Solid State Communications 151 (2011) 1894–1898

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Solid State Communications

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On the extendedness of eigenstates in a hierarchical lattice: A critical viewBiplab Pal ∗, Arunava Chakrabarti, Nitai BhattacharyaDepartment of Physics, University of Kalyani, Kalyani, West Bengal 741 235, India

a r t i c l e i n f o

Article history:Received 11 July 2011Accepted 28 September 2011by Y.E. LozovikAvailable online 2 October 2011

Keywords:A. Hierarchical latticesC. Tight binding HamiltonianD. Localization

a b s t r a c t

We take a critical view at the basic definition of extended single particle states in a non-translationallyinvariant system. For this, we present the case of a hierarchical lattice and incorporate long rangeinteractions that are also distributed in a hierarchical fashion. We show that it is possible to explicitlyconstruct eigenstates with constant amplitudes (normalized to unity) at every lattice point for specialvalues of the electron-energy. However, the end-to-end transmission, corresponding to the above energyof the electron in such a hierarchical system depends strongly on a special correlation between thenumerical values of the parameters of the Hamiltonian. Keeping the energy and the distribution of theamplitudes invariant, one can transform the lattice from conducting to insulating simply by tuning thenumerical values of the long range interaction. The values of these interactions themselves display afractal character.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The problem of localization of single particle states that wasinitially raised and solved by Anderson and others [1] still remainsvery much alive [2,3] and, have given birth to an enormous andhighly rich literature [4–13]. The fundamental result is that, inone and two dimensions all the single particle eigenstates areexponentially localized in presence of uncorrelated disorder [1],while in three dimensions one witnesses the possibility of ametal–insulator transition.

Interesting twist in the concept of electron localization in lowdimensional systems came up with the work of Dunlap et al. [6],Dutta et al. [8] who discussed that a short range positionalcorrelation between the constituents of a one dimensionaldisordered chain could lead to resonant (extended) eigenstateswith high transmittivity. The basic cause was traced back to a localresonance in a cluster of impurities, and the case was referredto as the random dimer model (RDM). The idea was tested in lowdimensions with various forms of disorder by many others [9–11].Later, it was argued, based on numerical diagonalization of theHamiltonian that, by choosing suitable long range positionalcorrelations between the potentials assigned to the atomic sitesin a linear chain of atoms, one could initiate a metal–insulatortransition even in one dimension [12,13].

The idea of generating extended eigenstates in low dimensionalsystems without any translational order (disordered systemsprovide one such class) was further extended to the infinitequasi-periodic chains, where one could unravel an infinite variety

∗ Corresponding author. Tel.: +91 33 2582 0184; fax: +91 33 2582 8282.E-mail address: biplabpal2008@gmail.com (B. Pal).

0038-1098/$ – see front matter© 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2011.09.028

of these unscattered states owing to the self-similarity of thelattices [14,15]. Similar studies were carried out on fractalnetworks [16–20] where, even in the absence of any localresonating clusters such as in the case of the RDM [6] or in thecase of a quasi-periodic chain [14] one finds an infinite number ofextended single particle states.

In all these exciting conceptual developments, a critical issueis practically overlooked. This is the question of categorizingan extended (resonant) state. More explicitly, let us raise thequestion, ‘‘when do we call an eigenstate extended?’’. An obviousanswer is obtained by looking at the amplitude of the wavefunction at a given energy. A nontrivial distribution of theamplitude throughout an infinite lattice very legitimately pointsto an extendedness of the eigenfunction. A second way tocharacterize an extended wave function is through a calculationof the electronic transmission across the lattice at the concernedenergy. Usually, an extended state should give rise to a ballistictransmission. This second criterion is compatible to the first one inall systems where translational symmetry prevails. However, it isnot apparent whether these two demands always go hand in handin systems where extended states exist even in the absence of anytranslational order (such as the fractals for example), and a seriousintrospection of the issue is in order.

