Experimental study of the electronic and lattice contributions to the VO2 transition

The contributions of electronic and crystallographic components to the semiconductor → metal transition in VO₂ have been estimated from resistivity, E.P.R., and calorimetric measurements of the electronic and thermodynamic properties of Ga_xV_?xO₂, where 0 < x < 0.0130. E.P.R. and resistivity measurements indicate a decrease in the metallic character of the high-temperature R phase with increasing x, and calorimetric measurements of the energetics of the transition show a decrease in the enthalpy and entropy of the transition with increasing levels of doping. This concomitant decrease in enthalpy and metallic character with increasing x implies a strong contribution of the electronic entropy to the transition. An extrapolation of the combined electronic and calorimetric data for Ga_xV_?xO₂ to pure VO₂ suggest that the electronic entropy comprises about 60% of the total entropy of transition in VO₂.     

Magnetic and electrical properties of intercalated phases in the (Cr, Cu)-HfSe2 system

It has been shown that intercalation of the HfSe₂ compound with chromium atoms results in an increase in the magnetic susceptibility and a decrease in the resistivity while retaining the semiconductor conductivity type. The Cr _x HfSe₂ compounds exhibit a paramagnetic behavior at temperatures above 2 K in the entire concentration range 0 ≤ x ≤ 0.25. It has been revealed that an increase in the electron density due to additional introduction of copper can cause the appearance of a cluster-spin-glass-type state in Cr _x Cu _y HfSe₂ compounds. The data obtained indicate a large role of the indirect exchange interaction via conduction electrons in the formation of the magnetic state in layered intercalated compounds based on transition metal dichalcogenides.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Effect of pressure on optical and electrical properties of tungsten diselenide single crystals

Optical absorption spectra of tungsten diselenide (WSe₂) single crystals subjected to different values of pressure (0, 2, 4 and 6 GPa) were obtained in the spectral range 700–1450 nm with the help of a UV–VIS–NIR spectrophotometer. The spectra were thoroughly analyzed in the absorption edge region for obtaining direct as well as indirect band gaps in this material. The high temperature resistivity and thermoelectric power on WSe₂ single crystals at various pressures were also studied. The results and their implications are discussed in the article.     

Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, T _PG and T _λ(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, T _PG. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature T _λ(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T ^*(x) ≈ T _λ(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La_2?x Sr_xCUO₄.     

Effect of Te doping on superconductivity and charge-density wave in dichalcogenides 2H-NbSe2-xTex （x = 0, 0.1, 0.2）

Single crystals of Te-doped dichalcogenides 2H-NbSe2-xTex （x ---- 0, 0.10, 0.20） were grown by vapour transport method. The effect of Te doping on the superconducting and charge-density wave （CDW） transitions has been investigated. The sharp decrease of residual resistance ratio, RRR = R（3OOK）/R（SK）, with increasing Te content was observed, indicating that the disorder in the conducting plane is induced by Te doping. Meanwhile the superconducting transition temperature, Tc, decreases monotonically with Te content. However, the CDW transition temperature, TCDW, shown by a small jump in the temperature dependence of the resistivity near 30 K, increases slightly. The results show that the suppression of superconductivity might be caused by the enhancement of CDW ordering. The disorder has little influence on the CDW ordering.     

Structure and superconductivity of Mg1−xPbxB2

A series of polycrystalline samples of Mg_1−xPb_xB₂ (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg_1−xPb_xB₂ compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (T_c) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect.     

Low-temperature ferromagnetism in a new diluted magnetic semiconductor Bi2−x FexTe3

Single crystals of a new diluted magnetic semiconductor Bi_2?x Fe_xTe₃ (0≤x≤0.08) were grown. For all crystals, the transition into a ferromagnetic state with easy axis parallel to the C ₃ axis was observed at temperature T _c increasing with iron concentration and reaching 12 K at x=0.08. An increase in the Seebeck coefficient, anomalous Hall effect, and resistivity jump at T=T _c were also observed. The frequency of the de Haas-van Alphen oscillations decreases with increasing x, indicating the donor properties of Fe.     

