BSCCO Single Crystals Research Articles

1 fm/Hz noise level low temperature Fabry-Pérot atomic force/magnetic force microscope operating in 4–300 K temperature range

In this publication, we describe the design of a new fiber Fabry-Pérot interferometer and employ this to a low temperature atomic force/magnetic force microscope (LT-AFM/MFM) operating in the 4-300 K temperature range. A multilayer dielectric mirror coated optical fiber is used to achieve unprecedented 1 fm/Hz noise level, while the shot noise limit is 0.51 fm/Hz. The cavity length is adjustable, and the fiber can be brought within a very close proximity of the cantilever using a dedicated 2 mm stroke piezonanopositioner integrated on the piezotube scanner. The same nanopositioner also is used to park the fiber at a safe parking location during cantilever exchange. We demonstrate the performance of the LT-AFM/MFM by imaging the ITO thin film at 300 K, atomic steps on HOPG at 300 K, magnetic bits on the hard disk at 15 K, and the Abrikosov vortex lattice in the BSCCO(2122) single crystal at 4 K.

Radiation pressure excitation of a low temperature atomic force/magnetic force microscope for imaging in 4-300 K temperature range.

We describe a novel radiation pressure based cantilever excitation method for imaging in dynamic mode atomic force microscopy (AFM) for the first time. Piezo-excitation is the most common method for cantilever excitation, however it may cause spurious resonance peaks. Therefore, the direct excitation of the cantilever plays a crucial role in AFM imaging. A fiber optic interferometer with a 1310 nm laser was used both for the excitation of the cantilever at the resonance and the deflection measurement of the cantilever in a commercial low temperature atomic force microscope/magnetic force microscope (AFM/MFM) from NanoMagnetics Instruments. The laser power was modulated at the cantilever's resonance frequency by a digital Phase Locked Loop (PLL). The laser beam is typically modulated by ∼500 μW, and ∼141.8 nmpp oscillation amplitude is obtained in moderate vacuum levels between 4 and 300 K. We have demonstrated the performance of the radiation pressure excitation in AFM/MFM by imaging atomic steps in graphite, magnetic domains in CoPt multilayers between 4 and 300 K and Abrikosov vortex lattice in BSCCO(2212) single crystal at 4 K for the first time.

Metastable inhomogeneous vortex configuration with non-uniform filling fraction inside a blind hole array patterned in a BSCCO single crystal and concentrating magnetic flux inside it

Using magneto-optical imaging, we map the local magnetic field distribution inside a hexagonally ordered array of blind holes patterned in BSCCO single crystals. The nature of the spatial distribution of local magnetic field and shielding currents across the array reveals the presence of a non-uniform vortex configuration partially matched with the blind holes at sub-matching fields. We observe that the filling fraction is different in two different regions of the array. The mean vortex configuration within the array is described as a patchy vortex configuration with the patches having different mean filling fraction. The patchy nature of the vortex configuration is more pronounced at partial filling of the array at low fields while the configuration becomes more uniform with a unique filling fraction at higher fields. The metastable nature of this patchy vortex configuration is revealed by the application of magnetic field pulses of fixed height or individual pulses of varying height to the array. The metastability of the vortex configuration allows a relatively easy way of producing flux reorganization and flux focusing effects within the blind hole array. The effect of the magnetic field pulses modifies the vortex configuration within the array and produces a uniform enhancement in the shielding current around the patterned array edges. The enhanced shielding current concentrates magnetic flux within the array by driving vortices away from the edges and towards the center of the array. The enhanced shielding current also prevents the uninhibited entry of vortices into the array. We propose that the metastable patchy vortex configuration within the blind hole array is due to a non-uniform pinning landscape leading to non-uniform filling of individual blind holes.

Influence of spin injection on the in-plane and out-of-plane transport properties of BSCCO single crystal

We have studied the influence of spin injection on the in-plane and out-of-plane transport properties of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ . Two different types of samples have been fabricated. One is a mesa composed of $\hbox{Co/Au/Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ for the out-of-plane transport measurement, and another is $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ single crystal bridge for the in-plane transport measurement. For the former, the hysteretic field dependence of $I_{c} $ of intrinsic Josephson junctions (IJJs) in the mesa, due to the spin injection effect, was observed at 77 K. Moreover, it was observed that the hysteresis in the $I_{c}$ – $B$ curves decreased with increasing the number of IJJs in mesas. This suggests that the contribution of the Josephson effect to the $c$ -axis transport property becomes larger than that of the spin injection effect with the increase in the number of junction. On the other hand, for the latter, hysteretic field dependence of the magnetoresistance was observed. These results give us the evidence for the superconductivity suppression of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ due to the injection of spin-polarized quasiparticles.

