Dynamical Conductivity and Zero-Mode Anomaly in Honeycomb Lattices

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1 Journal of the Physical Society of Japan Vol. 7, No. 5, May,, pp. 8 # The Physical Society of Japan Dynamical onductivity and Zero-Mode Anomaly in Honeycomb Lattices Tsuneya ANDO, Yisong ZHENG and Hidekatsu SUZUURA Institute for Solid State Physics, University of Tokyo, 5--5 Kashiwanoha, Kashiwa, hiba (Received January 5, ) A topological anomaly present at zero energy is shown to give rise to a peculiar and singular behavior of the dynamic and static conductivity in two-dimensional honeycomb lattices. A calculation in a selfconsistent orn approximation reveals that this singular dependence prevails although being somewhat smoothed out even if level broadening effects are taken into account. KEYWORDS: graphite, carbon nanotube, effective-mass theory, erry s phase, topological singularity DOI:./JPSJ.7.8. Introduction Since the discovery of carbon nanotubes, ) transport properties of the carbon network of the nanometer scale have attracted much attention. There have been a lot of experimental works focusing on the transport measurement in various nanotube structures and theoretical calculation of the conductance of carbon nanotubes. ) A carbon nanotube consists of coaxially rolled two-dimensional graphite sheets characterized by a honeycomb lattice. Therefore, the theoretical investigation on transport properties of honeycomb lattices is instructive for the comprehensive understanding of the transport property of nanotubes. Further, a honeycomb lattice can be realized at semiconductor heterostructures hexagonal antidot arrays. In a previous work,,) the density of states and the static magnetoconductivity were calculated by the quantum transport theory, in which short- and long-range scatterers were taken into account. It was found that the quantum theory provides conductivity exhibiting almost singular behavior in the vicinity of vanishing Fermi energy. In this paper, this behavior is shown to be a manifestation of zeromode anomalies due to a topological singularity through the study of the dynamical conductivity. In an effective-mass scheme is reviewed briefly. In the dynamical conductivity is calculated in a simplest relaxation-time approximation and it is shown that its frequency dependence is scaled by the Fermi energy and therefore becomes singular for the vanishing Fermi energy. In effects of level broadening are included in a selfconsistent orn approximation. It is shown that singularities prevail even in the presence of level broadening in the case of weak scattering. A summary and conclusions are given in 5.. Effective-Mass Description In a honeycomb lattice such as a two-dimensional graphite, a unit cell contains two atoms denoted as A and as shown in Fig.. Two bands having approximately a linear dispersion cross the Fermi level at K and K points of the first rillouin zone, p whose wave vectors are given by K ¼ð=aÞð=; = ffiffi Þ and K ¼ð=aÞð=; Þ a being the lattice constant. Under the half-filled condition, electronic properties are governed by states in the vicinity of Fig.. The structure of a honeycomb lattice and its first rillouin zone. A unit cell contains two atoms denoted as A and (small circles). The corner points of the rillouin zone are denoted as K and K. A honeycomb lattice can be realized at a semiconductor heterostructure by fabrication of a shortperiod hexagonal antidot array (shadowed big circles). Present address: Department of Physics, Tokyo Institute of Technology, -- Ookayama, Meguro-ku, Tokyo

