Bismuth-based Cuprate Research Articles

Doping dependence of spin-momentum locking in bismuth-based high-temperature cuprate superconductors

Non-zero spin orbit coupling has been reported in several unconventional superconductors due to the absence of inversion symmetry breaking. This contrasts with cuprate superconductors, where such interaction has been neglected for a long time. The recent report of a non-trivial spin orbit coupling in overdoped Bi2212 cuprate superconductor, has re-opened an old debate on both the source and role of such interaction and its evolution throughout the superconducting dome. Using high-resolution spin- and angle-resolved photoemission spectroscopy, we reveal a momentum-dependent spin texture throughout the hole-doped side of the superconducting phase diagram for single- and double-layer bismuth-based cuprates. The universality of the reported effect among different dopings and the disappearance of spin polarization upon lead substitution, suggest a common source. We argue that local structural fluctuations of the CuO planes and the resulting charge imbalance may cause local inversion symmetry breaking and spin polarization, which might be crucial for understanding cuprates physics.

Universal Stripe Symmetry of Short-Range Charge Density Waves in Cuprate Superconductors.

The omnipresence of charge density waves (CDWs) across almost all cuprate families underpins a common organizing principle. However, a longstanding debate of whether its spatial symmetry is stripe or checkerboard remains unresolved. While CDWs in lanthanum- and yttrium-based cuprates possess a stripe symmetry, distinguishing these two scenarios is challenging for the short-range CDW in bismuth-based cuprates. Here, high-resolution resonant inelastic x-ray scattering is employed to uncover the spatial symmetry of the CDW in Bi2 Sr2 - x Lax CuO6 + δ . Across a wide range of doping and temperature, anisotropic CDW peaks with elliptical shapes are found in reciprocal space. Based on Fourier transform analysis of real-space models, the results are interpreted as evidence of unidirectional charge stripes, hosted by mutually 90°-rotated anisotropic domains. This work paves the way for a unified symmetry and microscopic description of CDW order incuprates.

NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA) x FeSe and (CTA) x FeSe] with superconducting transition temperatures (T c) above 40 K have been discovered. Due to the large interlayer distance (∼ 15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above T c is revealed by nuclear magnetic resonance (NMR) measurements on 77Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY) x FeSe, with a reduced interlayer distance (∼ 10 Å) compared to (TBA) x FeSe and (CTA) x FeSe. By performing 77Se NMR and transport measurements, we observe a similar pseudogap behavior well above T c of ∼ 40 K and a large resistivity anisotropy of ∼10 4 in (PY) x FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.

Observation of broken inversion and chiral symmetries in the pseudogap phase in single- and double-layer bismuth-based cuprates

One of the long-standing questions in the study of the high-${T}_{c}$ cuprate superconductors has been the understanding of the nature of the boundary between the strange metal phase and the so-called pseudogap phase, which is a precursor to superconductivity. Testing for a possible symmetry breaking transition through that boundary, a detailed study of the photogalvanic effect along a set of crystallographic axes in BSCCO reveals a transition to a state with broken inversion and chiral symmetries, thus strongly constraining the possible theoretical proposals for this phenomenon.

Crossover from two-dimensional to three-dimensional superconducting states in bismuth-based cuprate superconductor

To decipher the mechanism of high-temperature superconductivity, it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors1–4, including the pseudogap5,6, strange metal7,8 and anomalous Fermi liquid9 phases. A long-standing issue is how the superconducting pairing is formed and condensed in the strange metal phase, because this is where the superconducting transition temperature is highest. Here, we use state-of-the-art high-pressure measurements to report the experimental observation of a pressure-induced crossover from two- to three-dimensional (2D to 3D) superconducting states in optimally doped Bi2Sr2CaCu2O8 + δ bulk superconductor. By analysing the temperature dependence of the resistance, we find that the 2D superconducting transition exhibits a Berezinskii–Kosterlitz–Thouless-like behaviour10. The emergence of this 2D superconducting transition provides direct evidence that the strange metal state is predominantly 2D-like. This is important for a thorough understanding of the phase diagram of cuprate superconductors. Applying pressure to a cuprate reveals that the strange metal phase has a two-dimensional character, as shown by emerging Berezinskii–Kosterlitz–Thouless behaviour.

Conspiring interactions in a cuprate

Superconductivity More than 30 years after the discovery of high-temperature superconductivity in copper oxides, its mechanism remains a mystery. Electron pairing mediated solely by lattice vibrations—phonons—is thought to be insufficient to account for the high transition temperatures. He et al. found a rapid and correlated increase of the superconducting gap and electron-phonon interactions as the chemical composition of their bismuth-based cuprate samples was varied across a critical doping concentration. The interplay of electron-phonon with electron-electron interactions may lead to enhanced transition temperatures. Science , this issue p. [62][1] [1]: /lookup/doi/10.1126/science.aar3394

Resolving unoccupied electronic states with laser ARPES in bismuth-based cuprate superconductors

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to two final-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultra-high momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the final-state gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions.

