Anomalous Hall effect in sputter-deposited CoxTi1−xO2–δ films

Hall effect measurements were performed on epitaxial Co_xTi_1−xO_2–δ thin films grown on (0 0 1) LaAlO₃ by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M–H loop at room temperature for Co_xTi_1−xO_2–δ thin films for which x?0.02. An anomalous Hall effect was observed for Co_0.10Ti_0.90O_2−δ films grown with the partial pressure of water P(H₂O)=4×10⁻⁴ Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti_nO_2n−1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO₂ thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti_nO_2n−1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.     

Heterogeneous magnetic and electronic states in Ca1−xLaxMnO3−δ (x⩽0.12) single crystals with electron doping

The magnetic, transport, and optical properties of electron-doped Ca_1−xLa_xMnO_3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0$x=0$ and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10$x=0.10$ and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10$x=0.10$ and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.     

La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La_2−xSr_xCu_1−yZn_yO_4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.     

Superconductivity and critical fields in undoped and Sn-doped MoSr2YCu2O8−δ

Samples with nominal compositions MoSn_xSr₂YCu₂O_8−δ (0 ? x ? 0.075) were synthesized and their superconducting and magnetotransport properties were investigated. It was established that the optimum Sn-doping (x = 0.02 and 0.03) increases the T_c of the undoped MoSr₂YCu₂O_8−δ. The upper critical fields of the samples were determined using magnetization, susceptibility and resistivity measurements. It was established that the Sn-doping weakly affects the first and the intragrain second critical field of Mo-1212 but enhances the extrapolated to T = 0 (i.e. the intergrain) upper critical field. The observed phenomena were discussed on the basis of the Josephson-junction-arrays model. A phenomenological expression describing the temperature dependency of the susceptibility of Mo-1212 was found.     

Low-temperature synthesis of nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) powders

Nanocrystalline Ca_1−xHo_xMnO_3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO₃. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio and to the particle grain size.     

Y1−xCaxBa2Cu3O7−δ thin films: From phase fluctuations to a complex order parameter

This report presents a study of the temperature dependence of the penetration depth in Y_1−xCa_xBa₂Cu₃O_7−δ thin films, which shows that a contribution to the order parameter from phase fluctuations in optimally doped samples cannot be excluded, and demonstrates the appearance of an imaginary component in the order parameter in overdoped samples. Measurements were performed using two complementary techniques: the parallel plate resonator (PPR), operated at 10 GHz, and far infrared (FIR) transmission, where the absolute value of the penetration depth can be obtained.     

Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

High-temperature electronic transport properties of La1−xCaxMnO3+δ (0.0?x?1.0)

La_1−xCa_xMnO_3+δ (0.0?x?1.0) samples were prepared and their resistivity and Seebeck coefficients were measured in the high-temperature range. Ca doping changes the ratio of Mn³⁺/Mn⁴⁺ and influences the electronic transport behavior markedly. With the increase of Ca concentration, the samples change from a p-type semiconductor to an n-type one and Seebeck coefficient becomes increasingly negative. Low doping (x=0.2) and high doping (x=0.8) induces the drop of the resistivity compared with undoped LaMnO_3+δ and CaMnO_3+δ samples due to the rise of carrier concentration. However, the resistivity of moderate-doped samples (x=0.4, 0.6) is larger than low- and high-doped samples because dopant scattering decreases carrier mobility.     

High-Jc GdBa2Cu3O7−δ films fabricated by the metal–organic deposition process

We report a successful fabrication of high-J_c GdBa₂Cu₃O_7−δ (GdBCO) films by the metal–organic deposition process on the LaAlO₃ (LAO) (0 0 1) substrates. The coating solution was prepared by mixing Gd, Cu fluorine-free sources with Ba trifluoroacetate. Samples were dip-coated, pyrolized within 3 h at the temperature up to 400 °C in a humid oxygen atmosphere, and finally fired at various high temperatures in 100 ppm Ar/O₂ atmosphere. The GdBCO films fired at 775 and 800 °C exhibited J_c values of ∼2.1 MA/cm² at 77 K in a self-field, which are attributable to both high T_c,zero values of ∼89 K and high in-plane textures of 1.3–1.4°. Above 800 °C, however, the superconducting properties of GdBCO films are degraded due to the thermal decomposition of GdBCO film in 100 ppm Ar/O₂ atmosphere.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Critical RF losses in fine particles of La1−xAgyMnO3+δ: Prospects for temperature-controlled hyperthermia

