Bi-based Cuprate Superconductors Research Articles

Epitaxial Bi2Sr2CuOu y thin films as p-type transparent conductors

Development of p-type transparent conducting thin films is tireless due to the trade-off issue between optical transparency and conductivity. The rarely concerned low normal state resistance makes Bi-based superconducting cuprates the potential hole-type transparent conductors, which have been realized in Bi2Sr2CaCu2O y thin films. In this study, epitaxial superconducting Bi2Sr2CuO y and Bi2Sr1.8Nd0.2CuO y thin films with superior normal state conductivity are proposed as p-type transparent conductors. It is found that the Bi2Sr1.8Nd0.2CuO y thin film with thickness 15 nm shows an average visible transmittance of 65% and room-temperature sheet resistance of 650 Ω/sq. The results further demonstrate that Bi-based cuprate superconductors can be regarded as potential p-type transparent conductors for future optoelectronic applications.

Ab initio description of the Bi2Sr2CaCu2O8+δ electronic structure

Bi-based cuprate superconductors are important materials for both fundamental research and applications. As in other cuprates, the superconducting phase in the Bi compounds lies close to an antiferromagnetic phase. Our density functional theory calculations based on the strongly-constrained-and-appropriately-normed (SCAN) exchange correlation functional in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ reveal the persistence of magnetic moments on the copper ions for oxygen concentrations ranging from the pristine phase to the optimally hole-doped compound. We also find the existence of ferrimagnetic solutions in the heavily doped compounds, which are expected to suppress superconductivity.

Partial substitution of Ba for Sr in the cuprate superconductor Bi2(Sr,La)2CuO6+δ

Polycrystalline samples of the Bi-based cuprate superconductors Bi2Sr2-x-yBaxLayCuO6+δ (y(La) = 0, 0.4, 0.6) have been prepared by the conventional solid-state reaction method to increase the superconducting transition temperature, Tc, in the so-called Bi-2201 phase. It has been found that the solubility limit of Ba is x(Ba) = 0, 0.2 and 0.3 for y(La) = 0, 0.4 and 0.6, respectively. Accordingly, the La-substitution for Sr is effective for the Ba-substitution for Sr. Superconductivity has also been observed for the Ba-substituted samples, but the increase in Tc through the Ba-substitution is only a few Kelvins.

Structural Origin of Apparent Fermi Surface Pockets in Angle-Resolved Photoemission ofBi2Sr2−xLaxCuO6+δ

We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission. From detailed low-energy electron diffraction measurements and an analysis of the angle-resolved photoemission polarization dependence, we show that these pockets are not intrinsic but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from angle-resolved photoemission [Meng et al., Nature (London) 462, 335 (2009)] have a similar structural origin and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO₂ plane.

Enhanced Pairing Correlations near Oxygen Dopants in Cuprate Superconductors

Recent experiments on Bi-based cuprate superconductors have revealed an unexpected enhancement of the pairing correlations near the interstitial oxygen dopant ions. Here we propose a possible mechanism--based on local screening effects--by which the oxygen dopants do modify the electronic parameters within the CuO2 planes and strongly increase the superexchange coupling J. This enhances the spin pairing effects locally and may explain the observed spatial variations of the density of states and the pairing gap.

High-Temperature Optical Spectral Weight and Fermi-liquid Renormalization in Bi-Based Cuprate Superconductors

The optical conductivity σ(ω) and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500K. Above 300K, W(T) deviates from the usual T2 behavior in both compounds, even though σ(ω→0) remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation, as shown by the good agreement with dynamical mean field calculations.

