Structural and dielectric properties of giant dielectric Na_(1/2)Sm_(1/2)Cu_3Ti_4O_(12) ceramics prepared by reactive sintering methods

Na_(0.5)Sm_(0.5)Cu_3Ti_4O_(12)(NSCTO) ceramics have been prepared by reactive sintering of amorphous powder.Spark plasma sintering(SPS) for 10 min at 1025℃ and conventional sintering(CS) for 10 h at 1090℃ have been employed.X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics.The SPS ceramic showed an average grain size of 500 nm, which is much smaller than that of the CS(～ 5 μm) sample.The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics.While the resistivities of grains of both ceramic samples were in the same order of magnitude, the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic.Both of the samples showed giant dielectric constant( 10~3) over wide ranges of temperatures and frequencies.Nevertheless, the room-temperature dielectric loss of the SPS NSCTO(3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO(0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former.Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries.The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the internal barrier layer capacitor(IBLC) model.     

Influence of annealing treatment on the luminescent properties of Ta:β-Ga_2O_3 single crystal

Ta~(5+)doped β-Ga_2O_3 single crystals were grown by using the optical floating zone method, and then annealed in the air and nitrogen gas at 1400℃ for 20 hours.The transmittance spectra, photoluminescence(PL), x-ray irradiation spectra, and PL decay profiles of the samples were measured at room temperature.The relevant results show that the optical transmittance of the samples annealed in the air or nitrogen gas was improved.By drawing the(ahv)~2–hv graph,it can be seen that the band gap decreased after being annealed in the air, but increased in nitrogen gas.The PL spectra and x-ray irradiation spectra show that the luminescent intensity of the sample annealed in the air increased substantially,while decreased for the sample annealed in nitrogen.The PL decay time of the Ta:β-Ga_2O_3 annealed in the air increased significantly compared with that of the Ta:β-Ga_2O_3 sample without annealing, but the tendency after annealing in nitrogen gas was opposite.     

Effects of growth temperature and metamorphic buffer on electron mobility of InAs film grown on Si substrate by molecular beam epitaxy

The growth of the InAs film directly on the Si substrate deflected from the plane(100) at 4° towards(110) has been performed using a two-step procedure. The effect of the growth and annealing temperature on the electron mobility and surface topography has been investigated for a set of samples. The results show that the highest electron mobility is4640 cm~2/V·s in the sample, in which the 10-nm InAs nucleation layer is grown at a low temperature of 320 ℃ followed by ramping up to 560 ℃, and the nucleation layer was annealed for 15 min and the second layer of InAs is grown at 520 ℃.The influence of different buffer layers on the electron mobility of the samples has also been investigated, which shows that the highest electron mobility of 9222 cm~2/V·s at 300 K is obtained in the sample grown on a thick and linearly graded InGaAlAs metamorphic buffer layer deposited at 420 ℃.     

Characterization of the BaBiO3-doped BaTiO3 positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with Tc=155℃

BaBiO 3-doped BaTiO3(BB-BT) ceramic,as a candidate for lead-free positive temperature coefficient of resistivity (PTCR) materials with a higher Curie temperature,has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature,Tc=155℃,which is higher than that of BaTiO3(≤130℃). Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current (AC) resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data,the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature,indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency,the conduction mechanism is also discussed for a BB-BT ceramic system.     

A STUDY OF THERMAL DESORPTION OF HELIUM FROM HIGH-PURE α-Ti AND HYDROGENATED Ti SAMPLES

Helium ions have been implanted into highly pure α-Ti and hydrogenated Ti samples with energy of 140keV and at dose of 5.2×10¹⁷He·cm^-2. The retained amount and depth pro-files of helium in the samples were determined by means of proton-enhanced-backscattering (PEBS) at room temperature. The changes of He peaks during isochronical annealing at 100, 200, 300℃,… were monitored in-situ by PEBS for groups of samples. The process of temperature ramping was not stopped until He peaks almost disappeared and hence the release curves were obtained. It was found that ～ 60% of helium was released at 200℃ for high-pure α-Ti samples, whereas massive release occurred at 300℃ for the samples with H/Ti=0.9 and at 350℃ for the samples with H/Ti=0.2. The mechanism of He release and the effect of hydrogen were also discussed.     

