Novel relation between Tc and low-T energy gap 2Δo in Bi2212 and La214: an STS study
TL;DR: On the basis of electronic specific heat, magnetic susceptibility and STS data in La214, this article reported that a pseudogap of almost the same scale as the low-T (T
Abstract: On the basis of electronic specific heat, magnetic susceptibility and STS data in La214, we report that a pseudogap of almost the same scale as the low-T (T
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TL;DR: The use of tunneling microscopy and spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors as discussed by the authors.
Abstract: Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.
790 citations
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TL;DR: In this article, a review of the current situation in studies of materials with competing electron spectrum instabilities, namely Cooper pairing, on the one hand, and charge density waves (CDWs) or spin-density waves (SDWs), on the other.
197 citations
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TL;DR: In this paper, the effects of interlayer hopping and the resulting kz dispersion in the cuprates within the framework of the one-band tight-binding model Hamiltonian are discussed.
Abstract: We discuss the effects of interlayer hopping and the resulting kz dispersion in the cuprates within the framework of the one-band tight-binding model Hamiltonian. Specific forms of the dispersion relations in terms of the in-plane hopping parameters t, t, t, and t and the effective interlayer hopping tz in La2xSrxCuO4 LSCO and Nd2xCexCuO4 NCCO and the added intracell hopping tbi between the CuO2 bilayers in Bi2Sr2CaCu2O8 Bi2212 are presented. The values of the “bare” parameters are obtained via fits with the first-principles local-density-approximation- LDA- based band structures in LSCO, NCCO, and Bi2212. The corresponding “dressed” parameter sets which account for correlation effects beyond the LDA are derived by fitting experimental Fermi surface FS maps and dispersions near the Fermi energy in optimally doped and overdoped systems. The interlayer couplings tz and tbi are found generally to be a substantial fraction of the in-plane hopping t, although the value of tz in NCCO is anomalously small, reflecting absence of apical O atoms in the crystal structure. Our results provide some insight into the issues of the determination of doping from experimental FS maps in Bi2212, the role of intercell coupling in c-axis transport, and the possible correlations between the doping dependences of the binding energies of the Van Hove singularities and various prominent features observed in the angle-resolved photoemission and tunneling spectra of the cuprates.
132 citations
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TL;DR: It is proven, that the obtained s-wave model supplements the predictions based on the BCS van Hove scenario, and the anisotropic model explains the dependence of the ratio on doping for the considered superconductors.
Abstract: The pairing mechanism for the high- superconductors based on the electron-phonon (EPH) and electron-electron-phonon (EEPH) interactions has been presented. On the fold mean-field level, it has been proven, that the obtained s-wave model supplements the predictions based on the BCS van Hove scenario. In particular: (i) For strong EEPH coupling and the energy gap () is very weak temperature dependent; up to the critical temperature extends into the anomalous normal state to the Nernst temperature. (ii) The model explains well the experimental dependence of the ratio on doping for the reported superconductors in the terms of the few fundamental parameters. In the presented paper, the properties of the d-wave superconducting state in the two-dimensional system have been also studied. The obtained results, like for s-wave, have shown the energy gap amplitude crossover from the BCS to non-BCS behavior, as the value of the EEPH potential increases. However, for the energy gap amplitude extends into the anomalous normal state to the pseudogap temperature. Finally, it has been presented that the anisotropic model explains the dependence of the ratio on doping for the considered superconductors.
50 citations
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TL;DR: In this article, the Coulomb potential is defined within a kinematically allowed region allowing a negative eigenvalue, which provides superconducting pairing and weakly decaying, quasistationary, large-momentum pair states.
Abstract: Antiferromagnetic correlations in superconducting cuprates can lead to the mirror nesting of Fermi contour segments near saddle points of the electron spectrum and to a logarithmic singularity of the scattering amplitude for a large pair momentum. The Coulomb potential defined within a kinematically allowed region allows a negative eigenvalue, which provides superconducting pairing and weakly decaying, quasistationary, large-momentum pair states. The Ginzburg–Landau equations for the two-component superconducting order parameter provide pairs of coupled particles and pairs of coupled orbital current circulations, which explains the fundamental cuprate properties such as strong and weak pseudogaps, the superconducting transition temperature, the diamagnetic pseudogap state, and details of the isotope effect. A quantum critical point and a transition of two superconducting phases, one of which displays superconductivity with current circulations, are predicted.
31 citations
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TL;DR: In this article, the phase of the order parameter is not important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition, and the phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties.
