In-plane Position Research Articles

Direct determination of epitaxial interface structure in Gd2O3 passivation of GaAs.

Obtaining accurate structural information on epitaxial films and interfaces is nowhere more critical than in semiconductor passivation layers, where details of the atomic structure and bonding determine the nature of the interface electronic states. Various non-destructive methods have been used to investigate the structure of films and interfaces, but their interpretation is model-dependent, leading occasionally to wrong conclusions. We have developed a new X-ray method for the direct determination of epitaxial structures, coherent Bragg rod analysis (COBRA). The usefulness of our technique is demonstrated by mapping, with atomic precision, the structure of the interfacial region of a Gd2O3 film grown epitaxially on a (100) GaAs substrate. Our findings reveal interesting behaviour not previously suggested by existing structural methods, in particular a lock-in of the in-plane Gd atomic positions to those of the Ga/As atoms of the substrate. Moreover, we find that the bulk stacking of the Gd2O3 atomic layers is abandoned in favour of a new structure that is directly correlated with the stacking sequence of the substrate. These results have important implications for Gd2O3 as an effective passivation layer for GaAs (ref. 7). Our work shows that the COBRA technique, taking advantage of the brilliance of insertion device synchrotron X-ray sources, is widely applicable to epitaxial films and interfaces.

The Effects of Process Variations on Residual Stress Induced by Laser Peening

Measurements of the distribution of residual stress with depth from the surface in laser peened coupons were made in a high-strength aluminum alloy. Residual stresses were measured using slitting (also known as the crack compliance method). Measurements were made on several coupons to: compare laser peening (LP) and shot peening residual stresses; ascertain the influence of LP parameters on residual stress; determine whether tensile residual stress existed outside the peened area; assess the variation of residual stress with in-plane position relative to the layout of the laser spots used for peening; and, determine the importance of a uniform spatial distribution of laser energy within the spot. Residual stress 0.1 mm from the surface due to LP and shot peening were comparable and the depth of the compressive stress for LP was far greater than for shot peening. Variations of most LP parameters did not significantly alter residual stress at shallow depths. Residual stresses adjacent to the peened area were found to be compressive. Decreased levels of surface residual stress were found when laser spots had a non-uniform distribution of laser intensity.

X-ray diffraction of heavy-atom labelled two-dimensional crystals of rhodopsin identifies the position of cysteine 140 in helix 3 and cysteine 316 in helix 8

We have used site-specific heavy-atom labelling and X-ray diffraction to localize single amino acid residues in the cytoplasmic domain of the integral membrane protein rhodopsin, the dim-light photoreceptor of retinal vertebrate rod cells. Two-dimensional orthorhombic crystals of the space group p22 12 1 ( a = 59.5(±1) Å and b = 82.7(±1.5) Å) were produced from detergent-solubilized, partially delipidated rhodopsin. To obtain milligram amounts of two-dimensional crystals, which are required for X-ray diffraction, the yield of the crystalline material was significantly increased by reconstitution of rhodopsin in the presence of cholesterol (1:2 to 1:10 mol/mol) and by adding polar organic solvents to the dialysis buffer. The native cysteine residues C140 and C316 were then selectively labelled with mercury using the sulphydryl-specific reagent p-chloromercuribenzoate (1.6–2.1 mol Hg per mol rhodopsin). The labelling did not affect the unit cell dimensions. Optical absorption spectra of labelled and native two-dimensional rhodopsin crystals showed the characteristic 11- cis-retinal peak at 498 nm, which corresponds to the dark state of rhodopsin. The in-plane position of the mercury label was calculated at 9.5 Å resolution from the intensity differences in the X-ray diffraction patterns of labelled and native crystals using Fourier difference methods and the phase information from electron crystallography. The label positions were in excellent agreement with the positions of C140 at the cytoplasmic end of helix 3 and of C316 in the cytoplasmic helix 8 recently obtained from three-dimensional rhodopsin crystals. Whereas these high-resolution diffraction studies were performed under cryogenic conditions (100 K), our results were obtained at room temperature with fully hydrated membranes and in the absence of loop-loop crystal contacts. To study the structural changes of the cytoplasmic loops involved in activation and signal transduction, our more physiological conditions offer important advantages. Furthermore, the localization of C316 is the first direct proof that the electron density on top of helix 1 observed by cryo-electron microscopy is a part of the C-terminal loop. Our approach is of particular interest for investigations of other membrane proteins, for which 3D crystals are not available. Structural constraints from heavy-atom labels at strategic sites enable the assignment of a position in the amino acid sequence to features visible in a low-resolution density map and the study of conformational changes associated with different functional states of the membrane protein.