In the present communication, we specifically address thisproblem.We provide explicit example of a hierarchical latticewithlong range interactions where eigenstates with identical (and non-zero) amplitudes can be constructed at all the lattice points for aspecial energy of the electron. This construction demands a welldefined correlation between the numerical values of a subset ofthe Hamiltonian parameters. The energy of the electron can bechosen independent of all or, a sub-set of the hierarchical longrange interactions. It is then shown that the corner-to-corner

B. Pal et al. / Solid State Communications 151 (2011) 1894–1898 1895

propagation of an electron depends crucially on the strengths ofthe long range interactions rendering, in some cases, the latticebecomes completely transparent to an incoming electron. In othersituations, with a different choice of the hierarchical parameter, atopologically identical lattice with the same constant distributionof amplitudes of the eigenfunction, becomes completely opaqueto an electron with the same energy as in the first case. Thisobservation, to our mind, introduces in a possible conceptualconflict between the extendedness of an eigenstate and a ballistictransmission, particularly in such hierarchical systems.

We discuss two kinds of hierarchical interaction in a fractalnetwork designed in the line of the well known Berker lattice [21].Hierarchical structures using superconductingwire networks havealready been fabricated and studied experimentally [22]. Withthe present day nano-technology, tailor-made geometries withquantum dots are also possible to fabricate using scanning tunnelmicroscope (STM) as tweezers. Our proposed structures aretherefore not far from reality. Also, by controlling the proximityof the dots one can induce tunnel hopping almost at will.

We work within a tight binding approach. A real space renor-malization group (RSRG) decimation scheme [23] is employedto calculate the corner-to-corner electronic transmission in finitebut arbitrarily large lattices, and to analyze the character of theelectron states. Incidentally, a similar analysis has been made re-cently [24] on a Koch fractal with hierarchical interactions, andsimilar questions have been raised. The present lattice is a non-trivial generalization of the Koch fractal [25], and exhibits a richerspectrumof results, yetmaintaining the basic issue, viz., the natureof extended eigenstates in such fractal lattices.

In what follows, we describe the results. In Section 2, wedescribe the model and the essential method in resolving theproblem. Numerical results and related discussion are presentedin Section 3, and we draw our conclusions in Section 4.

2. The model and the method

2.1. The Hamiltonian and the decimation scheme

We begin by referring to Figs. 1 and 2. We have designed ahierarchical lattice with two different configurations, in one ofwhich the long range interaction is along the axial direction (Fig. 1)and in the other one, it is along the transverse direction (Fig. 2).We adopt a tight binding formalism, and incorporate only thenearest neighbor hopping. The tight binding Hamiltonian for non-interacting electrons in the Wannier basis is given by

H =

−i

ϵi|i⟩⟨i| +

−⟨ij⟩

[tij|i⟩⟨j| + tji|j⟩⟨i|] (1)

where ϵi is the on-site potential of an electron on the i-th atomicsite and tij = tji is the nearest neighbor hopping integral betweenthe i-th and j-th sites. For the nearest neighboring sites of the latticewe assume tij = t , while the long range hopping integrals aretij = τ , and are assumed to follow a particular rule: τ(n) = λntwhere n represents the level of hierarchy, and λ is the hierarchyparameter. For such a hierarchical lattice of finite but arbitrarilylarge size, the on-site potential ϵi will be assigned values ϵA for thetwo extreme sites and ϵB(n), ϵC(n) etc. for the bulk sites dependingon their positions on the lattice as explained in Figs. 1 and 2.

Exploiting to the self-similarity of the lattice, we can apply thereal space renormalization group (RSRG) decimation scheme [23]to decimate an appropriate subset of sites. In decimating thosesubsets of sites, we have used the standard difference equation,which is an equivalent description of the Schrödinger equation,viz.,

(E − ϵi)ψi =

−j

tijψj. (2)

a

b

Fig. 1. (Color online). (a) Schematic diagram of the 2nd generation of a hierarchicallatticewith long range interaction along the ‘axial’ direction. ‘A’ denotes the extremesites, while B(n) and C(n) represent the bulk sites depending on their positions inthe lattice, n being the hierarchy index. ‘t’ represents the nearest neighbor hoppingintegral and τ(n) represents the value of the long range hopping integral in the n-thlevel of hierarchy. (b) The renormalized version of (a).

a

b

Fig. 2. (Color online). (a) Schematic diagram of the 2nd generation of a hierarchicallattice with long range interaction along the ‘transverse’ direction. ‘A’ denotes theextreme sites, while B(n) and C(n) represent the bulk sites depending on theirpositions in the lattice, n being the hierarchy index. ‘t’ represents the nearestneighbor hopping integral and τ(n) represents the value of the long range hoppingintegral in the n-th level of hierarchy. (b) The renormalized version of (a).

