CaCu2 O8 Research Articles

On-Demand Local Modification of High-Tc Superconductivity in Few Unit-Cell Thick Bi2 Sr2 CaCu2 O8+δ.

High-temperature superconductors (HTSs) are important for potential applications and for understanding the origin of strong correlations. Bi2 Sr2 CaCu2 O8+δ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, is of immense interest for superconducting electronics. A route to modify superconductivity locally by depositing metal on the surface is reported here by transport studies on few unit-cell thick BSCCO. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the underlying region. Cr locally depletes oxygen in CuO2 planes and disrupts the superconductivity in the layers below. This technique of modifying superconductivity is suitable for making sub-micrometer superconducting wires and more complex superconductingdevices.

Growth of high-quality Bi2Sr2 CaCu2O8+\u03b4 whiskers and electrical properties of resulting exfoliated flakes

In this work, we demonstrate a simple technique to grow high-quality whiskers of Bi2 Sr2 CaCu2 O8+δ – a high Tc superconductor. Structural analysis shows the single-crystalline nature of the grown whiskers. To probe electrical properties, we exfoliate these whiskers into thin flakes (~50 nm thick) using the scotch-tape technique and develop a process to realize good electrical contacts. We observe a superconducting critical temperature, Tc, of 86 K. We map the evolution of the critical current as a function of temperature. With 2-D materials emerging as an exciting platform to study low-dimensional physics, our work paves the way for future studies on two-dimensional high-Tc superconductivity.

A Reproducible Approach of Preparing High-Quality Overdoped Bi2Sr2CaCu2O8+δ Single Crystals by Oxygen Annealing and Quenching Method**Supported by the National Natural Science Foundation of China under Grant Nos 11190022, 11334010 and 11534007, the National Basic Research Program of China under Grant No 2015CB921000, and the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant No XDB07020300.

We report a reproducible approach in preparing high-quality overdoped Bi2Sr2 CaCu2 O8+δ (Bi2212) single crystals by annealing Bi2212 crystals in high oxygen pressure followed by a fast quenching. In this way, high-quality overdoped and heavily overdoped Bi2212 single crystals are obtained by controlling the annealing oxygen pressure. We find that, beyond a limit of oxygen pressure that can achieve most heavily overdoped Bi2212 with a Tc ∼63 K, the annealed Bi2212 begins to decompose. This accounts for the existence of the hole-doping limit and thus the Tc limit in the heavily overdoped region of Bi2212 by the oxygen annealing process. These results provide a reliable way in preparing high-quality overdoped and heavily overdoped Bi2212 crystals that are important for studies of the physical properties, electronic structure and superconductivity mechanism of the cuprate superconductors.

Diamagnetic fluctuations inYBa2Cu3O7−xandBi2Sr2CaCu2O8+x: Possible observation of phase correlations persisting aboveTc

We report on isofield curves of -M vs T, where M is the reversible magnetization, of YBa2 Cu3 O6.95, YBa2 Cu3 O6.65, and Bi2 Sr2 CaCu2 O8+x with the magnetic field, H, applied parallel to the c axis of the samples (and also parallel to the ab planes for YBaCuO). For temperatures close to the critical temperature, Tc, the quantity -M is proportional to the order parameter amplitude |ψ|. Curves of -M vs T allowed to study the asymptotic behavior of the form (Ta -T) m of |ψ| near Tc, as a function of field. Results for the studied samples produced values of Ta (H) lying above Tc, suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above Tc. The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudogap region of YBaCuO. -M vs T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above Ta (H) which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a total-energy cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5<m<0.7, are interpreted as due to phase fluctuations of the d -wave pairing symmetry of the order parameter in the ab planes. © 2009 The American Physical Society.

