Pr-doped Samples Research Articles

Pr-doping effect on the depinning of vortices in Bi2Sr2Ca1−xPrxCu2Oy single crystals

The irreversibility lines of Pr-doped Br2Sr2Ca1−xPrxCu2Oy single crystals have been studied systematically. It was found that for the Pr-doped samples as compared with the undoped Bi2Sr2CaCu2Oy the effective superconducting area shrinks, the irreversibility line Birr decreases, and the anisotropic parameter γ increases strongly due to Pr doping. It is evident that the substitution of Pr for Ca in the system effectively reduces the hole concentration on CuO2 layers to the underdoped state and destroys the Josephson coupling of the two superconducting regions of adjacent CuO2 layers. The vortex depinning is responsible for the irreversibility and the displacement value for vortex depinning is similar to that for vortex lattice melting, but with different physical meaning.

Growth and ultraviolet optical properties of KGd1–xREx(WO4)2 single crystals

The suitable conditions for growth of KGd(WO4)2 (KGW) and KGd1–xREx(WO4)2— RE = Nd, Er, Yb, Ho, Tm, Pr—by the top-seeded-solution-growth method, using K2W2O7 as solvent, are discussed. The relation between crystal size, mean growth rate, distribution coefficient of the substituting element, and the presence of macrodefects is analyzed. The optical absorption corresponding to the band-gap transition of KGW has been found to be temperature dependent; the absorption threshold energy changes from 34405 cm−1 at 300 K to 35330 cm−1 at 7 K. Narrow pre-edge absorption bands at about 32000 and 32600 cm−1 have been ascribed to Gd3+ intraconfigurational transitions. The photoluminescence of most RE3+ ions has been observed under ultraviolet (UV) excitation close to the absorption threshold of KGW. This suggests the contribution of charge transfer bands. In Pr-doped samples the presence of a minor concentration of Pr4+ could also contribute in this region. The irradiation with UV light does not introduce any significant coloration of our samples.

Contributions of CuO 2 planes and CuO chains on the transport properties of YBa 2Cu 4O 8

We have investigated the contributions of CuO 2 planes and CuO chains on the transport properties of YBa 2Cu 4O 8 by measuring the electrical resistivity and thermoelectric power ( S) of Y 1− x Pr x Ba 2(Cu 1− y Zn y ) 4O 8, Y 1− x Ca x Ba 2Cu 4O 8 and YBa 2(Cu 1− x Ga x ) 4O 8. Pr reduces the mobile carrier concentration in the CuO 2 planes in YBa 2Cu 4O 8, as a consequence, the superconducting transition temperature is progressively decreased with increasing Pr. The S( T) data suggest that the metallic behavior at lower temperature of the Pr and Zn doped compounds is contributed by the double CuO chains. On the other hand, the S( T) behavior of Ca doped samples is opposite to that of the Pr doped samples because the doping of Ca in Y-site introduces holes in the CuO 2 planes. However, the substitution of Ga in Cu(1)-site suppresses the carrier concentration in CuO chains and increases the normal-state resistivity revealing the important contribution from CuO chains on the metallic behavior of YBa 2Cu 4O 8 and PrBa 2Cu 4O 8.

Magnetic dimensional crossover in Pr-doped single crystal Bi 2Sr 2Ca 0.89Pr 0.11Cu 2O 8+δ

Magnetic hysteresis measurements on Pr-doped single crystal Bi 2Sr 2Ca 0.89Pr 0.11Cu 2O 8+δ show a dimensional crossover from the three-dimensional (3D) vortex glass to the two-dimensional (2D) pancake of vortices. The observed crossover field H 2D for field applied parallel to the c-axis decreases from 6 kG at 5 K to 410 G at 25 K and 120 G at 45 K. The substitution of 11% of magnetic Pr 3+ ions for Ca 2+ ions effectively decreases the hole concentration on the CuO 2 bi-layers but pushes the superconducting state from the overdoped region to the optimum composition with a superconducting transition temperature T c of 88 K. The lower crossover field H 2D values of the Pr-doped sample as compared with the undoped Bi 2Sr 2CaCu 2O 8+δ imply that the Josephson coupling between the two superconducting regions of adjacent CuO 2 bi-layers can be reduced by the presence of Pr.

