Structural, transport, and magnetic properties in the Ti-doped manganites LaMn1−xTixO3 (0≤x≤0.2)

The magnetism and transport properties of the samples LaMn_1−xTi_xO₃ (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn_1−xTi_xO₃ (0.05≤x≤0.2) is believed to be consistent with the Mn²⁺-O-Mn³⁺ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.     

Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

Magnetic phase transitions of antiperovskite Mn3−xFexSnC (0.5≤x≤1.3)

The magnetization and electrical resistivity of Mn_3−xFe_xSnC (0.5≤x≤1.3) were measured to investigate the behavior of the complicated magnetic phase transitions and electronic transport properties from 5 to 300 K. The results obtained demonstrate that Fe doping at the Mn sites of Mn₃SnC induces a more complicated magnetic phase transition than that in its parent phase Mn₃SnC from a paramagnetic (PM) state to a ferrimagnetic (FI) state consisting of antiferromagnetic (AFM) and ferromagnetic (FM) components, while, with the change of Fe-doped content and magnetic field, there is a competition between the AFM component and FM component in the FI state. Both the Curie temperature (T_C) and the saturated magnetization M_s increase with increasing x. The FM component region becomes broader with further increasing Fe-doped content x. The external magnetic field easily creates a saturated FM state (and increased T_C) when . Fe doping quenches the negative thermal expansion (NTE) behavior from 200 to 250 K reported in Mn₃SnC.     

Magnetocaloric properties of DyCo2−xGax alloys

The influence of the substitution of Ga atoms for Co atoms in DyCo₂ compounds on magnetocaloric properties has been investigated. A series of DyCo_2−xGa_x alloys with x=0, 0.03, 0.06, 0.1, 0.15, and 0.2 was prepared by the arc-melting method for this investigation. Experimental results revealed that the Ga substitution for Co in DyCo₂ can form a single phase with the cubic Laves phase structure up to x=0.2. As the Ga content x increases, the lattice parameter and the Curie temperature T_c increases from 143 to 196 K linearly. The maximum magnetic entropy changes in a low field change of 0-1.5 T, increasing from 8.24 to 10.61 J/K kg when the Ga content x increases from 0 to 0.03, but decreasing gradually to 3.51 J/K kg as the Ga content further increases to x=0.2. All the samples show a relatively large magnetic entropy change with very small hysteresis loss.     

Quadratic temperature dependence of electron-phonon scattering in disordered V1−xPdx alloys

We report the results of a comprehensive study of weak localization and electron-electron interaction effects in disordered V_1−xPd_x alloys whose compositions are close to the (low T_c) A15 V₃Pd compound. Magnetoresistivity and zero field resistivity have been measured within the temperature range 1.5≤T≤300 K. The low-temperature resistivity obeys a law, which is explained by electron-electron interaction. We have determined the electron-phonon scattering time (τ_e-ph) for V_1−xPd_x alloys. Our results indicate an anomalous electron-phonon scattering rate obeying quadratic temperature dependence. This observation is interpreted by the existing theories of electron-phonon interactions.     

Low temperature magnetization and magnetoresistance studies on the layered manganite system La1.2Sr1.8Mn2−xRuxO7 (x=0, 0.1, 0.5, 1.0)

We report the detailed results of magnetization and magnetoresistance measurements in the Ru doped layered manganite system La_1.2Sr_1.8Mn_2−xRu_xO₇ (x=0, 0.1, 0.5, 1.0). High-resolution measurements of magnetization and magnetoresistance were carried out as functions of temperature, magnetic field and time. We find evidence for the existence of competing ferromagnetic and antiferromagnetic interactions resulting in the formation of a frustrated spin-glass-like state at low temperatures. The time dependent magnetization follows the relation very well. We find that Ru doping enhances the coercive field and drives the system towards a magnetically mixed phase at low temperatures. Large negative magnetoresistance values are observed in all samples and at low temperatures the magnetoresistance varies as the square root of the applied magnetic field.     

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe_1−xTe_x, at doping levels of x=0.5 and 0.7, with critical temperatures . Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within . These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above T_c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T_c, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe_1−xTe_x, and in terms of its characteristic band structure.     

Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

We have measured the specific heat of crystals of (Ca_1−xSr_x)₃Ru₂O₇ using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel () and structural () phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L∼0.3R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T_N, but its magnitude () is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with the full ordering of localized spins. The T_N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T_c transition decreases rapidly with Sr substitution, and is not observable for our x=0.58 sample.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

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.     

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.     

Effect of cation size and disorder on the power loss of La0.7(Ba1−xSrx)0.3MnO3

The power loss P_L of La_0.7(Ba_1-xSr_x)_0.3MnO₃ (0≤x≤1) as a function of temperature, frequency (f) and amplitude of flux density (B_p), has been studied to analyze the effect of A-site cation mean radius <r_A> and size-disorders σ². It is found that as f is lower than 1 MHz, P_L is proportional to in the low induction region, and in the high induction region, indicating that P_L is dominated by hysteresis losses. For f approaches 1 MHz, P_L of the majority of samples obeys a law, which may be attributed to eddy current loss. The temperature dependence of P_L displays a peak around the transition from ferromagnetic-metal to paramagnetic-insulator, resembling the feature of the Rayleigh constant η. This case further demonstrates that in the low frequency region, P_L predominantly originates from hysteresis loss. A decrease in σ² with increasing x, will relieve local strain, raising η, thus give rise to an increase in P_L.     

Differences between surface and bulk refractive indices of a-InxSe1−x

Thin films of amorphous indium selenide compounds (a-In_xSe_1−x) are important, e.g. for photovoltaics. The feature of merit in such applications is also the real part of refractive index n of this material. The data on n in literature are divergent. In this paper, the results of investigations on n in the bulk as well as in the interface layers of thin films of a-In_xSe_1−x are presented. The measurements had been performed using optical transmittance and reflectance in spectral range from 1.24 to 1.96 eV of linear polarized radiation that hit the samples with angles of incidence from 0° to 80°. Investigations had been done for sample temperatures from 80 to 340 K. It was found that the refractive index for areas at the free surface n_f is bigger than the refractive index n_b at the interface of thin film-substrate. The averaged over thin film thickness value of real part refractive index have the biggest value in all spectral range. Values of these coefficients increase with increasing the temperature.     

Magnetic and magnetocaloric properties of Y bMn6Sn6−xInx

We have synthesized three Y bMn₆Sn_6−xIn_x representatives (x=0.45, 0.80, 1.20), the first pseudo-ternary RMn₆Sn_6−xX′_x compounds involving a divalent R metal. The crystal structure is found to evolve with the In concentration without modification of the Yb valency: Y bMn₆Sn_5.55In_0.45 is isotypic with HoFe₆Sn₆ (Immm) while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 crystallize in the TbFe₆Sn₆-type (Cmcm). The In content is also determining as regards the magnetic and magnetocaloric properties: Y bMn₆Sn_5.55In_0.45 () almost behaves like a simple ferromagnet while Y bMn₆Sn_5.20In_0.80 and Y bMn₆Sn_4.80In_1.20 also order ferromagnetically but at significantly lower temperatures ( and 129 K, respectively) and are further characterized by the interference of low temperature antiferromagnetic interactions. The results are discussed and compared to previously published data.     

Electrical switching and thermal studies on Ge22Te78−xIx chalcohalide glasses: The effect of iodine on network-topology

Investigations on the electrical switching behavior and thermal studies using Alternating Differential Scanning Calorimetry have been undertaken on bulk, melt-quenched Ge₂₂Te_78−xI_x (3≤x≤10) chalcohalide glasses. All the glasses studied have been found to exhibit memory-type electrical switching. The threshold voltages of Ge₂₂Te_78−xI_x glasses have been found to increase with the addition of iodine and the composition dependence of threshold voltages of Ge₂₂Te_78−xI_x glasses exhibits a cusp at 5 at.% of iodine. Also, the variation with composition of the glass transition temperature (T_g) of Ge₂₂Te_78−xI_x glasses, exhibits a broad hump around this composition. Based on the present results, the composition x=5 has been identified as the inverse rigidity percolation threshold at which Ge₂₂Te_78−xI_x glassy system exhibits a change from a stressed rigid amorphous solid to a flexible polymeric glass. Further, a sharp minimum is seen in the composition dependence of non-reversing enthalpy () of Ge₂₂Te_78−xI_x glasses at x=5, which is suggestive of a thermally reversing window at this composition.