Bi-O Planes Research Articles

Apparent color and Raman vibrational modes of the high-temperature superconductor Bi2Sr2CaCu2O 8+δ exfoliated flakes

Studying and controlling the properties of individual exfoliated materials is one of the first steps towards the fabrication of complex van der Waals systems. In this work, we present a systematic study of optical properties and micro-Raman spectroscopy on exfoliated flakes of the high-temperature superconductor Bi2Sr2CaCu2O 8+δ (BSCCO-2212). We demonstrate that these are quick and non-invasive techniques for studying air sensitive materials. The apparent color of BSCCO-2212 exfoliated flakes on SiO2/Si has allowed a rough and fast identification of the number of layers. By analyzing the optical contrast of different flakes we determined the complex refractive index in the visible range and found the optimal combination of illumination wavelength and substrate properties for the identification of flakes with different thickness. In addition, we report the hardening of the most characteristic Raman modes as flake thickness decreases, possibly caused by strain in the BiO and CuO2 planes. Moreover, the evolution of the Raman modes establishes a second approach to determine the thickness of BSCCO-2212 thin flakes. As BSCCO-2212 is a challenging material due to its sensitivity to ambient conditions, the present work provides a guide for the fabrication and characterization of complex van der Waals systems paving the way for studying heterostructures based on unconventional superconductors in the 2D limit.

Visualization of Dopant Oxygen Atoms in a Bi2Sr2CaCu2O8+δ Superconductor

AbstractA proper amount of excess oxygen plays a significant role in hole‐doped cuprate high‐Tc superconductivity. Here, the dopant oxygen in Bi2Sr2CaCu2O8+δ is directly imaged via integrated differential phase contrast combined with state‐of‐the‐art scanning transmission electron microscopy. The location of dopant oxygen is observed to be consistent with the position inferred from local strain analysis of the incommensurate structure. The influence of dopant oxygen on the local atomic lattice and electronic structure is further explored using first‐principle calculations. The dopant oxygen atoms not only aggravate the distortions of the local atomic arrangement but also alter the electronic states by transferring charge from the BiO planes to the CuO2 planes. The underlying mechanism of charge transfer is resolved. The results may also be applicable to other oxygen‐doped cuprates with high‐Tc superconductivity.

Electronic structure of the ingredient planes of the cuprate superconductorBi2Sr2CuO6+δ: A comparison study withBi2Sr2CaCu2O8+δ

By means of low-temperature scanning tunneling microscopy, we report on the electronic structures of BiO and SrO planes of Bi2Sr2CuO6+{\delta} (Bi-2201) superconductor prepared by argon-ion bombardment and annealing. Depending on post annealing conditions, the BiO planes exhibit either pseudogap (PG) with sharp coherence peaks and an anomalously large gap of 49 meV or van Hove singularity (VHS) near the Fermi level, while the SrO is always characteristic of a PG-like feature. This contrasts with Bi2Sr2CaCu2O8+{\delta} (Bi-2212) superconductor where VHS occurs solely on the SrO plane. We disclose the interstitial oxygen dopants ({\delta} in the formulas) as a primary cause for the occurrence of VHS, which are located dominantly around the BiO and SrO planes, respectively, in Bi-2201 and Bi-2212. This is supported by the contrasting structural buckling amplitude of BiO and SrO planes in the two superconductors. Our findings provide solid evidence for the irrelevance of PG to the superconductivity in the two superconductors, as well as insights into why Bi-2212 can achieve a higher superconducting transition temperature than Bi-2201, and by implication, the mechanism of cuprate superconductivity.

Mapping the Electronic Structure of Each Ingredient Oxide Layer of High-T\{c} Cuprate Superconductor Bi{2}Sr{2}CaCu{2}O{8+δ}.