Temperature and frequency dependencies of the real (χ′) and imaginary (χ″) parts of the dynamic magnetic susceptibility were studied experimentally in fine particles of La-Ag manganites prepared by various methods. The samples under study have the Curie points in the range T_C=42-48 °C, which is a medical hyperthermia range of interest. When approaching T_C from below, a critical peak of χ″ was revealed, followed by a steep drop while passing to the paramagnetic phase. The experiments on the magnetic radio-frequency (RF) heating of the manganite aqueous suspensions demonstrated good autostabilization of the temperature near T_C. Peculiar instability is found in the heating kinetics, caused by the observed critical behavior of the RF losses. The prospects of the La-Ag manganites as candidates for application in the temperature-controlled hyperthermia are discussed.     

Spin transistor and quantum spin Hall-effects in CdBxF2−x–p-CdF2–CdBxF2−x sandwich nanostructures

Planar CdB_xF_2−x–p-CdF₂–CdB_xF_2−x sandwich nanostructures prepared on the surface of the n-type CdF₂ bulk crystal are studied to register the spin transistor and quantum spin Hall-effects. The current–voltage characteristics of the ultra-shallow p⁺–n junctions verify the CdF₂ gap, 7.8 eV, and the quantum subbands of the 2D holes in the p-type CdF₂ quantum well confined by the CdB_xF_2−xδ-barriers. The temperature and magnetic field dependencies of the resistance, specific heat and magnetic susceptibility demonstrate the high temperature superconductor properties for the CdB_xF_2−xδ-barriers. The value of the superconductor energy gap, 2Δ = 102.06 meV, determined by the tunneling spectroscopy method appears to be in a good agreement with the relationship between the zero-resistance supercurrent in superconductor state and the conductance in normal state, πΔ/e, at the energies of the 2D hole subbands. The results obtained are evidence of the important role of the multiple Andreev reflections in the creation of the high spin polarization of the 2D holes in the edged channels of the sandwich device. The high spin hole polarization in the edged channels is shown to identify the mechanism of the spin transistor and quantum spin Hall-effects induced by varying the top gate voltage, which is revealed by the first observation of the Hall quantum conductance staircase.     

Pressure tuned crystal structure in NdBa2Cu3O6+δ superconductor

The high-pressure angle-dispersive X-ray diffraction experiments on the NdBa₂Cu₃O_6+δ superconductor were performed from ambient to above 30 GPa at room temperature. The structure analysis based on the Rietveld refinement methods shows the different pressure dependence for the bond length between the basal-plane copper of the pyramids to the apical oxygen (denoted Cu(2)–O(1)) and bond length between basal-plane copper to plane oxygen (denoted Cu(2)–O(2,3)). The ambient bulk modulus B₀ is derived as 127 GPa. A possible correlation between Cu(2)–O(1) and T_c was discussed.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.     

Structural investigation of the BaSnO3–YBa2Cu3O7−x system

We report the microstructure change of BaSnO₃ (BSO)–YBa₂Cu₃O_7−x (YBCO) thin film system grown on SrTiO₃ single crystal substrates by pulsed laser deposition with using the “surface-modified-target” and “mixture-target” methods. Although it was confirmed that the thick BSO nanorods incorporated to YBCO films act as strong artificial pinning centers, the formation mechanism of the nanorods is still unclear. The purpose of this work is to extend the structural investigation to higher contents of BSO (up to 71 vol.%) in order to enlighten the relationship among interfacial energy, morphology and pinning performance in binary BSO–YBCO films.     

