Hiroaki Matsui

The origin of multiple superconducting gaps in MgB2

Magnesium diboride, MgB2, has the highest transition temperature (Tc = 39u2009K) of the known metallic superconductors. Whether the anomalously high Tc can be described within the conventional BCS (Bardeen–Cooper–Schrieffer) framework has been debated. The key to understanding superconductivity lies with the ‘superconducting energy gap’ associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments; this is in contrast to both conventional and high-Tc superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the σ and π bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor.

Ultrahigh-Resolution Photoemission Study of h-ZrRuP

We have performed ultrahigh-resolution photoemission spectroscopy on the intermetallic superconductor hexagonal h -ZrRuP ( T c = 13 K). We observed the opening of a superconducting gap below T c , and found that the most probable order parameter is s -wave with a gap size (Δ) of 1.8 meV at 6 K. This gap size gives 2 Δ 0 / k B T c ∼3.5, suggesting the weak-coupling nature of the superconductivity in h -ZrRuP. We also observed a pseudogap-like suppression of spectral weight up to ∼45 meV from the Fermi level, indicative of the strong coupling of electrons with phonons and/or the charge density wave fluctuation.

Three-dimensional Fermi-surface nesting in 1T-VSe2 studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) with synchrotron radiation on layered transition-metal dichalcogenide 1 T -VSe 2 to study the mechanism of the three-dimensional charge-density-wave (CDW) transition. We found that the Fermi surface around the M(L) and K(H) points shows considerable wiggling along the wave vector normal to the layer ( k z ) despite the apparently two-dimensional crystal structure. ARPES spectra below the CDW transition temperature show a signature of pseudogap opening at a specific k z -region on the Fermi surface, demonstrating that the CDW transition of 1 T -VSe 2 is driven by three-dimensional Fermi-surface nesting.

Universality of Low-Energy Mass Renormalization in the Superconducting State of Hole-Doped High-Tc Superconductors

We have performed high-resolution angle-resolved photoemission spectroscopy on high- T c superconductors La 1.85 Sr 0.15 CuO 4 (LSCO) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) to study the universality of many-body interaction in hole-doped high- T c superconductors. We found that the emergence of kink in the dispersion in the off-nodal region at the superconducting state is a common feature, while the energy scale of kink is remarkably different between LSCO and Bi2212. We conclude that the coupling of electrons with spin excitations at the off-nodal region is universal in hole-doped high- T c cuprates.

Direct evidence for superconducting quasiparticle in triple-layered high-Tc superconductor

We have performed angle-resolved photoemission spectroscopy (ARPES) on triple-layered high-T-c cuprate superconductor Bi2Sr2Ca2Cu3O10+delta. We found a direct experimental evidence for Bogoliubov quasiparticle in high-T-c cuprates by observing the full energy dispersion below and above the Fermi level. The excellent quantitative agreement in the dispersion as well as the coherence factors between the ARPES experiment and Bardeen, Cooper and Shrieffer (BCS) theory establishes the validity and universality of the basic framework of BCS theory in high-T-c cuprates