Novel IV-Group Based Diluted Magnetic Semiconductor: CrxGe1?x

Cr_xGe_1–x (x 0.08) films were grown on GaAs (001) and Ge (001) substrates using molecular beam epitaxy (MBE) method, and their magnetic and transport properties have been studied in the temperature range between 1.8 and 300 K. All of the films exhibited weak paramagnetic behavior. Transport measurements showed that magnetoresistance ratio and the anomalous Hall resistance depend on the Cr concentration x and the kinds of substrate.     

Structural, electronic and optical properties of Sn1−xSbxO2

The goal of this paper is to undertake a detailed first principle calculation of the structural, electronic and optical properties of Sn_1−xSb_xO₂. The results show that the stability of Sn_1−xSb_xO₂ in the full range of Sb content points to the probability of a continuous solid solution, where the increasing Sb content leads to volume expansion with different variation trends in the lattice constants. The increase of Sb concentration in the semiconductor–metal–semimetal transition occurs in consonance with the corresponding changes in its structural, electronic and optical properties. Two competing mechanisms play essential roles in this transition, namely; the many body effect and the atom disorder. Our calculations concur with previous X-ray diffraction, sheet resistance, resistivity and optical parameters detections. The studies present a practical way of tailoring the physical behaviors of Sn_1−xSb_xO₂ through the alloying technique.     

Transport of high-temperature superconductors in the normal state

The impurity mechanism proposed earlier is used to interpret the transport properties of high-temperature superconductors in the normal state. Temperature and impurity concentration dependences of the resistance and the Seebeck and Hall coefficients are discussed. An explanation for the sharp drop in the Seebeck and Hall coefficients with impurity concentration and reducing them to zero in La_2–x Sr _x CuO₄ at the maximumx at which superconductivity exists is put foward.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

Abstract

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1?xTe_x) and Fe(Se_1?xS_x). In the case of Fe(Se_1?xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1?xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1?xCo_x)AsF. The T_c of Fe(Se_1?xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

The thermoelectric properties of Co4Sb12-xTex synthesized at different pressure

Skutterudite compounds Co₄Sb_12-xTe_x (0.1 ≤ ×≤0.8) was synthesized successfully by high temperature and high pressure (HTHP) method and characterized with X-ray diffractometry and thermoelectric properties measurements. The samples prepared by HTHP are nearly with the single phase CoSb₃. The electrical resistivity, Seebeck coefficient and thermal conductivity were all depending on synthetic pressure and the Te content of the Skutterudite compounds were performed at room temperature. As our expected, the Seebeck coefficient increased with an increase of the synthetic pressure and the thermal conductivity decreased with an increase of the synthetic pressure. These results indicated that HTHP technique may be helpful to prepare thermoelectric materials with enhanced thermoelectric properties.     

Crystal growth and structural analysis of zirconium sulphoselenide single crystals

A series of zirconium sulphoselenide (ZrS _x Se_3−x , where x = 0, 0·5, 1, 1·5, 2, 2·5, 3) single crystals have been grown by chemical vapour transport technique using iodine as a transporting agent. The optimum condition for the growth of these crystals is given. The stoichiometry of the grown crystals were confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and the structural characterization was accomplished by X-ray diffraction (XRD) studies. The crystals are found to possess monoclinic structure. The lattice parameters, volume, particle size and X-ray density have been carried out for these crystals. The effect of sulphur proportion on the lattice parameter, unit cell volume and X-ray density in the series of ZrS _x Se_3−x single crystals have been studied and found to decrease in all these parameters with rise in sulphur proportion. The grown crystals were examined under optical zoom microscope for their surface topography study. Hall effect measurements were carried out on grown crystals at room temperature. The negative value of Hall coefficient implies that these crystals are n-type in nature. The conductivity is found to decrease with increase of sulphur content in the ZrS _x Se_3−x series. The electrical resistivity parallel to c-axis as well as perpendicular to c-axis have been carried out in the temperature range 303–423 K. The results obtained are discussed in detail.     

