The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.     

Resistivity of high-T c, 1 at % Ti-substituted Nb3Ge

Measurements are presented of the resistivity in samples of (Nb_0.99Ti_0.01)_1–x Ge_x, 0.17 x 0.24, from the superconducting transition temperature T _cto 300 K. The T _cattains its maximum value of above 22 K and the residual resistivity its minimum value at the same off-stoichiometric composition when x = 0.205. At this same value of x the transition width is narrowest and the residual resistivity extrapolates to a finite value, indicating a comparatively low resistance ratio is to be expected. The resistance ratio achieved in these samples with Ti substitution is comparable to that achieved in the best samples of pure Nb₃Ge and is indicative of the short mean free path in this system.Supported by NSF DMR 78-03217 and PSC-BHE Research Award Program.     

Superconductivity in bulk A15 Nb3Si under pressure

We have determined the effect of hydrostatic pressureP on the superconducting transition temperatureT _c of bulk, A15 Nb₃Si. For 0P20 kbar (2 GPa),T _c decrease linearly with increasing pressure at a rate T _c/P=–2.67×10^–5 K/bar. From an estimate of T _c/P obtained using recent band structure calculations for the density-of-electronic-states change as a function of lattice parameter in Nb₃Si, we conclude that the pressure dependence of the electron-phonon interaction primarily determines T _c/P.Work performed under the auspices of the U.S. DOE.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Electronic specific heat of La2−xSrxCu1−yMyO4(M=Ga,Zn): Impurity effects on a D-wave superconductor

The electronic specific heat C_el was studied in Ga- and Zn-doped La_2–xSr_xCuO₄ (0.16x0.22) at T10K. Partial substitution of Ga or Zn for Cu suppresses T_c and revives the T-linear electronic specific heat, T, markedly. The (y)/n vs T_c/T_c0 relation for Zn-doped samples with x0.2 is in good agreement with the theoretical one for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x 0.2 deviate slightly from the theoretical curve. The deviation will be discussed in relation to changes in the magnetic properties of 3d electrons.     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

IR Spectra of La1.85Sr0.15Cu1?xMxO4?δ(M = Zn, Ni, Mg)

Polycrystalline samples La_1.85Sr_0.15Cu_1–xZn_xO_4– (0 x 0.3), La_1.85Sr_0.15Cu_1–xNi_xO_4– (0 x 0.3), and La_1.85Sr_0.15Cu_1–xMg_xO_4– (0 x 0.3) were synthesized by the solid-state reaction method. The crystal structure and phonon vibration were investigated by means of X-ray diffraction (XRD) and infrared spectrum. Zn, Ni, and Mg doping results in the lattice parameter c decreasing and a (b) increasing. The change of the phonon modes around 504 cm^–1 and 681 cm^–1 can be satisfactorily interpreted in terms of the change of the crystal structure and the itinerant nature of charge carrier in CuO₂ sheet. The relation between the structure and phonon vibration is discussed.     

Valence Charge Fluctuations and Tc Retention in Tl1?xPbxSr1+xLa1?xCuO5?y (0.0 ≤ x ≤0.5) System

Structural and x-ray photoelectron spectroscopy (XPS) studies of the Tl_1–xPb_xSr_1+xLa_1–xCuO_5–y (0.0x0.5) system have been carried out. The unit cell parameters increase with x. Irrespective of x the T_c is retained in this series and the optimum hole concentration (n_h) is maintained for x 0.5. An increase in Tl 4f and O ls binding energy with x suggests a reduction in their oxidation state. Origin of holes is discussed in terms of charge transfer between Tl, Pb and CuO₂ layers.     

Niboium-titanium nitride thin films for superconducting rf accelerator cavities

The development of sputtering-magnetron techniques makes possible the manufacturing of thin-film coated rf cavities for high-energy accelerators of various superconducting alloys or compounds. In this context high-quality (Nb_1–xTi_x)N films have been produced by dc magnetron sputtering and carefully characterized. These films, for x0.5, exhibit a critical temperature T_c as high as NbN (T_c=17 K) but show a markedly lower normal state resistivity. The calculated BCS surface impedance of the films is also lower than for NbN. The overall properties of the (Nb_1–xTi_x)N films are compared with those of other superconductors, and the suitability of this material for the production of superconducting sputter-coated copper cavities for particle physics accelerators is demonstrated. The potential interest of (Nb_1–xTi_x)N films for superconducting electronics is also briefly outlined.     

