Multilayered cuprate superconductor Ba2Ca5Cu6O12(O1−x,Fx)2 studied by temperature-dependent scanning tunneling microscopy and spectroscopy (original) (raw)
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Physica C: Superconductivity, 2010
The atomic-scale surface electronic states on the multi-layered apical-fluorine cuprate superconductor Ba 2 Ca 5 Cu 6 O 12 (O 1Àx , F x ) 2 ðT C ' 70 KÞ are investigated by using low temperature scanning tunneling microscopy/spectroscopy (STM/STS). The spatial gap distributions show the patch scale of 0:5nm,whichisquiteshorterthanthesuperconductingcoherencelength0:5 nm, which is quite shorter than the superconducting coherence length 0:5nm,whichisquiteshorterthanthesuperconductingcoherencelength2-3 nm of other cuprate superconducters of a few nm. The high-bias ($1 V) conductance map contains some characteristic spots with contrasts reverse to those at low bias, which are considered to be due to the charge unbalance of apical atoms such as O 2À and F À .
Physica C-superconductivity and Its Applications, 2009
Scanning tunneling microscopy/spectroscopy (STM/STS) measurements on multi-layered cuprate superconductor Ba 2 Ca 5 Cu 6 O 12 (O 1-x F x) 2 are carried out. STM topographies show randomly distributed bright spot structures with a typical spot size of ~ 0.8 nm. These bright spots are occupied about 28% per one unit cell of c-plane, which is comparable to the regular amount of apical oxygen of 20% obtained from element analysis. Tunneling spectra simultaneously show both the small and the large gap structures. These gap sizes at 4.9 K are about ∆ ~ 15 meV and ~ 90 meV, respectively. The small gap structure disappears at the temperature close to T C , while the large gap persists up to ~200 K. Therefore, these features correspond to the superconducting gap and pseudogap, respectively. These facts give evidence for some ordered state with large energy scale even in the superconducting state. For the superconducting gap, the ratio of 2∆ S /K B T C = 4.9 is obtained with T C = 70 K, which is determined from temperature dependence of the tunneling spectra.
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Physical review, 2003
The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa 2 Cu 3 O 7-δ thin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally. The crucial and yet controversial issues involved in the superconducting gap symmetry of the cuprate superconductors remain unsettled. Numbers of theories and experiments have been proposed and deployed to track down the nature of the order parameter symmetry of the cuprate superconductors. In general, the obtained results can be roughly classified into two sects 1. Majority of experimental results obtained by surface-sensitive experiments 2-6 , all point to the widely-accepted d-wave symmetry scenario. Nonetheless, recently some bulk-sensitive experiments 7-10 , e.g. the investigation of quasiparticle dynamics by the ultrafast time-resolved experiments, have revealed some disputable features of s-wave symmetry or s-d mixed characters. In particular, very recently, Kabanov et al. 7 compared the calculations on the temperature dependence of the photo-induced transmission amplitude below T c and claimed that the results were more consistent with an isotropic gap in YBa 2 Cu 3 O 7-δ (YBCO). This apparently has revived the extensive interest on this matter. The fact that, in addition to its bulk-sensitive characteristic, there exist intimate correlations between the superconducting gap opening and the amplitude as well as relaxation time of the transient reflectivity change (∆R/R) has made the femtosecond pump-probe technique one of the most powerful tools in studying the ultrafast carrier dynamics relevant to high-T c superconductivities 11-19. It is, thus, desirable to reexamine this issue with the improved femtosecond laser and more carefully characterized samples. In this study, based on the general consensus that the amplitude and relaxation time of the transient reflectivity in picosecond scale below T c are directly associated with the opening of the superconducting gap, we have measured the
Physica C: Superconductivity and its Applications, 2015
We have found a quantitative connection between the evolution of the inhomogeneous nanoscale electronic gaps (INSEG) state detected in Bi 2 Sr 2 CaCu 2 O 8+δ by scanning tunneling microscopy/spectroscopy (STM/S) and the two universal, the upper and the lower, pseudogaps in high-temperature cuprate superconductors (HTCS). When the doping and temperature dependent IN-SEG map were analyzed by using our proposed hole-scale, we find that the two pseudogaps are connected to two specific coverages of the CuO 2 plane by INSEG: the 50% and 100% coverages of the CuO 2 planes by INSEG correspond to the upper and lower pseudogaps, respectively. This quantitative connection to the two pseudogaps indicates that the origin of the measured pseudogap energies and temperatures are intimately related to the geometrical coverage of the CuO 2 planes by the INSEG state. We find that INSEG and superconductivity coexist in the underdoped to the overdoped regimes. We suggest that pseudogap states are microscopically inhomogeneous and 100% coverage of the CuO 2 planes by the INSEG is a necessary condition for the high-T c superconductivity.