Anomalous magnetostrictive effects in （La1—xTbx）2／3Sr1／3 MnO3

A series of (La_{1-x}Tb_x)_{2/3}Sr_{1/3}MnO_3 polycrystalline samples has been studied by means of x-ray diffraction, magnetostriction, and thermal expansion measurements. It has been found that this series undergoes a phase transition from a rhombohedral to an orthorhombic form at the doping level x≈0.20 at room temperature accompanied by an anisotropic magnetostriction up to －50×10^{-6} under a magnetic field of 1T. The linear and volume magnetostrictions vary with chemical composition, even change sign. At T=80K, the magnetostrictions for the samples of x=0.20 and 0.40 exhibit different behaviours. The sample of x=0.20 has positive volume and linear magnetostrictions and a negative anisotropic magnetostriction, while the sample of x=0.40 has an opposite behaviour. The magnitude of volume magnetostriction for both the samples is essential (～10^{-4}) at T=80K under a magnetic field of 4T. We conclude that these anomalous effects are due to the charge delocalization and the structural phase transition between orthorhombic and rhombohedral forms induced by the applied magnetic field.     

Structural evolution of a-Si:H/SiO2 multilayers upon step by step thermal annealing

a-Si:H/SiO_2 multilayers were prepared by alternatively changing plasma enhanced chemical vapour deposition of a-Si:H layers and in situ plasma oxidation process. Subsequently, as-grown samples were annealed at temperatures from 350℃ to 1100℃ in N_2 ambient with an increment of 100℃. The evolution of bonding configurations and structures with annealing treatments was systematically investigated by Fourier-transform infrared spectroscopy. The peak position of Si－O stretching vibration of SiO_2 layers shift to 1087cm^{-1} after annealing at 1100℃, which demonstrates that the SiO_2 films fabricated by plasma oxidation after high temperature annealing can have similar properties to the thermal grown ones. A Si－O vibration from interfacial SiO_x was identified: the value x was found to increase as increasing the annealing temperature, which is ascribed to the cooperation of hydrogen effusion and reordering of the oxygen bond in SiO_x networks. The H-related bonds were observed in the form of H－Si－O_3 and H－Si－Si_{3-n}O_n (n=1－2) configurations, which are supposed to be present in SiO_2 and interfacial SiO_x layers, respectively. The H atoms bonded in different bonding configurations effuse at different temperatures due to their different desorption energies.     

Magnetoresistance and Kondo Effect in Nodal-Line Semimetal VAs_2

We performed calculations of the electronic band structure and the Fermi surface,measured the longitudinal resistivity ρxx(T,H),Hall resistivity ρxy(T,H),and magnetic susceptibility as a function of temperature at various magnetic fields for VAs₂ with a monoclinic crystal structure.The band structure calculations show that VAs₂ is a nodal-line semimetal when spin-orbit coupling is ignored.The emergence of a minimum at around11 K in ρxx(T) measured at H=0 demonstrates that some additional magnetic impurities(V⁴⁺,S=1/2)exist in VAs₂ single crystals,inducing Kondo scattering,evidenced by both the fitting of ρxx(T) data and the susceptibility measurements.It is found that a large positive magnetoresistance(MR) reaching 649% at 10 K and 9 T,its nearly quadratic field dependence,and a field-induced up-turn behavior of ρxx(T) also emerge in VAs₂,although MR is not so large due to the existence of additional scattering compared with other topological nontrivial/trivial semimetals.The observed properties are attributed to a perfect charge-carrier compensation,which is evidenced by both the calculations relying on the Fermi surface and the Hall resistivity measurements.These results indicate that the compounds containing V(3d³4s²) element can be as a platform for studying the influence of magnetic impurities to the topological properties.     