Influence of magnetic (Fe) and non-magnetic (Ga) ion doping at Mn-site on the transport and magnetic properties of La0.7Ca0.3MnO3

The modifications in electrical and magnetic properties of polycrystalline bulk La_0.7Ca_0.3Mn_1−xT_xO₃ (T=Fe, Ga) samples at relatively higher doping concentration (x=0.08-0.12) are investigated. All the synthesized, single phase samples were subjected to resistivity measurements in the temperature range 50-300 K. No insulator-metal transition (T_P) was observed for Fe doped samples with x=0.12. For all the other samples the transition temperature decreased with increase in doping concentration. The small polaron hoping energy was found to increase, rather slowly, with increase in doping concentration. The effect on magnetic properties is also prominently observed with respect to doping element and doping concentration. Interestingly, with the increase in doping concentration, the Curie temperature (T_C) and T_P separate out significantly indicating decoupling of electric and magnetic properties. Changes in these properties have been analyzed on the basis of magnetic disorder introduced in the system due to the magnetic and nonmagnetic nature of these ions rather than strong lattice effects which is insignificant due to similar ionic radii of Fe⁺³ and Ga⁺³ when compared to that of Mn⁺³.     

Effect of disorder on the transport properties of the high-T c superconductor Nd2− x CexCuO4+δ

The temperature dependences ρ_ab(T) of Nd_2?x Ce_xCuO_4+δ single crystals with 0≤x≤0.20 are studied and analyzed on the basis of the concepts in the theory of disordered 2D systems. The results are compared with the data obtained for other copper-oxide HTSC. It is found that a transition to the superconducting state in the optimal doping region 0.14≤x≤0.18 occurs only in crystals with a fairly small degree of disorder (k _Fl≥2, where l is the mean free path). This transition is compatible with the weak 2D-localization mode as long as the localization radius is longer than the characteristic size of a Cooper pair. The superconducting transition temperature in the optimal doping region increases monotonically with the parameter k _Fl characterizing the degree of disorder in the crystal. The degradation of superconducting properties upon a further increase in the doping level (x>0.18) is apparently associated with a transition from 2D to 3D conductivity in the single crystal.     

Study of structural, transport and magneto-resistive properties of La0.7Ca0.3−xCexMnO3 (0≤x≤0.2)

We have synthesized a series of La_0.7(Ca_0.3−xCe_x)MnO₃ (0≤x≤0.2) by standard solid-state reaction method. X-ray diffraction (XRD) measurement was carried out for structural studies and Rietveld refinement was done for structural analysis. The transport properties were studied using four probe technique. The temperature dependence of the resistivity was measured in the temperature range of 20 K to room temperature. It is found that all samples show a systematic variation in metal to insulator transition at transition temperature (T_P) and resistivity (ρ) with the relative concentration of hole and electron doping in the system. The samples showed varying amounts of colossal magnetoresistance depending upon temperature and applied magnetic field. The magnetoresistance values as high as 72% were observed in x=0 sample.     

VO2-WO3 nanocomposite thin films synthesized by pulsed laser deposition technique

Pure VO₂ and VO₂-WO₃ composite thin films were grown on quartz substrate by pulsed laser deposition (PLD) technique. The influence of varying WO₃ molar concentration in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of VO₂-WO₃ nanocomposite thin films has been systematically investigated. X-ray diffraction studies reveal the single crystalline monoclinic VO₂ phase (m-VO₂) up to 10% of WO₃ content whereas both m-VO₂ as well as h-WO₃ (hexagonal WO₃) phases were present at higher WO₃ content (0.2 ≤ x ≤ 0.4). Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO₃ content. Temperature dependence of resistivity (R-T) measurements indicates significant variation in metal-insulator transition temperature, width of the hysteresis, and shape of the hysteresis curve. Cyclic Voltammetry measurements were performed on VO₂-WO₃ thin films. A direct correlation between V/W ratio and structure-property relationship was established. The present investigations reveal that doping of WO₃ in VO₂ is effective to increase the optical transmittance and to reduce the semiconductor to metal phase transition temperature close to room temperature.     

Optical properties of layered superconductor LixZrNCl

We investigated band-insulator-to-superconductor transition in Li_xZrNCl driven by carrier doping by means of magnetization, resistivity, and optical reflectivity measurements. The magnetization and the resistivity measurements showed that the transition occurs at around x=0.05. The pristine β-ZrNCl exhibited reflectivity and optical conductivity spectra typical of an insulator, whereas in the spectrum of Li_0.37ZrNCl, Drude-like high reflectance band and associated plasma edge are apparently observed. This is the direct spectroscopic evidence of insulator-to-metal transition of Li_xZrNCl.     