Here,ψi is the amplitude of thewave function at the i-th atomic siteand the index j represents the nearest neighbors of i. The recursionrelations for the site energies and hopping integrals for the two

1896 B. Pal et al. / Solid State Communications 151 (2011) 1894–1898

a b

Fig. 3. (Color online). Energy eigenvalue spectrum of a hierarchical fractal lattice as a function of the hierarchy parameter λ, obtained from the trace of the transfer matrixfor the 7th generation fractal, taken as the ‘unit cell’. We have set ϵA = ϵB(n) = ϵC(n) = 0 with n = 7 and t = 1. (a) The axial case and (b) The transverse case.

cases are given by:I. The axial case

ϵ′

A = ϵA +αt2

β(1 − γ 2)

ϵ′

B(n) = ϵB(n+1) +3αt2

β(1 − γ 2)

ϵ′

C(n) = ϵC(n+1) +2αt2

β(1 − γ 2)

t ′ =αγ t2

β(1 − γ 2)

τ ′(n) = τ(n + 1) for all n ≥ 1 (3)whereα = E − ϵC(1), β = α(E − ϵB(1))− 2t2 and

γ = [ 2t2 + ατ(1) ] /β. (4)

II. The transverse case

ϵ′

A = ϵA +δµt2

(µ2 − 4t4)

ϵ′

B(n) = ϵB(n+1) +3δµt2

(µ2 − 4t4)

ϵ′

C(n) = ϵC(n+1) +2δµt2

(µ2 − 4t4)

t ′ =2δt4

(µ2 − 4t4)

τ ′(n) = τ(n + 1) for all n ≥ 1 (5)where

δ = E − ϵC(1) − τ(1) and µ = δ[E − ϵB(1)] − 2t2. (6)We present our numerical results in the next section using theabove set of recursion relations.

2.2. The transmission coefficient

To get the end-to-end transmission coefficient of an ℓ-thgeneration fractal, we clamp the systembetween two semi-infiniteordered leads. The leads, in the tight binding model, are describedby a constant on-site potential ϵ0 and a nearest neighbor hoppingintegral t0. We then renormalize the lattice ℓ times to reduce itinto an effective dimer consisting of the ‘renormalized’ A-atoms,

each having an on-site potential equal to ϵ(ℓ)A and the effective A–Ahopping integral t(ℓ). The transmission coefficient is then obtainedby the well known formula [26],

T =4 sin2 ka

[(P12 − P21)+ (P11 − P22) cos ka]2 + [(P11 + P22) sin ka]2(7)

where

P11 =[E − ϵ

(ℓ)A ]

2

t0t(ℓ)−

t(ℓ)

t0

P12 = −[E − ϵ

(ℓ)A ]

t(ℓ)P21 = −P12

P22 = −t0t(ℓ). (8)

Here,

cos ka = (E − ϵ0)/2t0‘a’ is the lattice spacing in the leads, and throughout the calculationwe shall set ϵ0 = 0, and t0 = 1.

3. Numerical results and discussion

3.1. The eigenvalue spectrum

Before discussing the precise point of interest, we prefer tohave a look at the eigenvalue spectra of the proposed hierarchicalstructures. In particular, we examine the formation of the bandsand the gaps as a function of the hierarchy parameter λ both forthe axial and the transverse cases. To this end, we use a wellknown trick used frequently in the case of a quasi-periodic lat-tice [27], and later used for hierarchical structures as well [24,28].We sequentially generate periodic approximants of the original hi-erarchical structure, calculate the trace of the transfer matrix cor-responding to a ‘unit cell’ and extract those values of energy forwhich the magnitude of the trace of the transfer matrix remainsbounded by 2 [27]. The results are shown for the axial and thetransverse cases in Fig. 3(a) and (b) respectively. The common fea-ture in both the figures is the presence of multiple bands and gaps.A variation of the hierarchy parameterλ leads to band overlapping.Specifically speaking, in the axial case, the density of allowed en-ergy values is larger in the range−0.2t ≤ λ ≤ 0.2t . Band crossingsmaximize in this area. An increase in the numerical value of λ leadsto a thinning of the spectrum. Influence of λ on the spectrum of thetransverse model is also similar.

