Bitter Decoration Research Articles

Observations of vortex chains in Bi2Sr2CaCu2O8+y

Vortex states in Bi2Sr2CaCu2O8+y under tilted fields were studied by means of magneto-optical imaging and Bitter decoration. One-dimensional arrangements of pancake vortices (vortex chains) interacting with underlying Josephson vortices are visualized. From the spacing of vortex chains we directly determined the variation of the anisotropy parameter of the electronic mass (γ) from 800 in slightly underdoped to 180 in highly overdoped samples. These results indicate that the crossing-lattices model for highly anisotropic superconductors remains relevant at least for γ≥180, contrary to what was predicted theoretically by Dodgson [Matthew J. W. Dodgson, Physica C369, 182 (2002).].

Bitter decoration of vortex structure in MgB 2 single crystals

The Bitter decoration technique has been used to observe vortices in the superconducting state of single crystal MgB 2. Experiments in low applied magnetic field ( B≈4.4 G) allow for the estimation of the London penetration depth, λ, to be approximately 1900 Å for T≈6 K. Experiments in higher applied fields ( B≈200 G) unambiguously show a triangular vortex lattice in both real space (a larger than 10 μm×10 μm image of over 1000 vortices) as well as in reciprocal space.

Low-field reorientation transition and anisotropic orientational order of vortex lattices inLuNi2B2C

Vortex lattices in as-grown and annealed ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ single crystals were studied by high resolution Bitter decoration in fields up to 2000 Oe. The reorientation transition between two different rhombic structures is recorded at ${H}_{1}\ensuremath{\approx}250 \mathrm{Oe}$ in annealed crystals in which the transition to the square lattice occurs at ${H}_{2}\ensuremath{\approx}500 \mathrm{Oe},$ which is lower than for as-grown crystals. A strongly anisotropic orientational order is discovered in the vicinity of ${H}_{2}.$ These results are discussed in the framework of the nonlocal London model.

Vortex dynamics in Bi 2Sr 2CaCu 2O 8+ δ single crystals with planar defects

We have studied the effects of c-axis correlated planar defects on the vortex lattice of Bi 2Sr 2CaCu 2O 8 single crystals using Bitter decoration, transverse ac permeability and X-ray measurements. Our results show that the low field low temperature phase coherent vortex lattice exhibits anisotropic pinning in the presence of these planar defects. Frequency dependent ac permeability measurements show that the ordered solid–liquid transition is first order as in samples free of correlated defects. However, the melting point is detected only when the excitation field is along the defects.

Interplay between pinning energy and vortex interaction inYBa2Cu3O7−δwith oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements

Interplay between pinning energy and vortex interaction in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mi>−</mml:mi><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements

Repetition of the disordered pattern in successive solidifications of vortex matter observed by Bitter decoration

Repeated images of the vortex structure in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystals have been obtained, using the Bitter technique. By removing the iron dots between each decoration experiment, it is possible to image the vortices in different cooldown runs done under the same field and temperature conditions. The images show that these different realizations of a disordered vortex state found in twinned crystals are very similar at long range and differ only in small-scale detail. This is unusual behavior in glassy systems, where expectations are that successive configurations will differ over many scales. In contrast, ordered vortex crystals in clean samples can form with different orientations in different runs, therefore differing at long ranges. Double-sided decorations have been performed in twinned samples, and the correlation between images in both sides of the sample is similar in magnitude but slightly different qualitatively than that found in successive decorations.

Bitter pattern study of flux distributions in melt-processed YBCO

A high-resolution Bitter decoration technique has been used toobserve the microstructure of flux-line lattices in melt-processed YBa2Cu3Oy (YBCO). In particular, we studied both static anddynamic flux distributions at low (4-23 K) and high temperatures (68-69 K)after field-cooling in the presence of 150 G. We could observe the flux motionafter a long deposition time. There were remarkable differences in thebehaviour of the flux motion, depending on the temperature.

Dynamic studies of vortices in NbSe 2, from single flux lines to lattices

We present different techniques to study the dynamics of the vortex lattice in NbSe 2 single crystal samples. In the first set of experiments, we applied a field parallel to the c crystalline axis and extended the Bitter decoration technique to the study of dynamic structures by imaging the lattice during creep (“decoration during motion”). This method exposed a new smectic phase and an anisotropic “crystal” at higher fields. In the second set, we used a micro-mechanical high- Q oscillator to study the mesoscopic flux dynamics along the crystallographic ab plane. In this case, we were able to observe single vortex penetration, well described by a ‘1+1’-dimensional model.