Imaging nanoscale Fermi-surface variations in an inhomogeneous superconductor

A variant on Fourier-transform scanning tunnelling spectroscopy enables spatial variations in the Fermi surface of bismuth-based cuprate superconductors to be probed. This technique reveals that these variations take place over nanometre distances. Particle–wave duality suggests we think of electrons as waves stretched across a sample, with wavevector k proportional to their momentum. Their arrangement in ‘k-space’, and in particular the shape of the Fermi surface, where the highest-energy electrons of the system reside, determine many material properties. Here we use a novel extension of Fourier-transform scanning tunnelling microscopy to probe the Fermi surface of the strongly inhomogeneous Bi-based cuprate superconductors. Surprisingly, we find that, rather than being globally defined, the Fermi surface changes on nanometre length scales. Just as shifting tide lines expose variations of water height, changing Fermi surfaces indicate strong local doping variations. This discovery, unprecedented in any material, paves the way for an understanding of other inhomogeneous characteristics of the cuprates, such as the pseudogap magnitude, and highlights a new approach to the study of nanoscale inhomogeneity in general.

Nanoscale Modulation of Local Barrier Height on Bi-based Cuprate Superconductors Observed by Scanning Tunneling Microscopy/Spectroscopy

The atomically resolved local barrier height (LBH) of a Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) superconducting single crystal is investigated by using scanning tunneling microscopy/spectroscopy (STM/STS). The LBH distribution is found to be strongly related with the well-known supermodulation structure of the Bi2212 surface with a period of ∼2.7 nm. In particular, the magnitude of the LBH is anticorrelated to the Bi–Bi atomic distance that is affected by supermodulation. A comparison with simultaneously obtained conductance spectra shows that there is a small correlation between the spatial modulation of the gap amplitude (Δ) and the LBH, this indicates that the measured Δ is slightly affected by the topmost BiO surface, but dominantly reflects information on the inner CuO 2 layer, which is responsible for the superconductivity.

C-axis lattice dynamics in Bi-based cuprate superconductors

We present results of a systematic study of the c axis lattice dynamics in single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both experimental data obtained by spectral ellipsometry on single crystals and theoretical calculations. The calculations are carried out within the framework of a classical shell model, which includes long-range Coulomb interactions and short-range interactions of the Buckingham form in a system of polarizable ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve good agreement between the calculated A2u eigenfrequencies and the experimental values of the c axis TO phonon frequencies which allows us to make a reliable phonon mode assignment for all three Bi-based cuprate superconductors. We also present the results of our shell model calculations for the Gamma-point A1g symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is based on the published experimental Raman spectra. The superconductivity-induced phonon anomalies recently observed in the c axis infrared and resonant Raman scattering spectra in trilayer Bi2223 are consistently explained with the suggested assignment.

Intercalative route to heterostructured nanohybrid ☆

We have successfully synthesized inorganic–inorganic, organic–inorganic and bio-inorganic nanohybrids by applying a chimie douce intercalation technique systematically to layered titanate, to Bi-based cuprate superconductors, Bi 2Sr 2Ca m−1 Cu m O y ( m=1, 2, and 3; BSCCO), and to layered double hydroxides (LDHs), those which are of high importance in terms of basic understanding of intercalation reactions and also having practical application. The inorganic/inorganic hybrids were synthesized by exfoliation-restacking methods to obtain TiO 2-pillared titanate. A novel pillaring procedure using an osmotic swelling had been developed to prepare TiO 2-pillared layered titanate with a large surface area, high thermal stability, and enhanced photocatalytic activity. On the other hands, the organic/inorganic hybrids were achieved via intercalative complexation of iodine intercalated BSCOO with an organic salt of Py–C n H 2 n+1 I (Py=pyridine). The high- T c superconducting intercalate with its remarkable lattice expansion can be applied as a precursor for superconducting colloids when dispersed in an appropriate solvent. We were also able to demonstrate that the biomolecules stabilized in the interlayer space of layered double hydroxides (LDH), bio-inorganic hybrids, retain their chemical and biological integrity. If necessary, LDH, as a reservoir, can be intentionally removed by dissolving it in an acidic medium or interlayer biomolecules in LDH can be released via ion-exchange reaction in electrolyte. It can, therefore, be concluded that the inorganic LDH can play a role as a good host lattice for gene reservoir or carrier. These intercalative methods will provide remarkable synthetic routes to design new heterogeneous hybrid materials, which have the intrinsic properties starting materials and create synergetic effects by hybridization.