Synthesis of lead-based 1212 and 3212 superconductors by an aqueous sol-gel method

Abstract The aqueous sol-gel synthesis technique for the preparation of (Pb,Sr)Sr2(Y,Ca)Cu2O7±x (Pb-1212) and (Pb2,Cu)Sr2(Y,Ca)Cu2O8±x (Pb-3212) superconductors using two different complexing agents, namely 1,2-ethanediol and tartaric acid was studied. The phase transformations, composition and micro-structural features in the polycrystalline samples were studied by powder X-ray diffraction analysis (XRD), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). XRD analysis of the ceramic samples obtained by calcination of Pb-Sr-Y-Ca-Cu-O acetate-glycolate precursor gels in air, for 10 hours at 800°C and at 825°C, showed the presence of homogeneous Pb-1212 and Pb-3212 crystallites as major phases. The XRD patterns of the ceramics obtained from Pb-Sr-Y-Ca-Cu-O acetate-tartrate precursor gels, however, showed multiphasic character. The critical temperature of superconductivity (TC (onset)) observed by resistivity measurements were found to be 91 K and 75 K for Pb-1212 and Pb-3212 samples, respectively.

Controlling the dynamical behaviors of Josephson vortices

Using high Tc superconductor (HTSC) Ei2 Sr2 CaCu2 O8+y, which have intrinsically stacked Josephson junctions (IJJs), we observed periodical oscillations in the flow resistance of JVs against parallel magnetic field to the superconducting layers. We have introduced pancake vortices to control the dynamical motion of JVs, which have a role for pinning of JVs. Applying a time-asymmetric input current with biharmonics, we have observed a reversible rectification effect with changing the phase and the frequency difference in the biharmonics. The output signal is much larger than that using the single JJ. Utilizing a multiple-function of IJJs, new types of the junctions will make the new functional devices of HTSCs possible.

Single-crystal growth and superconductivity of (La 1−xSr x) 2CaCu 2O 6+δ

The effects of growth conditions and composition on the crystal growth of La-2126-phase superconducting materials have been studied by the floating-zone method. The composition of La-2126-phase single crystals was (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 – 0.2 5 ). The crystals were grown under oxygen pressures from 1 bar to 11 bar. A smooth planar solid–liquid growth interface of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h, or the Sr concentration x > 0.0 7 5 . When the planar solid–liquid growth interface broke down into a cellular interface, the single-crystal size in an as-grown rod decreased abruptly and the as-grown rod was not of single phase. Various small sizes of CaO second phases were found inside the as-grown single crystals of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x ⩾0.125). Based on the occurrence of these CaO second phases inside the as-grown crystals, the estimated solubility of as-grown single crystals is x = 0.0 7 5 for (La 1− x Sr x ) 2CaCu 2O 6+ δ . The melting point of the La-2126 materials increases with increasing oxygen pressure. Cubic-centimeter-size single crystals were successfully grown for (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ). The superconducting transition temperature of the as-grown (La 1− x Sr x ) 2CaCu 2O 6+ δ single crystals increases from T c = 2 8 K to 46 K with increasing the Sr concentration x from 0.075 to 0.25 at 11 bar oxygen pressure.

Interlayer magnetotransport study in electron-doped Sm2−x Ce x CuO4−δ

Vortex and pseudogap states in electron-doped Sm2−xCexCuO4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On cooling, the c-axis resistivity ρc of the mesa structures reveals a semiconductive upturn above Tc, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρc(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap, whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi2Sr2 CaCu2 O8+y.

Thallium and lead NMR studies of (Tl 0.5Pb 0.5)(Ba 0.2Sr 0.8) 2Ca 2Cu 3O y superconductors