Observation of periodic vortex pinning induced by Bitter decoration

In this paper we report on a type of vortex pinning, Bitter pinning. In this technique, a vortex lattice in a type II superconductor, ${\mathrm{NbSe}}_{2},$ is decorated with magnetic particles which form clumps on the surface of the sample where the vortices exit. Because of the van der Waals forces, these magnetic dots stay well fixed after the sample is returned to room temperature. We present data showing that these patterns can act as pinning sites for vortices in subsequent experiments. This method allows one, in a simple and effective way, to produce periodic, quasiperiodic, or disordered pinning structures for static and dynamic vortex experiments.

Superficial Periodic Pinning Induced by Bitter Decoration Applied to the Study of Vortex Structure Nucleation and Growth

We demonstrate that the use of the dynamic memory effect combined with vortex pinning induced by Bitter decorations allows the study of nucleation and propagation of vortex structures. It is shown that despite its low critical currents the static vortex structure in NbSe2 has an intrinsic granular topology. On the other hand the BSCCO crystals seem to be excellent candidates to examine the behavior of recent theories where the existence of a Bragg glass is predicted.

Local magnetic probes of superconductors

Investigations of the magnetic properties of high temperature superconductors (HTSs) have revealed the existence of striking new vortex phenomena due, in part, to their strong crystalline anisotropy, very short coherence lengths and the much larger thermal energies available at high temperatures. Some of these phenomena, for example vortex lattice 'melting', pose serious problems for technological applications of the most anisotropic HTS materials and a fuller understanding of them is of considerable importance. The most direct information regarding vortex structures and dynamics is obtained through local measurement of the magnetic field within or at the surface of a superconducting sample. A detailed review of such local magnetic probes is presented here including Lorentz microscopy, magnetic force microscopy, Bitter decoration, scanning Hall probe microscopy, magneto-optical imaging, and scanning superconducting quantum interference device microscopy. In each case the principles underpinning the technique are described together with the factors that limit the magnetic field and the spatial and temporal resolution. A range of examples will be given, emphasizing applications in the area of HTSs. In addition the ways in which the existing techniques can be expected to develop over the next few years will be discussed and new approaches that seem likely to be successful described.

Direct observation of vortex structure in a high-{Tc} YBa{sub 2}Cu{sub 3}O{sub 7{minus}y} thin film by Bitter decoration method

The Bitter decoration technique is one of the most powerful techniques to study the vortex structure of superconductor. We report the observation of vortex structure in a high TcYBa2Cu3O7−y(YBCO) thin film by Bitter decoration method. The image of vortex structure was monitored by SEM, AFM and high resolution optical microscope. For magnetic field about 4-6mT,a vortex structure is seen. The vortex image varied with changing magnetic field. As compared with the vortex image of a Bi2Sr2CaCu2O8+ysingle crystal, the observed image appeared to be more randomly distributed.

Study of pinning effects for vortices in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals using a Bitter decoration technique

Distribution of vortices has been analyzed to study on the pinning effects of the vortices in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals, observed with a Bitter decoration technique. On the cleaved surfaces of the samples, vortices are pinned in the disordered configurations at lower magnetic fields, which change to a hexagonal lattice structure with increasing a magnetic field. Furthermore, a dense concentration of vortices has been observed in the voids and on the lower terrace of the steps. These distributions of the vortices are found to be very stable from the estimation of the pinning energy.

Real Space Images of the Vortex Lattice Structure in a Type II Superconductor during Creep over a Barrier

We report on Bitter decoration studies of the magnetic flux line lattice in the type II superconductor ${\mathrm{NbSe}}_{2}$ during flux creep experiments over barriers, a surface step. We find that these steps can act as ``vortex diodes.'' By imaging the real space structures of the vortex lattices as they flow over these steps and measuring the change in vortex density, we are able to measure the energetic height of the barrier as well as the elastic correlation length of the vortices along their length. At low fields, we find that the vortices are correlated over distances of roughly $5\ensuremath{\mu}\mathrm{m}$.

Neutron standing wave resonator to measure the flux line lattice in thin film superconducting samples (abstract)

Flux line lattices in superconductors have been extensively studied in bulk samples by a variety of techniques including neutron diffraction. We present a technique that is useful for studying the flux lattice in a thin film in the configuration where the field (and hence the flux lines) is parallel to the sample surface. This configuration provides information about the in-plane order and is not accessible by other means (e.g., scanning tunnel microscope, Bitter decoration). A 5000-Å-thick niobium film epitaxially grown on an MgO substrate is studied by polarized neutron reflectometry (PNR) in this parallel field geometry. In order to enhance the signal, a neutron standing wave resonator is created in the superconducting film by spin casting a thin layer of deuterated polystyrene on the Nb. The increased scattering caused by the presence of flux lines in the sample leads to a depletion of the reflected neutron beam at the resonant wave vector. Our results show the expected depletion in the superconducting state for fields at which flux lines are expected to be present. The field and temperature dependence of the data are consistent with scattering arising from flux lines. We discuss plans to prove that the source of the increased scattering is indeed the flux lines. The most conclusive evidence for a well-ordered flux lattice in the sample is the presence of off-specular diffraction, strongly enhanced by the standing wave in the resonator. Measurements to see this off-specular signal have proved unsuccessful, leading us to conclude that we may not have a well-ordered lattice in our superconducting sample.