Understanding the mechanism of high transition temperature (T{c}) superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy (STM), we report on layer-by-layer probing of the electronic structures of all ingredient planes (BiO, SrO, CuO{2}) of Bi{2}Sr{2}CaCu_2}O{8+δ} superconductor prepared by argon-ion bombardment and annealing technique. We show that the well-known pseudogap (PG) feature observed by STM is inherently a property of the BiO planes and thus irrelevant directly to Cooper pairing. The SrO planes exhibit an unexpected van Hove singularity near the Fermi level, while the CuO{2} planes are exclusively characterized by a smaller gap inside the PG. The small gap becomes invisible near T{c}, which we identify as the superconducting gap. The above results constitute severe constraints on any microscopic model for high T{c} superconductivity in cuprates.

Structural modulation in Bi2Sr2Ca0.4Y0.6Cu2O8+δ

The neutron diffraction study has been performed on a single crystal specimen of yttrium doped compound Bi2Sr2Ca0.4Y0.6Cu2O8+δ, which is a structural analog of the Bi-2212 superconducting cuprates. The average structure is orfhorhombic of Fmmm symmetry (a = 5.448 Å, b = 5.439 Å and c = 30.416 Å). Two kinds of distortion are observed. The first one, manifested by satellite reflections of wavevector [τ,0,1] with τ = 0.220, is associated with an incorporation of extra oxygen (δ∼0.44) into BiO bilayer. The respective modulation consists of a flexing of the bilayer and corrugation of other structural layers with period 4.5a. The second distortion is associated with displacements in the BiO planes due to Bi3+ bonding and lone pair ordering, which results in common cases in a formation of infinite ribbons of Bi-O-Bi- chains and decrease of average symmetry to Amaa. In the present system, the reflections characteristic for Amaa are absent and, instead, a diffuse scattering along the [001] direction is observed, pointing to a presence of 2-dimensional superstructure with in-plane period 9.0a – twice the one of the regular modulation. Based on these data, we propose a structural model in which bonding chains in the BiO planes of Bi2Sr2Ca0.4Y0.6Cu2O8.44 are segmented and rearranged in a tweed pattern.

Coherent generation of phonon-polaritons in Bi2Sr2CaCu2O8+xintrinsic Josephson junctions

The Bi2Sr2CaCu2O8+x high-temperature superconductor represents a natural metamaterial, composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and detection of electromagnetic waves in Bi2Sr2CaCu2O8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to strong electronphonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm2 power densities.

First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite

We present a first-principles density functional study of the structural, electronic and magnetic properties of the ferroelectric domain walls in multiferroic BiFeO3. We find that domain walls in which the rotations of the oxygen octahedra do not change their phase when the polarization reorients are the most favorable, and of these the 109 degree domain wall centered around the BiO plane has the lowest energy. The 109 degree and 180 degree walls have a significant change in the component of their polarization perpendicular to the wall; the corresponding step in the electrostatic potential is consistent with a recent report of electrical conductivity at the domain walls. Finally, we show that changes in the Fe-O-Fe bond angles at the domain walls cause changes in the canting of the Fe magnetic moments which can enhance the local magnetization at the domain walls.

Structure optimization effects on the electronic and vibrational properties ofBi2Sr2CaCu2O8

We present detailed first-principles calculations for the normal state electronic properties of the high T$_C$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$, by means of the linearized augmented plane wave (LAPW) method within the framework of density functional theory (DFT). As a first step, the body centered tetragonal (BCT) cell has been adopted, and optimized regarding its volume, $c/a$ ratio and internal atomic positions by total energy and force minimizations. The full optimization of the BCT cell leads to small but visible changes in the topology of the Fermi surface, rounding the shape of CuO$_2$ barrels, and causing both the BiO bands, responsible for the pockets near the \textit{\=M} 2D symmetry point, to dip below the Fermi level. We have then studied the influence of the distortions in the BiO plane observed in nature by means of a $\sqrt{2}\times\sqrt{2}$ orthorhombic cell (AD-ORTH) with $Bbmb$ space group. Contrary to what has been observed for the Bi-2201 compound, we find that for Bi-2212 the distortion does not sensibly shift the BiO bands which retain their metallic character. As a severe test for the considered structures we present Raman-active phonon frequencies ($q = 0$) and eigenvectors calculated within the frozen-phonon approximation. Focussing on the totally symmetric A$_{g}$ modes, we observe that for a reliable attribution of the peaks observed in Raman experiments, both $c$- and a-axis vibrations must be taken into account, the latter being activated by the in-plane orthorhombic distortion.

