Conservation of the particle-hole symmetry in the pseudogap state in optimally-doped Bi2Sr2CuO6+δ superconductor

The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi₂Sr_1.6La_0.4CuO_6+δ superconductor, we report the observations of the particle-hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These results provide key insights in understanding the nature of the pseudogap and its relation with high temperature superconductivity.     

Superconductivity of Bi1.6Pb0.4Sr2Ca3Cu4O12

The superconducting ceramics Bi1.6Pb0.4Sr2Ca3Cu4O12 have been prepared by the melt-casting method.A zero resistance temperature at 60K has been observed.It has been found that the superconducting phase temperature Tc increases with increasing sintering temperature,The effect of Pb content on the superconductivity of the ceramic has been studied.The microstructure of the Sample has been investigated by scanning electron microscopy.Phase analysis has been carried out by x-ray diffraction patterns and energy dispersive analysis through x-ray spectroscopy.     

X-ray photoelectron spectroscopy analysis of (Ln1−xSrx)CoO3−δ (Ln: Pr,Nd and Sm)

The chemical states of the surface of (Ln_0.5Sr_0.5)CoO_3?δ (Ln (lanthanides) = Pr, Nd and Sm) used for cathode materials of intermediate temperature operating solid oxide fuel cells (IT-SOFCs) were investigated by X-ray photoelectron spectroscopy (XPS). Oxygen peaks comprised of lower binding energy (LBE) and higher binding energy (HBE) peaks from (Ln_0.5Sr_0.5)CoO_3?δ and Pr_0.3Sr_0.7CoO_3?δ (PSC37) showed that some merged oxygen peak behavior is a function of the Sr and lanthanide concentrations. By investigating the oxygen peaks, it was determined that more oxygen vacancies were generated on the surface of the cathodes when the lanthanides and Sr were substituted into perovskite oxides. When comparing the binding energies (BEs) of PSC37 with Pr_0.5Sr_0.5CoO_3?δ (PSC55), the LBE and HBE of the Sr peaks both increased when Sr was substituted at the A-site of a perovskite. Surface analysis of the Co peak on the surface of the cathode materials showed that the Co exists mainly as Co³⁺ and partially oxidized to Co⁴⁺ on the cathode materials. The partial existence of Co⁴⁺ can provide some polaron hopping providing electronic conduction for the solid oxide fuel cell.     

Raman Active Phonons in RCoO3 （R=La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy） Perovskites

We examine RCoO3 （R=La, Ce, Pr, Nd, Sin, Eu, Gd, and Dy） perovskites prepared with the solid-state reaction method by Raman spectroscopy, and report the Raman active phonons in the RCoO3 perovskites crystallized in cubic symmetry for RCoO3 （R=La, Ce, Pr and Nd） and orthorhombic symmetry for RCoO3 （R=Sm, Eu, Gd, and Dy）. It is found that the Raman spectra of RCoO3 perovskites are strongly dependent on the ionic radius of the rare earth elements, and the frequency shift of the most intense modes of the orthorhombic samples are correlated with some structural parameters such as Co-O bond distances, ionic radius of the rare earth elements and Jahn-Teller distortion. It is clear that Raman spectroscopy has the advantage of sensitivity to structure distortion and oxygen motion.     

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y,La, Eu,Sm; M′ = Ba,Sr)

Compounds of the form (M_1−xM′_x)₂CuO_4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La_1−xBa_x)₂CuO_4−δ, (La_1−xSr_x)₂CuO_4−δ and (Y_1−xBa_x)₂CuO_4−δ with superconducting transition temperature T_c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu_1−xBa_x)₂CuO_4−δ and (Sm_1−xBa_x)₂CuO_4−δ also exhibit high temperature superconductivity with T_c onsets of 95 K and 65 K, respectively. The highest T_c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y_0.33Ba_0.67Cu₂O_4−δ. Measurements of the specific heat C as a function of temperature T on a La_0.8Sr_0.2CuO_4−δ sample reveal a break in slope in the C/T vs T curve at the T_c midpoint, but no clearly discernable jump in C at T_c. A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.     

μSr and vortex pinning in Bi2Sr2Ca1−x Gd x )Cu2O y

We present transverse fieldSR and bulk magnetisation studies on powders of Bi₂Sr₂(Ca_1{itxGd _x )Cu₂O _y for 0x<0.5. We have found that the effective pinning decreases rapidly withx even for values ofx for whichT _c remains unchanged. We have interpreted this decrease in terms of changes in _ab (obtained fromSR) which cause a decrease in vortex-vortex interactions. The field distribution spectra in these superconductors have been obtained using a maximum entropy analysis and are found to agree with the calculations by Harshmanet al.     

Raman Shift of RMnO3 （R=La, Pr, Nd, Sm） Manganites

Raman-active phonons in orthorhombic perovskite-like RMnO3 were studied by measuring Raman spectra in various scattering configurations. The experimental Raman line wave numbers and the expected shapes for the phonon modes were compared to those reported for other perovskite-like compounds with the Pnma structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of RMnO3 were assigned to definite atomic motions. The remaining spectral weight can be explained by the presence of dynamic John-Teller distortions that lower the symmetry of the cubic perovskite.     