X-ray absorption studies of some doped mercury cuprate superconductors with varied oxygen stoichiometry

The x-ray edges of Pr, Pb and Cu in the mixed (Hg, Pr)- and (Hg, Pb)-1212 and 1201 superconductors were studied in order to obtain information about the electronic structure of the (Hg, Pr) - and (Hg, Pb) - O planes and their dependence on O stoichiometry. It has been found that, in Pr doped samples, the average valence of Pr increases with increasing O content. Concomitantly, the density of doping holes increases in the Cu - O planes. In the case of Pb doped samples, the Pb valence was found to be unchanged. We have estimated the density of doping holes per Cu - O plane for these doped Hg superconductors. The results are discussed in view of the band structure calculations in this system.

Characterization of Praseodymium-doped InP Epilayers Grown by Liquid-Phase Epitaxy

The various degrees of incorporation of praseodymium (Pr) into InP during liquid-phase epitaxial growth from an In-rich solution were investigated by double-crystal X-ray diffraction, Hall effect, and low-temperature photoluminescence (PL) measurements to investigate the effect of Pr concentration on structural, electrical, and optical properties. The lattice mismatch varies slightly with increasing Pr concentration in the growth melts. This variation in lattice mismatch with increasing Pr concentration is observed together with the shift of the PL emission energy. Examinations of the electrical property reveal that all the Pr-doped samples exhibit n-type conduction. Depending on the extent of Pr doping in the growth melts, InP epilayers have carrier concentrations ranging from 1017 cm-3 to 1016 cm-3 and mobilities varying from 1330 to 3800 cm2/V·s. It is also revealed that the impurities are gettered by Pr ions during liquid-phase epitaxial growth but the Pr-related defects formed in high Pr-doped melts may act as scattering centers and cause carries mobilities to decrease. It is found that epitaxial growth from the melt with 0.25–0.29 wt% Pr produces of the epitaxial layers highest quality. No intra-4f-shell transition line is observed from InP layers even at very high Pr concentrations.

Thermal conductivity of Pr doped YBa2Cu3Oy and a quasiparticle contribution

The thermal conductivity κ of Pr doped YBa2Cu3Oy (Y1−xPrxBa2Cu3Oy) polycrystals has been studied as a function of temperature from 10 K up to 150 K. A pure YBCO crystal exhibits a clear enhancement in κ belowT c but for highly Pr doped samples (x>0.2) the temperature dependence of κ monotonically decreases with reducing temperature. Furthermore, the value of κ of insulating PBCO (x=1.0) is strongly suppressed compared with that of superconducting YBCO. Such behaviour is not explained by the electron-phonon interaction model proposed by Bardeen, Rickayzen and Tewordt but suggests a picture in which an observed peak belowT c in κ of the pure YBCO sample is ascribed to a quasiparticle contribution.

Photoluminescent properties of undoped and Pr-doped GaAs epilayers

The various degrees of incorporation of praseodymium (Pr) into GaAs during liquid-phase epitaxy (LPE) growth from a Ga-rich solution were investigated by low temperature photoluminescence (PL) measurements. The PL spectrum of the undoped (background) GaAs epilayer shows four different peaks which are considered to be due to an intense transition between exciton and neutral donor (DX), an intense transition between exciton and neutral acceptor (AX), a transition of electron-acceptor (eA) recombination and a weak transition from recombination of donor-acceptor (DA), respectively. Variations of the different peaks with amount of Pr doping are detailed in the text. Generally we found that the PL spectra of the low Pr-doped GaAs epilayer demonstrate an obvious gettering effect on the donor-related peaks. As to the spectra of high Pr-doped epilayers, a defect-related (dX) emission peak was produced. Thus, for the purpose of purification, low Pr-doped ( X Pr ratio less than 0.05%) is suggested. Examination of electrical properties illustrates that all the Pr-doped samples exhibit p-type conduction. Depending on the extend of Pr doping in the growth melts, GaAs epilayers have hole concentrations ranging from 10 15 to 10 16 cm −3 and with corresponding hole mobilities varying from 380 to 50 cm 2/V · s. The electrical data also reveal that the undesired donor impurities are gettered by Pr ions during LPE growth, but the Pr-related defects formed in high Pr-doped melts may act as scattering centers and cause their mobilities to decrease.