Cavity phenomenon and terahertz radiation of a tall stack of intrinsic Josephson junctions wrapped by a dielectric material

In order to enhance the radiation power in the terahertz band based on intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ single crystal, we investigate a long cylindrical sample embedded in a dielectric material. Tuning the dielectric constant, the radiation power has a maximum which is achieved when it equals the dissipation caused by the Josephson plasma. This yields the optimal dielectric constant of the wrapping material in terms of the properties of a BSCCO single crystal. The maximal radiation power is found proportional to the product of the critical superconducting current squared and the typical normal resistance, which offers a guideline for choosing superconductors as a source of strong radiation.

Thin-film-like BSCCO single crystals made by mechanical exfoliation

Mechanical exfoliation has been proven to be an effective technique for producing one to several layers of strong anisotropic materials such as graphite, NbSe2 and Bi2Te3. Using this technique, thin-film-like BSCCO (Bi2Sr2CaCu2O8+δ and Bi2Sr2Ca2Cu3O8+δ) single crystals with typical thickness of a few to a few tens of nanometres are successfully produced. The thinnest crystal is only 0.5 unit cell in the c-axis. The measured resistivity–temperature properties of a 30nm-thick crystal show excellent superconductivity. It is observed that the topology of exfoliated thin-film-like single crystal can copy the structure of the substrate in AFM and profilometer measurements. With the aid of a dual-beam Focused Ion Beam, the interface between the thin-film-like BSCCO single crystal and a step-structured substrate is visible in a cross-section view, which shows that there is a gap between the step substrate and the thin-film-like single crystal. The gap size is sensitive to the thickness of the thin-film-like BSCCO single crystal as well as the structure of the substrate.

Direct observation order–disorder transition in flux line lattice BSCCO(2212) at low temperatures

The mixed state of highly anisotropic high-temperature superconductors like BSCCO has a very complicated phase diagram. The vortex structure has been investigated in optimally doped BSCCO single crystals at helium temperatures and magnetic fields up to 600 Oe by high- resolution Bitter technique. The transition from regular triangular vortex lattices to disordered state was revealed within an interval of magnetic fields 230–400 Oe for different samples. The anisotropy parameter was measured from observation of chain vortices at tilted magnetic fields. The results are discussed in framework a model taking in to account the influence of anisotropy and defects on vortex phase diagram.

Frequency dependence of the resistive switching effect in Bi 2Sr 2CaCu 2O 8+ y/Ag film heterocontacts

In this work we have performed the relaxation studies “ in situ” of the electron instability effect (EIE) in the heterostructures based on BSCCO single crystals. The new effect of suppression of EIE or colossal electroresistance via application of an alternating low frequency electric field to the heterojunctions in the BSCCO-based single crystals has been found. It has been shown that the top possible frequencies for observation of the effect are of the order of 10 3 Hz. This fact is interpreted as accumulation of the oxygen ions driven by the electric field to the interface. On the other hand, it has been shown that the switching events are limited by two time processes: t ≈ 1 ms and about ten seconds. The first ones are caused by rearrangement of a charge net in the degraded surface at the electric field switching. The latter are caused by oxygen diffusion to vacancies under electric field above some threshold value. The considered experimental data confirm the correlation character of the HTSC properties as Mott systems, which appears in extreme sensitivity to the doping level, in the tendency to phase separation under external actions, in the hysteresis character of the metal–insulator transition.

Fabrication and Transport Properties for Cleaved Thin Film BSCCO Single Crystals

We have fabricated truly single-crystal BSCCO thin films of the quality of traveling-solvent-floating-zone single crystals by using a double side cleaving technique. The thicknesses of the cleaved thin films thus obtained ranges from 20 nm to 100 nm. These films are found to adsorb or desorb oxygen very easily at a relatively low annealing temperature of 200 to 300degC. When a cleaved film is annealed in Argon at 300degC for 10 min, it changes to an insulator without any trace of superconducting transition. When this film is subsequently annealed in oxygen at 300degC , it exhibits a sharp superconducting transition at about 90 K. When the film is subjected to these process repeatedly, the film changes from an insulator to a superconductor quite reversibly. We have measured in-plane resistivity and the Hall coefficient systematically in a wide doping range for this thin film BSCCO single crystals.

