A Tunable Fano System Realized in a Quantum Dot in an Aharonov-Bohm Ring K. Kobayashi, H. Aikawa, S. Katsumoto, and Y. Iye Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Chiba 277-8581, Japan Abstract. We report a tunable Fano system realized in a quantum dot embedded in an Aharonov-Bohm interferometer on a two-dimensional electron gas. In the Coulomb oscillation, clear asymmetric line shapes were observed, which manifest the formation of the Fano state, i. e., a coherent mixture of localized-continuum states. The Fano interference can be tuned through the phase of electrons by the electrostatic gate and the magnetic flux piercing the ring. In addition, this effect was found to significantly affect the phase evolution of electrons. 1. Introduction A system where coexist a discrete state and the continuum energy state possesses a characteristic transition probability around the discrete energy level [1]. This is the Fano effect, reflecting the quantum interference between the two configurations in the transition process into the final states with the same energy: one directly through the continuum and the other through the resonance level arising around the discrete state, as sketched in Fig. 1 (a). This “configuration interaction” yields the peculiar asymmetric line shape in the transition probability T from an arbitrary initial state, which is expressed as T (˜ ǫ) ∝ (˜ ǫ + q ) 2 ˜ ǫ 2 +1 , ˜ ǫ = ǫ - ǫ 0 Γ/2 , (1) where ǫ 0 is the energy level of the resonance state and Γ is its width. The parameter q , which is the ratio of the matrix elements linking the initial state to the discrete and continuum parts of the final state, serves as a measure of the degree of coupling between them. When q →∞, Eqn. (1) falls into a Lorentzian corresponding to an ordinary Breit-Wigner-type resonance. For a finite q , Eqn. (1) gives various line shapes as shown in Fig. 1 (b). The Fano effect is a ubiquitous phenomenon observed in wide-ranging spectroscopy including neutron scattering [2], atomic photoionization [3], Raman scattering [4], and optical absorption [5]. It can be viewed as a theory describing how a localized state embedded in the continuum acquires itinerancy over the system [6]. An experiment on a single site, therefore, would reveal this fundamental process in a more transparent way. While the single- site Fano effect has been reported in the scanning tunneling spectroscopy study of an atom on the surface [7, 8] or in transport through a quantum dot (QD) [9], it has been difficult to obtain a Fano system with variable parameters. In the present work, we report a tunable Fano experiment performed in a QD embedded in an Aharonov-Bohm (AB) interferometer [10]. Especially, we focus on the result of the electrostatic and magnetic tuning of the Fano interference.