Superconductivity of barium-doped Bi2Sr2CaCu2Oy

Sintered ceramic samples of Bi₂Sr_2–x Ba _x CaCu₂O_y with nominal barium fraction 0x0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Effect of mechanical pressure and sintering temperature on the electrical properties of the rare earth ferrite Li0.6Co0.1Dy x Fe2.3−x O4; 0.0 ≤ x ≤ 0.2

Polycrystalline samples of mixed ferrite Li_0.6Co_0.1Dy _x Fe_2.3–x O₄; 0.0 x 0.2 were prepared by the standard ceramic technique. X-ray diffraction analysis was carried out to assure the formation of the samples. The resistance was measured at different temperatures (300–800 K) as a function of frequency (200 kHz–5 MHz) for the investigated samples prepared at different sintering temperatures (1100°C T _s 1250°C). It is found that this type of rare earth ferrite gives a distinguishable behavior at T _s = 1200°C where the resistance of the samples at room temperature reduced to 90% of their values of the others sintering temperatures. The data showed also that the resistance of these samples sintered at 1100°C is changed at 100 kHz from 12 M without applying pressure to 19 k with 0.8 MPa. This change occurs in a reversible manner (i.e., after removing the mechanical pressure effect the resistance of the samples well return to their initial values again immediately). Thus this type of rare earth ferrite could be more applicable in several advanced electronic devices can be used as a sensor for the mechanical pressure.     

Differential scanning calorimeter studies on Pb modified Ge–Se–Te glasses

Bulk glasses Pb _x Ge_{42 – x} Se₄₈Te₁₀(0 x 15) and Pb₂₀Ge _x Se_{70 – x} Te₁₀(17 x 24) have been prepared by quenching the melt. The non-isothermal properties of these glasses have been determined using a Differential scanning calorimeter (DSC). The composition dependence of the glass transition temperature, crystallisation temperature, excess heat capacity at glass transition, glass forming tendency and activation energy of glass transition and crystallisation show anomalous features near the composition at which the p- to n-type transition occurs. These observations reflect the changes occurring in the chemical bonding and the nature of the glassy network in these glasses. The results have been interpreted in terms of existing models and the majority charge carrier reversal phenomenon occurring in these glasses.     

Spin Gap Effects on Bulk R1+xba2?x Cu3O7?δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2–x Cu₃O_7– (R = Eu or Nd and 0 x 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Dielectric and Piezoelectric Properties of NaNbO3-Based Solid Solutions

Detailed data are presented on the structure and electrical properties of Na_{1 – x} Li _x NbO₃ (0 x 0.145), Na_{1 – x} K _x NbO₃ (0 x 1.0), and (Na_{1 – x} Pb _x )(Nb_{1 – x} Ti _x )O₃ (0 x 1.0) solid solutions prepared by solid-state reactions followed by hot pressing. The sequences of phase changes and the stability ranges of the resulting phases are identified. The results are used to map out the compositional phase diagrams of the NaNbO₃-based binary systems.     