(Tl 0.5Pb 0.5)(Ba 0.2Sr 0.8) 2Ca 2Cu 3O y (Tl,Pb-1223) has been prepared, characterized and oriented for NMR studies. For comparison, we also fabricated (Tl 0.5Pb 0.5)(Ba 0.1Sr 0.9) 2CaCu 2O y (Tl,Pb-1212). In the case of (Tl 0.5Pb 0.5)(Ba 0.2Sr 0.8) 2Ca 2Cu 3O y the 205Tl NMR spectra and the spin–lattice relaxation time were measured in the temperature range from 68 to 300 K. For (Tl 0.5Pb 0.5)(Ba 0.1Sr 0.9) 2CaCu 2O y the 205Tl spectra were measured at room temperature only. The 207Pb NMR spectra for both compounds were also obtained at room temperature. From the chemical shift measurements it was concluded that the valence states of Tl and Pb were 3+ and 4+, respectively. In addition, a linear relationship was found between the shift and the critical temperatures. At low temperatures an anisotropic melting of the vortex state was observed.

Synthesis of superconductive Cu2Ba2(Y, Ca)Cu2O8±δphase by novel encapsulation technique

An efficient synthesis technique has been developed to obtain theCu2Ba2(Y, Ca)Cu2O8±δ (Cu-2212 or `124') phase in areasonably short firing period under ambient or `semiambient' pressure. TheCu-2212 phase forms in a single-phase, highly crystalline form from astoichiometric mixture of CuBa2(Y, Ca)Cu2O7-δ (Cu-2212 or`123') and CuO at 850-880 °C in 24 h by encapsulating the precursormixture together with Ag2O2 in a fused-quartz ampoule. Under theseconditions, Ag2O2 decomposes into Ag2O/Ag acting as an efficientexcess-oxygen source in the synthesis. By changing the amount of Ag2O2added it is possible to control the partial pressure of oxygen inside thesynthesis ampoule up to ~40 atm. The Ca-free Cu-2212 phase forms evenwithout Ag2O2, but obtaining the 10% Ca-substituted phase in asingle-phase form requires 20 mol% Ag2O2. The synthesizedsuperconductive phases are found to be free from Ag contamination within thedetection limit of EDX analysis. The obtainedCu2Ba2YCu2O8±δ andCu2Ba2(Y0.9Ca0.1)Cu2O8±δ samples showed asingle-step superconductivity transition at 81 and 90 K, respectively.

Electric properties of planar Ag/HgBa 2CaCu 2O 6+ δ interface

Silver coatings were magnetron sputtered onto the surface of highly c-axis-oriented HgBa 2CaCu 2O 6+ δ superconducting thin films exhibiting zero resistance T c at about 120 K and the critical current density ∼10 5 A/cm 2 at 100 K. Electrical resistance of the Ag/HgBa 2CaCu 2O 6+ δ planar interface and current–voltage dependencies were investigated in a course of post-deposition annealing of the Ag/HgBa 2CaCu 2O 6+ δ at 50–350°C in both pure oxygen and oxygen-free ambient. Results of the electrical measurements are explained in terms of the insulating layer formation at the Ag/HgBa 2CaCu 2O 6+ δ interface. It is assumed this insulator is a product created by oxygen non-stoichiometry and decomposition of the superconductor at the interface between both materials.

Tl/Pb and Sr/Ba cuprates of type 1212: compositional effect on the purity and on the superconducting properties

The effect of Sr/Ba and Tl/Pb substitutions has been investigated for Tl 1− x Pb x (Sr 1− y Ba y ) 2CaCu 2O 7− δ (0≤ x and/or y≤1) compounds produced by conventional solid state reaction. The observed phases were characterized by X-ray powder diffraction, scanning electron microscopy and their nominal composition has been controlled by plasma emission spectroscopy. A tentative single phase field is proposed at T≤960°C. For x=0.5, the highest weight fraction of the (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase was found to be 96.4% and 94% when Ba increases from 0 to 0.2. It drastically decreases when Ba content is over 0.3. For Ba over 0.3, (Tl/Pb)(Sr/Ba)-1223 and perovskite (Tl/Pb)(Sr/Ba)O 3 are observed as main impurities and their content increases with Ba stoichiometry. The unit cell parameters of (Tl 0.5Pb 0.5)(Sr/Ba)-1212 phase increase when Sr atoms are progressively replaced by Ba atoms. Structural refinement leads to conclude that 10 at% of Tl would be substituted on Ca sites and less than 2 at% of Ca could be substituted on Sr sites. The distance between the Cu atom and the apical O atom of the square pyramid increases only slightly when the Ba content raises. The critical temperature remains between 80 and 90 K and seems to be affected by the substitution Sr/Ba.