B. Pal et al. / Solid State Communications 151 (2011) 1894–1898 1897

a b

Fig. 4. (Color online). (a) Transmission coefficient across a 7th generation fractal network (the axial case) for E = ϵA + t with ϵB(n) + τ(n) = ϵA − 2t , and ϵC(n) = ϵA − t .(b) Fine scan of a selected part of (a) to reveal the self-similar distribution of λ. We have set ϵA = 0, and t = 1.

a b

Fig. 5. (Color online). (a) Transmission coefficient across a 7th generation fractal network (the axial case) for E = ϵ′

A + t ′ and (b) a fine scan of a selected part of (a) to revealthe self-similar distribution of λ. We have set ϵA = ϵB(1) = ϵC(1) = 0, and t = τ(1) = 1.

3.2. The unusual states

We now examine some special situation which is the focus ofthis article.

3.2.1. The axial modelLet us begin with the axial model. It is easy to verify that, if we

choose the energy of the electron E = ϵA + t , then one can con-struct, by hand, a wave function with amplitude equal to unity atevery lattice point, provided one also fixes, ϵB(n) + τ(n) = ϵA −

2t and ϵC(n) = ϵA − t, for all n ≥ 1. Two points are worth noting.First, the distribution of amplitude thus constructed is independentof the individual values of ϵB(n) and τ(n), and only requires the spe-cial correlation in their numerical values as suggested above. Thus,in principle, ϵB(n) can be chosen out of any sequence of uncorre-lated random numbers. The values of the hopping integrals τ(n)only then need to be selected accordingly. Thus the constructedamplitude distribution remains valid even for a random choice ofa subset of the on-site potentials ϵB(n), and presents a new kind ofextended wave function. However, this construction cannot auto-matically be related to the good transmission property of any largebut finite lattice. This is easily understood when one appreciatesthat the choice of the energy did not depend on the hierarchicallydistributed hoppings viz., τ(n), but the end-to-end transmission isbound to be sensitive to the individual values of τ(n).

This is illustrated in Fig. 4(a), where we have worked outthe transmission coefficient across a 7th generation hierarchicalnetwork and for τ(n) = λτ(n − 1), with τ(1) = λt . Energy is

set at E = ϵA+t . The corresponding eigenstate has equal amplitude(normalized to unity) at all lattice points. The transmissionspectrum shows that the 7th generation network is transparent toan incoming electron with the above energy only when λ assumesa specific set of values. The transmission in this range of λ iscompletely ballistic for certain values. We may assign the wavefunction in this case the status of an extended state. Interestingly,the samedistribution of the amplitudes of thewave function, at thesame energy E = ϵA + t makes the lattice completely opaque tothe incoming electron for other ranges of λ. The energy definitelycorresponds to an eigenstate of the system, as has been verifiedby calculating the local density of states at the sites of an infinitehierarchical lattice for the axial case. Also, the very fact that, one isable to construct such a state on a lattice, no matter how large itis, automatically confirms that it is an eigenstate. This observationleads to the question of a proper identification of an extended statein a non-translationally invariant system.

The procedure can be implemented on say, a one steprenormalized lattice. That is, we can extract an energy eigenvalueby solving the equation E = ϵ′

A + t ′. The energy turns out to bea function of ϵA, t , ϵC(1), ϵB(1) and τ(1), but remains independentof ϵB(n), ϵC(n) and τ(n) for n ≥ 2. Once again we are to choose,ϵB(n)+τ(n) = ϵ′

A−2t ′ and ϵC(n) = ϵ′

A− t ′, for all n ≥ 2. The entirescheme works as before. In Fig. 5(a) we illustrate the transmissioncoefficient for E = 0. This energy is extracted by solving theequation E = ϵ′

A+t ′, wherewehave chosen ϵA = ϵB(1) = ϵC(1) = 0,and t = τ(1) = 1. A fine scan of a selected portion of Fig. 5(a) ispresented in Fig. 5(b) to show the self-similar distribution of the

1898 B. Pal et al. / Solid State Communications 151 (2011) 1894–1898

values of λ. The process can, in principle, be continued and one canextract energy eigenvalues for such unusual eigenstates by solvingthe equation E = ϵ

(ℓ)A + t(ℓ) at any ℓ-th stage of renormalization.