Crystal structure of (Ba1−x Lax) [Mg(1 + x)/3Nb(2−x)/3]O3 ceramics

The crystal structure of (Ba_1–x La _x )[Mg_{(1 + x)/3}Nb_(2–x)/3]O₃ (BLMN) ceramics with 0 x 1 was investigated using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). When the La content, x, was above 0.1, the 1:2 ordered hexagonal structure found in Ba(Mg_1/3Nb_2/3)O₃ (BMN) was transformed into the 1:1 ordered cubic structure. The 1:1 ordered cubic structure was maintained up to x = 0.7. When x exceeded 0.7, however, BLMN exhibited a 1:1 ordered monoclinic structure, rather than a 1:1 ordered cubic structure. La(Mg_2/3Nb_1/3)O₃ (LMN) has a 1:1 ordered monoclinic P2₁/n structure with a = 5.6004 Å, b = 5.6414 Å, c = 7.9346 Å, and = 89.9819°. The monoclinic LMN has the in-phase and the anti-phase tilt of oxygen octahedra. The anti-parallel shift of A-site cations was also found in LMN.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7?y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1–x Zn _x )₃O_7–y withy0.1 has been measured forx0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7–y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx 0.10. YBa₂(Cu_1–x Zn _x )₃O_7–y retains the YBa₂Cu₃O_7-y orthorhombic structure forx0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,(T), is metallic-like (d/dT>0) and increases gradually with increasing Zn content. However, forx 0.10,(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Effect of copper (II) content and metal vanadate formation on the structure,grain morphology and electrical conductance behaviour of Mg1.0−x -Cu x -V2.0 (0.0≤X≤1.0) mixed oxide systems

A series of Mg_1.0–x -Cu _x -V_2.0 oxides as well as the corresponding metal vanadates (0.0x1.0) were prepared by calcination at temperatures between 500 and 1000 °C. X-ray diffraction and infrared spectral studies were used to identify the different phases present. Scanning electron microscopic investigations showed that the grain morphology not only depends on the sample calcination temperature, T _c, but also on its composition, x. The electrical conductance properties of the samples were studied between 100 and 350 °C. The variation of the conductance/and the activation energy values with x or T _c is discussed in terms of the oxides or vanadate semiconducting properties. A conduction mechanism involving hopping of electrons or polarons through M ⁿ /M ^n–1 lattice sites is proposed.     

Magnetic Susceptibility Investigations on the Layered Superconductors Y2C2Br2:RE (RE = Gd, Dy, Er)

We investigated the magnetic properties of pure and lanthanoid (RE = Gd, Dy, or Er) doped samples of Y₂C₂Br₂ (T _c = 5.04 K) by means of SQUID magnetization measurements. The analysis of the high-field susceptibility of the undoped material allows the determination of the Pauli susceptibility, the electronic density of states, and the Stoner enhancement. T _c decreases with increasing spin momenta S and doping concentrations x (0 x 1.2 at.%/Y) of the paramagnetic lanthanoid dopants. We estimate the exchange integral |I _ex| of the lanthanoid spin and the conduction electron spin as well as the critical concentrations x _c , relying on the Ginzburg–Landau–Abrikosov–Gor'kov (GLAG) theory of impurity pair breaking for dilute alloys (x 0).     

Synthesis and characterization of indium intercalation compounds of molybdenum sulphoselenide

The synthesis, structure and properties of indium-intercalated compounds of molybdenum sulphoselenide, In_1/3MoS_xSe_2–x (0x 2) compounds were investigated. X-ray analysis shows that all these compounds possess layer-type structure with hexagonal symmetry. Studies of room-temperature magnetic susceptibility, thermoelectric power experiments and two-probe conductivity measurements in the temperature range 25–350 °C, confirmed that these are diamagnetic semiconductors with In_1/3MoS₂, In_1/3MoS_1.5Se_0.5 and In_1/3MoS_xSe_2–x (0x1.5) exhibiting n- and p-type conductivity, respectively. These results have been explained on the basis of existing band models. Thermal stability behaviour of these compounds in an air atmosphere and X-ray analysis of the oxidized product were also studied.     

Electrical resistivity and Seebeck coefficient of La1−x M x MnO3 (M = Ca,Sr) single crystals

The temperature dependence of the electrical resistivity and Seebeck coefficient was measured on single crystals of La_1–x Ca _x MnO₃(0 <x 0.3) and La_1–x Sr _x MnO 3 (0 <x 0.4) grown by the arc-image floating zone method. The electrical conduction for La_1–x crystals withx 0.2 was of the activation type aboveT _c and of the degenerate type belowT _c, while that for the crystal withx = 0.1 was of the activation type over the whole measured temperature range between –170 and 400°C. The conduction behaviour of La_{1– x} Sr _x MnO₃ was essentially the same as that of La_1–x Ca _x MnO₃ except that the conduction of the crystals withx = 0.3 and 0.4 was of the degenerate type aboveT _c. A distinct difference in Seebeck data was observed between the calcium and the strontium compounds.