Observation of an "Extended" Van Hove Singularity in YBa2Cu4

Abstract

We characterize the nature of the Van Hove singularity near the Fermi energy (E+) in YBa2Cu40& using ultrahigh energy resolution (10 meV) angle-resolved photoemission spectroscopy, together with corresponding first-principles spectra intensity computations.The singularity, related to Cu02 planes, is located about 19 meV below FF at the Y point in the Brillouin zone and arises from a band which is nearly dispersionless along the I to Y direction. Such an "extended" saddle point would render the band structure quasi-1D and yield a power law divergence in the density of states, allowing the occurrence of T, 's of order 100 K even in a weak coupling BCS scheme. PACS numbers: 74.72.Bk, 74.25.Jb, 79.60.Bm Ever since the discovery of high-T, superconductivity, one of the most intensely debated questions has been: What is the reason for the critical temperature in the new materials to be much higher than in the earlier superconductors such as Hg, Pb, Nb, and the A15's'? Many mechanisms have been proposed to explain these high transition temperatures, some exotic and others of a more conventional nature [1]. Of interest to this Letter is the so-called Van. Hove scenario [2 -9], in which the occurrence of high-T, 's is tied to the presence of Van Hove singularities in the density of states (DOS) close to the Fermi energy. So far, there has only been indirect evidence for the existence of such peaks in the DOS, for example, from the discontinuity of the specific heat at T, [6], studies of the isotope effect [7], of the thermopower [8], and of the quasiparticle lifetime broadening [9].In this Letter, for the first time, we give direct evidence for the presence of a Van Hove singularity in YBa2Cu40s near the Fermi energy.The nature of the singularity is characterized by determining its location in the Brillouin zone and the dispersion of the associated spectral density peak by ultrahigh resolution angle-resolved photoemission spectroscopy (ARPES). Here we show data at an energy resolution better than 10 meV. Most of the previously published synchrotron-based work has involved resolutions of 25 -30 meV. Since the Van Hove singularity lies very close to the Fermi energy, a high energy resolution is essential for these measurements.The experimental spectra are compared with corresponding first-principles ARPES intensity calculations, and it is clearly established that the measured spectral feature arises from bulk energy bands of the crystal related to the Cu02 planes. The conclusions are therefore generally relevant to the cuprate superconductors, particularly since we have found similar behavior in YBa2Cu307 -$, Bi2CaSr2Cu20s, and T128a2Cu06 [10].The common Van Hove scenario assumes that the diverging DOS arises from a saddle point in the underlying 2D Cu02 plane bands (i.e., a minimum along one direction and a maximum along the perpendicular direction). Our results provide a new twist to this picture in that the bands are measured to be nearly fiat (within 1 meV) in one direction, so that the situation may be described as...

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