First-principles study of the normal state electronic properties of the Bi-2212 cuprate superconductor

We present detailed first-principles calculations of the normal state electronic properties of the Bi 2Sr 2CaCu 2O 8 (Bi-2212) high T C superconductor by means of the linearized augmented plane wave (LAPW) method within the framework of density functional theory (DFT). We describe the crystal structure by the body-centered tetragonal (BCT) cell with space group I4/ mmm and optimize its volume, c/ a ratio and atomic positions by total energy and atomic-force minimization. The optimization of the cell leads to small but visible changes in the topology of the Fermi surface, rounding the shape of the CuO 2 barrels, and moving both the BiO bands, responsible for the pockets near the M̄ point, below the Fermi level. We have then studied the influence of the large distortions in the BiO plane observed in nature by means of an 2 × 2 orthorhombic cell (with Bbmb space group). Differently to what has been observed in the literature for the Bi-2201 compound, we find that for Bi-2212 the orthorhombic distortion does not induce relevant changes to the binding energies of the BiO bands, conserving their metallic character.

STRUCTURAL ASPECTS OF THE FALSE 2212 IN A (Bi,Pb):2223 SUPERCONDUCTOR

The full pattern decomposition method by Le Bail was applied to the X-ray diffraction pattern of a ( Bi 1.8 Pb 0.46) Sr 1.88 Ca 2.06( Cu 0.97 Fe 0.01 Ca 0.02)3 O y superconductor presenting the same characteristics as the false 2212 reported by Masqood et al. (ICDD file 42-514). By using the Cmmm space group, AB5 coordination in the BiO planes was evidenced, yielding an explanation for the apparent cell colapse. Orthorombic-pseudohexagonal settings for the a and b lattice constants may be derived from the group-supergroup relationships, i.e. instead of [Formula: see text] one may take a=2×3.8Å and [Formula: see text]. Indexing with these values avoids the assumption for the strange [Formula: see text] peak, and yields the refined values of a=7.664Å, b=3.356Å and c=16.172Å, space group Pmmm.

Inhomogeneousd-wave superconducting state of a doped Mott insulator

Recent scanning tunneling microscope (STM) measurements discovered remarkable electronic inhomogeneity, i.e. nano-scale spatial variations of the local density of states (LDOS) and the superconducting energy gap, in the high-Tc superconductor BSCCO. Based on the experimental findings we conjectured that the inhomogeneity arises from variations in local oxygen doping level and may be generic of doped Mott insulators which behave rather unconventionally in screening the dopant ionic potentials at atomic scales comparable to the short coherence length. Here, we provide theoretical support for this picture. We study a doped Mott insulator within a generalized t-J model, where doping is accompanied by ionic Coulomb potentials centered in the BiO plane. We calculate the LDOS spectrum, the integrated LDOS, and the local superconducting gap, make detailed comparisons to experiments, and find remarkable agreement with the experimental data. We emphasize the unconventional screening in a doped Mott insulator and show that nonlinear screening dominates at nano-meter scales which is the origin of the electronic inhomogeneity. It leads to strong inhomogeneous redistribution of the local hole density and promotes the notion of a local doping concentration. We find that the inhomogeneity structure manifests itself at all energy scales in the STM tunneling differential conductance, and elucidate the similarity and the differences between the data obtained in the constant tunneling current mode and the same data normalized to reflect constant tip-to-sample distance. We also discuss the underdoped case where nonlinear screening of the ionic potential turns the spatial electronic structure into a percolative mixture of patches with smaller pairing gaps embedded in a background with larger gaps to single particle excitations.

