Fermi Surface and Superconducting Gap of Triple-Layered Bi2Sr2Ca2Cu3O10 + δ

We present a comprehensive study performed with high-resolution angle-resolved photoemission spectroscopy on triple-layered Bi₂Sr₂Ca₂Cu₃O_{10 +} single crystals. By measurements above T _C the Fermi surface topology defined by the Fermi level crossings of the CuO₂-derived band was determined. A hole-like Fermi surface as for single and double-CuO₂ layered Bi-based cuprates is found, giving new input to the current debate of the general Fermi surface topology of the high-T _C superconductors. Furthermore, we present measurements of the superconducting gap of Bi-2223 and show that there are clear indications for a strong anisotropy of the superconducting gap. The universal properties of this phase in comparison to the other Bi-based cuprates will be discussed.     

Atomic-plane-selective STM imaging and STS on the Bi2Sr2CaCu2O8+δ cleaved surface

In STM experiments on Bi₂Sr₂CaCu₂O₈₊ .     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

XRD and PES Studies of the Bi2Sr2Ca1−xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

High-temperature X-ray Study of Melting and Solidification of the Bi2Sr2CaCu2O8+δ on Silver

The melting and solidification of a Bi ₂ Sr ₂ CaCu ₂ O ₈₊ compound on Ag were studied by in-situ high-temperature x-ray analysis. Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag samples were heated by the heating schedule known as partial melting and slow cooling under a 1-100%-O ₂/He gas flow. A strong c-axis alignment of the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ crystals was observed at the incongruent-melting temperature where the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and the molten phase coexist. Similarly, a strong c-axis alignment was observed at the initial stage of solidification. However, the subsequent solidification of the residual molten phase caused less aligned Bi ₂ Sr ₂ CaCu ₂ O ₈₊ on Ag. An improved heating schedule is proposed which increases the critical current of Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag superconducting tapes.     

Oxygen diffusion in epitaxial Bi2Sr2CaCu2O8+δ thin films

Superconducting Bi ₂ Sr ₂ CaCu ₂ O ₈₊ thin films (T _c 85K) were prepared on off-c-axis (001) SrTiO ₃ substrates by pulsed laser deposition. High quality oriented film growth with a c-axis inclined with respect to the surface normal by a tilt angle is found up to 15° from SEM, XRD and resistivity measurements. Isothermal oxygen diffusion is monitored by in-situ electrical resistance measurements at temperatures ranging from 200°C...400°C. From the temporal evolution of the resistance the in-plane diffusion constant D _ab is determined.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Anomalous Scattering Rate and Microwave Absorption in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7?δ

We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two-fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as YBa₂Cu₃O_7–. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.     

Anisotropic 3D fluctuations conductivity in Bi2Sr2CaCu2O8+δ films

In zero applied magnetic field the fluctuation conductivity is measured and analyzed for a high-quality thin film (100 nm) of Bi ₂ Sr ₂ CaCu ₂ O ₈₊ grown on a MgO substrate. The two-band model formulas derived by Konsin et al. for the temperature dependence of the anisotropic three-dimensional (3D) paraconductivity in the vicinity of a superconducting phase transition are used. One of the two correlation lengths is critical and along with the other parameters of the model it determines the fluctuation conductivity (T).The global result of two-band model mimics closely d-wave-type pairing. The theory is compared with the experiment, and we obtained reasonable values and the slope of (T), the superconducting critical temperature T _c and the coherence length (0).     

Impurity Resonance States in Bi2Sr1.6La0.4CuO6+δ

Using scanning tunneling spectroscopy (STS) at 4.2?K, we observed impurity resonance in single crystals of Zn doped Bi₂Sr_1.6La_0.4CuO_6+?? (T _C =28?K) (Bi2201). Many impurity resonances characterized by the peak structure near E _F were observed. We found that the number of the resonances was less than that of the doped impurities (0.45?% determined by inductively coupled plasma optical emission spectrometry). The resonance peaks were observed only in the region having smaller gap less than ??_cut??26?meV. This ??_cut was about half of that reported in Bi₂Sr₂CaCu₂O_8+?? .     

Infrared spectra of bismuth family of superconductors Bi2Sr2Can−1CunO4+2n+y

FTIR spectra of bismuth family of superconductors are studied. Polycrystalline samples of bismuth superconductors are prepared choosing an off-stoichiometric composition for phase stabilisation and reproducibility. Vibrational modes are strongly affected by the holes introduced into the CuO planes. Mid IR spectra of the Bismuth system was recorded using Bruker IFS 66v and the high frequency oxygen vibrations of the system is discussed based on the factor group analysis for both symmorphic and non-symmorphic space groups I4/mmm, F_mmm and A_maa of different structures based on the modulated superstructure.     

