Electronic spectrum of the high-temperature superconducting state

Abstract

Improved experimental conditions enabled us to increase the signal-to-noise ratio of the photoemission spectra for the superconducting state of Bi2Ca2SrCu20s, taken with high angular and energy resolution. This also enabled us to reveal a pronounced minimum that separates the two basic features of the spectrum, the narrow quasiparticle excitation peak and the still controversial broad band at lower kinetic energies. The minimum is approximately 3A below the Fermi level.PACS numbers: 74.70.Vy, 79.60.Cn High-resolution photoemission spectroscopy has recently emerged as a fundamental probe of the superconducting state for high-temperature superconductivity [1], Early experiments clearly established the presence of a gap in the electronic spectrum [2][3][4][5]. The spectrum in the superconducting state is dominated, of course, by the narrow peak corresponding to quasiparticle excitations [2,6]. This peak, however, is accompanied by a broad excitation band at lower kinetic energies, whose nature has not been clarified and whose relevance to the mechanism of high-temperature superconductivity has been emphasized by several authors [7][8][9].We carefully analyzed the spectrum with angular and energy resolution similar to or better than those of previous high-resolution studies-but with enhanced signalto-noise ratio. This enabled us to clearly reveal an additional feature that appears important for the general interpretation of the spectra and of the corresponding superconducting state: a pronounced minimum that separates the broad band from the narrow peak. The minimum could not be easily seen in previous studies because of the relatively large noise, and therefore its position could not be determined. We found that the position is close to 3A below the Fermi level, where A is the gap parameter (i.e., 2A is the gap). A broad-band threshold at 3A is predicted, for example, by Littlewood and Varma [7,8] based on the "marginal-Fermi-liquid" model, by Miiller, Arnold, and Swithart [10], and by other theorists [11].Our experimental procedure was a standard synchrotron-radiation photoemission approach, enhanced in order to achieve high angular and energy resolution with a high signal-to-noise level; the results are, to the best of our knowledge, state of the art in solid-state photoemission spectroscopy. The main factors for simultaneously reaching high resolution and high signal-to-noise levels were as follows: the use of very high-quality single crystals of Bi2Ca2SrCu2C>8 cleaved in situ under ultrahigh vacuum, whose structural and transport properties were carefully characterized; the use of a 4-m normal-incidence photon monochromator with resolving power up to 4.3 x 10 3 ; the use of a carefully magnetically shielded high-resolution VSW electron analyzer; and the use of synchrotron radiation.In order to test reproducibility, the experiments were performed on twenty different samples from different batches. In each case, many different points of the Brillouin zone were explored by varying the photoelectron collection ge...

Temperature Dependence of the Superconducting Gap Anisotropy in Bi 2 Sr 2 CaCu 2 O 8+x

“…Already, results establishing a marked anisotropy in the gap at low temperatures have ruled out an isotropic s-wave symmetry order parameter. [8][9][10][11] Our main result is that, contrary to anisotropic-gap conventional superconductors such as lead, [12][13][14] the gap anisotropy of Bi 2 Sr 2 Ca 1 Cu 2 O 8+x increases with increasing temperature as one approaches the superconducting transition temperature, T c . This result was discovered by estimating the gap from angle-resolved photoemission data, using the BCS-like lineshape [15,16] and computer code of Olson, Lynch and Liu.…”

“…The count rate in photoemission superconducting condensate spectral area at 36K was about 1 kHz. The main temperature-dependent features of these spectra include: A shift of the leading edge that reveals the opening of the superconducting gap; a photoemission superconducting condensate spectral area with an energy full width at half maximum of 25 meV is observed immediately below the leading edge; a dip at binding energy of about 80 meV [9,10] for temperatures up to 0.84T c . Note that along the Γ −M direction, the photoemission superconducting condensate spectral area has been reproducibly observed at temperatures within 2K of the transition temperature (T c ) of 83K, and disappears above T c .…”

“…[7][8][9][10][11] Angle-resolved photoemission has the advantage of directly investigating the momentum-dependence of the gap. Already, results establishing a marked anisotropy in the gap at low temperatures have ruled out an isotropic s-wave symmetry order parameter.…”

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

“…Photoemission measurements have been carried out by a number of groups [67,7,[68][69][70], and there is broad agreement on a number of features of the energy dispersion and Fermi surfaces. Figure 6 shows a representative collection of the Fermi surfaces reported by different groups.…”

Anomalous Electron Spectrum and Its Relation to Peak Structure of Electron Scattering Rate in Cuprate Superconductors

“…The early ARPES experiments observed that around the antinodal region, a sharp electron quasiparticle excitation peak develops at the lowest binding energy corresponding to the SC gap, and is followed by a dip and then a hump in the higher energies, giving rise to a striking peak-dip-hump (PDH) structure in the electron quasiparticle excitation spectrum [13][14][15][16][17][18][19][20][21] . In particular, this similar PDH structure has been observed in tunneling and Raman spectra [22][23][24][25] .…”