Bi-Sr-Ca-Cu-O Research Articles

Magnetic properties of Sm-doped Bi-2223 superconductor studied by low field local Hall generator ac susceptibility

We studied the grain boundary properties of Sm doped Bi(Pb)SrCaCuO using the Hall generator ac susceptibility ( χ = χ′ + iχ″). Hall generator is a convenient tool to investigate the shielding effect of superconductors under extremely low magnetic fields. Using a Hall generator system, we have investigated diamagnetic shielding behavior of Bi 1.6Pb 0.4Sr 2Ca 2− x Sm x Cu 3O y samples with x = 0.0, 0.0005, 0.001, 0.005, 0.01 and 0.1. The susceptibility measurements were carried out for ac field amplitudes of 47, 94, 141, 188 and 235 A/m with the frequency of 20 Hz. The critical temperatures were estimated from the temperature dependence of the ac susceptibility curves. We have also employed X-ray diffraction, energy dispersive X-ray fluorescent and scanning electron microscopy techniques to investigate the effect of Sm doping.

C-axis resistivity of Bi 2Sr 2CaCu 2O y thin films at various oxygen doping: Phase diagram and scaling law

We compare the temperature-dependence of the in-plane and c-axis resistivity, ρ ab ( T) and ρ c ( T), of Bi 2Sr 2CaCu 2O y . Measurements have been made on a thin film deposited on a vicinal substrate, examined in various doping states by removing oxygen in steps from the overdoped to very underdoped states. ρ c ( T) exhibits a metallic behaviour at high T followed by a semiconducting-like behaviour at lower T, interrupted by the superconducting transition: ρ c ( T) is minimum at T min and maximum at T P, T min is defined as the temperature where the pseudogap opens. T min, 1.4 larger than the temperature T ∗, below which the pseudogap signature is seen in ρ ab ( T), exhibits the same dependence on hole doping p. A phase diagram ( T, p) is established with these characteristic temperatures. Interestingly ρ c ( T) normalized by its value at T min obeys a scaling law vs T/ T min. This confirms that T min is the important energy scale in the normal state c-axis transport.

Oscillating flux instability in vortex matter

We explain theoretically an oscillatory behaviour in space and time of the magnetic induction in Bi 2Sr 2CaCu 2O 8+ δ crystals during magnetic relaxation. This new “flux waves” phenomenon appears near the order–disorder vortex phase transition, under specific conditions of temperature and induction gradient. Our theory is based on two coupled equations: The Landau–Ginzburg equation for the order parameter of the vortex phase transition and the diffusion equation for the magnetic induction. Linear stability analysis of these equations shows an appearance of oscillatory instabilities characterized by a period and wavelength that are in accordance with the experimental results.

Microwave generation by Josephson vortex motion in stacked high- Tc intrinsic junctions

We report the observation of the collective transverse plasma (CTP) modes induced by the Josephson vortex lattice motion and the excitation of corresponding electromagnetic waves in a stack of intrinsic Josephson junctions (IJJs) in Bi 2Sr 2CaCu 2O 8+ x . The existence of the CTP modes was confirmed by observing the sub-branches in the tunneling current–voltage curves of staked junctions in the Josephson vortex-flow region. For a proper bias in the sub-branches, the emission of the electromagnetic waves by the collective vortex resonance motion in a stack of IJJs (the oscillator stack) was examined using another on-chip stack of IJJs (the detector stack). The microwave emission from the oscillator stack and the resulting irradiation onto the detector stack was evidenced by the increase of Josephson vortex-flow voltages in the detector stack.

Magneto-optical imaging of vortex lensing in Bi 2Sr 2CaCu 2O 8+ y

In a highly anisotropic superconductor Bi 2Sr 2CaCu 2O 8+ y , crossing-lattices state of pancake and Josephson vortices is realized under tilted magnetic fields. In this state, one kind of vortices can drag another, opening a possibility to control the arrangements and motion of vortices in a novel way. We have demonstrated the focusing of pancake vortices in the center of the sample, vortex lensing, by differential magneto-optical imaging in Bi 2Sr 2CaCu 2O 8+ y . The final arrangement of vortices changes depending on the ramping rate of the in-plane field. Such a behavior is discussed in relation with the competition between the drag force by Josephson vortices and the vortex dome formation due to the surface barrier.