Abstract: THE superconducting state of a metal is characterized by a complex order parameter with an amplitude and a phase In the BCS-Eliashberg mean-field theory1, which is a very good approximation for conventional metals, the phase of the order parameter is un-important for determining the value of the transition temperature Tc and the change of many physical properties brought about by the transition Here we argue that superconductors with low super-conducting carrier density (such as the organic and high-Tc oxide superconductors) are characterized by a relatively small phase 'stiffness9 and poor screening, both of which imply a significantly larger role for phase fluctuations As a consequence, in these mat-erials the transition to the superconducting state may not display typical mean-field behaviour, and phase fluctuations, both classical and quantum, may have a significant influence on low-temperature properties For some quasi-two-dimensional materials, notably underdoped high-temperature superconductors, the onset of long-range phase order controls the gross value of Tc as well as its systematic variation from one material to another
1,533 citations
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TL;DR: In this paper, a study of underdoped Bi2Sr2CaCu2O8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES) is presented.
Abstract: IT is well known that BCS mean-field theory is remarkably successful in describing conventional superconductors. A central concept of BCS theory is the energy gap in the electronic excitation spectrum below the superconducting transition temperature, Tc. The gap also serves as the order parameter: quite generally, long-range phase coherence and a non-zero gap go hand-in-hand1. But in underdoped high-Tc superconductors there is considerable evidence that a pseudogap (a suppression of spectral weight) is already formed in the normal state above Tc—first, from studies of the spin excitation spectrum2–5,24, which measure a 'spin gap', and later from a variety of other probes6–10. Here we present a study of underdoped Bi2Sr2CaCu2O8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES), which directly measures the momentum-resolved electron excitation spectrum of the CuO2 planes. We find that a pseudogap with d-wave symmetry opens up in the normal state below a temperature T* > Tc, and develops into the d-wave superconducting gap once phase coherence is established below Tc.
1,019 citations
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TL;DR: In this article, the temperature dependence of the Fermi surface in underdoped Bi2Sr2CaCu2O8+δ superconductors was investigated using angle-resolved photoemission spectroscopy.
Abstract: The Fermi surface—the set of points in momentum space describing gapless electronic excitations—is a central concept in the theory of metals. In this context, the normal ‘metallic’ state of the optimally doped high-temperature superconductors is not very unusual: above the superconducting transition temperature, Tc, there is evidence for a large Fermi surface1,2,3, despite the absence of well-defined elementary excitations. In contrast, the normal state of underdoped high-temperature superconductors differs in that there is evidence for a ‘pseudogap’ above Tc (4–7). Here we examine, using angle-resolved photoemission spectroscopy, the temperature dependence of the Fermi surface in underdoped Bi2Sr2CaCu2O8+δ. We find that, on cooling the sample, the pseudogap opens up at different temperatures for different points in momentum space. This leads to an initial breakup of the Fermi surface, at a temperature T*, into disconnected arcs, which then shrink with decreasing temperature before collapsing to the point nodes of the superconducting ground state below Tc. This unusual behaviour, where the Fermi surface does not form a continuous contour in momentum space as in conventional metals, is unprecedented in that it occurs in the absence of long-range order. Moreover, although the superconducting gap below Tc evolves smoothly into the pseudogap above Tc, the pseudogap differs in its unusual temperature-dependent anisotropy, implying an intimate but non-trivial relationship between the pseudogap and the superconducting gap.
820 citations
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TL;DR: In this paper, the authors present tunneling spectroscopy of single crystals as a function of oxygen doping and temperature, with a quasiparticle gap that is reduced with increasing oxygen concentration.
Abstract: We present tunneling spectroscopy of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals as a function of oxygen doping and temperature. The doping dependence amounts essentially to an energy scaling of the tunneling spectra, with a quasiparticle gap that is reduced with increasing oxygen concentration. This superconducting gap is temperature independent up to the superconducting transition where the superconducting spectra merge continuously into another gaplike feature at the Fermi level. This pseudogap is found to be present both in underdoped and overdoped samples, and it scales with the superconducting gap.
683 citations
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TL;DR: The powder magnetic susceptibility chi(T) of La/sub 2-//sub 2/Sr/sub x/Sub x/CuO/sub 4-// sub y/ is found to scale with doped hole concentration p = x-2y according to a law of corresponding states for 0less than or equal topless than orequal to0.20, thereby allowing chi/sup Pauli/(p) of the holes and chi/Sup 2D/(p,T)
Abstract: The powder magnetic susceptibility chi(T) of La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is found to scale with doped hole concentration p = x-2y according to a law of corresponding states for 0less than or equal topless than or equal to0.20, thereby allowing chi/sup Pauli/(p) of the holes and chi/sup 2D/(p,T) of the Cu/sup +2/ spin sublattice to be separated and precisely evaluated. chi/sup Pauli/ increases with p. The shape of chi/sup 2D/(T) is that of the spin-(1/2 square-lattice Heisenberg antiferromagnet; however, by p = 0.20, the in-plane Cu-Cu superexchange coupling constant and effective magnetic moment per Cu ion are both largely suppressed.
275 citations