Generalized London free energy for high-Tcvortex lattices

We generalize the London free energy to include fourfold anisotropies that could arise from d-wave pairing or other sources in a tetragonal material. We use this simple model to study vortex lattice structure and discuss neutron scattering, scanning tunneling microscopy, Bitter decoration, and muon-spin-rotation experiments.

Nucleation of vortices inside open and blind microholes.

The critical field of a thin superconducting film with a blind circular hole is found theoretically. It is shown that the value of the critical field is sensitive to the bottom thickness, but the orbital momentum, i.e., the number of vortices which nucleate inside the hole, is not sensitive. A simple boundary condition for a steplike thin film is derived and used for comparative numerical analysis of the superconductivity nucleation in a microdisk and near the hole. By increasing the thickness of the bottom of a blind hole one can transform the hole into a disk of the same radius which rests on top of the film. We show that such transformation leads to a jump in the number of vortices which nucleate at the critical magnetic field inside the perimeter of the hole (the disk). We report also the results of the Bitter decoration experiments of a thin superconducting film with a lattice of open or blind holes. It is found (in accordance with the calculation) that the bottom thickness has only a weak influence on the number of vortices captured by a hole during the cooling of the sample at a constant perpendicular magnetic field. All the experimental results are explained under the assumption that the vortices nucleated inside a hole rest inside during the cooling process and no additional vortices enter the hole. \textcopyright{} 1996 The American Physical Society.

Vortex decoration in untwinned and twinned YBCO single crystals. Study of the influence of the magnetic field angle and comparison with theory

The vortex lattice in untwinned and twinned YBCO single crystals has been observed in function of the magnetic field orientation, using the high-resolution Bitter decoration technique [6]. In untwinned samples and in tilted fields, a chain structure appears, which is independent of crystallographic orientation. In weekly twinned samples, pinning along the twin boundaries is still present in addition to the vortex-chain state. A quantitative analysis of these observations and comparison to theory is presented.

Edge superconducting states

Surface superconductivity usually refers to the nucleation of superconctivity near the outer surface of a bulk specimen. The same intrinsic phenomena takes place in a patterned thin film giving rise to localized superconducting edge states near any free boundary, thickness jump or local variation of the vector potential. We have investigated a thin superconducting film with an array of patterned microholes. A splitting of the critical field line is found when the field is perpendicular to the film. The new transition line (Hc3) exhibits two types of fine structure due to the quantization of the multiply connected edge states. We discuss these states in terms of multiquanta vortices nucleated inside the holes. We report Bitter decoration of the the corresponding vortex configuration. Because of the enhancement of the order parameter near the hole edges, the interaction between the vortices and the microholes is repulsive slightly below the bulk critical field (Hc2) and is much stronger than the intervortex interaction. We find a "localization transition": all vortices drop inside holes when the coherence length becomes larger than the interhole spacing. This transition explains the two types of oscillation found in the transition line and leads to a qualitative change of the creep below the second critical field.

Role of edge superconducting states in trapping of multi-quanta vortices by microholes. Application of the bitter decoration technique

The Bitter decoration technique is used to study the trapping of single and multiple quanta vortices by a lattice of circular microholes. By keeping a thin superconducting layer (the bottom) inside each hole we are able to visualise the trapped vortices. From this we determine, for the first time, the filling factor FF, i.e. the number of vortices captured inside a hole. In all cases the sample is cooled at a constant field before making the decoration. Two qualitatively different states of the vortex crystal are observed: (i) In case when the interhole distance is much larger than the coherence length, the filling factor averaged over many identical holes (〈(FF〉)) is a stepwise function of the magnetic flux (of the external field) through the hole, because each hole captures the same number of vortices. The density of fluxoids inside the openings is higher than in the uniform film, but much lower than it should be in the state of equilibrium. We claim that the number of trapped vortices is determined by the edge superconducting states which appear around each hole at the modified third critical fieldH c3 * >H c2 . BelowH c2 such states produce a surface barrier of a new type. This barrier for the vortex entrance and exit is due to the strong increase of the order parameter near the hole edge. It keeps constant the number of captured vortices during the cooling at a fixed field, (ii) An increase of the hole density or of the hole radius initiates a sharp redistribution of fluxoids: all of them drop inside holes. This first order transition leads to a localization of all vortices and consequently to a qualitative change of the transport properties (TAFF in our case). In the resulting new state the filling factor is not any more the same for neighbouring holes and its averaged value is equal to the frustration of the hole network.