Systematic Study of Vortex Pinning and a Liquid–Glass Phase Transition in BaFe<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> Single Crystals

The vortex pinning and liquid-glass transition have been studied in BaFe_2–xNi _x As₂ single crystals with different doping levels (x = 0.065, 0.093, 0.1, 0.14, 0.18). We found that Ni-doped Ba-122 has rather narrow vortex-liquid state region. Our results show that the temperature dependence of the resistivity as well as I?V characteristics of Ni-doped Ba-122 is consistent with 3D vortex-glass model. It was found that -pinning gives the main contribution to overall pinning in 122 Ni-doped system. The vortex phase diagrams for different doping levels were built based on the obtained data of temperature of the vortex-glass transition T_g and the upper critical magnetic field H_c2.     

Magnetic properties of Sn1-xCrxTe crystals

Magnetization measurements of magnetic semiconductor Sn_1-xCr_xTe (x <5 at%) crystals with the Curie temperature T_c = 150–300 K were made down to 2 K. The magnetic properties are sensitive to isothermal annealing under Zn vapor. The overall magnetizations of the Zn-annealed crystals have paramagnetic and ferro- or antiferromagnetic contributions.     

Magnetic and charge ordering properties of Bi0.6−xEuxCa0.4MnO3 (0.0≤x≤0.6)

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6−xEu_xCa_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) perovskite manganites. Magnetic measurements show that the charge-ordering temperature (T_CO) decreases with increasing x up to x=0.4 and then slightly increases with further increasing x up to x=0.6. Further, the antiferromagnetic (AFM) ordering temperature (T_N) decreases with increasing x. At T<T_N a transition to metamagnetic glass like state is also seen. Eu doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.2 and then an increase in resistivity up to x=0.5. We propose that the local lattice distortion induced by the size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron are responsible for the observed variation in physical properties.     

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Nanostructural,magnetic and electrical properties of Ag doped Mn-ferrite nanoparticles

Nano-crystalline MnFe_2−xAg_xO₄ (x = 0, 0.1, 0.2, 0.3 and 0.6) samples with average grain size of 4–7 nm were synthesized by a simple method based on decomposition of metal nitrates in presence of citric acid. The samples were characterized by different structural, magnetic and electrical measurements. Rietveld refinement of X-ray diffraction data confirmed cubic spinel structure of the samples. Results show that Ag doping decreases the crystallite size, magnetization and coercivity of nanoparticles. By increasing the Ag content in the samples the saturation magnetization shows interesting temperature dependent behavior. It was realized that magnetization of smaller particles show higher sensitivity to temperature variations than larger particles. DC electrical resistivity measurements in the temperature range of 300–650 K show that the resistivity first increases and then decreases by increasing the Ag content in the samples. Curie temperature (T_c) and polaron activation energy in ferromagnetic and paramagnetic regions were estimated by using resistivity curves.     

Magnetic and electrical properties of amorphous PdCoP alloys

Magnetic susceptibility and electrical resistivity measurements were performed (Pd_100?xCo_x)₈₀P₂₀ alloys where 15 < x < 50. The magnetic properties show that these alloys undergo a ferromagnetic transition between 272 and 399 K as the cobalt concentration increases from 15 to 50 atomic %. Below 20 atomic % Co the short-range exchange interactions which produce the ferromagnetism are unable to establish a long-range magnetic order and a peak in the magnetization shows up at the lowest temperature range under an applied field of 6.0 kOe. The electrical resistivity of these alloys has been measured from 4.2 K up to the vinicity of the melting point (900 K). The electrical resistivity data could be interpreted by the coexistence fo a Kondo-like minimum and ferromagnetism. The minimum becomes less important as the transition metal concentration increases. The coefficients of In T and T² become smaller and concentration dependent. The spin ordering in such alloys can be simulated as either the ordering due to an applied “external field” or as an increase in “internal fields”. These are due to an increase in transition metal concentration. The negative magnetoresistivity is a strong indication of the existence of localized moment.     

Transport properties and the anisotropy of Ba1 − x K x Fe2As2 single crystals in normal and superconducting states

The transport and superconducting properties of Ba_{1 ? x} K _x Fe₂As₂ single crystals with T _c ≈ 31 K were studied. Both in-plane and out-of-plane resistivity was measured by a modified Montgomery method. The in-plane resistivity is almost the same for all studied samples, unlike the out-of-plane resistivity, which differs considerably. We have found that the resistivity anisotropy γ = ρ _c /ρ _ab is almost independent of temperature and lies in the range 10–30 for the studied samples. This indicates the extrinsic nature of high out-of-plane resistivity, which may be due to the presence of flat defects along Fe-As layers in the samples. This statement is supported by comparatively small effective mass anisotropy, obtained from the upper critical field measurements, and from the observation of the so-called “Friedel transition,” which indicates the existence of some disorder in the samples in the c-direction.