 10-50 kW/cm², number of pulses N = 5-50, was studied by electron microscopy. An increase in laser influence promotes enlargement of the layer grains and transformation of their polycrystalline structure towards higher degree of stoichiometry. As a result of laser solid restructuring heterojunctions of In₄Se₃-In₄Te₃, being photosensitive within 1.0-2.0 μm and showing fast time of response, have been obtained. Laser modification of structure enables one to optimize electrical and optical properties of functional elements on the base of thin films and layers of In₄Se₃, In₄Te₃, widely used as infrared detectors and filters.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

Mn1−xFexIn2Se4: a new layered semiconductor system

Mn_1−xFe_xIn₂Se₄ compounds (x=0.1; x=0.7) were grown by the chemical vapor transport method. X-ray diffraction analysis data show that these compositions crystallize as different polytypes that belong to the hexagonal structure. The crystal symmetry of the sample with x=0.1 belongs to the space group Rm and for the sample with x=0.7 the space group is P6₃mc.The magnetic behavior of both samples has been investigated in the temperature range between 5 and 300 K. Spin-glass-like behavior below the freezing temperature T_f=9 K has been found for the sample with x=0.7. The sample with Fe content x=0.1 behaves as a paramagnet down to the lowest experimental measured temperature. High-temperature susceptibility data follow the Curie-Weiss law with a negative paramagnetic temperature indicating predominant antiferromagnetic interactions.Optical studies reveal that both samples (x=0.1; 0.7) are direct band gap semiconductors. The temperature dependence of the energy gap fits Varshni relation quite well.     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

Photoelectrical memory effect in ZnIn2Se4

The compound ZnIn₂Se₄ with n-type conductivity is shown to exhibit electro-optical memory effect and negative resistance effect similar to those previously reported for ZnIn₂S₄. At low temperature ZnIn₂Se₄ material will present a high conductivity during and after illumination by light of energy greater than the band gap. This high conductivity state can be quenched in three different ways (i) by heating the sample until it reaches room temperature, (ii) by illumination with monochromatic light of appropriate energy, (iii) by an electric field. In the latter case the material exhibits a negative resistance effect in the transition between the low and high conductivity conditions. These charge storage phenomena have been explained by assuming the presence of a level, localized in the forbidden gap, which is twofold negative charged and presents a repulsive barrier to the recombination of electrons.     

Antiferromagnetism and spin-glass transition in the FeInxCr2−xSe4 series of selenides

The magnetic behavior of the FeIn_xCr_2−xSe₄ system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe²⁺ oxidation state, with a minor (∼9%) Fe³⁺ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is T_N=208(2) K for FeCr₂Se₄ and decreases to 174(3) K for FeIn_0.4Cr_1.6Se₄. The effective magnetic moment μ_eff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe²⁺ (⁵D) and Cr³⁺ (⁴F) electronic states. A second, low-temperature transition is observed at T_G∼13 K, which has been assigned to the onset of a glassy state.     

Third-order nonlinear optical properties of GeSe2-Ga2Se3-PbI2 glasses

The third-order nonlinear optical (NLO) properties of new selenium-based GeSe₂-Ga₂Se₃-PbI₂ glasses have been measured using the optical Kerr effect (OKE) technique, with picosecond and femtosecond laser pulses. The 0.70GeSe₂-0.15Ga₂Se₃-0.15PbI₂ glass has the largest third-order optical nonlinear susceptibility in GeSe₂-Ga₂Se₃-PbI₂ glass system with χ⁽³⁾ of 5.28×10¹² esu. In addition, the response time of glasses is sub-picosecond, which is predominantly associated with electron cloud. Local structure of the glasses has been identified by using Raman studies, while the origins of the observed nonlinear optical response are discussed. The [Ge(Ga)Se₄] tetrahedral and lone-pair electrons from highly polarizable Pb atom in glasses play an important role in enhanced NLO response. These results as well as their good chemical stability indicate that GeSe₂-Ga₂Se₃-PbI₂ glasses are promising materials for photonic applications of third-order nonlinear optical signal processing.     

Strong polarization dependence and selenium lone-pairs in the photoemission spectra of Sb2Se3

The photoemission energy distribution curves (EDC's) of crystalline and amorphous Sb₂Se₃ were measured in the photon energy range hv=7 to 20 eV using polarized radiation from a synchroton storage ring. The EDC's show that the six electrons per Sb₂Se₃ molecule, attributed primarily to the selenium p-pairs, are clearly separated from the remaining part of the valence band of crystalline Sb₂Se₃. The optical transitions from these states occur with matrix elements strongly dependent on the orientation of the electrical vector of the polarized radiation as a result of crystal field effects. Model densities of states are constructed for both crystalline and amorphous Sb₂Se₃.     