118 Static and dynamic measurements on an aerospace composite beam by embedded fiber optic sensors

Two rectangular plates, assimilable to beams, one with embedded fiber optic sensors and one with the sensor glued on its surface have been manufactured using a prepreg carbon fiber fabric. The sensors, specifically two Fiber Bragg Gratings (FBGs), have been embedded in the same in-plane position but at different depths so as to be in opposite position with respect to the neutral axis. A second plate with lower natural frequency has been tested to prove the feasibility of a modal analysis by the use of a fiber Bragg Grating. The results from the static tests have shown excellent agreement with the theory. From the dynamic tests performed with the Fiber Bragg Grating the second mode of vibration was retrieved. The comparison with the results obtained with the accelerometer was very good.

Characterization of the oxygenated intermediate of the thermophilic cytochrome P450 CYP119

Using UV–Vis, resonance Raman, and EPR spectroscopy we have studied the properties of the oxygenated ferrous cytochrome P450 from Sulfolobus Solfataricus, (CYP119). The recently determined crystal structure of CYP119 is compared with other available structures of P450s, and detailed structural and spectroscopic analyses are reported. With several structural similarities to CYP102, such as in-plane iron position and a shorter iron-proximal ligand bond, CYP119 shows low-spin conformation preference in the ferric form and partially in the ferrous form at low temperatures. These structural features can explain the fast autoxidation of the oxyferrous complex of CYP119. Finally, we report the first UV–Vis and EPR spectra of the cryoradiolytically reduced oxygenated intermediate of CYP119. The primary reduced intermediate, a hydroperoxo–ferric complex of CYP119, undergoes a ‘peroxide shunt’ pathway during gradual annealing at 170–195 K and returns to the low-spin ferric form.

Effect of transition layers on the electromagnetic properties of composites containing conducting fibres

The approach to calculating the effective dielectric and magnetic response in bounded composite materials is developed. The method is essentially based on the renormalisation of the dielectric matrix parameters to account for the surface polarisation and the displacement currents at the interfaces. This makes it possible the use of the effective medium theory developed for unbounded materials, where the spatially-dependent local dielectric constant and magnetic permeability are introduced. A detailed mathematical analysis is given for a dielectric layer having conducting fibres with in-plane positions. The surface effects are most essential at microwave frequencies in correspondence to the resonance excitation of fibres. In thin layers (having a thickness of the transition layer), the effective dielectric constant has a dispersion region at much higher frequencies compared to those for unbounded materials, exhibiting a strong dependence on the layer thickness. For the geometry considered, the effective magnetic permeability differs slightly from unity and corresponds to the renormalised matrix parameter. The magnetic effect is due entirely to the existence of the surface displacement currents.

Evolution structurale de la nacrite en fonction de la nature des molécules organiques intercalees

Nacrite has been intercalated with two polar organic molecules: dimethyl sulfoxide (DMSO) andN-methylacetamide (NMA). The homogeneous nacrite complexes have been studied by X-ray diffraction (XRD) and infrared (IR) spectroscopy. The XRD study is based on a comparison between experimental and calculated patterns. The structures of the intercalated compounds have been determined, including the mutual positions of the layers after intercalation and the positions of the intercalated molecules in the interlayer space. It has been shown that the intercalation process causes not only a swelling of the interlayer space but also a shift in the mutual in-plane positions of the layers. This shift depends on the nature of the intercalated molecules and is related to their shape and the hydrogen bonds which are established with the surrounding surfaces. For a given molecule, the intercalation process is the same for the different polytypes of the kaolinite family. These XRD results are consistent with those of IR spectroscopy.

Growth and characterization of non-c-oriented epitaxial ferroelectric SrBi2Ta2O9 films on buffered Si(100)

We have grown non-c-oriented SrBi2Ta2O9 (SBT) epitaxial thin films with well-defined (116) orientation by pulsed laser deposition on yttria-stabilized zirconia-buffered (YSZ-buffered) Si(100) substrates covered with electrically conductive (110)-oriented SrRuO3 (SRO) bottom electrodes. The epitaxial growth of (110)-oriented SRO films on (100)-oriented YSZ on Si(100) was confirmed both by x-ray pole figures and transmission electron microscopy (TEM) analyses showing a diagonal-type rectangle-on-cube epitaxy of SRO on YSZ with respect to the substrate and yielding specific multiple twins which originate from the particular in-plane positioning of SRO on YSZ. Cross-sectional TEM analyses revealed a roof-like morphology at the SBT/SRO interface while the other interfaces are sharp. The ferroelectric measurements of the (116)-oriented SBT films show a remanent polarization (2Pr) of 6.8 μC/cm2 and a coercive field (2Ec) of 142 kV/cm for a maximum applied electric field of 283 kV/cm. A comparable hysteresis loop recorded from local piezoresponse by an atomic force microscope working in a piezoelectric mode has also been obtained.