Infrared- and Raman-active modes of Bi-based cuprate superconductors calculated in amicroscopic model

We present phonon frequencies and eigenvectors for thec-axis-polarized infrared- and Raman-active modes ofA2u andA1g symmetry in thesingle-layer Bi2Sr2CuO6 (Bi-2201)and the bilayer Bi2Sr2CaCu2O8 (Bi-2212) cuprate superconductors within our recently proposed microscopic formulation ofthe electronic density response and lattice dynamics in high-temperature superconductors(HTSCs). The model includes localized electronic charge-fluctuations as well as anisotropicdipole-fluctuations. The former have been shown to be the dominating polarization processin the CuO planes while the latter are particularly important for the ions in the ioniclayers of the HTSC. The effect of both types of screening contribution on thec-axislattice dynamics of the Bi-based cuprates is investigated. A characteristic and strong renormalization ofcertain A2u and A1g modes is shown to be related to the metallicity of the BiO layers. We obtain a good agreement between thecalculated A2u frequencies and new experimental results obtained by ellipsometric measurements onsingle crystals and propose an assignment of the modes. Moreover, the calculatedA1g Raman modes and eigenvectors are compared with measured frequencies and thecontradictory assignments of the modes found in the literature.

Intercalative route to heterostructured nanohybrids

Abstract We have successfully synthesized inorganic–inorganic, organic–inorganic and bio-inorganic nanohybrids by applying an intercalation technique systematically to layered titanate, molybdenum disulfide (MoS 2 ), Bi-based cuprate superconductors (Bi 2 Sr 2 Ca m −1 Cu m O y ( m =1, 2, and 3; BSCCO)), and to layered double hydroxides (LDHs), those which are of high importance in terms of basic understanding of intercalation reactions and of their practical applications. The inorganic–inorganic systems such as TiO 2 -pillared titanate, TiO 2 -pillared MoS 2 , and CdS–MoS 2 hybrids were synthesized by exfoliation–restacking method. A novel pillaring process using an osmotic swelling was developed to prepare TiO 2 -pillared layered titanate with a large surface area, high thermal stability, and enhanced photocatalytic activity. And the intercalation of TiO 2 and/or CdS nanocluster into the two dimensional MoS 2 lattice could be also realized by exfoliating and reassembling the lithiated molybdenum disulfide (LiMoS 2 ) in the presence of cationic TiO 2 and/or CdS nanocluster in an aqueous solution, respectively, to obtain the semiconductor–semiconductor hybrids. On the other hands, the organic–inorganic hybrids were achieved via intercalative complexation of iodine intercalated BSCOO with organic salt of Py–C n H 2 n +1 I (Py=pyridine). The high- T c superconducting intercalate with its remarkable lattice expansion can be applied as a precursor for superconducting colloids when dispersed in an appropriate solvent. This superconducting hybrid material had an unique structural feature of a superconducting-insulating-superconducting multilayer with atomically clean interfaces. Especially, this organic–inorganic nanohybrid is expected to be a promising precursor for preparing the superconducting colloidal suspension, which could be applied to the fabrication of superconducting films or wires. Recently, we were very successful in demonstrating in which the formation of bio-inorganic hybrids stabilized in the interlayer space of LDH retain their chemical and biological integrity. If necessary, LDH, as a reservoir, can be intentionally removed by dissolving it in an acidic media in such a way the interlayer biomolecules can be recovered or the intercalated biomolecules can be released from the LDH via ion-exchange reaction in electrolyte. It is, therefore, concluded that the inorganic LDH can play a role as a gene reservoir or carrier for various unstable organic or bio-molecules such as drugs and genes.

Estimation of matrix-element effects and determination of the Fermi surface inBi2Sr2CaCu2O8+δsystems using angle-scanned photoemission spectroscopy

The strong dependence of the momentum distribution of the photoelectrons on experimental conditions raises the question as to whether angle-resolved photoemission spectroscopy (ARPES) is able to provide an accurate reflection of the Fermi surface in Bi-based cuprate superconductors. In this paper we experimentally prove that the main contribution to the intensity variation comes from matrix elements effects and develop an approach to overcome this problem. We introduce a concept of 'self-normalization' which makes the spectra essentially independent of both the matrix elements and particular experimental parameters. On the basis of this concept we suggest a simple and precise method of Fermi surface determination in quasi-2D systems.

