Morphology and electrical properties of pulsed-laser deposited Bi2Sr2CaCu2O8+δ films on vicinal substrates

Thin films of Bi₂Sr₂CaCu₂O_8+δ have been grown on vicinal (001) SrTiO₃ substrates by pulsed-laser deposition. Transmission electron microscopy (TEM) and X-ray diffraction reveal well ordered films with the c axis of the film parallel with the c axis of the substrate for miscut angles up to θ_S≈15°. TEM also reveals the step-like film morphology due to step-flow growth. The in-plane and out-of-plane resistivities are independent of film thickness within the range 20–300 nm and agree quite well with Bi₂Sr₂CaCu₂O_8+δ single-crystal data. Received:15 May 2000 / Accepted:17 May 2000 / Published online: 9 August 2000     

The normal state scattering rate in high- cuprates

A detailed phenomenological model is developed for the normal state transport properties of optimal and overdoped high-T_c cuprates. In particular, an explicit form of scattering rate is identified that may account, qualitatively and quantitatively, for the normal state (magneto)-transport properties of Tl₂Ba₂CuO _{6 + δ} and Bi₂Sr₂CaCu₂O _{8 + δ} across the overdoped side of the phase diagram. The proscribed form of the scattering rate is consistent also with features seen in the photoemission spectroscopy of Bi₂Sr₂CaCu₂O _{8 + δ} and offers a new and intuitive way to understand the evolution of the temperature dependence of the inverse Hall angle in Bi-based cuprates with carrier concentration. Received 5 June 2002 / Received in final form 17 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: n.e.hussey@bristol.ac.uk     

High-resolution electron microscopy and electron diffraction on Bi2Sr2-x La x Ca n Cu1+n O6+2n+δ

Solid solutions, Bi₂Sr_2-x La _x CuO₆₊ withx< 1.3 and Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ shows sometimes the absence of the 0.54 nm period alonga, whereas the modulation remains, suggesting both features not to be correlated. The investigation of the solid solution Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ is severly hampered by the presence of the n=0 structure in the reaction product when one starts with stoichiometric mixture. This can be overcome by adding extra Ca and heating directly just below the melting point. However addition of Ca to the mixture leads to changes in the solid solution range, because Ca can replace Sr.     

Angle-resolved photoemission experiments on Bi2Sr2CaCu2O8+δ(001)

Core-level X-ray photoelectron-diffraction patterns have been measured from cleaved Bi₂Sr₂CaCu₂O₈₊ (001) surfaces for all elements present in this compound. The incommensurate modulation alongb ([010]) leads to a strong inequivalence ofa- andb-directions for Bi, Sr and Cu photoelectrons, while Ca and O emission show less effect. Ultraviolet-photoemission experiments recording the emission intensity at the Fermi energy over a large solid angle are also presented, providing a direct mapping of the Fermi surface. Ac(2×2) superstructure is observed on the Fermi surface suggesting antiferromagnetic correlations within the Cu–O planes. The effects of the lattice modulation are clearly observable at the Fermi energy, and they are enhanced for binding energies higher than a few tens of meV.     

Anomalies in the temperature dependence of the anisotropic gap in high-T c superconductors

In a model for an anisotropic superconductor with a multicomponent gap parameter and Coulomb repulsion predominating between electrons from different sections of a multiply-connected Fermi surface, anomalous temperature dependences for different components of the gap are obtained which are radically different from the temperature dependence of the gap in the BCS theory and which agree with the experimental data for the anisotropic gap in the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+x . Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 553–558 (10 April 1996)     

Extraction of tight binding parameters from <Emphasis Type="Italic">in-situ</Emphasis> ARPES on the continuously doped surface of cuprates

Recently we developed a technique of ozone/vacuum annealing to continuously change the doping level of the surface of Bi₂Sr₂CaCu₂O_8+x and measured a nearly whole superconducting dome on one surface by in-situ angle-resolved photoemission spectroscopy [arXiv: 1805.06450]. Here we study the evolution of the electronic structures of Bi₂Sr₂CaCu₂O_8+x using this technique together with tight binding fits. The tight binding parameters are extracted to study their evolution with doping.     

Electronic structure of a superconducting Bi2212 crystal by angle resolved ultra violet photoemission

The electronic structure of a high quality superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi2212) single crystal is studied by angle resolved ultra violet photoemission (ARUPS) using He I (21.2 eV). Our results appear to show two bands crossing the Fermi level in ΓX direction of the Brillouin zone as reported by Takahashiet al. The bands at higher binding energy do not show any appreciable dispersion. The nature of the states near the Fermi level is discussed and the observed band structure is compared with the band structure calculations.     