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x.

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are 'doped' away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers ('holes') into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of approximately 14 A. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

High-resolution electron microscopy investigation on stacking faults in SrBi2Ta2O9 ferroelectric thin films prepared by metalorganic deposition

A structural planar defect in the SBT film with 10 mol% excess bismuth grown on Pt/TiO2/SiO2/Si substrates by metalorganic deposition has been observed by means of high-resolution electron microscopy. It was found that the stacking defect was an isolated defect with extra inserted Bi-O planes normal to the c axis. This structural defect is expected to effectively improve the ferroelectric response and fatigue-resistance characteristics of SBT films because of the extra inserted Bi-O planes within the structural defect having higher structural flexibility and alleviating the mechanical stresses and strains, as well as injected-charge problems.

Interfacial structures and structural defects of SrBi2Ta2O9 ferroelectric thin films prepared by MOD and PLD

SrBi2Ta2O9 (SBT) films were deposited on Pt/TiO2/SiO2/Si (100) substrates by metalorganic deposition and pulsed laser deposition. Interfacial structures between SBT films and Pt electrodes (both top and bottom) were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The structural defects such as stacking faults were revealed by HRTEM image of SBT films with 10 mol% excess bismuth prepared by MOD. The stacking faults were examined as structural defects with extra inserted Bi-O planes normal to the c axis. The relationships between the ferroelectric properties and interfacial structures, and structural defects are discussed.

Atomic-scale microstructures of SrBi2Ta2O9 (SBT) ferroelectric thin films prepared by MOD and PLD for ferams applications

In this work, the microstructural defects in SrBi2Ta2O9 (SBT) ferroelectric thin films were investigated at the atomic-scale by high-resolution transmission electron microscopy (HRTEM). A stacking fault with an extra inserted Bi-O plane normal to the c-axis was observed in SBT film with 10mol% excess bismuth prepared by metalorganic deposition. Edge dislocations with an average space about 3nm were observed at the small misorientation angle (8.2°) tilt grain boundary of SBT film with (001)-orientation prepared by pulsed laser deposition. The Burgers vector b for the edge dislocation was determined to be 1/2[110]α0, where α0 is the parameter of SBT unit cell. Chemical compositions of grains and grain boundaries in SBT films annealed in forming gas at 450°C and 500°C for 60 minutes were analyzed by using energy dispersive spectra at the nano-scale. Effects of the microstructural defects and microchemistry of the grain boundaries on the leakage current of SBT films are briefly discussed.

Atomic image of aCuO2plane in the STM image of aBi2Sr2CaCu2O8+δcleaved surface

Clear atomic images of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (BSCCO) cleaved surface were obtained at room temperature by scanning tunneling microscopy (STM). The features of some observed images vary between two extreme cases, that is, the Bi-O plane image and the ${\mathrm{CuO}}_{2}$ plane image. And we have succeeded in obtaining a clear STM image of a ${\mathrm{CuO}}_{2}$ plane. From the ${\mathrm{CuO}}_{2}$ plane image, we found that this plane had two superstructures, one along the $\mathbf{a}$ axis and the other along the $\mathbf{b}$ axis. The periodicity of the superstructure along the $\mathbf{b}$ axis is $(4--5)\mathbf{b},$ the same as that in the Bi-O plane, and the periodicity along the $\mathbf{a}$ axis is $2\mathbf{a}.$ To explain these superstructures, we propose a lattice distortion model for the ${\mathrm{CuO}}_{2}$ plane of BSCCO.

