Pb-doped Bi Research Articles

Revised phase diagram of the high- Tc cuprate superconductor Pb-doped Bi2Sr2CaCu2O8+δ revealed by anisotropic transport measurements

Although phase diagrams can be leveraged to investigate high transition temperature (high-${T}_{c}$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-${T}_{c}$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the ``weak'' pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the ``strong'' pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-${T}_{c}$ superconductivity.

Tunneling STM/STS and break-junction spectroscopy of the Pb-doped Bi2223 superconductor

The combined scanning tunneling microscopy/spectroscopy (STM/STS) and the break-junction tunneling spectroscopy (BJTS) measurements of the three-layered Pb x Bi2-x Sr2Ca2Cu3O10+y cuprate superconductors were carried out. The averaged dI/dV spectrum obtained by the STS shows the gap Δ STS ≈ 45 meV, while that of BJTS shows Δ BJ ≈ 35 meV. In case of the BJTS measurements, we also referred the zero bias (ZB) peak as being due to the maximum Josephson current IJc . The product of IJc and normal resistance RN (IJcRN ) was obtained IJcRN ≈ 3.5 mV at T = 11 K, which is one order lower than that of the BCS-based Ambegaokar-Baratoff theory with Δ BJ =35 meV taken into account. With increasing the temperature, the IJcRN was reduced and vanished just below the Tc (≈ 106 K), indicating the bulk superconductivity. Simultaneously, from the temperature dependence of the gap features from 80 K to 120 K, the gap Δ was shown to persist across the Tc , indicating that the IJcRN product is a significant parameter for understanding the cuprate superconductivity.

Nano-scale periodic structures and gap distribution in Pb-doped Bi2223 cuprate superconductors observed by STM/STS

The first systematic study of the nano-scale periodic local density of states (LDOS) and the inhomogeneous gap distributions for the Pb doped Bi2Sr2Ca2Cu3O10 + y (PbBi2223) microcrystals was carried out using the low-temperature scanning tunneling microscopy/spectroscopy. The spatially inhomogeneous gap distributions with patchy structures of several nm size reveal the average gap magnitude Δave ≃ 46 meV with the standard deviation σ = 8 meV. Those features are similar to properties of the widely investigated two-layer Bi-based cuprates. The energy dispersive types of the LDOS modulations are identified as being due to the quasiparticle interference. The relevant scattering vectors seem to testify that the PbBi2223 superconductors are ones, which was shown earlier to be most probable for Bi2212. Three types of energy non-dispersive modulations manifest themselves. Among them, the modulation in the diagonal direction with the period of ∼2.1a0 [ (2π/a0)] seems to be observed for the first time.

Refinement of the Modulated Structures of Pb-Free and Pb-Doped Bi-2223 HTSC

Refinement of the Modulated Structures of Pb-Free and Pb-Doped Bi-2223 HTSC

Textured Pb-Doped Bi-2212 Superconductors for Current Limiters

Bi2−x Pb x Sr2CaCu2O y (x = 0.0, 0.3, 0.4, and 0.5) textured materials have been prepared by the laser floating zone technique. The E − I curves showed and increase of the slope of the normal to superconducting transition with Pb addition. The best values have been obtained for the 0.4 Pb-doped samples, around 16 at 77 K. In spite of this improvement, Pb addition decreased J C and T C in all cases. On the other hand, electrical characterization at lower temperatures has led to an impressive raise in the 0.4 Pb-doped samples, reaching n values around 31, and J C of about 3,000 A/cm 2, at 65 K. At the same time, the mechanical integrity of the vitreous precursor materials has been determined. All the obtained results clearly indicate that these precursors, when properly processed by the laser floating technique, are promising candidates for practical applications as current limiters.

Thermoelectric properties of p-type Pb-doped Bi 85Sb 15− xPb x alloys at cryogenic temperatures

In this study we have investigated the effect of the Pb on the thermoelectric propertied of Bi–Sb alloy with different Pb-content. The Pb-doped Bi 85Sb 15− x Pb x ( x = 0, 0.5, 1, 2, 3) alloys were synthesized by mechanical alloying followed by pressureless sintering. The crystal structure was characterized by X-ray diffraction. The Seebeck coefficients, electrical conductivities, and thermal conductivities were measured in the temperature range of 77–300 K. The results show that all the Pb-doped alloys are p-type thermoelectric materials in the whole measurement temperature range. A minimum thermal conductivity of 1.7 W/mK was obtained for Bi 85Sb 12Pb 3 sample at 150 K. A maximum ZT value of 0.11, which is higher than those previous reported, was obtained for Bi 85Sb 14Pb 1 at 210 K.