Quaternary system SrO-CaO-BaO-CuO at 950 °C: II. Phase equilibria related to the substitution Sr/Ba in (Sr/Ba) 2Ca 2Cu 2Cu 3O 7 and (Sr/Ba) 2CaCu 2O 5 samples

A partial study of the solid-solid equilibria in the quaternary system BaOSrOCaOCuO has been undertaken in order to determine the observed phases and their evolution when Ba is substituted for Sr in mixtures (Sr/Ba) 2CaCu 2O x and (Sr/Ba) 2Ca2Cu 3O x. The temperature was fixed at 9SO °C and samples were elaborated under 1 bar of flowing oxygen. It has been shown that two four-phased domains exist in this quantemary system, including the following phases: ( Ba /Sr/Ca)CuO 2, ( Sr/Ca /Ba) 2CuO 3, ( Sr/Ca /Ba) 14Cu 24O x and Ba 4CaCu 3O 8 substituted Sr/Ca for the one and ( Ba/Sr /Ca)CuO 2, ( Sr/Ca /Ba) 2CuO 3, ( Sr/Ca /Ba) 24Cu 24O x and ( Sr/Ca /Ba)CuO 2 for the other.

Studies of the residual absorption of a Bi2Sr2CaCu2O8+? single crystal at THz frequencies

We measured the residual absorption of a Bi2Sr2 CaCu2 O8+δ single crystal at low temperature in the frequency range between 0.6 THz and 4.2 THz using a new method, photothermal interference spectroscopy. By combining with reflectivity data obtained with a Fourier-transform IR spectrometer (frequency range: 3 THz to 100 THz) we have calculated the dynamical conductivity using a Kramers-Kronig-transformation. The results are compared with calculations of the far infrared conductivity and show qualitative features expected for a superconductor with nodes in the gap.

Preparation of the (La, A) 2CaCu 2O 6− y [A: Sr, Ca]/(La, Sr) 2CuO 4− y superlattice by laser ablation

We prepared c-axis oriented (La, A) 2CaCu 2O 6− y [A: Sr, Ca]/(La, Sr) 2CuO 4− y (La(2126)/La(214)) superlattices on (100) oriented SrTiO 3 substrates using laser ablation. Satellite peaks in a large-angle X-ray diffraction pattern and a peak of a small-angle X-ray diffraction pattern indicate that the superlattice structure was obtained. Cross-sectional transmission electron microscopy (TEM) reveals that the superlattice was grown with the orientation of the c-axis perpendicular to the substrate surface. The temperature dependences of the resistivity of La 1.6Sr 0.4CaCu 2O 6− y and La 1.6Sr 0.4CuO 4− y exhibit semiconducting and metallic behavior, respectively. However, the superlattice in which four half-unit cells of La 1.6Sr 0.4CaCu 2O 6− y and ten half-unit cells of La 1.6Sr 0.4CaCu 2O 6− y were alternately stacked exhibits superconductivity. T c onset was 25 K, and T c zero was 8 K.

