Yunhui Xu

Bio: Yunhui Xu is an academic researcher from Forschungszentrum Jülich. The author has contributed to research in topics: Materials science & Supercapacitor. The author has an hindex of 7, co-authored 8 publications receiving 154 citations.

Preparation of a-axis oriented YBa2Cu3O7 and PrBa2Cu3−xGaxO7 thin films

Abstract: For Josephson junctions, a-axis oriented YBa 2 Cu 3 O 7 films and a suitable 123-structured barrier material are desirable. We have deposited a-axis oriented films of YBa 2 Cu 3 O 7 and PrBa 2 Cu 3− x Ga x O 7 on SrTiO 3 and LaAlO 3 by dc magnetron sputtering using a two step process: Nucleation at lowered deposition temperature is followed by deposition at higher temperature which improves the electrical properties of the films. We have shown that by substituting Ga for Cu the resistivity of PrBa 2 Cu 3 O 7 at 77K can be raised by several orders of magnitude. The practical range of substitution for dc sputtering was x ≤ 0.1. The effect of substitution has also been investigated by Raman spectroscopy which shows evidence for preferred substitution at the Cu(1)-sites.

Hopping conductivity in the PrBa2Cu3−xGaxO7−y system

Abstract: The temperature dependece of resistivity is reported for the PrBa 2 Cu 3− x Ga x O 7− y system with x =0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6. We find that the substitution of Cu by Ga increases the resistivity of PrBa 2 Cu 2.4 Ga 0.6 O 7− y to reach 10 10 Ωcm at 100 K. This is a 123-structural insulator and thus an ideal barrier material for fabrication of S-I-S junctions. The results are interpreted in terms of hopping conductivity (both nearest-neighbor hopping and variable-range hopping) for doped semiconductors.

Effect of Zn doping on the superconductivity of RBa2Cu3−xZnxO7−y (R = Yb, Er, Y, Dy, Gd, Eu, SmandNd)

Abstract: Zn, having a definite valence state and filled 3d levels, is used for substituting Cu sites in RBa 2 Cu 3− x Zn x O 7⇔ y ( R = Yb , Er , Y , Dy , Gd , Eu , Sm and Nd ). Substitution of Zn for Cu in these systems causes a rapid nearly linear suppression of the superconducting transition temperature T c . All the RBa 2 Cu 3− x Zn x O 7− y samples are essentially single phase and retain the orthorhombic structure in the range of substitution (0≤ x ≤0.1). The reduction of T c by Zn doping in RBa 2 Cu 3− x Zn x O 7− y was found to be strongly dependent on the ionic radius of the rare-earth element R. The superconducting-nonsuperconducting transition accompanied by a transition into the semiconducting phase in these systems is interpreted in terms of a Mott transition.

Ion size effect on Tc in RBa2Cu3−χGaχO7−y systems (R=Er, Y, Dy, Eu and Nd)

Abstract: The effect of Ga substitution for Cu in RBa2Cu3−χGaχO7−y (R=Er, Y, Dy, Eu and Nd) has been investigated for nominal composition χ=0, 0.05, 0.1, 0.15, 0.2 and 0.3. The superconducting transition temperature Tc falls with increasing Ga concentration, and at a constant Ga concentration, Tc decreases with increasing radius of the R ions. We suggest that the decrease of density of states N(EF) or the localization of carriers due to Ga substitution are the possible mechanisms of superconductivity suppression. The superconducting-nonsuperconducting transition accompanied by a transition into the semiconducting phase in these systems is interpreted in terms of the Mott transition.

Nonmagnetic-magnetic transition of Pr ion in (R1−xPrx)Ba2Cu3O7−y systems (R Er, Y, Dy, Gd, Eu)

Abstract: Detailed measurements of the superconducting transition temperature T c in (R 1− x Pr x )Ba 2 Cu 3 O 7− y (R  Er, Y, Dy, Gd, Eu) systems in the low Pr concentration regime ( x ⩽0.1 have revealed that when the Pr-doping concentration is sufficiently low, the local state of Pr ion is short-lived compared to thermal fluctuation lifetimes. The resulting magnetic moment averages over time to zero demagnetizing the Pr ions, thus superconductivity is no longer suppressed by the pair-breaking mechanism. We observed an interesting nonmagnetic-magnetic transition of the Pr ions in the (R 1− x Pr x )Ba 2 Cu 3 O 7− y systems induced either by increasing the Pr concentration in one of these systems or by changing the R ion in the host matrix RBa 2 Cu 3 O 7− y (R  Er, Y, Dy, Gd, Eu) in a fixed low Pr concentration.

