Band Gap Closing in a Synthetic Hall Tube of Neutral Fermions

We report the experimental realization of a synthetic three-leg Hall tube with ultracold fermionic atoms in a one-dimensional optical lattice. The legs of the synthetic tube are composed of three hyperfine spin states of the atoms, and the cyclic inter-leg links are generated by two-photon Raman transitions between the spin states, resulting in a uniform gauge flux $\phi$ penetrating each side plaquette of the tube. Using quench dynamics, we investigate the band structure of the Hall tube system for a commensurate flux $\phi=2\pi/3$. Momentum-resolved analysis of the quench dynamics reveals that a critical point of band gap closing as one of the inter-leg coupling strengths is varied, which is consistent with a topological phase transition predicted for the Hall tube system.Comment: 8 pages, 8 figure

Double resonance of Raman transitions in a degenerate Fermi gas

We measure momentum-resolved Raman spectra of a spin-polarized degenerate Fermi gas of $^{173}$Yb atoms for a wide range of magnetic fields, where the atoms are irradiated by a pair of counterpropagating Raman laser beams as in the conventional spin-orbit coupling scheme. Double resonance of first- and second-order Raman transitions occurs at a certain magnetic field and the spectrum exhibits a doublet splitting for high laser intensities. The measured spectral splitting is quantitatively accounted for by the Autler-Townes effect. We show that our measurement results are consistent with the spinful band structure of a Fermi gas in the spatially oscillating effective magnetic field generated by the Raman laser fields.Comment: 7 pages, 6 figure

Determinants Of Enforcement Action By The Financial Supervisory Service Of Korea From The Perspective Of Audit Firms

In this study, we examine the determinants of enforcement action by the Financial Supervisory Service of Korea from the perspective of audit firms. Enforcement action is an indication of audit failure. Both client- and audit firm-specific factors are involved in its occurrence. Most published studies of enforcement after audit failure focus on client characteristics because details about audit firms from financial statements and information about organizational structure are not publicly available. However, examining the issues surrounding enforcement from the perspective of audit firms may also be valuable in elucidating the potential determinants of audit failure resulting in enforcement action. Utilizing publicly available data from audit firms in South Korea, we identify several audit firm characteristics as determinants of enforcement action. The results of our empirical analysis reveal that the likelihood of audit failure is positively associated with the ratio of accounts receivable to total assets, the ratio of audit fees to total revenue, the ratio of partners to the total number of CPAs, CEO ownership, and age of audit firms. In addition, the likelihood of audit failure is negatively associated with ownership concentration and profitability. These associations are more pronounced in non-affiliated audit firms than affiliated audit firms. Several useful implications for regulators are described for improving audit quality by means of enforcement action

Creutz ladder in a resonantly shaken 1D optical lattice

We report the experimental realization of a Creutz ladder for ultracold fermionic atoms in a resonantly driven 1D optical lattice. The two-leg ladder consists of the two lowest orbital states of the optical lattice and the cross inter-leg links are generated via two-photon resonant coupling between the orbitals by periodic lattice shaking. The characteristic pseudo-spin winding structure in the energy bands of the ladder system is demonstrated using momentum-resolved Ramsey-type interferometric measurements. We discuss a two-tone driving method to extend the inter-leg link control and propose a topological charge pumping scheme for the Creutz ladder system. ©2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaf

Realization of a cross-linked chiral ladder with neutral fermions in an optical lattice by orbital-momentum coupling

We report the experimental realization of a cross-linked chiral ladder with ultracold fermionic atoms in an optical lattice. In the ladder, the legs are formed by the orbital states of the optical lattice and the complex inter-leg links are generated by the orbital-changing Raman transitions that are driven by a moving lattice potential superimposed onto the optical lattice. The effective magnetic flux per ladder plaquette is tuned by the spatial periodicity of the moving lattice, and the chiral currents are observed from the asymmetric momentum distributions of the orbitals. The effect of the complex cross links is demonstrated in quench dynamics by measuring the momentum dependence of the inter-orbital coupling strength. We discuss the topological phase transition of the chiral ladder system for the variations of the complex cross links.Comment: 8 pages, 8 figure