2 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. 9 K and K points. The Schrödinger equation for such states is given by 5) H F ¼ "F; ð:þ ð ^k x i ^k y Þ ð ^k x þ i ^k y Þ H ¼ ð ^k x þ i ^k y A ; ð ^k x i ^k y Þ ð:þ where is a band parameter, ^k (¼ ir) is the wave vector operator, and F is a four-component wave function. The eigenfunction of H is given by s F K sk ðrþ ¼ p e i ðkþ ffiffiffi expðik rþ A ; ð:þ and F K sk ðrþ ¼ p ffiffiffi expðik rþ e i ðkþ A ; ð:þ s where L is the area of the system, ðkþ is the angle of the wave vector k, and s denotes the bands (s ¼þ for the conduction band and s ¼ for the valence band). The corresponding energy is given by " sk ¼ sk; ð:5þ k ¼jkj. The first two components of the wave function F describe the amplitudes at a site A and associated the K point, and the next two those associated the K point. The ð; Þ Hamiltonian H is completely separable into ð; Þ components for the K and K points in the absence of scatterers. The ð; Þ Hamiltonian for the K point is written as ð ^kþ in terms of Pauli s spin matrices ¼ð x ; y Þ. We consider two different kinds of scatterers. First, the range of the scattering potential is smaller than the lattice constant of the two-dimensional graphite. When such a short-range scatterer is present at an A site ri A, the effective Hamiltonian has been calculated as 6) e ia i Ui A ðrþ e ia i A ua i ðr ra i Þ; ð:6þ A i ¼ðK KÞri A and u A i being the strength. Similarly, for a scatterer located at a site r i, Ui ðrþ ¼ e i A u i ðr r i Þ; ð:7þ e i i where i ¼ðK KÞr i. Next, the range is larger than the lattice constant but much smaller than the typical electron wavelength (which is infinite at " ¼ ). In this case matrix elements between K and K points can be neglected and the potential is given by a diagonal matrix, i.e., U i ðrþ A u iðr r i Þ; ð:8þ where r i is the impurity position. This type of scatterers is called long-range, although the potential is given by a function.. Dynamical onductivity The current operator is given by j ¼ e h ; ð:þ for the K point. The corresponding matrix elements are calculated as ðskj j x js k Þ¼ e se i ðkþ þ s e i ðkþ ; h ðskj j y js k Þ¼ e i se i ðkþ s e i ðkþ ð:þ ; h for the K point. The dynamical conductivity is isotropic and given by ð!þ ¼ e h L h i X f ½" s ðkþš f ½" s ðkþš ½" ss k s ðkþ " s ðkþš½" s ðkþ " s ðkþþh! þ iš ; ð:þ where f is the Fermi distribution function, is a positive infinitesimal, and a factor two has been multiplied because of the presence of the K and K points. When the Fermi energy lies in the conduction band (" F ¼ k F ) at zero temperature, we have e h j" F j ð!þ ¼ h i ðh! þ iþ! ð:þ X h! þ i L jkj½ðh! þ iþ : k Š jkj>k F The relaxation time in the absence of a magnetic field is defined as ¼ X X jh jskjuj j s k ij ð" sk " s h k Þ; ð:5þ j ¼K;K s k where U is the effective Hamiltonian for scatterers. In the case of short-range scatterers it is given by ¼ n A i hðu A i Þ iþn i hðu i Þ i j"j h ; ð:6þ where n A i and n i are the concentration of scatterers in a unit area and hi means the average. The relaxation time for long-range scatterers is given by ¼ n i hðu i Þ i j"j h ; ð:7þ

3 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. When we assume u ¼hðu A i Þ i¼hðu i Þ i¼hðu i Þ i and n i ¼ n A i þ n i and n A i ¼ n i, the relaxation time becomes same between short- and long-range cases and ¼ j"j ha ; ð:8þ where we have introduced a dimensionless parameter to characterize the scattering strength given by A ¼ n i u : ð:9þ With the use of the oltzmann transport equation, the transport relaxation time is given by ¼ X X jh jskjuj j s k ij ð cos Þð" sk " s k h Þ; j ¼K;K s k ð:þ where cos ¼ k k =k. We have ð"þ ¼ ð"þ ; ð:þ ¼ in the case of short-range scatterers and ¼ = in the case of long-range scatterers. In order to include effects of level broadening due to scatterers, we shall replace by h=, where is a relaxation time. ecause of the energy dependence of the relaxation time, we shall assume ¼ ð"þ " ¼ " F in the first term in the right hand side of eq. (:) and " ¼ h!= in the second term, corresponding to the energy of the states giving a major contribution to the transition. Then, the dynamical conductivity is calculated as ð!þ ¼ e h i" F h! þ i½h=ð" F ÞŠ þ þ i ln h! þ i½h=ðh!=þš " F h! þ i½h=ðh!=þš þ " F : ð:þ ecause h =ð"þ /j"j, the frequency dependence of the dynamical conductivity is scaled by h!=" F. Figure shows ð!þ as a function of h!=" F for several values of A. The scaling of the dynamical conductivity ðh!