Weiyan Guan

Bio: Weiyan Guan is an academic researcher from National Tsing Hua University. The author has contributed to research in topics: Ionic radius & Electrical resistivity and conductivity. The author has an hindex of 7, co-authored 11 publications receiving 155 citations.

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.

Ion-size effect on tn in (R1-XPrX)Ba2Cu3O7-Y systems (R=Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm, and Nd)

Abstract: We conducted a detailed study of the structure and magnetic properties of ([ital R][sub 1[minus][ital x]]Pr[sub [ital x]])Ba[sub 2]Cu[sub 3]O[sub 7] sintered samples, where [ital R]=Lu, Yb, Tm, Er, Y, Ho, Dy, Gd, Eu, Sm, and Nd for [ital x]=0.5--1.0. We found that the temperature dependence of the dc susceptibility follows the Curie-Weiss law in the temperature range 20--300 K and the paramagnetism of the Pr and [ital R] sublattices exist independently of one another. The antiferromagnetic ordering temperature [ital T][sub [ital N]] of Pr ions decreases monotonically with increasing [ital R] concentration (1[minus][ital x]). At a given [ital x], [ital T][sub [ital N]] is [ital R]-ion-size dependent. The slope in the [ital T][sub [ital N]] vs [ital x] curve is steeper for ions with smaller ionic radii. The observed results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO[sub 2] planes.

Ion-size effect on Tm and Tc in (R1-xPrx)Ba2Cu3O7 systems (R = Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, Nd and Y)

Abstract: The magnetic ordering temperatures T m of Pr ions in (R 1−x Pr x )Ba 2 Cu 3 O 7 systems (R = Yb, Tm, Er, Ho, Dy, Gd, Eu, Sm, Nd and Y) with x = 0.5 – 1.0 were measured. We observe that T m decreases monotonically with increasing R concentration. At constant x, T m is R ion-size dependent. The slope in the T m vs. x is steeper for ion with smaller ionic radius. In comparison with the ion-size effect on the superconducting transition temperatures T c in these systems, the observed results can be qualitatively interpreted in terms of the hybridization between the local states of Pr ion and the conduction band states of the CuO 2 planes.

Hole localization in Pr-doped RBa2Cu3O7−y systems

Abstract: The hole concentration per unit cell (p) was measured by using an iodometric titration technique for bulk Gd1xPrxBa2Cu3O7−y (x = 0.1−0.9) and R0.8Pr0.2Ba2Cu3O7−y (R = Yb, Er, Dy, Gd and Nd) samples. These data were compared with the Hall number per unit cell (nH) at 100 K and superconducting transition temperature (Tc) reported in the literature. We suggest that the hybridization between 4f states of the Pr ion and the conduction band states in CuO2 planes, leading to the hole localization are the mechanism for the suppression of superconductivity in these systems. The correlation between Tc and p cannot be described by the empirical relation observed in many high-Tc oxides

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.

Electronic transport properties of conducting polymers and carbon nanotubes

Abstract: We review and compare electronic transport in different types of conducting polymer: conjugated organic polymers, the inorganic polymer polysulphur nitride, alkali-metal fulleride polymers, and carbon nanotubes. In each case, the transport properties show some unusual features compared to conventional metals. In conjugated organic conducting polymers, electronic transport shows a systematic pattern involving both metallic and non-metallic character. We discuss the physical conduction processes that can account for this behaviour. Key roles are played by the metal-semiconductor transition as the doping level is varied, and by the limited size of crystalline regions in the polymers, which gives rise to heterogeneous conduction. Transport data provide indirect evidence that the intrinsic conductivity of doped crystalline polyacetylene, in the absence of disordered regions, is higher than that of copper at room temperature; this high conductivity is consistent with the expected suppression of backscattering in highly anisotropic ('quasi-one-dimensional') metallic conduction. Bundles of single-wall carbon nanotubes have also been found to exhibit metallic behaviour. The temperature dependence of the conductivity of bulk samples is remarkably similar to the pattern characteristic of organic conducting polymers, typically showing a crossover from metallic to non-metallic behaviour as temperature decreases. Quantized one-dimensional conductance and other quantum effects are seen in individual nanotubes.

High-Mobility Bismuth-based Transparent p-Type Oxide from High-Throughput Material Screening

Abstract: Transparent oxides are essential building blocks to many technologies, ranging from components in transparent electronics, transparent conductors, to absorbers and protection layers in photovoltaics and photoelectrochemical devices. However, thus far, it has been difficult to develop p-type oxides with wide band gap and high hole mobility; current state-of-art transparent p-type oxides have hole mobility in the range of < 10 cm$^2$/Vs, much lower than their n-type counterparts. Using high-throughput computational screening to guide the discovery of novel oxides with wide band gap and high hole mobility, we report the computational identification and the experimental verification of a bismuth-based double-perovskite oxide that meets these requirements. Our identified candidate, Ba$_2$BiTaO$_6$, has an optical band gap larger than 4 eV and a Hall hole mobility above 30 cm$^2$/Vs. We rationalize this finding with molecular orbital intuitions; Bi$^{3+}$ with filled s-orbitals strongly overlap with the oxygen p, increasing the extent of the metal-oxygen covalency and effectively reducing the valence effective mass, while Ta$^{5+}$ forms a conduction band with low electronegativity, leading to a high band gap beyond the visible range. Our concerted theory-experiment effort points to the growing utility of a data-driven materials discovery and the combination of both informatics and chemical intuitions as a way to discover future technological materials.

The question of Pr in HTSC

Abstract: Among all the rare earth elements, Pr is known to behave as a singularity in the high-Tc families. When high-temperature cuprates are doped with Pr the superconductivity is quenched, and a metal insulator transition occurs. The origin of this behavior, being of fundamental interest, may also shed light on the mechanism of high-temperature superconductivity, since any reasonable theory for this mechanism should explain the behavior of the Pr-based system. The physical properties and the several theories which have been proposed to describe the anomalous behavior of Pr-doped systems in the normal and superconducting states are 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.

Bismuthates: BaBiO3 and related superconducting phases

Abstract: BaBiO3 has the perovskite structure, but tilting of the BiO6 octahedra destroy the ideal cubic symmetry except at temperatures above 820 K. BaBiO3 is a diamagnetic semiconductor due to a charge density wave (CDW), which is equivalent to a Ba2Bi3+Bi5+O6 representation. Recent calculations and experimental results confirm that there is no significant deviation from the oxidation states of 3+ and 5+. Superconductivity with a Tc as high as 13 K occurs for BaPb1−xBixO3 phases where the 6s band is about 25% filled, and superconductivity with a Tc as high as 34 K occurs for Ba1−xKxBiO3 phases where the 6s band is about 35% filled. Structures in these two solid solutions can have cubic, tetragonal, orthorhombic, or monoclinic symmetry. However, superconductivity has only been observed when the symmetry is tetragonal.