Effective activation energy in melt-textured (Y 1− xPr x) Ba 2Cu 3O 7−δ

Flux creep experiments are conducted at μ 0 H = 1 T (/ / c-axis) on a set of MTG (Y 1− x Pr x )Ba 2Cu 3O 7−δ ( x ≤ 0.08) samples within the temperature region from 4.5 to 75 K. The effective activation energy U eff( J, T) is extracted by employing the procedure of Maley et al. In comparison with pure YBa 2Cu 3O 7−δ, the effective activation energy and current densities are enhanced for all the Pr-doped samples investigated with an optimal doping effect achieved at x = 0.05 or so. The zero-temperature effective activation energy U eff( J, 0) is obtained through a constructive scaling procedure for each sample and is found to be in qualitative agreement with the theoretical prediction from the collective-pinning model.

Structure and superconductivity in La 1.15-xPr 0.85Sr xCuO 4

A phase diagram of Pr-doped La 2-xSr xCuO 4 including structural and superconducting properties is presented. We find that Pr-doping shifts the phase boundary of the high temperature tetragonal to orthorhombic phase transition to higher Sr-concentrations without inducing the low temperature structural phase transition as other RE do. As superconductivity occurs in the same Sr-concentration range as in “pure” La 2-xSr xCuO 4 (LSCO), the disappearance of superconductivity is not related to the high temperature structural transition, but is determined predominantly by the hole-concentration in the CuO-planes. The decrease of T c of ≈ 5–10 K in the Pr-doped samples with respect to the undoped ones can mainly be attributed to the increase of the octahedra tilting angle.

Flux pinning and flux creep in melt-textured (Y1-xPrx)Ba2Cu3O7- delta

Flux pinning and flux creep behaviour for the H//c axis have been investigated on a set of MTG (Y1-xPrx)Ba2Cu3O7- delta (x<or=0.08) samples. The critical current densities Jc, apparent pinning potential U0* and effective activation energy Ueff(J,T) have been derived from the corresponding magnetic hysteresis and relaxation measurements. The qualitative temperature and field dependences of the critical current densities are found to be unaffected by the Pr doping, up to the highest concentration level, in the present study. The temperature-dependent profiles of the normalized relaxation rate S show similar features, all including the initial linear and so-called plateau regions (roughly) with a peak interposed between them. Compared with the pure YBa2Cu3O7- delta , the Jc, U0*, J and Ueff(J,T) values get enhanced for all the Pr-doped samples, with an optimal effect achieved at Pr concentration around x=0.05. A scaling relation Ueff(J,T)=UiG(T)F(Ji/J) at a given magnetic field is used to analyse the Ueff(J,T) data, where Ui and Ji are the scales of activation energy and current density, respectively, and G(T) is either determined by a constructive scaling procedure or empirically chosen as G(T)=(1-(T/T1)2)3/2. The current-density-dependent behaviour of Ueff(J,T)/G(T), together with the observed S(T) feature and field-independent phenomenon of Jc(H), is qualitatively consistent with the collective-pinning theory. Possible pinning natures as well as a possible explanation for the observed Pr concentration dependence of critical current density and activation energy are discussed, especially within the context of pinning induced by the local lattice mismatch due to the Pr doping.