Spin Glass Behaviour and Spin-Dependent Scattering in La0.7Ca0.3Mn0.9Cr0.1O3 Perovskites

The magnetic, electrical and thermal transport properties of the perovskite La0.7Ca0.3Mn0.9 Cr0.1O3 have been investigated by measuring dc magnetization, ac susceptibility, the magnetoresistance and thermal conductivity in the temperature range of 5-300 K. The spin glass behaviour with a spin freezing temperature of 70 K has been well confirmed for this compound, which demonstrates the coexistence and competition between ferromagnetic and antiferromagnetic clusters by the introduction of Cr. Colossal magnetoresistance has been observed over the temperature range investigated. The introduction of Cr causes the ““double-bump““ feature in electrical resistivity ρ(T). Anomalies on the susceptibility and the thermal conductivity associated with the double-bumps in ρ(T)are observed simultaneously. The imaginary part of ac susceptibility shows a sharp peak at the temperature of insulating-metallic transition where the first resistivity bump was observed, but it is a deep-set valley near the temperature where the second bump in ρ(T) emerges. The thermal conductivity shows an increase below the temperature of the insulating-metallic transition, but the phonon scattering is enhanced accompanying the appearance of the second peak of double-bumps in ρ(T). We relate those observed in magnetic and transport properties of La0.7Ca0.3Mn0.9Cr0.1O3 to the spin-dependent scattering. The results reveal that the spin-phonon interaction may be of more significance than the electron (charge)-phonon interaction in the mixed perovskite system.     

boldmath Characterization of the BaBiO3-doped BaTiO3 positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with Tc=155℃

BaBiO₃-doped BaTiO₃ (BB-BT) ceramic, as a candidate for lead-free positive temperature coefficient of resistivity (PTCR) materials with a higher Curie temperature, has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature, T_c=155 ℃, which is higher than that of BaTiO₃ ( ≤ 130 ℃). Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current (AC) resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data, the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature, indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency, the conduction mechanism is also discussed for a BB-BT ceramic system.     

Preparation and the physical properties of antiperovskite-type compounds Cd1-xInxNNi3 (0≤x≤0.2) and Cd1-yCuyNNi3 (0≤y≤0.2)

Two series of Cd1-xInx NNi3(0 ≤x≤ 0.2) and Cd1-yCuyNNi3(0≤y≤0.2) samples were prepared from CdO, In2O3 , CuO, and nickel powders under NH3 atmosphere at 773K. The structural and physical properties were investigated by means of X-ray powder diffraction temperature-dependent resistivity and magnetic measurements. X-ray powder diffraction results showed that the Cd 1 x In x NNi 3 and Cd 1 y Cuy NNi 3 compounds have a typical antiperovskite structure, and the CdNNi3, Cd0.9 In 0.1 NNi3 , and Cd0.9Cu0.1NNi3 compounds show metallic temperature-dependent resistivity and exhibit a Fermi liquid behavior at low temperature. In contrast to the paramagnetism previously reported, the CdNNi 3 sample exhibits very soft and weak ferromagnetism, and no superconductivity was found in the Cd 1 x In x NNi 3 and Cd 1 y Cu y NNi 3 samples down to 2 K. Each sample exhibited very soft and weak ferromagnetism, and the temperature dependence of the magnetization of the Cd 1-xInx NNi 3 and Cd1-y Cu y NNi 3 samples can be well fitted to the combination of a Bloch term and a Curie–Weiss term.     

Magnetism and magnetoresistance of Er1-xSmxMn6Ge6(x=0.2－1.0) compounds

Magnetic and transport properties of Er_{1-x}Sm_xMn_6Ge_6(x=0.2-1.0) have been investigated by x-ray diffraction (XRD) and magnetization measurement. Analysis of the XRD patterns indicates that the samples with x≤0.4 mainly consist of HfFe_6Ge_6-type phase and the samples with 0.6≤x≤1.0 mainly consist of YCo_6Ge_6-type phase (P6/mmm). The lattice constants and the unit cell volume increase with increasing Sm content. The antiferro－ferri－ferromagnetic transitions can be observed with increasing Sm content. The samples with x=0.2 and 0.4 order antiferromagnetically at 420 and 425K, respectively. The samples with x=0.6, 0.8 and 1.0 order from ferri- to ferromagnetically over the whole magnetic ordering temperature range. The corresponding Curie temperatures are 435, 441 and 446K, respectively. The magnetoresistance (MR) isotherms of the sample with x=0.8, measured at various temperatures, are analysed. The magnitude of MR is found to be positive below 55K and gradually increases to a relatively large value of about 5.02% at 5K in a field of 5T as the temperature is lowered. A possible explanation for the positive MR is given.     