Vortex glass scaling in Pb-doped Bi-2223 single crystal

We report on study of the vortex liquid in Pb-doped Bi-2223 single crystal using the in-plane resistivity measurements as a function of temperature and magnetic field up to 6T applied perpendicular to CuO planes. Below Tc at the upper part of superconducting transition we found Arrhenius-like resistivity behavior. With further temperature decrease close to resistivity onset resistivity shows power law dependence on temperature signaling approaching vortex-glass transition. The critical exponents nu(z-1)=4.6 plus-minus 0.5 are found to be field independent within experimental errors. We also present magnetic phase diagram defining region of nonzero critical current for Pb-doped Bi-2223 single crystal.

Magnetic shielding properties of high-Tc superconducting hollow cylinders: model combining experimental data for axial andtransverse magnetic field configurations

Magnetic shielding efficiency was measured on high-Tc superconducting hollow cylinders subjected to either an axial ora transverse magnetic field in a large range of field sweep rates,dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce themain features of the field penetration curves by using a minimum number of freeparameters suitable for both magnetic field orientations. The field penetrationmeasurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applyinglinearly increasing magnetic fields with a constant sweep rate ranging between10 µT s−1 and10 mT s−1 for bothdirections of the applied magnetic field. The experimental curves of the internal field versus the appliedfield, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs,Blim, is lower forthe transverse configuration than for the axial configuration. A power law dependence with large exponent,n′, is foundbetween Blim and dBapp/dt. Thevalues of n′ are nearly the same for both configurations. We show that the main features of the curvesBin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving aE(J) power lawwith an n-exponent and a field-dependent critical current density,Jc(B), (following theKim model: Jc = Jc0(1+B/B1)−1). In particular, a linear relationship between the measuredn′-exponents andthe n-exponentof the E(J) power law is suggested by taking into account the field dependence of the critical currentdensity. Differences between the axial and the transverse shielding properties can be simplyattributed to demagnetizing fields.

Static magnetic order and anisotropy of the layered cobalt dioxides and

Abstract The magnetism of a Pb-doped Bi 2 Sr 2 Co 2 O y (BSCO) crystal has been investigated by positive muon-spin rotation and relaxation ( μ + SR ) spectroscopy. Weak transverse field (wTF)- μ + SR measurements show that the whole sample enters into a magnetic state below ∼ 4.5 K . Combining the results of zero-field (ZF)- μ + SR experiment with susceptibility measurements, it is clarified that the sample is in a ferromagnetic ordered phase with a Curie temperature ( T C ) of 4.7 K and with the ordered internal magnetic field parallel to the c-axis within ∼ ± 15 ∘ . On the other hand, a pure BSCO crystal is also found to exhibit a bulk magnetic transition at 1.0 K by μ + SR . Since the relationship between the reduced T and reduced internal magnetic field for BSCO is almost equivalent to that for Pb-doped BSCO, the origin of the magnetic transition for both crystals is thought to be explained by common physics.

Critical current density and flux pinning in aBi1.7Pb0.4Sr2−xLaxCa1.1Cu2.1Oy system

The critical current density of Bi2Sr2CaCu2O8+δ (Bi-2212) superconductors can be enhanced either by changing the charge carrierconcentration (holes) to the optimum value, from the overdoped state, or by creatingflux pinning centres to inhibit the flux flow in magnetic fields. In this work wesubstitute the rare earth La at the Sr site of Pb-doped Bi-2212 [(Bi, Pb)-2212].La3+ ionsat the Sr2+ site decrease the hole concentration to the optimum value and enhance the superconductingproperties. Also La at the Sr site creates point defects which act as flux pinningcentres. These dual effects of La at the Sr site highly enhance the transportJC in self and applied magnetic fields.

Growth, structure and physical properties of single crystals of pure and Pb-doped Bi-based high Tc superconductors

Abstract Single crystals of (Bi 1− x Pb x ) 2 Sr 2 Ca 2 Cu 3 O 10+ δ ( x = 0 and 0.16) (sizes up to 3 × 2 × 0.1 mm 3 ) have been grown by means of a newly developed “vapour-assisted travelling solvent floating zone” technique (VA-TSFZ). Post-annealing under high pressure of O 2 (up to 10 MPa at T = 500 °C) was applied to enhance T c (up to 111 K) and improve the homogeneity of the crystals (Δ T c ⩽ 1 K). The structure of both Pb-free and Pb-doped Bi-2223 was refined for the first time from single crystal X-ray diffraction (XRD) data. The unit cell of the average structure is pseudo-tetragonal with a = 5.4210(7), b = 5.4133(6) and c = 37.010(7) A, and a = 5.395(1), b = 5.413(1) and c = 37.042(11) A, for the Pb-free and the Pb-doped phase, respectively. An incommensurate modulation in the direction of one of the short cell vectors has been defined ( q ∼ 0.21 a ∗ ), however, the structure can be conveniently described in a supercell with a fivefold volume ( a = 27.105(4) A). With respect to the “non-modulated” structure, one additional oxygen atom for ten initial O was found to be inserted into the BiO layers. The superconducting anisotropy of Bi-2223 was found to be ∼50, from measurements of the lower critical field. The anisotropy of Bi-2223 is significantly reduced compared to that of Bi-2212, and this accounts for the enhanced irreversibility fields in Bi-2223. Furthermore, Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, which are both signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.