Refinement of the Crystal Structures of Strontium Cobalt Oxychlorides by Neutron Powder Diffraction

The crystal structures of strontium cobalt oxychlorides of general formula Sr2CoO3Cl (Co-1L), Sr3Co2O5Cl2 (Co-2L), and Sr4Co3O7.5Cl2 (Co-3L) have been determined from neutron powder data. The compounds crystallize in tetragonal symmetry, space group P4/nmm (Co-1L) or I4/mmm (Co-2L and Co-3L). Their structures are analogous to the Ruddlesden−Popper phases of general formula Srn+1MnO2n+1. All three structures contain layers of highly distorted CoO5Cl octahedron. The Co-3L structure, in addition, has a layer of regular CoO6 octahedra. Because the in-plane oxygen position of the latter is partially occupied, some of the Co atoms adopt pyramidal coordination, with their basal planes parallel to the c-axis. The temperature dependence of the magnetic susceptibility is also reported. The moment per mole of Co atom decreases with increasing number of (CoO2) layers.

Dynamic behavior of nickel atoms in graphitic networks

The dynamic behavior of nickel atoms in graphitic carbon onions, observed by in situ atomic-resolution electron microscopy, shows the formation of stable new C-Ni phases. Nickel is observed to take substitutional in-plane positions in graphene layers, forming a planar graphitelike C-Ni lattice. Evidence is furthermore seen for the formation of a cubic C-Ni phase, suggesting a possible phase transformation in C-Ni from a graphitelike to a diamondlike structure. The stability of the planar phases is shown by first-principles calculations which also indicate that the C-Ni planes are metallic.

Supramolecular Structures of Peptide Assemblies in Membranes by Neutron Off-Plane Scattering: Method of Analysis

In a previous paper (Yang et al., Biophys. J. 75:641–645, 1998), we showed a simple, efficient method of recording the diffraction patterns of supramolecular peptide assemblies in membranes where the samples were prepared in the form of oriented multilayers. Here we develop a method of analysis based on the diffraction theory of two-dimensional liquids. Gramicidin was used as a prototype model because its pore structure in membrane in known. At full hydration, the diffraction patterns of alamethicin and magainin are similar to gramicidin except in the scale of q (the momentum transfer of scattering), clearly indicating that both alamethicin and magainin form pores in membranes but of different sizes. When the hydration of the multilayer samples was decreased while the bilayers were still fluid, the in-plane positions of the membrane pores became correlated from one bilayer to the next. We believe that this is a new manifestation of the hydration force. The effect is most prominent in magainin patterns, which are used to demonstrate the method of analysis. When magainin samples were further dehydrated or cooled, the liquid-like diffraction turned into crystal-like patterns. This discovery points to the possibility of investigating the supramolecular structures with high-order diffraction.

A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt 25Rh 75(100)

Pt 25Rh 75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23 L O 2, 600°C). A previous STM study on O/Pt 50Rh 50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant (“shifted rows”). We present a LEED I( E) analysis of Pt 25Rh 75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I( E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95 Å. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08 Å, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38 Å. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt 25Rh 75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt 25Rh 75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt 25Rh 75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt 25Rh 75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.

Radiation Products of Thymine, 1-Methylthymine, and Uracil Investigated by Density Functional Theory

The geometries, relative energies, and hyperfine coupling constants of dehydrogenated, hydrogenated, and hydroxylated radiation products, as well as the anion and the cation, of thymine (T), 1-methylthymine (1-MeT), and uracil (U) are calculated through the use of density functional theory. The results for T and 1-MeT are subsequently compared to accurate ENDOR experiments performed by Sagstuen and co-workers (Sagstuen, E.; Hole, E. O.; Nelson, W. H.; Close, D. M. J. Phys. Chem. 1989, 93, 5974 and references therein). The theoretical coupling constants support the experimental assignment of the observed radicals. It is suggested that the hydrogen added to form the O4-hydrogenated product is located at an angle of 50° out of the molecular plane in T. Unfavorable interactions of this hydrogen with the N3 and the methyl hydrogens are shown to lead to this out-of-plane position for O4−H. For the similar radical in 1-MeT, hydrogen bonding in the crystal leads to an in-plane hydrogen position. Notable geometry ...