Ideally, we thus have an infinite number of such eigenstates.

3.2.2. The transverse modelFor the transverse model, we can use an identical string of

arguments to construct an eigenstate with an amplitude equal tounity at every lattice point. For this construction we fix the energyE = ϵA + t , and demand, ϵB(n) = ϵA − 2t and ϵC(n) + τ(n) =

ϵA−t, for all n ≥ 1. This choice of parameters leads to a completelynew scenario in comparison to the axial case. Here, for values ofthe hierarchy parameter λ > 1, it is found that at any ℓ-th stage ofrenormalization the on-site potential at the C-sites grow followingthe rule:

ϵ(ℓ)

C(n) = λϵ(ℓ)

C(n−1) − ξ (ℓ)(λ) (9)

where ξ (ℓ)(λ) is a constant, function of λ, and the on-site potentialat the B-sites at any ℓ-th stage of renormalization ϵ(ℓ)B(n) remainsconstant. Thus an incoming electron with energy E = ϵA + twill face effectively higher and higher potential barriers, offered bythe ‘C ’ sites while traveling through the lattice. This will lead to agradual decay of the end-to-end transmission as the system growsin size. In the thermodynamic limit the hierarchical lattice willremain non-conducting. For λ ≤ 1, no regular pattern in the on-site potentials is observed. However, the hopping integral t alwaysdecays to zero for E = ϵA + t .

We have also examined the case, where the energy is extractedfrom a one step renormalized lattice by solving the equation E =

ϵ′

A + t ′. For this we are free to choose ϵA, ϵB(1), ϵC(1), t and τ(1)freely, that is, in an uncorrelated fashion. The correlation now setsin from the hierarchy level n = 2 onwards. For example, we haveexamined the special situation when ϵA = ϵB(1) = ϵC(1) = 0,and t = τ(1) = 1. The roots are, E = −1.90321, 0.193937,and 2.70928. The transmission in each case drops fast as the latticegrows in size. The amplitude-distribution on a one step renormal-ized lattice is such that ψi = 1 on every vertex of the renormal-ized lattice. Therefore, the states are still strictly localized. Thus wecan say that, a hierarchically distributed long range hopping in thetransverse direction does not allow the lattice to be conducting.

Thus, once we appreciate that a long range tunnel hopping canbe associated with the proximity of the atoms in the structure, wesee that a proximity along the principal axis of the fractal allowsboth conduction and localization, whereas, a proximity in thetransverse direction makes the lattice non-conducting in general.

Before we end, it should be emphasized that, all thesediscussions are made with reference to special values of theenergy E for which one can construct a unique spatially extendeddistribution of the eigenfunctions. However, hierarchical lattices,as already discussed in the introduction, may possess bothlocalized and extended eigenstates coexisting in the spectrum.For example, we have worked out a special situation in thetransverse case, where a different set of values for the Hamiltonianparameters and the electron-energy may lead to a one cycle fixed

point of the parameter space. The eigenstate in this particularcase is definitely extended and the transport is high. However, werefrain from entering into this aspect to save space.

4. Conclusions

In conclusion, we have addressed the issue of extendednessof an eigenstate and the corresponding transport property of ahierarchical lattice that lacks translational symmetry. We come tothe conclusion that, a state that looks extended by constructionis not necessarily conducting, and the mobility of the state isstrongly sensitive to a correlated choice of a subset of the systemparameters. Even within the same basic lattice topology, a longrange correlated tunnel hopping along the principal axis is foundto lead to both extended and localized states depending on thevalue of the hierarchy parameter. The hierarchically long rangehopping in the transverse direction, on the other hand, makes theeigenfunction localized in the lattice.

Acknowledgments

One of the authors (B. Pal) acknowledges financial assistancefrom DST, India through the INSPIRE fellowship.

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