Intrinsic Josephson junctions in mesas and ultrathin BSCCO single crystals: Ultimate control of shape and dimensions

We describe experiments which are only possible through an ultimate control of sample shape and dimensions down to nanometer scale whereby transport measurements can be done in various restricted geometries. We use photolithography patterning together with a flip–chip technique to isolate very thin (d∼100nm) pieces of Bi2Sr2CaCu2O8+δ (BSCCO) single crystals. Ar-ion milling allows us to further thin these crystals down to a few nanometers in a controlled way. With decreasing thickness below two to three unit cells, the superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor–insulator transition in these ultrathin single crystals. In a refined technique, a precise control of the etching depth from both sides of the crystal makes it possible to form stacks of intrinsic Josephson junctions (IJJs) inside the ultrathin single crystals. The stacks can be tailor-made to any microscopic height (0–9nm<d), i.e. enclosing a specific number of IJJs (0–6). In certain geometries, by feeding current into the topmost Cu2O4-layer of a mesa on the surface of a BSCCO single crystal, we measured the critical value of this current by detecting a sharp upturn or break in the current–voltage characteristics. From this, we estimate the sheet critical current density of a single Cu2O4 plane to be ∼0.3–0.7A/cm at 4.5K, corresponding to a bulk current density of ∼2–5MA/cm2. These values are among the largest ever reported for BSCCO single crystals, thin-films and tapes.

Twisting waves in bulk YBCO

The formation of “twisted” vortex structure under the action of rocking magnetic field, and its stability are studied on a set of single crystal YBCO plates with varied thickness. The result are compared with those, received on melt-textured YBCO plates and on doped BSCCO single crystals. Based on our results we argue the helical nature of twisters.

Search for in-phase Josephson vortex solutions in BSCCO type junctions: modelling andnumerical simulations

The Josephson fluxon behaviour in layered BSCCO-type single crystals is modelled bycoupled long Josephson junctions described by coupled sine–Gordon equations. Among allthe possible solutions, particularly interesting are the so-called in-phase fluxon solutionswith coherent motion of all the fluxons. Recently, generation of electromagnetic waves atalmost terahertz (THz) frequencies in BSCCO single crystals was reported. In order tomaximize the power of such generation, these in-phase fluxon solutions are essential.Two different avenues to obtain in-phase fluxon motion have been suggested,i.e. either using the interaction with a cavity to phase lock the fluxon motion orapplying a large magnetic field to create in-phase flux flow solutions. We havenumerically considered both mechanisms. The results provide an understanding of thesystem and a guidance to experimentalists looking for THz generation in BSCCO.

Intrinsic Josephson junctions formed inside ultra-thin BSCCO single crystals

Ultra-thin pieces of Bi2Sr2CaCu2O8+δ single crystals (d<100 nm) can be isolated by cleaving pre-etched mesas off the main crystal using aflip-chip technique. Further thinning of the pieces by Ar-ion milling down to afew nanometres reveals the superconductor–insulator transition in the samples.In a refined technique, a precise control of the etching depth from both sidesof the crystal makes it possible to form stacks of intrinsic Josephson junctions(IJJs) inside these ultra-thin single crystals. The stacks can be tailor-made toany small height (0–9 nm) thus assuring any low number of IJJs in the stacks(N = 0–6). Several features can be seen in the current–voltage(I–V) characteristics of these stacks, and thereforedI/dV (V) curves contain several peaks that behave differently withN. While the high-bias peaks shift in the normalized voltageV/N, the low-lyingpeaks at V/N≈32.4 ± 0.8 mV are N-independent. These peaks are reproducibly found in stacks of various lateral dimensions(0.5–11 µm2,N = 1...5) in a widetemperature range (T<0.8Tc). We briefly discuss possible reasons for the small values of the peak voltage.

Self-consistent estimations of heating in stacks of intrinsic Josephson junctions

We use the temperature dependentc axis critical current and characteristic gap voltage as temperature gauges to assess theself-heating in stacks of intrinsic Josephson junctions etched out from the ultrathinBi2Sr2CaCu2O8+δ (BSCCO) single crystals. The thermal resistance which characterizes the overall cooling efficiency is foundto be ∼120–160 K mW−1 for the two stacks studied. This value is in agreement with earlier reports for stacks ofsimilar geometry, while it is about two to three times higher than for simple mesas made onthe surfaces of mm-size BSCCO single crystals. The suggested method can guarantee thevalidity of the temperature estimations inside the stacks of various geometries anddifferent cooling efficiencies without the need for attaching additional thermometers.

Measurement of fluctuation-induced diamagnetism in BSCCO-2212 single crystals using magneto-optics

Fluctuation-induced diamagnetism in BSCCO single crystals with different doping levels (i.e., different oxygen stoichiometry) was measured using magneto-optics (MO) in a wide frequency range up to several MHz. A shift in Tc onset, measured by MO, of up to 6K towards higher temperatures was observed at high frequencies in those samples which are far from the optimally doped condition. An explanation of the observed effect in terms of the phase fluctuations of the superconducting order parameter is proposed.

