Logarithmic temperature dependence of the resistivity of Nd2−xCexCuO4−y
15 Apr 1990-Physica C-superconductivity and Its Applications (North-Holland)-Vol. 167, Iss: 1, pp 53-58
TL;DR: In this paper, single phase compounds of Nd 2− x Ce x CuO 4− y (0.14⩽ x ⩽0.18) were made by the standard solid state reaction.
Abstract: Single phase compounds of Nd 2− x Ce x CuO 4− y (0.14⩽ x ⩽0.18) were made by the standard solid state reaction. The as-prepared nonsuperconducting samples were further annealed in various reducing atmospheres to induce superconductivity. All of the samples, except that with a concentration x =0.18, exhibited superconductivity after an annealing treatment at 850°C for 6 h in a vacuum of 10 -5 Torr. Magnetisation measurments confirmed the Meissner fraction to be around 20% for these compounds. Resistivity measurements were performed on samples which were annealed under different conditions. The resistivity showed a ln T dependence, which was prominent in those samples in which superconductivity was not observed. In the samples showing zero resistivity, the ln T term is absent or present with a much smaller coefficient than in the nonsuperconducting samples.
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TL;DR: In this paper, a review of recent theoretical advances in the study of granular metals is presented, emphasizing the interplay of disorder, quantum effects, fluctuations, and effects of confinement.
Abstract: Granular metals are arrays of metallic particles of a size ranging usually from a few to hundreds of nanometers embedded into an insulating matrix. Metallic granules are often viewed as artificial atoms. Accordingly, granular arrays can be treated as artificial solids with programmable electronic properties. The ease of adjusting electronic properties of granular metals assures them an important role for nanotechnological applications and makes them most suitable for fundamental studies of disordered solids. This review discusses recent theoretical advances in the study of granular metals, emphasizing the interplay of disorder, quantum effects, fluctuations, and effects of confinement. These key elements are quantified by the tunneling conductance between granules $g$, the charging energy of a single granule ${E}_{c}$, the mean level spacing within a granule $\ensuremath{\delta}$, and the mean electronic lifetime within the granule $\ensuremath{\hbar}∕g\ensuremath{\delta}$. By tuning the coupling between granules the system can be made either a good metal for $gg{g}_{c}=(1∕2\ensuremath{\pi}d)\mathrm{ln}({E}_{c}∕\ensuremath{\delta})$ ($d$ is the system dimensionality), or an insulator for $gl{g}_{c}$. The metallic phase in its turn is governed by the characteristic energy $\ensuremath{\Gamma}=g\ensuremath{\delta}$: at high temperatures $Tg\ensuremath{\Gamma}$ the resistivity exhibits universal logarithmic temperature behavior specific to granular materials, while at $Tl\ensuremath{\Gamma}$ the transport properties are those generic for all disordered metals. In the insulator phase the transport exhibits a variety of activation behaviors including the long-puzzling $\ensuremath{\sigma}\ensuremath{\sim}\mathrm{exp}[\ensuremath{-}({T}_{0}∕T{)}^{1∕2}]$ hopping conductivity. Superconductivity adds to the richness of the observed phases via one more energy parameter $\ensuremath{\Delta}$. Using a wide range of recently developed theoretical approaches, it is possible to obtain a detailed understanding of the electronic transport and thermodynamic properties of granular materials, as is required for their applications.
544 citations
TL;DR: In this article, measurements of the in-plane and c-axis resistivity in a superconducting Nd 1.85 Ce 0.15 CuO 4−δ single crystal are reported.
Abstract: Measurements of the in-plane and c -axis normal-state resistivity in a superconducting Nd 1.85 Ce 0.15 CuO 4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, ϱ ab ( T ) and ϱ c ( T ) displays a metallic-like positive temperature coefficient of resistivity with a basic T 2 dependence. We discuss some possible origins of this peculiar temperature dependence.
30 citations
TL;DR: In this paper, the electron-doped Nd 2-x Ce x CuO y superconducting system in the range 0.120⩽ x ⩽0.190 with Δx = 0.005 was investigated.
Abstract: We report a refined phase diagram for the electron-doped Nd 2- x Ce x CuO y superconducting system in the range 0.120⩽ x ⩽0.190 with Δx =0.005. It is found that the highest T c ( ϱ =0) ∼17 K is confined to the very narrow range 0.145⩽ x ⩽0.150; a T conset ∼ 24 K with sharp superconducting transition is also observed. An attempt is made to understand the T c dependence on the variation of the oxygen content ( y ) in the Nd 2- x Ce x CuO y system.
27 citations
TL;DR: In this paper, the superconducting state is completely destroyed by the magnetic field when the field is applied along the c-axis and the normal electrical resistivity is obtained at all temperatures.