Te concentration dependent photoemission and inverse-photoemission study of FeSe1−xTex

Abstract

We have characterized the electronic structure of FeSe_1?xTe_x for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (E_F): its splitting near E_F and filling of the pseudogap in FeSe. IPES shows two features, near E_F and approximately 6 eV above E_F; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near E_F for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.     

Structural and Electrical Transport Properties of Al1?xMxB2 (M = Li, Vacancy)

AlB₂ and MgB₂ are neighboring compounds and have similar crystal structures. However, their superconducting properties are very different because the holes which play a key role in the superconductivity of MgB₂ do not exist in AlB₂. In this paper we reported our attempts to dope holes into AlB₂ by substituting Li and vacancy for Al, and these holes were expected to induce superconductivity in the system. However, experimental results showed that Li was hard to substitute Al in AlB₂. The variation of resistivity with temperature and x for samples with the nominal composition of Al_1–xLi _x B₂ was analyzed by considering the phase variation of the samples with x. The temperature dependence of resistivity for Al_1–xB₂ was also studied and orders of magnitude change in resistivity was observed when x was up to 0.4 and 0.5, similar to that observed in Mg_1–xB₂ system by Sharma et al. (A. Sharma, N. Hur, Y. Horibe, C. H. Chen, B. G. Kim, S. Guha, Marta Z. Cieplak, and S. W. Cheong, Phys. Rev. Lett. 89, 167003 (2002)). It is proposed that phase separation may exist in Al_1–xB₂, leading to the dramatic change of resistivity because of percolation.     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Transport properties of MoSe x Te2−x (0 ≤x ≤ 2) single crystals

The layer type MoSe _x Te_2−x (0 ≤x ≤ 2) have been grown in single crystalline form by chemical vapour transport technique using bromine as the transporting agent. The electrical resistivity and Hall mobility perpendicular to thec-axis of the crystals were measured at room temperature. The variation of the Seeback coefficient with temperature was also investigated.     

Magnetic and Electrical Properties of Random and Digital Alloys of GaSb:Mn

We have investigated the effect of Sb/Ga flux ratio on the magnetic and electronic properties of Mn-incorporated GaSb random and digital alloys grown by low-temperature molecular beam epitaxy. The magnetic and magnetotransport properties of Ga_1–xMn_xSb random alloys are strongly dependent on Sb/Ga flux ratio. Clear square-like hysteresis loops were observed for Sb/Ga flux ratios between 4.60 and 5.25. The coercive field and negative magnetoresistance increase with decreasing Sb/Ga flux ratio, while the Curie temperature remains constant at approximately 23 K, with no systematic dependence on the hole density. In contrast, the Curie temperatures for the GaSb:Mn digital alloys with different Mn surface coverages depend significantly on the Sb/Ga flux ratio, and it is also directly correlated with the hole density.     

New magnetic implant material for interstitial hyperthermia

Magnetic properties and heat generation characteristics of a bio-compatible ceramic Mg_1+xFe_2–2xTi_xO₄ system have been investigated as an implant material for the magnetic induction hyperthermia. Curie temperature (T_c) of the ceramic decreases with increasing x; and become T_c, 350 K at x ç0:35 and, 315 K at x ç 0:38; which is suitable Curie temperature for implant material. The temperature of ceramic as a function of time under the high frequency alternating magnetic field is self-controlled at T_c.

The surface temperature of a powder injection sphere cancer model, which was a mixture of the agar phantom and the ceramic powder implant, and the temperature distribution around the sphere set in the pure agar phantom matrix have been measured. The result is in good agreement with calculation using a finite element method (FEM). It was found that the temperature distribution inside of the sphere and the minimum quantity of Mg_1+xFe_2–2xTi_xO₄ necessary for hyperthermia could be estimated by the FEM calculation.     

