Magnetocaloric Effect in Nanopowders of Pr0.67Ca0.33Fe x Mn1−x O3

Magnetocaloric effect in nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ (x=0,0.1,0.3,0.4), in the vicinity of magnetic phase transitions, was investigated. It is shown that Pr_0.67Ca_0.33MnO₃ exhibits the largest magnetic entropy change (ΔS _M ) of 0.61 J/kg/K at 208 K upon 0.5 T magnetic field variation. Furthermore, the ΔS _M distribution of the Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ is much more uniform than that of gadolinium. Because of these results, nanopowders of Pr_0.67Ca_0.33Fe _x Mn_1?x O₃ have some potential applications for magnetic refrigerants in an extended high-temperature range. Moreover, it can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Normal and Anomalous Pseudogap of Superconducting Y0.9−x Pr x Ca0.1Ba2[Cu1−y Zn y ]3O7−δ

Electrical resistivity measurements on the superconducting oxides of the compositions Y_0.9−x Pr _x Ca_0.1Ba₂[Cu_1−y Zn _y ]₃O_7−δ (0≤x≤0.20 and 0.0≤y≤0.10) sintered in oxygen atmosphere were carried out to obtain the normal and anomalous pseudogaps in underdoped and overdoped samples. It is observed that pseudogap temperature T ^* decreases with increasing doping level p in the underdoped case. For the overdoped sample with y=0.06, T ^* shows no p dependence.      

Anomalous Behavior in Y1−x Pr x Ba2Cu3O7−δ

It is found that the samples of x=0.25 and 0.5 in the series samples of Y_1?x Pr _x Ba₂Cu₃O_7?δ demonstrate some anomalous behavior. First, the changes of T _c value of them are a bit different from their neighbor samples. Second, under a small magnetic field, i.e. 5–100 Oe, a little rise in the T _c value with the rise of the field is observed. The anomalous behavior is primitively explained by the local structure changes and the magnetic properties. It is attributed to local structural changes, i.e. the chemical bonds in the Cu–O₂ plane.     

Magnetization reversal and ferrimagnetism in Pr1−x Nd x MnO3

Detailed magnetic properties of Pr_{1? x} Nd _x MnO₃ (x = 0.3, 0.5 and 0.7) have been reported. All the samples crystallize in orthorhombic perovskite structure with Pnma space group. Magnetization measurements under field cooled (FC) protocal reveal magnetization reversal at low temperatures and low magnetic field. This indicates clear evidence of two magnetic sublattices aligned opposite to each other. There is a well-defined maximum around 48 K in the x = 0.7 sample (i.e. Pr_0.3Nd_0.7MnO₃) in the χ′ value which is identified as paramagnetic to ferrimagnetic transition. The peak value shifts to higher temperature with decrease of x and width of the maximum broadened. It is also observable that with decrease of Nd, both the value of χ′ and χ″ decrease. An attempt is made to explain the magnetization reversal within the framework of available models.     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.     

Dielectric studies of Ca x Sr1−x F2 mixed crystals

The dielectric constant (K) and loss (K) of CaF₂, SrF₂ and their mixed crystals as a function of frequency in the range 10² to 10⁵ Hz at room temperature were measured. Except at 10² Hz, the value of the dielectric constant was found to be independent of frequency. At any frequency the value ofK for mixed crystals was found to vary non-linearly as a function of composition, showing a maximum at about equimolar composition. The value ofK was less in mixed crystals compared to the end members. An attempt is made to explain the results.     

The solid solution structure of BiSr x Ca1−x O2.50

The formation and solid-solution structure of BiSr _x Ca_1–x O_2.50 were studied by X-ray powder diffraction analysis, and the crystallography data of the phase are given. The crystal structure of BiSr _x Ca_1–x O_2.50 belongs to the monoclinic system with space group P1m1 or P12/m1, and the cell parameters for BiCaO_2.50 (x=0.00) are a=1.8363 nm, b=0.5366 nm, c= 1.4670 nm and =100.26°. BiSr _x Ca_1–x O_2.50 is a solid solution of Bi-Ca-O and Bi-Sr-O. When the strontium solid solubility Sr/Sr+Ca=0.50, the strontium dissolved in the phase reaches saturation, while the strontium solid solubility limit of the phase is between 0.67 and 0.75, and beyond this limit the crystal structure is greatly distorted.     

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below T _c and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^?1. This decrease occurs below T _c and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ ₂(ω)at 10 cm^?1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

Specific heat and magnetocaloric effect of Pr1−x Ag x MnO3 manganites

This paper presents experimental results of the specific heat and magnetocaloric effect in Pr_1?x Ag _x MnO₃ (x = 0.05–0.25) manganites. A maximum value of magnetic entropy change for the sample with x = 0.15 was observed in magnetic field of 18 kOe at T _C = 125 K and was ΔS _max ≈ 2.96 J/kg K. In Pr_0.95Ag_0.05MnO₃ sample, a sign inversion of the magnetocaloric effect was observed at low temperatures (~80 K) in low magnetic fields of 750 Oe. The coexistence of ferromagnetic and canted antiferromagnetic phases with closely spaced critical temperatures was found to force the magnetocaloric effect into abrupt sign inversion.     