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

Abstract: The properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

Crystal chemical substitutions and doping of YBa2CU3Ox and related superconductors

Abstract: The 90 K superconductor, YBa 2 Cu 3 O δ (YBCO), has proved to be highly adaptable chemically as it can accommodate a wide variety of cationic and anionic substitutions. Indeed, the majority of the chemical elements, excluding noble gases and actinides, have been reported to substitute to some extent into the YBCO structure. This review covers the literature on such substitutions and their effect on the properties of YBCO. Reported solubility limits are given, together with crystal symmetry and trends in unit cell parameters with dopant concentration. The dopant site is considered; this is additionally complex in the case of copper substitution because of the two distinct copper sites in the crystal structure. The effect of the dopant on the critical temperature, T c , is reviewed; the literature is often contradictory due to the dual effects of variable oxygen content and the nature of the dopant. Preparation methods appear to have an effect on solubility limits, crystal symmetry and T c , Also, the methods used to determine solubility limits are often imprecise which can lead to contradictions. The magnetic properties of doped materials are reviewed; for some dopants, particularly the magnetic lanthanides, antiferromagnetism and superconductivity co-exist. The related RBa 2 Cu 3 O δ phases (R = lanthanide), their structure, properties and behaviour on doping are reviewed in a similar way. For the larger rare earths, the related systems R 1+ x Ba 2- x CU 3 O δ , are reviewed; as x increases, the transition temperature decreases and compositions R 1.5 Ba 1.5 Cu 3 O δ are semiconducting. The upper and lower solubility limit changes with R, and for R = Dy, the upper limit is x = 0 composition, LaBa 2 Cu 3 O δ , cannot be prepared in air since substitution of La onto the Ba site occurs, forming the Ba-deficient solid solutions. The discovery of superconductivity above liquid nitrogen temperatures in YBa 2 Cu 3 O 7 , has generated a vast quantity of research; to date, > 20 000 papers have appeared on the 123 materials (source: BIDS). This review, therefore, cannot be comprehensive, but attempts to highlight important substitutions, their effect on the properties of YBCO and any contradictions in the literature.

Ramp type HTS Josephson junctions with PrBaCuGaO barriers

Abstract: Ramp type Josephson junctions have been fabricated using DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as electrode material and PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// with x=0, 0.10 and 0.40 as junction barriers. Barrier thickness lie between 6-30 nm. Several junctions without barrier were made in order to find ways to minimize the damage of the ramp interface. In total about 40 chips were fabricated each containing several junctions and their I-V characteristics measured for various temperatures down to 4.2 K. Only those junctions showing clear RSJ-like curves were selected to be analyzed. In some cases we also measured I/sub c/ as a function of a small applied field and obtained a clear Fraunhofer pattern, but there is a tendency to flux trapping as evidenced by LTSEM. It was found at 4.2 K that the critical current density J/sub c/ scales with the specific resistance R/sub n/A as J/sub c/=C/sub bar/(R/sub n/A)/sup -m/ (m=1.8/spl plusmn/0.5). The barrier material dependent constant C/sub bar/ increases with x, whereas, for a given d, J/sub c/ is constant and R/sub n/A increases. >

Epitaxial quality of c-axis and a-axis oriented YBa2Cu3O7 films. Characterization by Raman spectroscopy

Abstract: We present a model that describes the epitaxial quality of YBa2Cu3O7 films by three parameters — the c-axis oriented fraction of the film, its degree of in-plane orientation and the degree of in-plane orientation of the remaining a-axis oriented fraction. These parameters can be specified by the relative phonon intensities in polarized Raman spectra. We have determined the epitaxy parameters of a-axis and c-axis oriented laser-deposited YBa2Cu3O7 films on MgO (100) substrates. The accuracy of these parameters has been tested by performing Raman measurements at various rotation angles. The ratio of the intensities of the B2g and B3g modes observed on the a-axis oriented film indicates that a-axis growth is preferred to b-axis growth.

Oxygen content and disorder in a‐axis oriented YBa2Cu3O7−δ thin films

Abstract: Using results of Raman spectroscopy, it is shown that the transition temperature Tc (R=0) of a‐axis oriented YBa2Cu3O7−δ thin films strongly depends on the disorder in the oxygen sublattice. Regardless of deposition temperature Td, the films have an oxygen deficiency δ<0.1. However, at low Td, films show Tc≊50 K and a significant disorder in the distribution of chain and/or apex oxygen atoms. Using a template growth procedure we show that, at a sufficiently high Td, the oxygen disorder vanishes and Tc≳80 K, although the crystalline disorder, measured by the width of the (200) x‐ray rocking curve, is increasing with Td.