A 89Y NMR study of Pr- and Nd-doped YBa 2Cu 3O 7

High resolution NMR data for the shift and spin-lattice relaxation time of the 89Y resonance have been obtained for up to 70% Pr and up to 30% Nd substitution in Y 1− x R x Ba 2Cu 3O 7. The shift data show that changes in spin susceptibility are small and there is no evidence in this data for hole filling in Pr-doped samples being the main cause of the quenching of T c. The relaxation data demonstrate that the lifetime of the Pr moment must be very short (⩽10 -14 s for x<0.4).

Hole filling and hole creation in the superconducting compounds Bi2Sr2-xRxCuO6+y (R=La, Pr, Nd, and Sm).

The superconducting properties, hole content, and lattice constants of several rare-earth-doped compounds ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{R}}_{\mathit{x}}$${\mathrm{CuO}}_{6+\mathit{y}}$ have been investigated as a function of rare-earth doping and oxygen content. The maximum superconducting transition temperature, ${\mathit{T}}_{\mathit{c}}$, obtained for each of the rare-earth dopants was 23 K (La), 25 K (Pr), 21 K (Nd), and 12 K (Sm). The ${\mathit{T}}_{\mathit{c}}$'s of these compounds exhibited a parabolic dependence on the La and Pr concentration (0x1), with the maximum ${\mathit{T}}_{\mathit{c}}$ occurring for x=0.4 for samples with low oxygen content. For La- and Pr-doped samples treated in pure oxygen, the position of the maximum shifted to x=0.6 and 0.5, respectively, with a concurrent reduction in the maximum value for ${\mathit{T}}_{\mathit{c}}$ (17 K for La and 18 K for Pr). For x&gt;0.2, the samples were single phase and exhibited unusually sharp superconducting transitions (width of ac-susceptibility transition less than 2 K). The high quality of the samples allowed a precise study of the variation of the orthorhombic cell parameters with the rare-earth and oxygen content.The oxygen content of the samples was systematically varied using oxygen partial pressures that ranged from 100 ppm to 100% oxygen during the heat treatments. Absolute values of the oxygen content and the changes of the oxygen content with heat treatments were determined using thermo- gravimetric analysis. The qualitative features of the dependence of ${\mathit{T}}_{\mathit{c}}$ on x and y can be understood by assuming an excess of holes in the Cu-O planes of the undoped compound that are filled by the extra electrons contributed with rare-earth doping or increased with the addition of oxygen. For La-doped samples treated in pure oxygen there was a smooth variation of ${\mathit{T}}_{\mathit{c}}$ with hole content, with the maximum ${\mathit{T}}_{\mathit{c}}$ (18 K) occurring at a hole content of 0.2\ifmmode\pm\else\textpm\fi{}0.02. For La-doped samples treated in 0.1% oxygen, the maximum ${\mathit{T}}_{\mathit{c}}$ occurred at the same hole content (0.2), but the value of ${\mathit{T}}_{\mathit{c}}$ was significantly higher (23 K). These data illustrate that, even within one alloy system, composition can affect ${\mathit{T}}_{\mathit{c}}$ as well as hole content. Finally, lattice-constant and oxygen-content data from the Pr-doped materials suggest that Pr substituted onto the Sr site has a formal valence of approximately 3.3.

High energy crystal field excitations in Pr x Y1−x Ba2Cu3O7−δ

We have measured the high energy crystalline electric field transitions in samples of Pr x Y1−x Ba2Cu3O7−δ withx ≈ 0·5 andx ≈ 1. The fully Pr-doped samples exhibits three strong transitions with excitation energies of about 65, 85 and 105 meV, plus three more weaker ones at 113, 123 and 132meV. Magnetic intensity is observed in the same energy range in thex=0.5 sample but is largely structureless in character, with only one clear peak at about 109meV. The large number of transitions would seem to indicate a Pr valence of 3+, rather than 4+, but the intrinsically broad character of the excitations is evidence for a dynamically mixed valence state.