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**|**Nanoscale Res Lett**|**v.6(1); 2011**|**PMC3253270

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- Abstract
- Introduction
- Model
- Scaling analysis
- Renormalization of energy level
- Renormalization of exchange coupling
- Conclusions
- Abbreviation
- Competing interests
- Authors' contributions
- References

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Nanoscale Res Lett. 2011; 6(1): 604.

Published online 2011 November 23. doi: 10.1186/1556-276X-6-604

PMCID: PMC3253270

Ryosuke Yoshii: festina.lente/at/a3.keio.jp; Mikio Eto: eto/at/rk.phys.keio.ac.jp

Received 2010 September 3; Accepted 2011 November 23.

Copyright ©2011 Yoshii and Eto; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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).

PACS numbers:

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 [4], the SU(4) Kondo effect with *S *= 1/2 and orbital degeneracy [5], 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 [8], Friedel oscillation around a magnetic impurity in metal [9], 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 [12], the numerical renormalization group method [13], the Bethe ansatz [14], the density-matrix renormalization group method [15], etc. This Fano-Kondo resonance was observed experimentally [16]. The interference effect on the value of *T*_{K}, however, has not been fully understood [17,18].

In our previous work [19], we examined this problem in the small limit of AB ring using the scaling method [20]. 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 [21]. 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).

Our model is shown in Figure Figure1a.1a. A quantum dot with an energy level *ε*_{0 }is connected to two external leads by tunnel couplings, *V*_{L }and *V*_{R}. Another arm of the AB ring (reference arm) and external leads are represented by a one-dimensional tight-binding model with transfer integral -*t *and lattice constant *a*. The size of the ring is given by *L *= (2*l *+ 1)*a*. The reference arm includes a tunnel barrier with transmission probability of *T*_{b }= 4*x*/(1 + *x*)^{2 }with *x *= (*W*/*t*)^{2}. The AB phase is denoted by *ϕ *= 2*π*Φ/Φ_{0}, with flux quantum Φ_{0 }= *h*/*e*. The Hamiltonian is

(1)

(2)

(3)

(4)

where and *d _{σ }*are creation and annihilation operators, respectively, of an electron in the quantum dot with spin σ. and

We consider the Coulomb blockade regime with one electron in the dot, -*ε*_{0}, *ε*_{0 }+ *U * Γ, where Γ = Γ_{L }+ 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*.

We create an equivalent model to the Hamiltonian (1), following Ref. [19]. First, we diagonalize the Hamiltonian *H*_{leads+ring }for the outer region of the quantum dot. There are two eigenstates for a given wavenumber *k*; |*ψk*,→ represents an incident wave from the left and partly reflected to the left and partly transmitted to the right, whereas |*ψ _{k}*,← represents an incident wave from the right and partly reflected to the right and partly transmitted to the left. Next, we perform a unitary transformation for these eigenstates

where *A _{k }*and

Neglecting the decoupled mode, we obtain the equivalent model in which a quantum dot is coupled to a single lead. In a wide-band limit, the Hamiltonian is written as

(5)

with and density of states in the lead

(6)

Here, *D*_{0 }is the half of the band width, *k*_{F }is the Fermi wavenumber, *R*_{b }= 1 - *T*_{b}, and

(7)

where *α *= 4Γ_{L}Γ_{R}/(Γ_{L }+ Γ_{R})^{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),

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.

In the first stage, the charge fluctuation is taken into account. We denote *E*_{0}, *E*_{1}, and *E*_{2 }for the energies of the empty state, singly occupied state, and doubly occupied state in the quantum dot, respectively. Then the energy levels in the quantum dot are given by *ε*_{0 }= *E*_{1 }- *E*_{0 }for the first electron and *ε*_{1 }= *E*_{2 }- *E*_{1 }for the second electron. When the bandwidth is reduced from *D *to *D *- |d*D*|, *E*_{0}, *E*_{1}, and *E*_{2 }are renormalized to *E*_{0 }+ d*E*_{0}, *E*_{1 }+ d*E*_{1}, and *E*_{2 }+ d*E*_{2}, where

within the second-order perturbation with respect to tunnel coupling *V*. For *D * |*E*_{1 }- *E*_{0}|, |*E*_{2 }- *E*_{1}|, they yield the scaling equations for the energy levels

(8)

where *i *= 0, 1 and

(9)

By the integration of the scaling equation from *D*_{0 }to , we renormalize the energy levels in the quantum dot *ε _{i }*to :

(10)

where

Si(*x*) goes to 0 as *x *→ 0 and *π*/2 as *x *→ ∞.

From Equation 10, we conclude that

(11)

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*.

In the second stage, we consider the spin fluctuation at low energies of *D < D*_{1}. For the purpose, we make the Kondo Hamiltonian via the Schrieffer-Wolff transformation,

(12)

(13)

(14)

where , and are the spin operators in the quantum dot. The density of states in the lead is given by Equation 6 and half of the band width is now . *H _{J }*represents the exchange coupling between spin 1/2 in the dot and spin of conduction electrons, whereas

By changing the bandwidth, we renormalize the coupling constants *J *and *K *so as not to change the low-energy physics within the second-order perturbation with respect to *H _{J }*and

(15)

(16)

The energy scale *D *where the fixed point (*J *→ ∞) is reached yields the Kondo temperature.

Scaling equations (15) and (16) are analyzed in two extreme cases. In the case of *D * *ε*_{T}, the oscillating part of the density of states *ν*(*ε _{k}*) is averaged out in the integration [22]. Then the scaling equations are effectively rewritten as

(17)

(18)

In the case of *D * *ε*_{T}, the expansion around the fixed point [23] yields

(19)

(20)

where *ξ *= *D*/*T*_{K }- 1 and

(21)

Now we evaluate the Kondo temperature in situations (i) *L*_{c } *L*_{K } *L*, (ii) *L*_{c } *L * *L*_{K}, and (iii) *L * *L*_{c } *L*_{K}, where *L*_{K }= *ν*_{F}*ħ*/*T*_{K}. In situation (i), *ε*_{T } *T*_{K }and thus *J *and *K *follow Equations 17 and 18 until the scaling ends at *D * *T*_{K}. Integration of Equation 17 from *D*_{1 }to *T*_{K }yields

(22)

where .

In situation (iii), *D*_{1 } *ε*_{T}. Then the scaling equations (19) and (20) are valid in the whole scaling region (*T*_{K }<*D *<*D*_{1}). From the equations, we obtain

(23)

where *f*(*ϕ*) = [1 - *f*(*k*_{F}*L *+ *π*/2, *ϕ*)]^{-1}.

In situation (ii), *T*_{K } *ε*_{T } *D*_{1}. The coupling constants, *J *and *K*, are renormalized following Equations 17 and 18 when *D *is reduced from *D*_{1 }to *ε*_{T }and following Equations 19 and 20 when *D *is reduced from *ε*_{T }to *T*_{K}. We match the solutions of the respective equations around *D *= *ε*_{T }and obtain

(24)

where *γ *≈ 0.57721 is the Euler constant.

The different expressions of *T*_{K}(*ϕ*) in the three situations can be explained intuitively. In situation (i), *ε*_{T } *T*_{K}. Then the oscillating part of the density of states *ν*(*ε _{k}*) with period

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).

AB: Aharonov-Bohm.

The authors declare that they have no competing interests.

All authors read and approved the final manuscript.

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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