Dielectric properties of Cu0.5Tl0.5Ba2Ca3Cu4O12−δ bulk superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12−δ superconductor samples were studied at 79 and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 kHz to 10 MHz. A negative capacitance (NC) phenomenon has been observed, which is most likely arising due to higher Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge located at the multiple insulator–superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 79 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Magnetic properties of low-doped bulk Sr(Ti, Co)O3−δ perovskites

Magnetic properties of bulk SrTi_1−xCo_xO_3−δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO₃ matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.     

Magnetic and transport properties in double-doping La(2+4x)/3Sr(1−4x)/3Mn1–xCuxO3 (0⩽x⩽0.20) systems

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1–xCu_xO₃(0?x?0.2)$(0?x?0.2)$with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 and the single-doping samples La_2/3Sr_1/3Mn_1–xCu_xO₃(0?x?0.2)$(0?x?0.2)$ have been investigated. For the double-doping samples, though the ratio Mn³⁺/Mn⁴⁺=2:1 has been generally recognized the optimum ratio, the Curie temperature _TC$T_{\mathrm{C}}$ and metallic–insulator transition temperature _Tp1$T_{\mathrm{p}1}$ are more rapidly decreased by Cu substitution than that corresponding to single-doping samples. And the resistivity ρ$\rho$ value for the double doping is larger about two or three orders of magnitude than that corresponding to single doping. At the same time, two resistivity peaks and two magnetoresistance (MR) peaks appear. We suggest that for the double-doping samples the A-site cation size 〈r_A〉 and the A-site mismatch factor σ²${\sigma }^2$ decreases with increasing doping level, which leads to the system microstructural distortion. This microstructural distortion makes the Mn³⁺–O–Mn⁴⁺ cut off more cluster-spin except for the clusters induced by Cu. These cluster interfaces contribute to ρ$\rho$, which exceeds far the contribution of e_g electron decreasing with doping increasing in the single doping. At the same time, such interface scattering also gives rise to the appearance of second peak for the double-doping samples. The experimental results shows that double doping could be also a potential way in tuning colossal MR (CMR), which can give a guide for the adequate selection of CMR materials.     

Fermi-arc superconductivity, pseudogap and charge order in Bi2Sr2CaCu2O8+δ

We report STM/STS observations on the 4a×4a charge order in the pseudogap (PG) and superconducting (SC) states of Bi₂Sr₂CaCu₂O_8+δ, and suggest that the charge order is associated with incoherent quasiparticle or pair states around the antinodal fermi surface (FS), which are also responsible for the PG, and it can coexist with the superconductivity caused by the pairing of coherent quasiparticles on the nodal fermi arc. We also suggest that the nanometer-scale gap inhomogeneity in the SC state, reported previously [Pan, et al., Nature 413 (2001) 282; Mommo, et al., J. Phys. Soc. Jpn. 74 (2005) 2400; Hashimoto, et al., Phys. Rev. B74 (2006) 064508] arises from that in the PG state, which occurs around the antinodal FS.     

Sr doping effects on the transport and magnetic properties of GdBaCo2O5+δ

Studies on some new members of the cobalt perovskite Gd_1−xSr_xBaCo₂O_5+δ with low strontium concentrations (0<x<0.1) have been carried out with the aim of investigating possible metallization of the GdBaCo₂O_5+δ system by hole doping. Low temperature electrical resistivity, magnetic susceptibility and thermopower of the above system have been studied. The pristine compound with x=0 and δ∼0.5 exhibits a semiconductor-like behavior and two magnetic transitions below room temperature. Upon Sr²⁺ substitution, there is a fall in resistivity by 2-3 orders of magnitude at low temperature and also a dramatic reduction in the ferromagnetic to antiferromagnetic transition temperature. These changes can be explained on the basis of hole doping (and increase in the Co⁴⁺content). Evidence for an increase in Co⁴⁺ with Sr²⁺ substitution is provided by iodometric analysis.