Measurement of anomalous nuclear reaction in deuterium-loaded metal

This paper reports on an experiment for testing natural nuclear fusion at low temperature searching for evidence of the origin of ³He from natural nuclear fusion in deep Earth. The experiment was carried out using deuterium-loaded titanium foil samples and powder sample. Detection of charged particle was carried out using a low-level charged particle spectrometer. An Al foil was used as an energy absorber for identification of charged particle. Although the counting rate is very low in the experiment, the emission of energetic particle from the sample is observed and the particle is identified as a proton having energy about 2.8 MeV after exiting the titanium sample. This work provides a positive result for the emission of charged particle in the deuterium-loaded titanium foil samples at low temperature, but a negative result for the deuterium-loaded titanium powder sample. The average reaction yield is deduced to be (0.46±0.08) protons/h for the foil samples. With the suggestion that the proton originates from d--d reaction, we calculate the reaction rate for d--d reaction, and the obtained result is 1.4× 10^-24 fusion/d--d\cdot sec. The negative result of the deuterium-loaded titanium powder sample suggests that the reaction yield might be correlated with the density or microscopic variables of deuterium-loaded titanium materials. The negative result also indicates that d--d reaction catalysed by μ-meson from cosmic ray can be excluded in the samples in this experiment.     

Effect of Different Substrate Temperature on Phosphorus-Doped ZnO Thin Films Prepared by PLD on Sapphire Substrates

Phosphorus-doped ZnO （ZnO：P） thin films are deposited on a c-plane sapphire in oxygen at 350℃, 450℃, 550℃ and 650℃, respectively, by pulsed laser deposition （PLD）, then all the ZnO：P samples are annealed at 650℃ in oxygen with a pressure of 1 × 10^5 Pa. X-ray diffraction measurements indicate that the crystalline quality of the ZnO：P thin films is improved with the increasing substrate temperature from 350℃ to 550℃. With a further increase of the deposition temperature, the crystalline quality of the ZnO：P sample is degraded. The measurements of low-temperature photoluminescence spectra demonstrate that the samples deposited at the substrate temperatures of 350℃ and 450℃ show a strong acceptor-bound exciton （A^0X） emission. The electrical properties of ZnO：P films strongly depend on the deposition temperature. The ZnO：P samples deposited at 350℃ and 450℃ exhibit p-type conductivity. The p-type ZnO：P film deposited at 450℃ shows a resistivity of 1.846Ω·cm and a relatively high hole concentration of 5.100 × 10^17 cm^-3 at room temperature.     

Quantum critical behavior in an antiferromagnetic heavy-fermion Kondo lattice system(Ce_(1-x)La_x)_2Ir_3Ge_5

The measurements on temperature dependences of magnetic susceptibility χ(T), specific heat C(T), and electrical resistivity ρ(T) were carried out for the antiferromagnetic(AFM)(Ce_(1-x)La_x)_2Ir_3Ge_5(0 ≤ x ≤ 0.66) system. It was found that the Neel temperature TNdecreases with increasing La content x, and reaches 0 K near a critical content xcr =0.6. A new phase diagram was constructed based on these measurements. A non-Fermi liquid behavior in ρ(T) and a log T relationship in C(T) were found in the samples near xcr, indicating them to be near an AFM quantum critical point(QCP) with strong spin fluctuation. Our finding indicates that(Ce_(1-x)La_x)_2Ir_3Ge_5 may be a new platform to search for unconventional superconductivity.     