Correlation between modulation structure and electronic inhomogeneity on Pb-doped Bi-2212 single crystals

The correlation between nanometer-size electronic states and surface structure is investigated by scanning tunneling microscopy/spectroscopy (STM/S) on Pb-doped Bi 2− x Pb x Sr 2CaCu2O 8+ y (Pb–Bi-2212) single crystals. The advantage of the Pb–Bi-2212 samples is that the modulation structure can be totally or locally suppressed depending on the Pb contents and annealing conditions. The superconducting gap ( Δ) distribution on modulated Pb–Bi-2212 samples showed the lack of correlation with modulation structure except a slight reduction of superconducting island size for the b-axis direction. On the other hand, the optimal doped Pb–Bi-2212 ( x = 0.6) samples obtained by reduced-annealing showed totally non-modulated structure in topography, however, the spatial distribution of Δ still showed inhomogeneity of which features were quite similar to those of modulated samples. These results suggest that the modulation structure is not the dominant origin of inhomogeneity although it modifies the streaky Δ structure sub-dominantly. From the gap structure variation around the border of narrow gap and broad gap regions, a trend of the coexistence of two separated phases i.e., superconducting phase and pseudogap like phase, is detected.

Growth and Superconducting Properties of Pb-Free and Pb-Doped Bi-2223 Crystals

High quality single crystals of both Pb-free and Pb-doped Bi-2223 (with sizes up to 3/spl times/2/spl times/0.1mm/sup 3/) were grown by means of a newly developed "Vapor-Assisted Travelling Solvent Floating Zone" technique in an image furnace. This technique allowed us to grow large Pb-doped Bi-2223 crystals for the first time. Post-annealing in high pressure oxygen (up to 10 MPa) enhanced T/sub c/ (up to 111 K) and sharpened the superconducting transition. The crystal structures of both Bi-2223 and Bi,Pb-2223 were refined and are reported in this paper. The unit cell of the Pb-free average structure is pseudo-tetragonal with a=5.4210(7), b=5.4133(6) and c=37.009(7) /spl Aring/. An incommensurate structural modulation along the direction of one of the short cell vectors has been defined (q/spl sim/0.21 a/sup */); however, the structure can be conveniently described in a supercell with a 5-fold volume (a=27.105(4) /spl Aring/). One additional O atom per translation unit of the modulation wavelength was found to be inserted into the BiO layers. Approximately 8% of the Ca sites are occupied by Bi atoms, and the refined composition is found to be Bi:Sr:Ca:Cu:O=2.16:2:1.84:3:10.16. Similar features are found in the Pb-doped phase. The magnetization of Pb-free Bi-2223 crystals has been studied. The vortex phase diagram has been traced and the anisotropy of Bi-2223 is found to be /spl gamma/=53 from measurements of the lower critical field, which is much smaller than that of Bi-2212 (/spl gamma/=165). Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, both of which are signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.

STM/STS observations of Pb-doped Bi-2212 single crystals

The surface states of Bi 2− x Pb x Sr 2CaCu 2O 8+ y (Pb–Bi-2212, x=0.6) single crystals are investigated by scanning tunneling microscopy/spectroscopy in order to clarify the origin of inhomogeneous superconducting states. The topography obtained at a cleaved surface of Pb–Bi-2212 showed the coexistence of two (modulated and non-modulated) phases with domain structure of several tens of nanometers. The spatial distribution of gap amplitude observed on a modulated region showed the lack of the correlation between the gap distribution and the modulation structure. On the other hand, the weak correlation detected between the gap amplitude and bright spots on the topography obtained at 2 V bias implies the non-negligible influence of the local atomic distortion on the superconductivity.

The interpretation of improved flux pinning behaviour and second magnetization peaks observed in overdopedCu-rich Bi2Sr2CaCu2O8+x single crystals

The irreversibility line and second magnetization peak of heavily overdoped Cu-richBi2Sr2CaCu2O8+x (Bi2212) single crystals are studied by accurate magnetization measurements with a field. Compared with optimally-doped and Pb-doped Bi2212 single crystals, the present systemexhibits an enhanced pinning behaviour and higher dimensional crossover field. Thetemperature dependences of the critical current densities at the first and thesecond peaks exhibit different behaviour before and after the dimensional crossover.Above the crossover field, the irreversibility line obeys an exponential temperaturedependence. Our results suggest that the second peak is caused by the switch of the fluxpinning from three-dimensional flux line pinning to two-dimensional (2D) collectivepinning, and the irreversibility line is the depinning line of the 2D vortex lattice.