Distributed injection of transient vortex states in a prism-shaped Bi 2Sr 2CaCu 2O 8+ δ crystal

Magneto-optical measurements in Bi 2Sr 2CaCu 2O 8+ δ demonstrate enhanced effects of transient disordered vortex states (TDVS) in a prism-shaped sample. The existence of such transient states is indicated by the appearance of a sharp change (‘break’) in the slope of the induction profiles. As the external magnetic field is swept up or down, the break, which signifies the front of the TDVS, moves towards the sample center at a constant velocity determined by the rate of change of the external field. We discuss the results in terms of a competition between injection and annealing processes, the latter being ineffective in the prism due to direct injection of disorder into the bulk through surface barriers distributed along the tilted facets of the prism.

MQT observation in Bi2212 intrinsic Josephson junctions

The quantum dynamics of Bi 2Sr 2CaCu 2O 8+ δ intrinsic Josephson junctions (IJJ’s) is studied based on the escape rate measurements. The saturations observed in the escape temperature and the width of the switching current below 0.45 K (= T ∗) indicate the transition of the switching mechanism from the thermal activation to the macroscopic quantum tunneling at T ∗. It is shown that most of the switching properties are consistently explained in terms of the underdamped Josephson junction with quality factor of about 70 in spite of possible damping due to d-wave superconductivity. The present result gives the upper limit of the dissipation of IJJ’s.

Vortex flow in micro-fabricated Bi 2Sr 2CaCu 2O 8+ y single-crystal thin films

To study the influence of sub-micron artificial pinning centers on vortex phases in high- T c superconductors, we have introduced holes array in Bi 2Sr 2CaCu 2O 8+ y single-crystal thin films using a focused ion beam. To preserve the quality of the crystals we made film samples from bulk crystals by the cleaving, and the surface of the Bi2212 films were covered by Au protection layer. The vortex-flow resistance shows a matching effect in the samples having triangular holes array which appears in vortex liquid phase of the pristine sample. This suggests that an in-plane symmetry is induced in the vortex liquid by regular holes array. Additionally, sub-harmonics of the matching effect are newly observed near the superconducting transition temperature ( T c). This suggests that the correlation length of pancake vortices is longer with approaching T c since the in-plane penetration depth diverges toward T c.

Terahertz-pulse radiation phenomena of high- TC superconductors by ultrashort optical pulse illumination

We have studied the terahertz (THz)-pulse radiation properties of high- T C superconductors. Up to now, it was found that the THz-pulse radiation property strongly depends on the kind of high- T C superconducor material and also its carrier-doping level. The bandwidth in the Fourier spectrum of the THz-pulse decreases with an increase of the two-dimensionality of high- T C superconductor material in the order of YBa 2Cu 3O 7− δ , Tl 2Ba 2CaCu 2O 8+ δ , and Bi 2Sr 2CaCu 2O 8+ δ . These results indicate that the ultrafast carrier dynamics of supercarriers along the crystal c-axis is an important factor to determine the Fourier components of THz-pulse.

Vortex liquid correlations induced by in-plane field in underdoped Bi 2Sr 2CaCu 2O 8+ δ

By measuring the Josephson plasma resonance, we have probed the influence of an in-plane magnetic field on the pancake vortex correlations along the c-axis in heavily underdoped Bi 2Sr 2CaCu 2O 8+ δ ( T c = 72.4 ± 0.6 K) single crystals both in the vortex liquid and in the vortex solid phase. Whereas the in-plane field enhances the interlayer phase coherence in the liquid state close to the melting line, it slightly depresses it in the solid state. This is interpreted as the result of an attractive force between pancake vortices and Josephson vortices, apparently also present in the vortex liquid state. The results unveil a boundary between a correlated vortex liquid in which pancakes adapt to Josephson vortices, and the usual homogeneous liquid.