Joys and Pitfalls of Fermi Surface Mapping inBi2Sr2CaCu2O8+δUsing Angle Resolved Photoemission

On the basis of angle-scanned photoemission data recorded using unpolarized radiation, with high (E,k) resolution, and an extremely dense sampling of k space, we resolve the current controversy regarding the normal state Fermi surface (FS) in Bi(2)Sr(2)CaCu(2)O(8+delta). The true picture is simple, self-consistent, and robust: the FS is holelike, with the form of rounded tubes centered on the corners of the Brillouin zone. Two further types of features are also clearly observed: shadow FSs, which are most likely to be due to short range antiferromagnetic spin correlations, and diffraction replicas of the main FS caused by passage of the photoelectrons through the modulated Bi-O planes.

Properties of Bi 2+ xSr 2− xCuO 6+ δ thin films obtained by MBE

We report on the fabrication of epitaxial, c -axis oriented Bi 2+ x Sr 2− x CuO 6+ δ (2201) thin films deposited by molecular beam epitaxy on MgO and SrTiO 3 substrates, using co-evaporation method. reflection high energy electron diffraction (RHEED) has been employed to monitor the surface structure of the samples during the growth. The morphology and the electronic properties of the films surface have been analyzed after the deposition by scanning tunneling microscopy (STM) at room temperature in ultra high vacuum conditions. The presence of well defined strikes in the RHEED patterns indicates a 2D nucleation growth, confirmed by STM analysis which shows very smooth surfaces on micron scales with the presence of half or one unit cell high terraces. STM spectroscopic measurements shows that the 2201 terminating layer is formed by BiO plane. The RHEED patterns show the presence of a superstructure along the b direction, due to an excess of oxygen in the BiO surface layer, whose periodicity is about twice that usually observed for BSCCO compound. The dependence of the c -axis length, obtained by X-ray diffraction patterns, has been studied as a function of the sample stoichiometry. It has been observed that a change in the Bi/Sr ratio strongly influence both the c -axis length and the electrical properties of the samples. Superconductivity has been detected in the samples with a little excess of Bi respect to Sr independently on the oxygenation conditions.

Ｉ２あるいはＦｅＩ２を反応物としたＢｉ２Ｓｒ２Ｃａ（ｎ−１）ＣｕｎＯｙ（ｎ＝２，３）超伝導体へのヨウ素インターカレーション

We have investigated the influence on the superconductivity of the iodine intercalation into sintered samples of Bi2212 and Bi2223 superconductors by using I2 or FeI2 as reactant. It has been found from X-ray diffraction that iodine intercalated compound of IXBi2Sr2Ca2Cu3Oy was prepared by intercalation by using FeI2. The iodine intercalation using FeI2 as a reactant into Bi2223 phase is reported for the first time as long as we know.The decrease of TConset, ΔTCon, for Bi2223 was smaller than one for Bi2212. This is likely due to the number of the CuO2 planes between BiO planes. In the case using FeI2, I2 originates from FeI2 with reduction of host.

Correlations Between Physical and Crystallographic Parameters in the (Bi, Pb)2Sr2 (Ca, Y)Cu2O8+δ Superconductors

Systematic studies of the lead and yttrium substituted Bi-2212 phase were performed on various compositions ranging between 0 and 0.8 for Pb and between 0 and 0.9 for Y. Crystallographic measurements reveal that the two modulations observed in unleaded and highly Pb-substituted samples are closely linked, the main differences being that the commensurate component disappears in type II (lead type) modulation, along with the modulation of the Bi 3+ ion. We argue that this is due to a covalent bonding between Bi-O planes. The study of internal parameters, including modulation amplitudes, of the Bi2Sr2 ( Ca 1-x, Y x) Cu2O 8+x/2+δ system reveals three distinct regimes with very different behaviors, corresponding to the overdoped, underdoped and insulating regimes. By comparing these results to the variation of the diamagnetic shielding with Y and Pb contents, we conclude that yttrium and lead affect more the mobility of the holes than the hole concentration itself. Y seems to induce, rather than a metal/insulator transition, a switch from a three-dimensional to a two-dimensional regime with a hole localization in the CuO 2 planes.