Rotated stripe order and its competition with superconductivity inLa1.88Sr0.12CuO4

We report the observation of a bulk charge modulation in La$_{1.88}$Sr$_{0.12}$CuO$_4$ (LSCO) with a characteristic in-plane wave-vector of (0.236, $\pm \delta$), with $\delta$=0.011 r.l.u. The transverse shift of the ordering wave-vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal "214" cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families.

三軸結晶配向技術としての磁場配向法

Magnetic alignment technology using the modulated rotating magnetic fields (MRF) enables formation of tri-axial grain arrangement of the substances with tri-axial magnetic anisotropy. In this review, we introduce a high potential of magnetic alignment technique as the fabrication process of tri-axially grain-oriented cuprate superconductors. As the first topic, we reported the tri-axial magnetic alignment of rare-earth-based and bismuth-based cuprate superconductors operated at room temperature. In the second topic, we show the importance of single-ion magnetic anisotropy of the rare-earth ions as a determination factor of magnetization axes and the tri-axial magnetic anisotropy in rare-earth-based cuprate superconductors. The final topic is the fabrication of tri-axially grain-aligned ErBa2Cu4O8 (Er124) ceramics under a modified MRF. To date, the Er124 ceramic with the degree of in-plane orientation with ~11 degree has been successfully fabricated by controlling the viscosity of slurry and introducing the oscillation type of MRF.

Scanning tunnelling microscopy imaging of symmetry-breaking structural distortion in the bismuth-based cuprate superconductors

A complicating factor in unravelling the theory of high-temperature (high-T(c)) superconductivity is the presence of a 'pseudogap' in the density of states, the origin of which has been debated since its discovery. Some believe the pseudogap is a broken symmetry state distinct from superconductivity, whereas others believe it arises from short-range correlations without symmetry breaking. A number of broken symmetries have been imaged and identified with the pseudogap state, but it remains crucial to disentangle any electronic symmetry breaking from the pre-existing structural symmetry of the crystal. We use scanning tunnelling microscopy to observe an orthorhombic structural distortion across the cuprate superconducting Bi(2)Sr(2)Ca(n-1)Cu(n)O(2n+4+x) (BSCCO) family tree, which breaks two-dimensional inversion symmetry in the surface BiO layer. Although this inversion-symmetry-breaking structure can impact electronic measurements, we show from its insensitivity to temperature, magnetic field and doping, that it cannot be the long-sought pseudogap state. To detect this picometre-scale variation in lattice structure, we have implemented a new algorithm that will serve as a powerful tool in the search for broken symmetry electronic states in cuprates, as well as in other materials.

Diamagnetism of real-space pairs aboveTc in hole doped cuprates

The nonlinear normal state diamagnetism reported by Li et al (2010 Phys. Rev. B 81 054510)is shown to be incompatible with a claimed Cooper pairing and vortex liquidabove the resistive critical temperature. However, it is perfectly compatible withthe normal state Landau diamagnetism of real-space composed bosons, whichprovides a description of the nonlinear magnetization curves of the less anisotropiccuprates La–Sr–Cu–O (LSCO) and Y–Ba–Cu–O (YBCO) as well as for stronglyanisotropic bismuth-based cuprates over the whole range of available magnetic fields.

Imaging nanoscale Fermi-surface variations in an inhomogeneous superconductor

A variant on Fourier-transform scanning tunnelling spectroscopy enables spatial variations in the Fermi surface of bismuth-based cuprate superconductors to be probed. This technique reveals that these variations take place over nanometre distances. Particle–wave duality suggests we think of electrons as waves stretched across a sample, with wavevector k proportional to their momentum. Their arrangement in ‘k-space’, and in particular the shape of the Fermi surface, where the highest-energy electrons of the system reside, determine many material properties. Here we use a novel extension of Fourier-transform scanning tunnelling microscopy to probe the Fermi surface of the strongly inhomogeneous Bi-based cuprate superconductors. Surprisingly, we find that, rather than being globally defined, the Fermi surface changes on nanometre length scales. Just as shifting tide lines expose variations of water height, changing Fermi surfaces indicate strong local doping variations. This discovery, unprecedented in any material, paves the way for an understanding of other inhomogeneous characteristics of the cuprates, such as the pseudogap magnitude, and highlights a new approach to the study of nanoscale inhomogeneity in general.

Magnetic orientation and magnetic anisotropy in paramagnetic layered oxides containing rare-earth ions

The magnetic anisotropies and easy axes of magnetization at room temperature were determined, and the effects of rare-earth (RE) ions were clarified for RE-based cuprates, RE-doped bismuth-based cuprates and RE-doped Bi-based cobaltite regarding the grain orientation by magnetic field. The easy axis, determined from the powder orientation in a static field of 10 T, depended qualitatively on the type of RE ion for all three systems. On the other hand, the magnetization measurement of the c-axis oriented powders, aligned in static or rotating fields, revealed that the type of RE ion strongly affected not only the directions of the easy axis but also the absolute value of magnetic anisotropy, and an appropriate choice of RE ion is required to minimize the magnetic field used for grain orientation. We also studied the possibility of triaxial grain orientation in high-critical-temperature superconductors by a modulated oval magnetic field. In particular, triaxial orientation was attempted in a high-oxygen-pressure phase of orthorhombic RE-based cuprates Y2Ba4Cu7Oy. Although the experiment was performed in epoxy resin, which is not practical, in-plane alignment within 3° was achieved.