Interlayer tunnel and thermal activation in c-axis transport in Bi 2Sr 2CaCu 2O 8+ δ

We present a phenomenological model for the electrical transport in double-layered cuprates, based on the existence of two energy barriers along the c-axis. The model is applied to measurements of the dc resistivity in Bi 2Sr 2CaCu 2O 8+ δ single crystals. We find that the model gives a comprehensive description of the out-of-plane normal resistivity ρ c at various doping δ. Once the normal state has been determined, it is possible to obtain the fraction η( T, δ) of the carriers that, below the pseudogap temperature T ∗, do not participate to the c-axis conductivity. We find that all the curves η( T, δ) collapse onto a single universal curve η[ T/ T ∗( δ)].

Angular field dependence of the Nernst effect in Bi 2Sr 2CaCu 2O 8+ δ

The anomalous Nernst effect in hole-doped cuprates has been interpreted in terms of the existence of a superconducting order parameter well above the transition temperature, T c( H). In order to improve the understanding of this problem, we have measured the angular field dependence of the Nernst signal in Bi 2Sr 2CaCu 2O 8+ δ single crystals. The experiment was done tilting the magnetic field with respect to the direction of the temperature gradient, above and below T c( H), with applied fields up to 16 T. We have found that all the curves are scaled on one curve against the reduced field H cos θ at a fixed temperature.

Systematic transport properties and their doping dependences for Bi 2Sr 2CaCu 2O 8+ δ single crystals in a wide doping range

We have investigated the temperature dependence of the in-plane resistivity and the Hall coefficient for high-quality single crystal Bi 2Sr 2CaCu 2O 8+ δ (Bi2212) in a wide doping range. Doping is controlled by successive annealing treatments from a nearly optimally doped region to an insulating region systematically. It is found that the residual resistivity increases notably with decreasing doping and undergoes the superconductor–insulator (SI) transition when the residual resistivity becomes ∼1 mΩ cm, which corresponds to the universal sheet resistance h/4 e 2. In addition, the Hall coefficient at which superconductivity disappears is found to be very small compared with the La- or Y-based system. These results imply that the transition temperature is anomalously suppressed in the underdoped region of Bi2212 while the SI transition is of the same origin for various systems.

Ratchet without spatial asymmetry: Controlling the motion of magnetic flux quanta using time-asymmetric drives

We report on vortex manipulation experiments which exploit the attractive interaction between ‘crossing’ pancake vortex (PV) and Josephson vortex (JV) lattices in Bi 2Sr 2CaCu 2O 8+ δ (BSCCO) single crystals under tilted magnetic fields. Deformations of the JV lattice allow us to indirectly move mutually interacting PVs. In dc-driven mode the in-plane field is varied slowly (adiabatically) while the PV density is monitored at different points in our single crystal BSCCO sample using a micro-Hall probe array. Variations in PV density as large as 40% have been demonstrated in this way and both enhancement and depletion are observed, depending on the location in the crystal, temperature and PV density. In ac-driven experiments, manipulation is achieved by applying trains of time-asymmetric (sawtooth) in-plane field pulses. We show how both flux focusing and defocussing can be readily achieved merely by varying the time-asymmetry of the drive. Our experimental results are well described by molecular dynamics simulations which consider the dragging of PVs by JVs.

Switching current distribution in large Bi 2Sr 2CaCu 2O y intrinsic Josephson junctions

We investigated the switching current distribution, P( I), from the zero-voltage state to a nonzero-voltage state in the intrinsic Josephson junctions of Bi 2Sr 2CaCu 2O y . Because of a reduction of the Josephson penetration depth attributed to an atomic-scale thickness of the superconducting layers, a depinning process of Josephson vortices was found to dominate the behavior of P( I) for the mesas with a lateral size larger than ∼10 μm.