Asymmetric local displacements in the Bi2Sr2CaCu2O8+β superconductor

Instantaneous in-plane Cu-O bond distribution in the Bi₂Sr₂CaCu₂O_8+β (Bi2212) superconductor has been investigated by high k-resolution Cu K-edge extended X-ray absorption fine structure (EXAFS) measured with polarized vector parallel to the two orthogonal Cu-O-Cu bonds of the CuO₂ square plane. The results show an anisotropic Cu-O distribution in the two directions and provide further information on the local atomic displacements in the lattice-charge stripes. Received 5 June 2000     

Transverse Seebeck effect in Bi2Sr2CaCu2O8

2 Cu₃O₇ has been observed in “off-c-axis” grown Bi₂Sr₂CaCu₂O₈ films. The films were prepared by pulsed laser deposition from Bi₂Sr₂CaCu₂O₈ targets and grown with a tilt angle of 10 degrees between the film c-axis and the film surface on off-axis cut SrTiO₃ substrates. From the thermoelectric response upon laser irradiation the anisotropy of the thermopower is estimated. Received: 15 April 1996/ Accepted:19 November 1996     

Extended -wave superconductivity. Flat nodes in the gap and the low-temperature asymptotic properties of high- superconductors

Remarkable anisotropic structures have been recently observed in the order parameter of the underdoped superconductor Bi₂Sr₂CaCu₂O . Such findings are strongly suggestive of deviations from a simple d _{x2 - y2} -wave picture of high- superconductivity, i.e. . In particular, flatter nodes in are observed along the directions in -space, than within this simple model for a d-wave gap. We argue that nonlinear corrections in the -dependence of near the nodes introduce new energy scales, which would lead to deviations in the predicted power-law asymptotic behaviour of several measurable quantities, at low or intermediate temperatures. We evaluate such deviations, either analytically or numerically, within the interlayer pair-tunneling model, and within yet another phenomenological model for a d-wave order parameter. We find that such deviations are expected to be of different sign in the two cases. Moreover, the doping dependence of the flatness of the gap near the nodes is also attributable to Fermi surface effects, in addition to possible screening effects modifying the in-plane pairing kernel, as recently proposed. Received 19 November 1999     

Electron spin resonance in manganese-doped BSCCO (2212) crystals

The electron spin resonance (ESR) spectra of undoped and manganese-doped BSCCO (2212) crystals (nominal composition Bi₂Sr₂CaCu_2–x Mn_xO_y, where x=0.1) are compared. Zh. Tekh. Fiz. 67, 137–138 (October 1997)     

Effects of the doping and the number of planes on the shadow bands of Bi-based cuprates

Angle resolved photoemission spectroscopy (ARPES) has been used to investigate the shadow bands present at the Fermi surface of bismuth based superconductors. Bi₂Sr₂CaCu₂O_8+δ (Bi2212) single crystals with different doping levels and Bi₂Sr₂CuO_6+δ (Bi2201) thin films have been studied by means of momentum distribution curves along the ΓY high symmetry direction in two different Brillouin zones and for an extensive photon energy range. The results show an enhancement of the shadow band intensity in the second Brillouin zone for photon energies around hν=30 eV. Furthermore, the ratio Shadow Bands/Main Band is shown to be constant as a function of both the doping level and the number of CuO₂ planes. An interpretation of this observation on the basis of the current theoretical models will be provided.     

Superconducting gap observed in Raman spectra of Bi2Sr2CaCu2O8+x

Investigation of the temperature behavior of electronic Raman scattering in Bi₂Sr₂CaCu₂O_8+x superconducting crystals indicates the frequency of the mode whose spectral position is customarily used to derive the width of the superconducting gap to be only weakly temperature dependent. Measurements carried out under different resonance conditions can be interpreted as due to spatial dispersion of the superconducting gap. Fiz. Tverd. Tela (St. Petersburg) 40, 998–1001 (June 1998)     

Lock-in transition in Bi2Sr2CaCu2O8+δ observed through 10GHz dissipation

We observe the lock-in transition in Bi₂Sr₂CaCu₂O_8+δ single crystals, by measuring the 10 GHz dissipation for various orientations of the magnetic field; various vortex lattice structures are revealed by different dissipation regimes.     