Sulfur Distribution in Bacteriorhodopsin from Multiple Wavelength Anomalous Diffraction Near the Sulfur K-Edge with Synchrotron X-Ray Radiation

Bacteriorhodopsin contains nine sulfur atoms from the nine methionine residues. The distribution of these sulfur atoms in the projected density map was determined from x-ray diffraction experiments using multiple wavelength anomalous diffraction (MAD) at the sulfur K-edge (5.02 Å) with synchrotron radiation. The experiments were performed with uniaxial samples of oriented purple membranes at room temperature and 86% relative humidity. For such samples only the real part f′ ( λ) of the resonant scattering amplitude of sulfur contributes to the observed scattering intensity. The sulfur density was determined from the difference in diffraction intensities detected at two wavelengths near the sulfur K-edge that were ∼0.004 Å apart. The measured change in f′ between these two wavelengths corresponds to 6 electron units. This shows that large anomalous dispersion effects occur near the sulfur K-edge. The in-plane positions of the sulfur atoms of Met 32, Met 56, and Met 209 were determined unambiguously. The difference density from Met 20, Met 60, Met 118, and Met 145 is concentrated in the interior of the seven α-helical bundle, overlaps strongly in the projected density map, and cannot be resolved at the resolution of these experiments (8.2 Å). This method of localizing individual sulfur atoms can be applied to other two-dimensional protein crystals and is promising in conjunction with the site-directed introduction of sulfur atoms by the use of cysteine mutants.

Structure and Reactivity of [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)](4+) Complexes. Quantum Chemical Calculations, X-ray Structural Characterization, and Raman Spectroscopic Measurements.

A series of compounds containing the [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)-(dtc)(3)](+) complex (dtc = diethyldithiocarbamate) with the anions I(-) (1), I(-) and Br(-) (2), S(2)(-) (3), ClO(4)(-) (4), NO(3)(-) (5), and SO(4)(2)(-) (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH(2)Cl(2) in the crystal having the composition [Mo(3)S(7)(dtc)(3)]I.0.5CH(2)Cl(2) (1a), which was in contradiction to the previous protocol. The corresponding ClO(4)(-) compound (4a) is isotypic. Crystal data: C(15.5)H(31)Cl(2)Mo(3)N(3)O(4)S(13), orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms S(ax) of the disulfido bridges. The Raman data showed a significant decrease of the S(eq)-S(ax) stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the S(eq)-S(ax) bond length and with a significant shortening of the X.S(ax) distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO(3)(-) and ClO(4)(-) the S(ax).X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I(-) and S(2)(-) significant covalency is observed. In addition, the general reaction of [Mo(3)S(7)](4+) complexes with a nucleophile was modeled. With regard to the side-on bonding of the &mgr;-S(2) groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to &mgr;(3)-S, are less pronounced.

Simulation of epitaxial growth of CeO 2 on YSZ(100) and SrTiO 3 on MgO(100) for YBa 2Cu 3O 7− x deposition

A simulation based on the optimization of the interaction energy between the atoms at both sides of the different film and film-substrate interfaces has been used to qualitatively predict their relative in-plane positioning and orientation. The method has been applied to YBa 2Cu 3O 7 (YBCO), CeO 2 and SrTiO 3 films on yttria stabilized zirconia (YSZ) (100) and MgO(100) single crystals. It allows to predict cube-on-cube epitaxial growth of CeO 2 on YSZ(100) and SrTiO 3 on MgO(100). [00h] oriented YBCO epitaxial growth should take place on the buffers with [0h0] axes rotated 45° away from [100] direction of CeO 2, and parallel to [100] direction on SrTiO 3. The computed relationships are in agreement to the results obtained in laser ablation deposited YBCO/YSZ(100), YBCO/CeO 2/YSZ(100), YBCO/MgO(100) and YBCO/SrTiO 3/MgO structures as well as other reported results.

Catheter tracking using continuous radial MRI.

The guidance of minimally invasive procedures may become a very important future application of MRI. The guidance of interventions requires images of the anatomy as well as the information of the position of invasive devices used. This paper introduces continuous radial MRI for the simultaneous acquisition of the anatomic MR image and the position of one or more small RF-coils (mu-coils), which can be mounted on invasive devices such as catheters or biopsy needles. This approach allows the in-plane tracking of an invasive device without any prolongation of the overall acquisition time. The extension to three-dimensional position tracking is described. Phantom studies are presented demonstrating the capability of this technique for real-time automatic adjustment of the slice position to the current catheter position with a temporal resolution of 100 ms. Simultaneously the in-plane catheter position is depicted in the actually acquired MR image during continuous scanning.