Structural studies and physical properties of novel Sm-doped Sb2Se3 nanorods

Sm³⁺ doped Sb₂Se₃ nanorods were synthesized by the co-reduction method at 180 °C and pH=12 for 48 h. Powder XRD patterns indicate that the Sm_xSb_2−xSe₃ crystals (x=0.00-0.05) are isostructural with Sb₂Se₃. The cell parameters increase for Sm³⁺ upon increasing the dopant content (x). SEM images show that doping of Sm³⁺ ions in the lattice of Sb₂Se₃ results in nanorods. High-resolution transmission electron microscopic (HRTEM) studies reveal that the Sm_0.05Sb_1.95Se₃ is oriented in the [1 0 −1] growth direction. UV-vis absorption reveals mainly electronic transitions of the Sm³⁺ ions in doped nanomaterials. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂Se₃, show other emission bands originating from f-f transitions of the Sm³⁺ ions. The electrical conductance of Sm-doped Sb₂Se₃ is higher than undoped Sb₂Se₃ and increase with temperature.     

High pressure optical study of KTbP2Se6

We have observed a pressure induced phase transition in KTbP₂Se₆ at about 9.2 GPa which was accompanied by a discontinuous jump of the absorption edge of about 0.5 eV into the red. We have proposed that the high pressure phase is due to charge transfer from the selenium to the phosphorous accompanied by the formation of a Se-Se bond. In order to exclude a model which involves a 4f-5d charge transfer in Tb, leading to the intervalence charge transfer from Tb³⁺P⁴⁺-Tb⁴⁺P³⁺,we have measured the pressure dependence of the luminescence of the crystal field split levels of the Tb-ion. A comparison of the pressure dependence of the luminescence spectra of KTb(MoO₄)₂ and KTbP₂Se₆ shows that such a charge transfer does not occur in KTbP₂Se₆.     

Suppression of the spin-glass state by indirect exchange via charge carriers in spinellides xCuCr2Se4-(1−x)Cu0.5Me0.5Cr2Se4 (Me = Ga,In)

Spin glass (SG) is observed in semiconducting solid solutions xCuCr₂Se₄-(1?x)Cu_0.5Me_0.5Cr₂Se₄ (Me = In, Ga) for 0?x?0.1. For x0.1 the material exhibits p-type metal conductivity. For x?0.6 the magnetic properties are purely ferromagnetic (FM), while for 0.1 <x?0.2 an unusual mixed two-phase SG+FM state is found. Indirect exchange via charge carriers is assumed to be responsible for SG suppression.     

Pressure-induced phase transition in defect Chalcopyrites HgAl2Se4 and CdAl2S4

Results of angle dispersive X-ray diffraction (ADXRD) measurements on the defect chalcopyrites (DCP), HgAl₂Se₄ and CdAl₂S₄ up to 22.2 and 34 GPa, respectively, are reported. The ambient tetragonal phase is retained in HgAl₂Se₄ and CdAl₂S₄ up to 13 and 9 GPa respectively. The values of the bulk modulus estimated from the Equation of State is 66(1.5) and 44.6(1) GPa for HgAl₂Se₄ and CdAl₂S₄ in the chalcopyrite phase. At higher pressure a disordered rock-salt structure and on pressure release a disordered zinc blende structure with broad X-ray diffraction lines are observed as is the case for several defect chalcopyrites.     

Changes of the magnetic interactions in TlCo2Se2 upon metal substitution

Various solid solutions TlCo_2−xMe_xSe₂ (Me=Fe, Ni and Cu) have been investigated by neutron powder diffraction, supplemented by magnetometry. The incommensurate spin-helix running along the c-axis in tetragonal TlCo₂Se₂ prevails for low concentrations of copper and iron but changes pitch. In the copper case, only cobalt carries a magnetic moment. On nickel substitution, however, collinear antiferromagnetic coupling between the ferromagnetic layers occurs. The magnetic moment distribution between the two transition metals in the solid solution TlCo_2−xNi_xSe₂ was tentatively probed with first principle calculations on fictive ordered TlCoNiSe₂, modelled by two types of superstructures. Also the ternary mother compounds, Pauli paramagnetic TlNi₂Se₂ and antiferromagnetic TlCo₂Se₂, were investigated with the same LMTO method.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

Crystal structure of the quaternary compound CuTa2InTe4 from X-ray powder diffraction

The crystal structure of the new quaternary compound CuTa₂InTe₄ was studied using X-ray powder diffraction data. The powder pattern refined by the Rietveld method indicates that this material crystallizes in the tetragonal system with space group I-4¯2m (No. 121), Z=2, and unit cell parameters a=6.1963(2) Å, c=12.4164(4) Å, c/a=2.00 and V=476.72(3) Å³. The structural and instrumental refinement of 28 parameters led to R_p=10.4%, R_wp=11.1%, R_exp=6.8% and χ²=2.7 for 96 independent reflections.