Abstract: Electron high- T c superconductor Nd 2- x Ce x CuO y single crystals are studied under high magnetic field of up to 20 tesla. The superconducting state is completely destroyed by the field when the field is applied along the c -axis and the normal electrical resistivity is obtained at all temperatures. The temperature dependence of the normal resistivity is plotted down to 1.3 K and a clear resistance minimum is found. The result is understood in terms of the two-dimensional weak localization model and this means that the materials are regarded as the intrinsic two-dimensional conduction system.
21 citations
TL;DR: In this article, the low temperature electronic transport of highly boron-doped nanocrystalline diamond films is studied down to 300mK, where the films show superconducting properties with critical temperatures Tc up to 2.1mK.
Abstract: The low temperature electronic transport of highly boron-doped nanocrystalline diamond films is studied down to 300 mK. The films show superconducting properties with critical temperatures Tc up to 2.1 K. The metal–insulator and superconducting transitions are driven by the dopant concentration and greatly influenced by the granularity in this system, as compared to highly boron-doped single crystal diamond. The critical boron concentration for the metal–insulator transition lies in the range from 2.3 × 1020 up to 2.9 × 1020 cm−3, as determined from transport measurements at low temperatures. Insulating nanocrystalline samples follow an Efros–Shklovskii (ES) type of temperature dependence for the conductivity up to room temperature, in contrast to Mott variable range hopping (VRH) in the case of insulating single crystal diamond close to the metal–insulator transition. The electronic transport in the metallic samples not only depends on the properties of the grains (highly boron-doped single crystal diamond), but also on the intergranular coupling between the grains. The Josephson coupling between the grains plays an important role for the superconductivity in this system, leading to a superconducting transition with global phase coherence at sufficiently low temperatures. Metallic nanocrystalline samples show similarities to highly boron-doped single crystal diamond. However, metallic samples close to the metal–insulator transition show a richer behavior. In particular, a peak was observed in the low-temperature magnetoresistance measurements for samples close to the transition, which can be explained by corrections to the conductance arising from superconducting fluctuations.
16 citations
References
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TL;DR: In this paper, a family of superconducting copper oxides in which the carriers are electrons was discovered, with the formula Ln2−xCexCuO4−y, where Ln stands for lanthanides Pr, Nd or Sm.
Abstract: Since the discovery of high-temperature superconductivity in cop-per oxide compounds1, there has been much effort to understand what is required for a compound to have a high transition tem-perature (Tc). Up to now, all of the families of high-Tc copper oxide superconductors have contained two-dimensional sheets of Cu–O pyramids or octahedra, and the carriers of the superconduct-ing current have been electron vacancies, or 'holes'. By contrast, we report here the discovery of a family of superconducting copper oxides in which the carriers are electrons. The new superconductors are Ce4+-doped compounds, with the formula Ln2–xCexCuO4–y, where Ln stands for the lanthanides Pr, Nd or Sm. The compounds have the Nd2CuO4 (T′-phase) structure2, which is composed of sheets of Cu–O squares (see Fig. la). This structure has no apical oxygen atoms, in contrast to the T-phase structure with Cu–O octahedra (Fig. 1b), as observed in La2–xSrxCuO4 (refs 1, 2), and the T*-phase structure with Cu–O pyramids (Fig. 1c), as in Nd2–x–z Cex Srz CuO4 (ref. 3 and Y. T. et al., manuscript in preparation).
1,402 citations
TL;DR: It is discovered that the Ce4+doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the Nd2CuO4-type structure with sheets of Cu-O squares, in contrast to the previously reported high-Tccuprates.
Abstract: We have discovered that the Ce4+doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the Nd2CuO4-type structure with sheets of Cu-O squares. In contrast to the previously reported high-Tccuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence ofTcon the concentration of doped electrons is shown for these electron-doped superconducting cuprates.
682 citations
TL;DR: In this paper, the growth of single crystals of Nd2-xCexCuO4-y, and measurements of their anisotropic superconducting properties are reported.
Abstract: ELECTRON-DOPED copper oxide superconductors have recently been discovered in the system Ln2–xCexCuO4–y, where Ln is Pr, Nd or Sin1,2. Here we report the growth of single crystals of Nd2–xCexCuO4–y, and measurements of their anisotropic superconducting properties. A sharp superconducting transition is observed at 22.5 K in Nd1.84Ce0.16CuO4–y. The resistivity shows a metallic temperature dependence both parallel and perpendicular to the basal plane. Applying a magnetic field perpendicular to the basal plane causes a parallel shift of the resistive transition curve to lower temperatures, as in conventional type II superconductors but unlike the hole-doped copper oxide superconductors. The Ginzburg–Landau coherence lengths estimated from the resistively defined upper critical magnetic field are 70.2 A in the basal plane and 3.4 A along the c axis, showing an anisotropy factor of 21. This value is much larger than that in La2–xSrxCuO4–y despite their apparent structural similarities. Crystallographic differences from the hole-doped systems and the concomitant changes in electronic properties may offer clues to the role of the Cu–O planes in the microscopic pairing mechanism of high-temperature superconductors.
203 citations