Effect of Sb substitution on the superconductivity of Bi2(Ca,Sr) n+1Cu n O4n+2 (n=1, 2 and 3) systems

Effect of incorporation of Sb in place of Bi in the bismuth cuprate superconductors has been examined. The nominal compositions studied are MCa_1·5Sr_1·5Cu₂O_8+δ and MCa₂Sr₂Cu₃O_10+δ, whereM=Bi_2−x Sb _x or Bi_1·5−x Sb _x Pb_0·5. Different preparative routes such as the ceramic method, the matrix route as well as the melt route were employed to prepare the materials. No indication of either Sb entering the lattice or enhancement ofT _c is noted from resistivity, magnetic susceptibility and microwave absorption measurements. Communication No. 162 from Materials Research Centre.     

Charge states of transition metal in “Cr, Co and Ni” doped Ln0.5Ca0.5MnO3 CMR manganites

The XAS study at Cr, Co, Ni and Mn K-edges was performed for the doped CMR manganites Ln_0.5Ca_0.5Mn_1-xB_xO₃ with Ln=La, Nd, Sm and B= Cr, Co, Ni (), on the samples that were studied previously for their ferromagnetic-metallic to antiferromagnetic-insulator transition. We observed that the formal charges of the doping elements are Ni²⁺, Co²⁺ and Cr³⁺. It is also evidenced that the average formal charge of the manganese is increased after doping, in agreement with the charge compensation keeping “O₃” stoichiometry. These results suggest that the doping elements participate directly to the band structure. Received: 9 January 1998 / Received in final form: 6 April 1998 / Accepted: 7 April 1998     

Effect of doping by low content of yttrium at Ca and Sr sites of Bi(Pb)-2212 superconducting ceramics

In this work, a comparative study of the effect of doping by low content of Y³⁺ between (Ca site) and out of (Sr site) the CuO₂ planes of Bi(Pb)-2212 phase is presented. Ceramics of Bi_1.6Pb_0.4Sr₂Ca_1−xY_xCu₂O_8+d (called CY series) and Bi_1.6Pb_0.4Sr_2−xY_xCaCu₂O_8+d (called SY series) with x=0, 0.025, 0.05, 0.075 and 0.1 are elaborated in air by conventional solid state reaction. They are characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM), density, Vickers microhardness and resistivity measurements. The refinement of cell parameters is done by considering the structural modulation. In comparison with the undoped sample (x=0), the cell parameters a, b and c are reduced by the doping for both series while the b component of the modulation vector increases. A good correlation between the variations of the bulk density and the Vickers microhardness with x is obtained. For both series, the SEM analysis shows that the doped samples exhibit a reduced grain size than that of the undoped one. The variation of resistivity with temperature shows that all samples exhibit a metallic-like character in the normal state. For all doping levels, the CY series presents higher onset critical transition temperature than that of the undoped sample, which is equal to 85.43 K. The opposite is obtained for SY series. The highest value of this temperature is obtained for x=0.075 in the doped samples and is about 92.15 and 79.96 K for CY and SY series, respectively. These values may correspond to a near optimally doped state since the slope (dρ/dT) of resistivity shows a maximum at the same value of x=0.075. For both series, when Y³⁺ is introduced into the samples, the residual resistivity decreases first for x=0.025 and increases gradually after this value until x=0.1.     

Effects of crossed columnar defects on vortex pinning in Bi2Sr2CaCu2O8

To investigate the existence of a splay effect in Bi₂Sr₂CaCu₂O₈ (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect. Received 27 December 1999     

Fluctuation conductivity in cuprate superconductors

We have measured the in-plane resistivity of Bi₂Sr₂CaCu₂O_8+δ and Tl₂Ba₂ CaCu₂O_8+δ single crystals in the temperature range 70–300 K. The thermodynamic fluctuations in the conductivity of both the samples start around ∼ 125 K. We find the Lawrence and Doniach [1] model to be inadequate to describe the fluctuation conductivity in these materials. The modification suggested by Ramallo et al [4] where by the conductivity is enhanced due to the presence of two superconducting layers in each unit cell is also not adequate. We suggest the fluctuation conductivity to be reduced due to the reduction in the density of states (DOS) of the quasiparticles which results due to the formation of Cooper pairs at the onset of the fluctuations. The data agrees with the theory proposed by Dorin et al [5] which takes into account this reduction in DOS.     

Influence of columnar defects on the thermodynamic properties of BSCCO

Amorphous columnar defects strongly affect the reversible magnetization of Bi₂Sr₂CaCu₂O_8+δ single crystals both in the vortex solid, where the change reflects the change in vortex energy due to pinning, and in the vortex liquid, where the randomly positioned columns disrupt the interaction between superconducting fluctuations.