Static magnetic and microwave properties of Li-ferrite films prepared by pulsed laser deposition

Highly textured Li-ferrite films have been synthesized by pulsed laser deposition from bulk targets onto c-plane sapphire substrates. For substrate temperatures approaching 1000 °C the growth mode was predominantly (333). A similar growth mode was recently reported for Ni-ferrite films grown onto c-plane sapphire substrates. The films were 4–8 μm thick and were grown at deposition rates of 3–5 Å/s at oxygen partial pressures of 200–400 mTorr. Magnetic measurements were made by SQUID and 9.5 GHz ferromagnetic resonance (FMR) on the as deposited films. Textured Li-ferrite films with x-ray diffractometer intensity ratios of I(333)/I(311)=10.3 for films made in 200 mTorr oxygen and 14.5 for films made in 400 mTorr oxygen exhibited room temperature in-plane intrinsic coercivities of <100 Oe for SQUID magnetometer measurements. Highly (333) textured Li-ferrite films made in 400 mTorr oxygen exhibited clean symmetric FMR absorption lines with a derivative peak-to-peak linewidth of 335 Oe for the in-plane static field configuration. The FMR profiles for the perpendicular-to-plane field configuration were highly distorted. The magnetization versus field measurements and the in-plane FMR field position yielded saturation flux density 4πMs and effective saturation flux density 4πMeff values of 3.6–4.0 kG and 3.56–3.70 kG, respectively. These values are in the same range as for bulk Li-ferrite. The in-plane FMR linewidths were significantly larger than expected for bulk single crystals or large grain size, dense polycrystals.

Characterization of the structure of YD 3 by neutron powder diffraction

Neutron-powder-diffraction (NPD) measurements of the hexagonal rare-earth trideuteride γ-YD 3 were undertaken between 10 and 400 K. Rietveld refinements indicated that the YD 3 structure possesses P 3 ̄ c1 ( D 3d 4) symmetry, in agreement with previous NPD studies of YD 3 and HoD 3. The unit cell is a (√3 × √3)R30 ° expansion of the conventional hcp unit cell in the ab plane. To accomodate the D atoms, the c axis for YD 3 is elongated by ~ 15% [ c 0 a 0 = 1.8019(1), a 0 = 3.6627(1) A ̊ , at 295 K] compared with that for Y. The D atoms occupy unusual interstitial positions of a slightly distorted, hcp metal lattice, instead of the ideal octahedral ( o) and tetrahedral ( t) sites. In particular, the hypothetical o-site D atoms are displaced vertically toward the Y-defined basal planes, with correlated placements at either near-plane or in-plane threefold metal ( m) sites. The t-site D(D t ) atoms are displaced horizontally, also in a correlated fashion, with respect to the m-site D(D m ) atoms. This structure is maintained over the entire temperature range from 10 to 400 K. Moreover, the data suggest the presence of an order-disorder equilibrium amongst the D m atoms. The disorder involves c-axis displacements of a fraction of D m atoms from near-plane to in-plane positions, concomitant with an equal number of compensating D m atoms shifting from in-plane to near-plane positions. The displacements are facilitated by the relatively open channels formed along the c direction by the Y and D t atoms.

Local order in YBa2Cu3−yCoyO6+2x studied by anomalous diffuse x-ray scattering

The local order in a single crystal of YBa2Cu2.53Co0.47O7.13 has been studied with anomalous diffuse x-ray scattering. (For such a Co concentration the compound is nonsuperconducting.) Intensity measurements were carried out at two energies below the Co edge. The difference data could then be expressed in terms of the local structure around a Co atom. The short-range order parameters (α's) indicate that the Co and Cu atoms are nearly randomly distributed on the Cu1 sites. The estimated size of the Co-free “domains” is 5-7 Å. The first neighbor in-plane Co-Co distances are significantly shortened, indicating that the Co atoms are displaced from their average positions. The data also show a significant decrease of the Co-O1 distance, leading to an increase of the Cu2-O1 distance. The lengthening of the Cu2-O1 distance implies a lowering of the Cu2 formal valence. The Co substitution also affects the in-plane Cu2 positions. The present study shows that the Cu-O2 structural coherence is altered on a scale smaller than the superconducting coherence length. As far as the superconductivity is concerned, the Cu2 valence remains one of the most important parameters in determining the superconducting properties of the Co-doped 123 compounds. On the other hand, there is some evidence that in order for superconductivity to occur in this and other doped cuprate compounds, the size of dopant-free regions in the basal plane may have to exceed the superconducting coherence length.