Analytical expression of Kondo temperature in quantum dot embedded in Aharonov-Bohm ring
© Yoshii and Eto; licensee Springer. 2011
Received: 3 September 2010
Accepted: 23 November 2011
Published: 23 November 2011
We theoretically study the Kondo effect in a quantum dot embedded in an Aharonov-Bohm ring, using the "poor man's" scaling method. Analytical expressions of the Kondo temperature T K are given as a function of magnetic flux Φ penetrating the ring. In this Kondo problem, there are two characteristic lengths, and L K = ħv F = T K, where v F is the Fermi velocity and is the renormalized energy level in the quantum dot. The former is the screening length of the charge fluctuation and the latter is that of the spin fluctuation, i.e., size of Kondo screening cloud. We obtain diferent expressions of T K(Φ) for (i) L c ≪ L K ≪ L, (ii) L c ≪ L ≪ L K, and (iii) L ≪ L c ≪ L K, where L is the size of the ring. T K is remarkably modulated by Φ in cases (ii) and (iii), whereas it hardly depends on Φ in case (i).
Since the first observation of the Kondo effect in semiconductor quantum dots [1–3], various aspects of Kondo physics have been revealed, owing to the artificial tunability and flexibility of the systems, e.g., an enhanced Kondo effect with an even number of electrons at the spin-singlet-triplet degeneracy , the SU(4) Kondo effect with S = 1/2 and orbital degeneracy , and the bonding and antibonding states between the Kondo resonant levels in coupled quantum dots [6, 7]. One of the major issues which still remain unsolved in the Kondo physics is the observation of the Kondo singlet state, so-called Kondo screening cloud. The size of the screening cloud is evaluated as L K = ħv F/T K, where v F is the Fermi velocity and T K is the Kondo temperature. There have been several theoretical works on L K, e.g., ring-size dependence of the persistent current in an isolated ring with an embedded quantum dot , Friedel oscillation around a magnetic impurity in metal , and spin-spin correlation function [10, 11].
We focus on the Kondo effect in a quantum dot embedded in an Aharonov-Bohm (AB) ring. In this system, the conductance shows an asymmetric resonance as a function of energy level in the quantum dot, so-called Fano-Kondo effect. This is due to the coexistence of one-body interference effect and many-body Kondo effect, which was studied by the equation-of-motion method with the Green function , the numerical renormalization group method , the Bethe ansatz , the density-matrix renormalization group method , etc. This Fano-Kondo resonance was observed experimentally . The interference effect on the value of T K, however, has not been fully understood [17, 18].
In our previous work , we examined this problem in the small limit of AB ring using the scaling method . Our theoretical method is as follows. First, we create an equivalent model in which a quantum dot is coupled to a single lead. The AB interference effect is involved in the flux-dependent density of states in the lead. Second, the two-stage scaling method is applied to the reduced model, to renormalize the energy level in the quantum dot by taking into account the charge fluctuation and evaluate T K by taking spin fluctuation . This method yields T K in an analytical form.
The purpose of this article is to derive an analytical expression of T K for the finite size of the AB ring, using our theoretical method. We find two characteristic lengths. One is the screening length of the charge fluctuation, with being the renormalized energy level in the quantum dot, which appears in the first stage of the scaling. The other is the size of Kondo screening cloud, L K, which is naturally obtained in the second stage. In consequence, the analytical expression of T K is different for situations (i) L c ≪ L K ≪ L, (ii) L c ≪ L ≪ L K, and (iii) L ≪ L c ≪ L K, where L is the size of the ring. We show that T K strongly depends on the magnetic flux Φ penetrating the AB ring in cases (ii) and (iii), whereas it hardly depends on Φ in case (i).
where and d σ are creation and annihilation operators, respectively, of an electron in the quantum dot with spin σ. and a i,σ are those at site i with spin σ in the leads and the reference arm of the ring. is the number operator in the dot with spin σ. U is the charging energy in the dot.
We consider the Coulomb blockade regime with one electron in the dot, -ε 0, ε 0 + U ≫ Γ, where Γ = ΓL + Γ is the level broadening. , with ν 0 being the local density of states at the end of semi-infinite leads. We analyze the vicinity of the electron-hole symmetry of -ε 0 ≈ ε 0 + U.
where A k and B k are determined so that with dot state |d〉. In consequence, mode |ψ k 〉 is coupled to the dot via H T, whereas is completely decoupled.
where α = 4ΓLΓR/(ΓL + ΓR)2 is the asymmetric factor for the tunnel couplings of quantum dot.
The AB interference effect is involved in the flux-dependent density of states in the lead, υ(ε k ) in Eq. (6). As schematically shown in Figure 1(b), υ(ε k ) oscillates with the period of ε T, where ε T = ħv F/L is the Thouless energy for the ballistic systems. We assume that ε T ≪ D 0.
We apply the two-stage scaling method to the reduced model. In the first stage, we consider the charge fluctuation at energies of D ≫ |ε 0|. In this region, the number of electrons in the quantum dot is 0, 1, or 2. We reduce the energy scale from bandwidth D 0 to D 1 where the charge fluctuation is quenched. By integrating out the excitations in the energy range of D 1 < D < D 0, we renormalize the energy level in the quantum dot ε 0. In the second stage of scaling, we consider the spin fluctuation at low energies of D < D 1. We make the Kondo Hamiltonian and evaluate the Kondo temperature.
Renormalization of energy level
Si(x) goes to 0 as x → 0 and π/2 as x → ∞.
when , and when . These results can be rewritten in terms of length scale. We introduce , which corresponds to the screening length of charge fluctuation. When L ≪ L c, the renormalized level is given by Equation 11. When L ≫ L c, the energy level is hardly renormalized and is independent of ϕ.
Renormalization of exchange coupling
The energy scale D where the fixed point (J → ∞) is reached yields the Kondo temperature.
where f(ϕ) = [1 - f(k F L + π/2, ϕ)]-1.
where γ ≈ 0.57721 is the Euler constant.
We have theoretically studied the Kondo effect in a quantum dot embedded in an AB ring. The two-stage scaling method yields an analytical expression of the Kondo temperature T K as a function of AB phase ϕ of the magnetic flux penetrating the ring. We have obtained different expressions of T K(ϕ) for (i) L c ≪ L K ≪ L, (ii) L c ≪ L ≪ L K, and (iii) L ≪ L c ≪ L K, where L is the size of the ring, is the screening length of the charge fluctuation, and L K = ħν F/T K is the screening length of the charge fluctuation, i.e., size of Kondo screening cloud. T K strongly depends on ϕ in cases (ii) and (iii), whereas it hardly depends on ϕ in case (i).
This study was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, and by Global COE Program "High-Level Global Cooperation for Leading-Edge Platform on Access Space (C12)."
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