Vibronic states in La{2-x}BaxCuO4BaxCuO4

The temperature dependences of the resistivity R(T) and the thermoelectric power α(itT) under different hydrostatic pressuresP for the system La_{2-x}BavCuO₄, 0.11 ≤ x < -1.6, are reported and interpreted as evidence for strong electron coupling to oxygen vibrations along the CuO-Cu bond axes of the CuO₂ sheets. The sensitivity of the transport properties to the bending Φ of the (180‡-Φ) Cu-O-Cu bond angle and their insensitivity to long-range magnetic order indicate that the segregation into hole-rich and magnetic stripe domains is driven primarily by electron-lattice interactions and not by electron-electron interactions alone.     

Study of electrical properties of CIGS thin films prepared by multistage processes

In this work, the dispersion mechanisms affecting the electric transport in CuIn_1−xGa_xSe₂ (CIGS) thin films grown by a chemical reaction of the precursor species, which are evaporated sequentially in two and three-stage processes are analyzed. It was found, through conductivity and Hall coefficient measurements carried out as functions of temperature, that the electrical conductivity of the CuIn_1−xGa_xSe₂ films is affected by the transport of free carriers in extended states of the conduction band as well as for variable range hopping transport mechanisms, each one predominating in a different temperature range.     

First principles investigation of electronic, optical and transport properties of α- and β-phase of arsenic telluride

Crystalline arsenic telluride exists in two stable phases. The monoclinic α-phase transforms to rhombohedral β-phase under high pressure. The electronic, optical and transport properties of the two phases has been investigated using full potential linear augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of DFT with generalized gradient approximation (GGA). We present the energy bands, density of states and optical properties like the complex dielectric functions and absorption coefficients. From the dynamic dielectric constant, the structural anisotropy for the monoclinic α-phase is clearly observed, whereas the longitudinal and transverse components are almost identical for the β-phase. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the infrared and visible regions compared to the α-phase. The rhomohedral phase which has the Bi₂Te₃ type structure has the possibility of thermoelectric properties, therefore transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.     

Morphology and properties of Mg2Si and Mg2(SixSn1−x) reinforcements in magnesium alloys

In this paper, the effects of Sr, Sb, Sr+Sb and Sn on Mg₂Si reinforcement phases in an Mg–Al–Zn–Si alloy are studied, and the structures and characteristics of Mg₂(Si_xSn_1?x) phases are analysed with first-principle calculations. The results show that the coarse eutectic Mg₂Si can be refined by modifying processes with Sr, Sb, and their combination. When alloying with Sn, a new reinforcement phase Mg₂(Si_xSn_1?x) forms by a substitution reaction, instead of Mg₂Si. Calculations indicate that Mg₂(Si_xSn_1?x) has a certain percentage of covalent bonds, which ensure it has sufficient hardness to act as a reinforcement phase. Calculated results for physical parameters, such as the bulk modulus and shear modulus, indicate that an Mg₂(Si_xSn_1?x) intermetallic exhibits greater ductility than Mg₂Si.

Highlights

• The coarse eutectic Mg₂Si can be refined by modifying processes.

• A new phase Mg₂(Si_xSn_1?x) forms by substitution reaction during solidification.

• Mg₂(Si_xSn_1?x) has certain covalent bound percentage.

• Mg₂(Si_xSn_1?x) has better plasticity than that of Mg₂Si.

     

Crystal growth and reflectivity studies of Zn1-xMnxTe crystals

Single crystals of Zn_1-xMn_xTe were prepared by vertical Bridgman crystal growth method for different concentrations of Mn. Chemical analysis and reflectivity studies were carried out for compositional and band structure properties. Microscopic variation in composition between starting and end compounds was observed from EDAX analysis. Linear dependence of fundamental absorption edgeE ₀as a function of Mn concentration (x) was expressed in terms of a straight line fit and a shift in E₀ towards higher energy was observed in reflectivity spectra of Zn_1-xMn_xTe.