Many theoretical models of high-temperature superconductivity focus only on the doping dependence of the CuO 2-plane electronic structure. However, such models are manifestly insufficient to explain the strong variations in superconducting critical temperature, T c, among cuprates that have identical hole density but are crystallographically different outside of the CuO 2 plane. A key challenge, therefore, has been to identify a predominant out-ofplane influence controlling the superconductivity, with much attention focusing on the distance d A between the apical oxygen and the planar copper atom. Here we report direct determination of how variations in interatomic distances within individual crystalline unit cells affect the superconducting energy-gap maximum ⌬ of Bi 2Sr2CaCu2O8؉␦. In this material, quasiperiodic variations of unit cell geometry occur in the form of a bulk crystalline ''supermodulation.'' Within each supermodulation period, we find Ϸ9 ؎ 1% cosinusoidal variation in local ⌬ that is anticorrelated with the associated d A variations. Furthermore, we show that phenomenological consistency would exist between these effects and the random ⌬ variations found near dopant atoms if the primary effect of the interstitial dopant atom is to displace the apical oxygen so as to diminish d A or tilt the CuO5 pyramid. Thus, we reveal a strong, nonrandom out-of-plane effect on cuprate superconductivity at atomic scale. apical oxygen ͉ dopant atoms ͉ out-of-plane influence ͉ superconducting energy gap ͉ supermodulation
Connection between the Electronic Inhomogeneity and the Pseudogap of Bi2Sr2CaCu2O8+d Superconductors
2013
We have found a quantitative connection between the evolution of the inhomogeneous nanoscale electronic gaps (INSEG) state detected in Bi 2 Sr 2 CaCu 2 O 8+δ by scanning tunneling microscopy/spectroscopy (STM/S) and the two universal, the upper and the lower, pseudogaps in high-temperature cuprate superconductors (HTCS). When the doping and temperature dependent IN-SEG map were analyzed by using our proposed hole-scale, we find that the two pseudogaps are connected to two specific coverages of the CuO 2 plane by INSEG: the 50% and 100% coverages of the CuO 2 planes by INSEG correspond to the upper and lower pseudogaps, respectively. This quantitative connection to the two pseudogaps indicates that the origin of the measured pseudogap energies and temperatures are intimately related to the geometrical coverage of the CuO 2 planes by the INSEG state. We find that INSEG and superconductivity coexist in the underdoped to the overdoped regimes. We suggest that pseudogap states are microscopically inhomogeneous and 100% coverage of the CuO 2 planes by the INSEG is a necessary condition for the high-T c superconductivity.
Subgap structures and pseudogap in cuprate superconductors: Role of density waves
Physical Review B
In scanning tunneling microscopy (STM) conductance curves, the superconducting gap of cuprates is sometimes accompanied by small sub-gap structures at very low energy. This was documented early on near vortex cores and later at zero magnetic field. Using mean-field toy models of coexisting d-wave superconductivity (dSC), d-form factor density wave (dFF-DW), and extended s-wave pair density wave (s PDW), we find agreement with this phenomenon, with s PDW playing a critical role. We explore the high variability of the gap structure with changes in band structure and density wave (DW) wave vector, thus explaining why sub-gap structures may not be a universal feature in cuprates. In the absence of nesting, non-superconducting results never show signs of pseudogap, even for large density waves magnitudes, therefore reinforcing the idea of a distinct origin for the pseudogap, beyond mean-field theory. Therefore, we also briefly consider the effect of DWs on a pre-existing pseudogap.
Physica C: Superconductivity, 2001
Scanning tunneling spectroscopy (STS) studies reveal long-range (~100 nm) spatial homogeneity in optimally and underdoped superconducting YBa 2 Cu 3 O 7−δ (YBCO) single crystals and thin films, and macroscopic spatial modulations in overdoped (Y 0.7 Ca 0.3)Ba 2 Cu 3 O 7−δ (Ca-YBCO) epitaxial films. In contrast, STS on an optimally doped YBa 2 (Cu 0.9934 Zn 0.0026 Mg 0.004) 3 O 6.9 single crystal exhibits strong spatial modulations and suppression of superconductivity over a microscopic scale near the Zn or Mg impurity sites, and the global pairing potential is also reduced relative to that of optimally doped YBCO, suggesting strong pair-breaking effects of the non-magnetic impurities. The spectral characteristics are consistent with d x 2 −y 2 pairing symmetry for the optimally and underdoped YBCO, and with (d x 2 −y 2+s) for the overdoped Ca-YBCO. The doping-dependent pairing symmetry suggests interesting changes in the superconducting ground state, and is consistent with the presence of nodal quasiparticles for all doping levels. The maximum energy gap ∆ d is non-monotonic with the doping level, while the (2∆ d /k B T c) ratio increases with decreasing doping. The similarities and contrasts between the spectra of YBCO and of Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) are discussed.
Fluctuating stripes at the onset of the pseudogap in the high-T-c superconductor Bi2Sr2CaCu2O8+x
2010
Doped Mott insulators have been shown to have a strong propensity to form patterns of holes and spins often referred to as stripes 1-5. In copper-oxides, doping also gives rise to the pseudogap state 6 , which transforms into a high temperature superconductor with sufficient doping or by reducing the temperature. A long standing question has been the interplay between pseudogap, which is generic to all hole-doped cuprates, and stripes, whose static form occurs in only one family of cuprates over a narrow range of the phase diagram 2,7. Here we examine the spatial reorganization of electronic states with the onset of the pseudogap state at T* in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+x using spectroscopic mapping with the scanning tunneling microscope (STM). We find that the onset of the pseudogap phase coincides with the appearance of electronic patterns that have the predicted characteristics of fluctuating stripes 8. As expected, the stripe patterns are strongest when the hole concentration in the CuO 2 planes is close to 1/8 (per Cu) 2-5,8. While demonstrating