Fabrication of intrinsic Josephson junction of bismuth-based cuprates

We have developed a new process for fabricating the intrinsic Josephson junctions of Bi-based cuprates with a size of 1 μm × 1.7 μm. The fabricated junctions have an S-shaped structure with four-terminal electrodes that enables us to measure the transport properties of the junctions without considering the effect of the contact resistance. The current–voltage characteristics exhibited clear hysteresis suggesting the absence of serious damage to the insulating layer during the fabrication process. The plasma frequency and crossover temperature were estimated from the parameters extracted from the transport measurements.

Morphological Effects of Grinding on Bismuth-Based Cuprate Superconductors

Morphological changes of bismuth-based cuprate superconductors, such as the 2223 phase, via a small mechanical force were studied mainly by means of high-resolution transmission electron microscopy and atomic force microscopy. Even light mechanical grinding of the sintered 2223 samples led to the formation of superfine particles that had a width of {approximately} 5 nm. Moreover, the authors found first that the superfine particles had a discrete thickness of odd numbers of a half unit length c/2 of the 2223 phase, e.g. 1.9 nm, 5.6 nm, and so on. The formation of the superfine particles was due to cleavage at Bi-O double layers that were bound weakly to each other in a unit cell.

Rietveld texture analysis of complex oxides: examples of polyphased Bi2223 superconducting and Co349 thermoelectric textured ceramics characterization using neutron and X-ray diffraction

Orientation distributions of cobaltite thermoelectric and polyphased bismuth-based cuprate superconducting textured materials are determined from neutron and X-ray diffraction analysis. Curved position-sensitive detectors coupled to a tilt-angle (χ) scan enable the treatment of the whole diffraction pattern using the combined Rietveld–WIMV–Popa algorithm. The textures of three phases of superconducting compounds are determined. The critical current densities, measured for four samples, are strongly dependent on the calculated texture strengths, crystallite sizes and phase ratios. For the cobaltite compounds, a comparison between X-ray and neutron analysis shows the advantages of the latter technique for avoiding the limitations of the X-ray analysis with respect to the defocusing effect. The results highlight the necessity and efficiency of the combined approach for a quantitative texture analysis of complex materials and exemplify the texture–anisotropic physical properties relationship for a better understanding and design of improved bulk superconducting and thermoelectric materials.

Dependence of upper critical field and pairing strength on doping in cuprates.

We have determined the upper critical field Hc2 as a function of hole concentration in bismuth-based cuprates by measuring the voltage induced by vortex flow in a driving temperature gradient (the Nernst effect), in magnetic fields up to 45 tesla. We found that Hc2 decreased steeply as doping increased, in both single and bilayer cuprates. This relationship implies that the Cooper pairing potential displays a trend opposite to that of the superfluid density versus doping. The coherence length of the pairs xi(0) closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature Tc0.

Relevance of cooling rates on formation from the melt of superconducting bismuth-based cuprate phases

This work presents a study on phase formation processes in the Bi–Sr–Ca–Cu–O system. A liquid phase is employed as nutrient in order to optimize the system’s components mixing and the cooling rate is the relevant experimental variable. Different cooling rates are employed to promote changes in component’s mobility inside the liquid, as well as to collect information regarding the metastable feature of the stoichiometrically ideal superconducting phases on this system. The solidification process is evaluated in real time by means of differential thermal analysis and thermogravimetry. Conventional X-ray crystallography is employed to characterize the samples at room temperature in powder form. Crystal structure refinement employs the Rietveld method. The results suggest that the amount of 2201 increases and of 2212 decreases as the cooling rate increases, and that formation of 2201 proceeds at high temperature.

The binding energies of various components in a bismuth-based cuprate superconductor

A structural analysis of a non-crystalline bismuth-based high-Tc ceramic superconductor (Bi,Pb)2Sr2Ca2Cu3O10 + δ (Bi-2223) has been carried out by secondary ion mass spectrometry and X-ray photoelectron spectroscopy techniques. The results have been compared with that of similar measurements on non-crystalline Bi2O3, CuO and CaCO3 samples so as to have detailed information about the surface binding energy as well as the chemical nature of the various individual components that exist inside this high-Tc compound.

Corrosion process of bismuth-based superconductor cuprates in a moisture atmosphere

Abstract Sintered pellets and single crystals of Bi 2 Sr 2 CaCu 2 O 8 (2212) and Bi 2 Sr 2 CuO 6 (2201) have been exposed to moistened air or just to water vapour. The following alterations observed have been studied by SEM, microanalysis, optical microscopic techniques, and FTIR. A possible reaction path for the alteration process is proposed. By means of optical and SEM observations, several morphologies of the alteration products have been observed. Two main steps, responsible for the degradation, have been found: the leaching of Sr and Ca cations from the matrix by fixed water, then a carbonate and hydroxide precipitation. In both cases, Ca and Sr extraction leads to Bi 2 CuO 4 .