Glass–ceramic route of BSCCO superconductors – Fabrication of amorphous precursor

It is well known that many Bi–Sr–Ca–Cu–O compositions are glass-forming and some Bi-based glasses such as Bi 2Sr 2CaCu 2O x and Bi 2Sr 2Ca 2Cu 3O x are converted into high critical temperature superconductors after proper annealing. In order to fabricate superconductors having high- T c and high critical current density using the glass–ceramic route, it is necessary to clarify the total chemical composition of the quenched glasses prepared in most cases by rapid quenching of melts from around 1200 °C in air. The total oxygen content measured directly reflects a significant oxygen deficit due to the melting process. We have also investigated the cation content in quenched Bi 2Sr 2Ca 2Cu 3O x precursors and found that there are substantial differences from the nominal composition to the quenched materials especially for calcium. Such glasses also show some CaO crystalline reflexes in the XRD patterns.

Magnetic isotope effect in Bi 2Sr 2CaCu 2O 8+ δ studied by high-resolution angle-resolved photoemission spectroscopy

We studied the impurity effect on the electronic structure near the Fermi level in Bi 2Sr 2CaCu 2O 8+ δ by high-resolution angle-resolved photoemission spectroscopy to clarify the origin of dispersion kink in high- T c superconductors. We found that the dispersion kink at off-nodal region is significantly suppressed by Zn or Ni impurity while that at the node is less affected, suggesting that electrons are strongly coupled to the spin excitations at the off-nodal region.

Comments on identity of gap anisotropy and nodal constant Fermi velocity in Bi 2Sr 2CaCu 2O 8+ δ

On the basis of both the inhomogeneity of a superconductor Bi 2Sr 2CaCu 2O 8+ δ and the effect of measurement, we reveal that an anomalously large gap anisotropy known as evidence of d x 2 - y 2 -wave symmetry [Z.X. Shen, D.S. Dessau, B.O. Wells, D.M. King, W.E. Spicer, A.J. Arko, D. Marshall, L.W. Lombardo, A. Kapitulnik, P. Dickinson, S. Doniach, J. DiCarlo, A.G. Loeser, C.H. Park, Phys. Rev. Lett. 70 (1993) 1553] is not intrinsic, and that the constant Fermi velocity at the node as an unsolved problem [X.J. Zhou, T. Yoshida, A. Lanzara, P.V. Bogdanov, S.A. Kellar, K.M. Shen, W.L. Yang, F. Ronning, T. Sasagawa, T. Kakeshita, T. Noda, H. Eisaki, S. Uchida, C.T. Lin, F. Zhou, J.W. Xiong, W.X. Ti, Z.X. Zhao, A. Fujimori, Z. Hussain, Z.-X. Shen, Nature 423 (2003) 398] is due to the d x 2 - y 2 -wave insulator–metal transition.

Understanding the origin of the shadow bands of Bi-based superconductors with angle resolved photoemission

Angle resolved photoemission spectroscopy (ARPES) has been used to investigate the shadow bands (SB) present at the Fermi surface (FS) of bismuth based superconductors. Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) single crystals and Bi 2Sr 2CuO 6+ δ (Bi-2201) thin films have been studied by momentum distribution curves (MDCs) along the ΓY high symmetry direction in two different Brillouin zones (BZ). The results show the ratio shadow bands/main band (SB/MB) to be constant as a function of both the doping level and the number of CuO 2 planes, which discards the antiferromagnetic (AF) scenario to explain their origin. On the other hand, the SB and the main bands (MB) have different initial state symmetry, which implies that the orthorhombicity exhibited by these materials cannot be responsible for their emergence at the FS. An interpretation on the basis of a c(2 × 2) surface reconstruction, recently observed by LEED, becomes be only plausible explanation compatible with all experimental data.

Origin of dips in tunneling d I/d V characteristics of cuprates

Extensive efforts have been made to understand the electronic properties of high- T c superconductors. One feature which has been discussed in the literature during the past few years is the dips in tunneling conductances obtained in cuprates. In this contribution, we focus our attention on the origin of these dips. On the basis of experimental data obtained in Bi 2Sr 2CaCu 2O 8 and YBa 2Cu 3O 7, we show that the dips appear naturally in tunneling spectra due to a superposition of the peaks and humps and, therefore, have no physical meaning.