Synthesis of Pb-doped Bi-2223 from Pb-doped Bi-2212 via partial melting

Nearly 100% Pb-doped Bi-2223 was synthesized from Pb-doped Bi-2212, Ca 2CuO 3 and CuO. The partially melted materials show a lamellar structure whose extent correlates with the amount of Bi-2223 phase. The fluid-like movement is attributed to the breaking of the BiO bonds and the rapid formation of the Bi-2223 phase attributed to the activation of the structure that enhances intercalation.

Magnetic hysteresis and relaxation in Bi2212 single crystals doped with Fe and Pb

Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviors for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Unlike pure and Fe doped Bi2212 crystals, heavily Pb doped crystal showed strong vortex pinning behavior. We interpret the strong pinning in heavily Pb doped Bi2212 single crystals as arising from the improved Josephson coupling in Bi2212 single crystal after heavy Pb doping. In heavily Pb doped single Bi2212 crystals, H/sub dis/(T) was observed to decrease with increasing T. Here, H/sub dis/(T) is an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. However, in pure and Fe doped Bi2212 single crystals, H/sub dis/(T) was observed to be temperature independent. We also report a significant shift of T/sub CR/, a crossover temperature separating two pinning regimes, toward higher temperatures with heavy Pb doping of Bi2212 single crystals. On the other hand T/sub CR/ did not shift with Fe doping of Bi2212 single crystals. It is argued that the temperature dependence of H/sub dis/(T) and the shift of T/sub CR/ in heavily Pb doped Bi2212 crystals was related to the enhanced c-axis conductivity caused by the Pb situated between the CuO/sub 2/ layers and imposing a 3D characteristic on the vortex lattice.

Characteristic properties of doped Bi2212 single crystals studied by magneto optical measurement

Local J/sub c/ distribution under H /spl par/ c was carefully investigated for lightly Pb-doped Bi2212 single crystals having uniform microstructure in order to clarify essential effect of Pb-doping on intra-grain J/sub c/ properties. An abrupt change of internal field gradient was observed at a constant field, H/sup */, for each Bi(Pb)2212 single crystal below 25 K. J/sub c/ is apparently higher above H/sup */. The H/sup */ was observed at several hundred Oe which was much lower than the second peak field, H/sub pk/, in the magnetization hysteresis loops. This suggested that dimensionality of vortex changed at H/sup */ from 3D to quasi-2D state with an increase of internal magnetic field, possibly due to an inhomogeneous distribution of lead ions in the crystal. Higher J/sub c/ above H/sup */ and temperature dependent H/sub pk/ indicated generation of field-induced pinning sites in the present system. Doping effects of cobalt for heavily Pb-doped Bi2212 single crystals on the flux pinning properties were also studied. In-plane J/sub c/ was found to become isotropic with increasing the Co-doping level.

Local magnetic properties of high-Tc superconductors probed by scanning SQUID microscopy

We have performed scanning SQUID (superconducting quantum interference device) microscopy (SSM) on the ac-surfaces of heavily Pb-doped Bi2212, Bi1.6Pb0.6Sr1.8CaCu2Oy, single crystals at low temperatures. The observed images demonstrated interlayer Josephson vortices below Tc. The value of c-axis penetration depth, λc, evaluated from the SSM images was 11.2±0.7 μm, which is approximately one order of magnitude smaller than that of pure Bi2212 in the optimal doping regime. This clearly indicates that Pb doping substantially reduces the anisotropy of Bi2212.

Anomalous magnetization in a heavily Pb-doped Bi 2Sr 2CaCu 2O 8+ δ as-grown single crystal

Anomalous magnetization is investigated on a heavily Pb-doped Bi 2Sr 2CaCu 2O 8+ δ as-grown single crystal [(Bi,Pb)-2212] with T c=92 K. Both the anomalous magnetization peak H 2p and the onset of the anomalous magnetization peak H on are detected in temperature region from 0.3 T c to 0.9 T c based on the descending field branch of magnetic hysteresis loops. At low temperatures below 20 K, the magnetic hysteresis loop behaves as an obvious asymmetry. The double magnetic hysteresis behavior is observed at high temperatures close to T c. The temperature dependence of H 2p can be described according to the thermal disorder induced decoupling theory. The double magnetization behavior may be ascribed to the existence of intra-grain granularity due to the inhomogeneity of oxygen distribution induced by heavily Pb-doping.