Tunneling in single-layer Bi2Sr2CuO6+δ single crystals in high magnetic field

In tunneling experiments with high-quality single crystals of a single-layer cuprate superconductor Bi₂Sr₂CuO_6+δ using the break junction and point-contact techniques at T<T _c, the coexistence of the superconducting-state gap and the normal-state gap was observed. The values of the superconducting energy gap 2Δ_p?p are in the range from 13.4 to 15 meV (Δ_p?p=6.7–7.5 meV). The values of 2Δ_p?p are similar for two samples with T _c=4 K and for two samples with T _c=9–10 K and are independent of the carrier concentration. The normal-state gap, with the magnitude approximately equal to 50 meV, persists at T<T _c and in the magnetic field H?H _c2 up to 28 T. After the transition of the sample to the normal state, the intensity of the tunneling conductance rapidly decreases with increasing magnetic field strength and temperature. The observed large broadening of the tunneling spectra and large zero-bias conductances can be caused by a strong angular dependence of the superconducting gap. The tunneling results are in full agreement with the data of the angle-resolved photoemission spectroscopy measurements.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Positron annihilation and magnetization studies of proton irradiated polycrystalline textured Bi2Sr2CaCu2O8+δ and Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10+δ superconductors

Proton irradiations on polycrystalline textured Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) and Bi_1.84Pb_0.34Sr_1.91Ca_2.03Cu_3.06O_10+δ (Bi-2223) samples have been carried out. The irradiation induced defect size increases with dose up to 5×10¹⁵ protons/cm² in Bi-2212, but remains effectively constant in Bi-2223 as revealed from positron lifetime measurements. Magnetization studies show that at moderate dose, there is no significant change in intergranular critical current density (J_c^int) in both systems. But intragranular critical current density (J_c) increases with dose in Bi-2212. A drastic fall in both J_c^int and J_c is observed at high dose in Bi-2223 as compared to Bi-2212.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

X射线衍射研究Bi2Sr2—yMyCuO6＋δ（M=La,Pr）晶体结构

作报道了用Ｘ射衍射法对Ｂｉ２Ｓｒ２－ｙＭｙＣｕＯ６＋δ（Ｍ＝ＬａＰｒ０．０＜ｙ＜１．０）系列单晶样品进行结构的研究，根据晶体结构的不同性质，我们将掺杂区分为三个区域，第一个区域的掺镧量在０．０＜ｙ＜０．１０之间，在此区域中晶体的平均结构为单斜结构，其调制波矢具有两个非公度分量ｑ＝βｂ＋γｃ，第二个掺杂区的掺镧量在０．１０＜ｙ＜０．６０之间，晶体的平均结构由单斜转变为正交结构，调制波矢具有单个非公     

The interplay of superconductivity and localization in Nd2−xCexCuO4+δ single crystal films

The influence of nonstoichiometric disorder on the in-plane resistivity and SC-transition has been investigated for Nd_2−xCe_xCuO_4+δ single crystal films (x=0.15 and 0.18). It is shown that with increasing of δ the in-plane normal state resistivity increases (the mean free path diminishes) and SC-transition temperature decreases with essential broadening of the transition region. The observed evolution from homogeneous metallic (and superconducting) Nd_2−xCe_xCuO_4+δ system to inhomogeneous dielectric one is described as Anderson-type disorder-induced transition in a two-dimensional electron system.     

Preparation,characterization, and conductivity of YLnTiZrO7 (Ln = La,Nd, Sm,and Eu)

The pyrochlore oxide of composition YLnTiZrO₇ (Ln?=?La, Nd, Sm, and Eu) was prepared by sol–gel method. All the samples were characterized by powder X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffused reflectance spectroscopy, and impedance spectroscopy. The powder XRD and Raman studies reveal that these samples were crystallized in cubic lattice with pyrochlore structure. The Rietveld analysis of the samples was carried out to obtain the unit cell parameters and reliability factors. The broad Raman bands observed for all the samples are due to cation/anion disorder in the lattice and nanosize. The XPS analysis of the samples shows the characteristic peaks belonging to Y³⁺, Ln³⁺ (Ln?=?La, Nd, Sm, and Eu), Ti⁴⁺, and Zr⁴⁺. Electrical conductivity of YLaTiZrO₇ (YLTZ) and YEuTiZrO₇ (YETZ) samples was calculated from the impedance as a function of frequency and temperature. These samples have shown conductivity of the order of 10^?5 scm^?1 at 500 °C.     

Local structure,magnetism, and superconductivity in Sr analogs of Fe-doped YBa2Cu3−y Fe y O6+x

The local structure, magnetism, and superconductivity of Y_1–z Ca _z Sr₂Cu_3–y Fe _y O_6+x have been investigated by magnetic susceptibility,⁵⁷Fe Mössbauer absorption spectroscopy, X-ray powder diffraction, and iodometric titration. By modification of the hole distribution between the Cu(1)O _x chains and Cu(2)O₂ sheets, we induced bulk superconductivity with a maximum superconducting critical temperatureT _c near 20 K in Y–zCa _z Sr₂Cu_2.5Fe_0.5O_6+x (0.05z0.18). We interpret the development of a broad maximum inT _c and the Meissner fractionV _f versusz nearz=0.12 to be due to an increasing transfer of holes from the Cu(1)O _x chains to the Cu(2)O₂ sheets induced by a shorterc-axis and reduced Coloumb repulsion of the positively charged Y_1–z Ca _z plane. The increase in hole transfer from the Cu(1)O _x chains to the Cu(2)O₂ sheets also modulates the magnetic ordering temperatureT _m. We show thatT _m of YSr₂Cu₂FeO_7.26 is approximately 65 K, which is much lower than that observed in YBa₂Cu_2.31Fe_0.69O_7.21 (T _m400 K).