X-ray absorption spectroscopy of Pr x Y1−x Ba2Cu3O7−y single crystals

Substituting Y in orthorhombic (Y,RE)Ba₂Cu₃O₇ by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of Pr_xY_1–xBa₂Cu₃O_7–y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y 0.9) to the doped materials (y 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f–02p hybridization.     

Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

Two distinct structural phases in Sr3−x Ca x Ru2O y

Non-cuprate layered perovskite Sr₂RuO₄, which is an analogue of (La,Sr)₂CuO₄, exhibits superconductivity below T_c1K. We have synthesized a new system Sr_3–xCa_xRu₂O_y and found the solubility of Ca to be at least as much as x = 2. These samples are not superconductive down to 2K but magnetically ordered. The x dependence of the lattice parameters and that of T_mag clearly indicate that there are at least two distinct structural phases in Sr_3–xCa_xRu₂O_y.On leave from Hiroshima University.     

The nonstoichiometry and physical properties of the Nd1−x Ca1+x FeO4−y system

The solid solutions of the Nd_1–x Ca_1+x FeO_4–y system for compositions ofx=0.000,0.125, 0.250,0.375, and 0.500 are prepared by drip pyrolysis. XRD analysis shows all the solid solutions are tetragonal I_4/mmm. The Fe⁴⁺ ratio to the total Fe ions or value has a maximum for the compositionx=0.375. From the X-ray powder diffraction analysis and the Mössbauer spectroscopy, the distortion and symmetry change of oxygen octahedra of Fe ions are observed. The structural change of oxygen octahedra of Fe ions strongly affects the physical properties. The solid solution whenx=0.000 shows a weak ferromagnetic behaviour due to the spin canting of the distorted octahedra. The other solid solutions withx=0.125, 0.250, 0.375, and 0.500 show a paramagnetic behaviour over room temperature. The decrease of the magnetic transition temperature is due to the distortion of oxygen octahedra of Fe ions and the existence of the Fe⁴⁺ ion. The formation site of oxygen vacancies plays an important role in the conductivity of the Nd_1–x Ca_1+x FeO_4–y system. Although the oxygen vacancies in [Nd, Ca]-O layer have little effect on conductivity, the oxygen vacancies in the FeO₂ plane of the perovskite layer act as electron trapping sites and thus increase the activation energy.     

Excess Conductivity Analysis of Tl1−x Y x Ba2Ca2Cu3O10−δ Superconductors

Excess conductivity analyses of resistivity data of Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) samples have been carried out by following Lawrence & Doniach (LD) and Maki-Thompson (MT) models. In the critical regime important superconductivity parameters have been elucidated by employing Ginzburg-Landau number N _G of Ginzburg Landau theory. Our samples have shown a decrease in the T _c (R=0) and magnitude of diamagnetism with increased Y-doping. The cell parameters and volume of the unit cell increase with doping of Y⁺³ in Tl_1?x Y _x Ba₂Ca₂Cu₃O_10?δ (x=0, 0.04, 0.06, 0.08, 0.1) which shows a decrease in the density of charge carriers in the conducting CuO₂ planes. Since the Fermi vectors of the carriers, K _F=[3π ² N/V]^1/3=[3π ² n]^1/3, their coherence length along the c-axis, ξ _c=?K _F/2mΔ, and the Fermi velocity, V _F=?K _F/m depend on density of mobile charge carriers, the doping of Y⁺³ suppresses it and hence the superconductivity parameters. We have confirmed these conjectures with the excess conductivity analyses (FIC) of our conductivity data. The FIC analysis of conductivity data has shown a decrease in the values of ξ _c, V _F, B _c(0), B _c1(0) and J _c(0) with increase doping of Y (except for the samples with x=0.04). The width of two dimensional conductivity regimes is shrunken with increased Y-doping. From these studies it is concluded that presence of Y⁺³ in the unit cell of TlBa₂Ca₂Cu₃O_10?δ impedes the flow of the mobile charge carriers to the conducting CuO₂ planes which induce suppression in the superconductivity parameters. The studies also stress the vital role of mobile charge carriers in the mechanism of high temperature superconductivity.     

Pb1−x Bi x /Bi/Pb1−x Bi x Josephson junction

A superconductor/semimetal/superconductor (S/SM/S) Josephson junction has been developed. We have used an alloy of Pb_1–x Bi _x (0x 0.6) as the superconductor and Bi as the semimetal. By irradiating at X-band microwave of 10 GHz, Shapiro steps were observed for various bismuth barrier thicknesses inÅ and bismuth weight ratiosx. Finally, we obtained the empirical relationship for barrier thickness, below which microwaves could be detected for various bismuth weight ratiosx at the temperature of 4.2 K.