Vortex pumps in the crossing lattices regime of highly anisotropic layered superconductors

It is now well established that vortex dynamics in samples with a spatially asymmetric pinning potential can lead to rectifying vortex ‘diode’ behaviour. Spatial asymmetry is not a fundamental requirement for the control of vortex motion, however, and we demonstrate that vortex ‘lensing’ is possible in highly anisotropic layered superconductors simply under the action of non time-reversible trains of in-plane magnetic field pulses. Our devices depend crucially on the existence of ‘crossing’ pancake vortex (PV) and Josephson vortex (JV) lattices in Bi2Sr2CaCu2O8+δ (BSCCO) single crystals under tilted magnetic fields. An attractive interaction between these two sub-lattices makes it possible to indirectly manipulate the PV distribution by modifying the JV lattice, and a number of functional devices based on this principle have been proposed. In our experiments a BSCCO single crystal is placed on a Hall probe array, and cooled below Tc in a small out-of- plane magnetic field. Trains of sawtooth in-plane field pulses are then applied to the system and different elements of the Hall array used to demonstrate PV lensing or antilensing behaviour, depending on the pulse shape. The mechanism leading to lensing will be discussed and results compared with molecular dynamics simulations.

Superconducting critical current of a singleCu2O4plane in aBi2Sr2CaCu2O8+xsingle crystal

By feeding current into the topmost ${\mathrm{Cu}}_{2}{\mathrm{O}}_{4}$ layer of a mesa etched into the surface of a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+x}$ (BSCCO) single crystal, we measured its superconducting critical value from a sharp upturn or break in the current-voltage characteristics of the mesa. From this, we estimate the sheet critical current density of a single ${\mathrm{Cu}}_{2}{\mathrm{O}}_{4}$ plane to be $\ensuremath{\sim}0.3--0.7\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕\mathrm{cm}$ at $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, corresponding to the bulk current density of $2--5\phantom{\rule{0.3em}{0ex}}\mathrm{MA}∕{\mathrm{cm}}^{2}$. These values are among the largest ever measured for BSCCO single crystals, thin films and tapes, and we argue that they represent the true intrinsic values of the material.

Microwave Oscillator Based on an Intrinsic BSCCO-Type Josephson Junction

The electrical behavior of anisotropic BSCCO single crystals is modeled by mutually coupled long Josephson junctions. For the basic fluxon modes with one fluxon per layer, the fluxons will arrange themselves in an anti phase configuration (triangular lattice) because of the mutual repulsion. We are interested in the in-phase modes (square lattice) desired for many potential applications. We consider two mechanisms (i) intrinsic locking by out of phase oscillations at the trailing edge and (ii) locking by an external high-Q resonator with a resonance frequency corresponding to fluxon in-phase motion. The resulting model is a set of coupled nonlinear partial differential equations. By direct numerical simulations we have demonstrated that the qualitative behavior of the combined intrinsic Josephson junction and cavity system can be understood on the basis of general concepts of nonlinear oscillators interacting with a resonator. For some region of the parameter space it is possible to reach the desired synchronous state, making the system potentially suitable for applications. We also consider the system in the flux flow mode under a high magnetic field.

STUDY OF ITINERANT HOLES IN PLANAR AND APICAL OXYGEN IN TWO DIFFERENT BSCCO (2212) SINGLE CRYSTALS USING POLARISED X-RAY ABSORPTION

X-ray absorption measurements have been made on Cu L 3 and O K-edge on BSCCO (2212) single crystals annealed under two different conditions at 1 bar O 2 (under-doped) and another annealed in 1:4 ratio of O 2: Ar (optimally doped) using the F. Y. detection technique. Our results clearly indicate that hole density decreases while going towards c-axis from the ab plane. Our results shows that the doping holes have predominantly x, y symmetry compared to those having pz symmetry. The T c for both the crystals does not depend on the density of doping holes in the apical oxygen. However, the matter needs to be further explored.

Interaction between a BSCCO-type intrinsic Josephson junction and a microwave cavity

The electrical behavior of anisotropic BSCCO single crystals is modeled by mutually coupled long Josephson junctions. We show that although the fluxons in the different layers do not a priori prefer the in-phase motion desired for many potential applications it is possible to induce such behavior by coupling the system to a high-Q resonator with a resonance frequency corresponding to fluxon in-phase motion. The resulting model is a set of coupled non-linear partial differential equations. By direct numerical simulations we have demonstrated that the qualitative behavior of the combined stacked long Josephson junctions and cavity system can be understood on the basis of the general concepts of nonlinear oscillators interacting with a resonator. For some region of the parameter space it is possible to reach the desired synchronous state, making the system potentially suitable for applications. We also look at the different dynamical states defined by different fluxon dynamical states in combination with different cavity properties.