Effect of ZrO2 Addition on Microstructure, Transport and Magnetic Properties of Bi2Sr2CaCu2O8+δ System

The influence of zirconium (Zr) addition to the Bi₂Sr₂CaCu₂O_8+δ (Bi2212) system on its phase formation, microstructure, transport, and magnetic properties is investigated. Samples have been characterized by means of X-ray diffraction analysis (XRD), scanning electron microscopy equipped with energy dispersive of X-ray analysis (SEM/EDX), and resistivity versus temperature measurement. The results show an increase of the critical temperature of superconductive transition T _c for x=0.01 and in all the samples containing Zr the Bi2212 phase is the majority. SEM observations show whiskers grains randomly distributed and microstructural change due to the addition of Zr. Resistivity measurement show that the temperature of the onset of transition T _c increases by 9 K for x=0.005, 0.02, and 5 K for x=0.01 of ZrO₂. An improvement of the normal state conductivity is obtained for the lowest content of Zr (x=0.005). In the normal state, all the samples exhibit a metallic behavior except the sample with x=0.01, which exhibit a semiconductor character.     

Studies Of Dynamic Critical Behavior in Bi2Sr2CaCu2O8+δ Single Crystals

Up to now, the value of the dynamic critical exponent z is still an open question. We have studied this exponent in the intermediate (XY) critical regime in zero field for a set of high quality Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals both by previously obtained critical exponents of the dc critical conductivity fluctuations [1] and in terms of Fisher-Fisher-Huse (FFH) dynamic scaling [2]. In this regime (i.e. the absence of Coulomb interactions), one would expect model-E dynamics (z = 1.5) to apply with the conserved charge-density conjugate to the phase fluctuations. However, we find the dynamic critical exponent to be z = 2, which is in agreement with the suggestion of FFH that due to the plasma fluctuation, fully relaxational dynamics, model A (z=2), should be appropriate.     

Josephson plasma resonance in Bi2Sr2CaCu2O8+δ under parallel magnetic field

Josephson plasma resonance in under-doped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals has been observed when magnetic field is applied parallel to the ab plane and its vicinity. The resonance mode splits into two branches at higher and lower temperature regions, and a definite gap appears in the temperature region between them. As the magnetic field is tilted from the ab plane, these branches come closer and finally merge to a single mode. This leads to a general interpretation that the resonance mode observed in a parallel field may be due to continuous extension of the c axis plasma mode. However, splitting of the mode as well as the peculiar temperature dependence of the resonance field, the line shape, and its intensity strongly imply that these behaviors may originate from the inherent resonance modes in parallel field. In particular, the high temperature mode fields goes even higher in temperature beyond the zero field resonance as the field increases, suggesting that a new explanation may be required for the case of parallel magnetic field.     

Substitution Effect of Pr for Ca in Bi2Sr2CaCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction, and photoemission experiments have been performed. Both X-ray diffraction and photoemission measurements show that Pr ion enters the lattice with a valence higher than 3⁺, which supports the hole-filling mechanism of the suppression of superconductivity.     

Optical Study of the Pseudogap State in Single Layer Bi2Sr2CuO6+δ

We present infrared reflectivity measurements of the ab-plane response of Pb- and La-doped Bi ₂ Sr ₂ CuO ₆₊ single crystals. A suppression of scattering rate is observed for La-doped samples below and above T _c , suggestive of a pseudogap. Compared with double-layer Bi ₂ Sr ₂ CaCu ₂ O ₈₊ , the suppression is weak and appears at lower frequencies. This may reflect a weak pairing interaction and/or smaller gap amplitude.     

XRD and PES Studies of the Bi2Sr2Ca1?xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

Systematic Magnetization Measurements on Single Crystalline Bi2Sr2CaCu2O8+δ with Columnar Defects

We have performed systematic magnetization measurements on single crystalline Bi ₂ Sr ₂ CaCu ₂ O ₈₊ with columnar defects of B = 0.005 to 1 T by using a SQUID magnetometer. Magnetization hysteresis curves of the pristine sample show a weak irreversible behavior in the vortex liquid state, suggesting the existence of the new vortex state in the vortex liquid state. This weak irreversible region persists systematically in the samples with columnar defects even up to B = 1 T. It is shown that the weak hysteresis of magnetization is sensitive to the disorder level of the sample and shifts systematically to higher temperature and field region with increasing the number of columnar defects. This behavior clearly indicates that effective pinning mechanism exists even in the vortex liquid state and generates a finite critical current.     

Theory of the Phase-Sensitive Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junction Experiments

We studied theoretically the phase-sensitive c-axis Bi₂Sr₂CaCu₂O_8+δ (BSCCO) bicrystal twist Josephson junction experiments of Li et al. (Phys. Rev. Lett. 83, 4160 (1999)), calculating the critical current as a function of the twist angle in a wide variety of models. The data provide strong evidence that the c-axis tunneling in BSCCO is incoherent, and that the order parameter is s-wave for T ≤ T _c.