Intercalative Route to Heterostructured Nanohybirds

ABSTRACTWe have successfully synthesized organic-inorganic and bio-inorganic nanohybrids by applying an intercalation technique systematically to Bi-based cuprate superconductors, Bi2Sr2Cam-1CumOy(m = 1, 2, and 3; BSCCO), and to layered double hydroxides(LDHs), those which are of high importance in terms of basic understanding of intercalation reactions and of their practical applications. The organic-inorganic hybrids were achieved via intercalative complexation of iodine intercalated BSCOO with organic salt of Py-CnH2n+1I (Py = pyridine). The high-Tcsuperconducting intercalate with its remarkable lattice expansion can be applied as a precursor for superconducting colloids when dispersed in an appropriate solvent. We were also able to demonstrate that the biomolecules stabilized in the interlayer space of LDH retain their chemical and biological integrity. If necessary, LDH, as a reservoir, can be intentionally removed by dissolving it in an acidic media or interlayer biomolecules in LDH can be released via ion-exchange reaction in electrolyte. It is, therefore, concluded that the inorganic LDH can play a role as a good host lattice for gene reservoir or carrier.

An In Situ Study on Phase Relation between Bi2SrO4 and SrCuO2 in the Bi2O3–SrO–CuO System

A pseudobinary phase relation of Bi2SrO4–SrCuO2 in the Bi2O3–SrO–CuO ternary system between 850 and 1100°C has been investigated in situ using an apparatus that combines high-temperature X-ray diffraction with an optical microscopy. The liquidus temperatures increase with decreasing Bi composition from 910°C to 1090°C. The lowest phase change temperature which corresponds to the solidus line of this system is 910°C in the compositions below Bi2Sr4Cu3O10. Above 910°C, the decomposition to Bi4Sr8Cu5Ox and liquefaction occurs. On further heating above 925°C, a partially molten phase with solid SrCuO2 appears in the Bi-deficient side. The results are discussed in comparison with stabilities of the Ca-added system of Bi2O3–SrO–CaO–CuO, in which Bi-based cuprate superconductors are formed.

Proton Induced Defects in Bi-Based Cuprate Superconductors

Proton Induced Defects in Bi-Based Cuprate Superconductors

X-Ray absorption studies of bi-based superconductors

X-Ray absorption (XANES and EXAFS) spectroscopic techniques have been applied to study the local structural environment of host and dopant cations in Bi-based high-Tc superconductors; the materials studied include the following: BaBiO3, BaBi1 –xPbxO3, Ba1 –xKxBiO3, Bi2Sr2CuO6, Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10. XANES spectroscopy has been used to determine the valence state of Bi, Pb and Cu in these materials. In BaBiO3 the valence state of Bi is found to be close to IV both in the pure and doped material, and a disproportionation of BiIV into BiIII and BiV is not indicated by analyses of these data. The valence state of Pb is also found to be IV in BaBi1 –xPbxO3. In Bi-based cuprate superconductors, the formal valence states of Bi and Cu are slightly higher than their stoichiometric values of III and II. EXAFS spectroscopy has been used to examine the local structure around Bi, Ba, Pb, Sr, Ca and Cu cations in these systems. In the BaBiO3 system, doping with Pb and K results in a decrease in the metal–oxygen bond lengths. The results indicate that the local environment of all cations in the Bi–Sr–Ca–Cu–O system is characterised by substantial static disorder.

Multiple phases or multiple flux lattice melting temperatures: Vibrating reed studies on Bi-based ceramics

We present vibrating reed measurements on Pb-substituted Bi-based cuprate superconductors in a wide range of temperature (4.2–120 K) and magnetic field (< 1 T). The resonance frequency and peaks in damping measured as a function of field and temperature are interpreted in terms of superconducting phases present and thermally assisted depinning of the flux line lattice.