Discovery of a new nontoxic cuprate superconducting system Ga-Ba-Ca-Cu-O

Superconductivity is observed in a new nontoxic cuprate system Ga-Ba-Ca-Cu-O,with T_c=82 K for Ga Ba_2Ca_5Cu_6O_(14+δ)(Ga-1256)and T_c=116 K probably for Ga Ba_2Ca_3Cu_4O_(10+δ)(Ga-1234)or Ga Ba_2Ca_2Cu_3O_(8+δ)(Ga-1223),respectively.All compounds are fabricated by solid state reaction method under high pressure and high temperature.Samples are characterized by resistivity,magnetization and X-ray diffraction(XRD)measurements.The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes on one sample(S1)shows two superconducting transitions at about 82 K and113 K.The estimated shielding fraction for the phase with T_cof 82 K is about 67%,while the fraction for another phase with T_(c )of 113 K is quite small.The XRD Rietveld refinement for S1 indicates two main phases existing in the sample,Ga-1256 with fraction of about 58%and non-superconducting Ca_(0.85)Cu O_2with fraction of about 42%respectively.Thus,we can conclude the superconducting phase with transition temperature of 82 K is due to Ga-1256.The resistivity measurement also confirms the superconductivity for S1,and the resistivity reaches zero at about 82 K.The temperature dependence of magnetization for another sample(S2)shows much higher superconducting shielding fraction for the phase with T_cof 116 K,which may be a promising prospective for the synthesis of Ga-1234 or Ga-1223 phase.     

Thermal Conductivity Anomalies Related to the Double-Bump of Resistivity in Nd0.7Sr0.3Mn1-xCrxO3

The temperature dependence of the thermal conductivity κ(T) and electrical resistivity ρ(T) has been measured for perovskite Nd0.7Sr0.3Mn1-xCrxO3 (0.01≤x≤0.15) in the temperature range of 10-300 K. The double-bump feature in p(T) is observed for x = 0.08 and 0.10 samples. Below the insulating-metallic transition temperature TIM, κ(T) shows a sharp increase and such an increase is depressed gradually by the increasing Cr content.While κ(T) displays an anomalous decrease around the temperature where the second bump of ρ(T) emerges.We suggest that the enhancement of κ(T) below TIM is related to both phonons and magnons, the ferromagnetic interaction of Mn^3 -O-Cr^3 should be super-exchange interaction rather than double-exchange interaction, and the Cr doping just weakens the DE interaction in this system.     

Chemical composition and magnetism of Ag doped LaMnO3

By using a sol-gel clue, a set of polycrystalline perovskite samples La1-xAgxMnO3 with a nominal doping level x ranging from 0.05 to 0.45 has been synthesized. The chemical composition and the magnetism of the samples were investigated. A little Ag was： found seeping from the samples in the sintering process when the doping level exceeded 0.05 and the sintering temperature was higher than 700℃ resulting in the samples being in multiphase. The magnetic transition points of the samples have been found to decrease with increasing sintering temperature. A concentration-dependent Tc similar to that of bivalent metal ion doped perovskite, has been obtained. We believe that the Ag seeping in the sintering process is responsible for those magnetic characteristics.     

Spin-Polarized Tunnelling Magnetoresistance Effects in La0.833K0.167MnO3／SrTiO3 Polycrystalline Perovskite Manganites

A sample of La0.833K0.167MnO3-SrTiO3 (LKMO/STO) is fabricated by the sol-gel method. The microstructure,magnetic and transportation properties have been studied. X-ray diffraction patterns indicate that the structure of LKMO/STO is a homogeneous solid solution phase. The resistivity of LKMO/STO shows the insulator behaviour, which is different from La0.833K0.167MnO3 (LKMO) whose resistivity shows metal-insulator transition with decreasing temperature. The low-field (moH = 0.02 T) magnetoresistance decreases from 11% to 0.2% with the increasing temperature from 4 K‘to 220K for the LKMO/STO sample. The magnitude of magnetoresistance in a strong field (μoH = 5.5 T) almost increases linearly with decreasing temperature and reaches the maximum of 65% at the low temperature of 4.2K, which is much higher than that oE LKMO (40%). The enhanced low-field magnetoresistance effects are quantitatively explained by the spin-polarized tunnelling at grain boundaries.     

Fabrication of transparent Tb_3Ga_5O_(12) ceramic

Tb_3Ga_5O_(12)(TGG) is an excellent material for magneto-optical applications,and is the key component in Faraday isolators(FIs).The preparation process of transparent TGG ceramics is experimentally studied.The optical quality and the microstructure of the samples are investigated.The results show that the transmittance of the sample sintered at 1550℃ is close to 72%in the region of 500-1500 nm.The Verdet constant at 632.8 nm measured at room temperature is —125.01 rad T~(-1) m~(-1),which is almost the same as that of a single crystal.