Dingsong Wu

Abstract:

The Planck constant, with its mathematical symbol h, is a fundamental constant in quantum mechanics that is associated with the quantization of light and matter. It is also of fundamental importance to metrology, such as the definition of ohm and volt and the latest definition of kilogram. One of the first measurements to determine the Planck constant is based on the photoelectric effect; however, the values thus obtained so far have exhibited a large uncertainty. The accepted value of the Planck constant, 6.626 070 15 × 10^-34 J s, is obtained from one of the most precise methods, the Kibble balance, which involves the quantum Hall effect, the Josephson effect, and the use of the international prototype of the kilogram or its copies. Here, we present a precise determination of the Planck constant by modern photoemission spectroscopy technique. Through the direct use of Einstein's photoelectric equation, the Planck constant is determined by accurately measuring the energy position of the gold Fermi level using light sources with various photon wavelengths. The precision of the Planck constant as measured in this work, 6.626 10(13) × 10^-34 J s, is improved by four to five orders of magnitude from the previous photoelectric effect measurements. We propose that this direct method of photoemission spectroscopy has potential to further increase its measurement precision of the Planck constant to be comparable to the most accurate methods available at present.

Abstract:

In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs (Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state. High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor (Ba_0.6K_0.4)Fe₂As₂. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at T_c. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.