希土類と鉛で同時置換した変調構造のない（Ｂｉ，Ｐｂ）２Ｓｒ２（Ｃａ，Ｒ）Ｃｕ２Ｏｚ＋ａ（Ｒ：ｒａｒｅ ｅａｒｔｈ＝Ｙ）のフォノンラマン散乱

Phonon Raman spectra of the modulation-free Bi1.2 Pb0.8 Sr2 Ca1-y, YyCu2Oz(0.2≤y≤0.8) have been investigated both in polycrystalline and single crystal forms. Polarized Raman spectroscopic measurements of single crystals made it possible to clarify some controversies on the Raman mode assignments for Bi2 Sr2 CaCu2 O8+d (Bi2212), most notably in high-frequency three prominent oxygen modes observed at 290, 460 and 630cm-1, which we assigned to AgOBi+B1gOCu, Ag OBi and Ag Oap _??_ x, respectively. The energies of the three oxygen modes show drastic change with a decrease of the hole concentration: (1)Ag OBi+B1g OCu. mode is shifted towards higher frequencies, (2)AgOBi mode hardly varies for y 0.6, and Ag Oap _??_ x mode is shifted towards lower frequencies. The observed changes in these oxygen modes are similar to those observed for the modulated Bi2 Sr2 Ca1-y Yy Cu2Oz. We propose that the Y/Ca substitution both in the modulated and modulation-free Bi2212 systems induced a hole filling on the CuO2 plane, which can explain the behaviour of the oxygen modes observed in their Raman spectra.

QUALITY CONTROLS ON HIGH-Tc BASED TUNNEL JUNCTIONS

We report on systematic studies to perform quality controls on planar tunnel junctions realized on high-T c superconductors (HTS). Tunneling spectra of the orthorhombic 90 K and 60 K YBa2 Cu 3O7−δ phases, of the magnetically doped, tetragonal YBa 2( Cu 0.96 Fe 0.04)3 O 7 compound and of the Bi 2 Sr 2 CaCu 2O8 system are analyzed. The traditional, low-bias barrier analysis as well as a number of tests useful to investigate the “status” of the HTS surface layer are carried out. Evidence is given that some anomalous tunneling data can be understood in terms of a Kondo-type scattering between tunneling electrons and magnetic impurities at the interface. The reproducibility of the tunneling spectra for different junction fabrication conditions is also discussed.

Study on Thermal Hysteresis of Elastic Modulus in Granular Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O

ABSTRACTA large thermal hysteresis ( &gt; 100 K) of elastic modulus was measured for coarse grained Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. Whereas, only a small thermal hysteresis (2∼3 K) near the velocity minima at 240 K, and 150 K was observed in Bi2 Sr2 CaCu2 O8 single crystal, which were found to be associated with phaselike transitions (PLT). The investigations of isothermal recovery of modulus for coarse grained YBCO and BSCCO disclose that the large thermal hysteresis may be attributed to the defects of pore size generated in the interconnecting region between grains as a result of PLT of weak first order but covering a wide temperature range. The mechanism of a large thermal hysteresis for coarse grained samples has been discussed in detail.

Structural study of undoped and doped Bi2Sr2CuO6 phases by transmission electron microscopy

Bi-based superconducting phases have been studied by transmission electron microscopy. The strong one-dimensional modulations in undoped and Ce-doped monoclinic Bi2 Sr2 CuO6 (2201) samples are different from one another. We have found a new 2201 phase containing Pb and Ca with orthorhombic symmetry in nominal (Bi0.90 Pb0.10 )2 Sr2 CaCu2 O8 crystals. Its structure with space group Bbmb and two-dimensional modulation are analogous to those of the Pb-doped higher members of the structural series Bi2 Sr2 Can−1 Cun O2n+4 . Hence, this new orthorhombic rather than the monoclinic 2201 phase should be the first member of the structural series.

Identification of Pb-doped Bi-Sr-Ca-Cu-O superconducting phases by convergent beam electron diffraction

Pb-doped Bi2Sr2Ca2Cu3O10 (2223) and Bi2 Sr2 Ca3 Cu4 O12 (2234) superconducting phases have been identified by convergent beam electron diffraction. The c parameters of the 2223 and 2234 phases are 37.6 and 44 Å, respectively. The two phases have the same symmetry; their common point group is mmm. Both phases are modulated, but the modulations are different from and considerably weaker than that in the unleaded Bi2 Sr2 CaCu2 O8 . This work confirms for the first time the existence of the 2234 phase.