=" F Þ shows that ð!þ exhibits a singular behavior at the point ð!; " F Þ¼ð; Þ. In fact, when we set! ¼ first, the static conductivity is given by ðþ ¼ e A h ; ð:þ independent of the Fermi energy " F in agreement the oltzmann result. ) When we set " F ¼ first nonzero!, on the other hand, the static conductivity at!! becomes e =8h which is much smaller than in the oltzmann limit A. The correct way is to let!! at each " F, leading to a singular jump of the static conductivity at " F ¼. The calculation in a self-consistent orn approximation shows that this anomaly manifests itself as a near singular dependence of on " F even if level broadening effects are included and that the conductivity at " F ¼ is given by a universal conductivity quantum e = h as will be discussed in the following. ). Self-onsistent orn Approximation. Green s function In the following we shall use Green s function technique Dynamical onductivity (units of e / h) 5 5 oltzmann onductivity A A/ Frequency (units of F /h) Fig.. The dynamical conductivity calculated using the oltzmann transport equation. The frequency is scaled by the Fermi energy. ¼ and / in the case of short- and long-range scatterers, respectively. to evaluate the dynamical conductivity. 7,8) This technique allows us to consider broadening effects self-consistently and is widely used. The Green s function (resolvent) is defined by Gð"Þ ¼ " H : ð:þ The matrix elements of unperturbed Green s function are diagonal: G ðþ ð"þ ¼ " H ¼ G ðþ " " ð"þ; ð:þ where ¼ðj; s; kþ j ¼ K or K and s ¼. For evaluating physical quantities, we need hg ð"þi, which is the average of the matrix element of Gð"Þ over all possible configuration of random distributions of impurities. The averaged Green s function is connected to the proper selfenergy ð"þ by Dyson s equation: hg ð"þi ¼ G ðþ ð"þþgðþ ð"þ X ð"þhg ð"þi: ð:þ Figure shows a diagramatic representation of the selfconsistent orn approximation. The self-energy is given by ð"þ ¼ X hu U ihg ð"þi; ð:þ where terms lowest-order in U are neglected because they can be absorbed into a shift in the energy origin. For the choice of the scattering parameters discussed in the previous section, the self-energy and Green s function become independent of short- and long-range scatterers. We have ð"þ ¼ ð"þ; ð:5þ hg ð"þi ¼ G ð"þ;

4 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. substitution of X ð" þ iþ ¼X ð" þ iþþix ð" þ iþ, D ð"þ ¼ j"j : ð:þ. Dynamical conductivity The conductivity is written as xx ð!þ ¼ h Z d" f ð"þ fð" þ h!þ X ðþ X L h! ðþ X ss k ðs kj j x jskþg sk ð" þ h! iþg s kð" iþ Fig.. Some diagrams in the self-consistent orn approximation. The top shows the diagram of the self-energy for intravalley scattering processes and for intervalley. The middle shows the diagram of the conductivity vertex corrections. The bottom shows the equation for the current vertex part. ð"þ ¼ n iu X G L ð"þ; ð:6þ and G ð"þ ¼ " " ð"þ : ð:7þ Explicitly, we have ð"þ ¼ n iu Z kc kdk ½" ð"þš ½" ð"þš k ; ð:8þ where k c is a cutoff wave number. Define Xð"Þ ¼" ð"þ: ð:9þ Then, the self-consistency equation is rewritten as " ¼ Xð"Þþ A Xð"Þ ln Xð"Þ ln½xð"þ " c Š ; ð:þ where " c ¼ k c : ð:þ The cutoff k c is given by k c =a where a is the lattice constant and the corresponding " c is of the order of the band width. 9,) The oltzmann result for scattering rate can be obtained by taking the limit Xð"Þ ¼X ð"þ " þ i in the second term of the right hand side of eq. (:) as X ð" þ iþ X ð" þ iþ ¼i A j"j ¼i h ð"þ ; ð:þ X ð" þ iþ ¼Re Xð" þ iþ and X ð" þ iþ ¼Im Xð"þ iþ. This lowest order result is valid in the case of weak scattering A and " 6¼. The density of states is given by Dð"Þ ¼ X ImXð" þ iþ ImhG L ð" þ iþi ¼ ; n i u ð:þ which gives that in the absence of impurities upon J ss k x ð" þ h! i;" iþ; ð:5þ where J ss k x ð" þ h! i;" iþ is the current vertex part as shown in Fig. and the factor two is due to the presence of the K and K points. In the case of short-range scatterers, vertex corrections vanish identically and we have J ss k x ð"; " Þ¼ðskjj x js kþ: ð:6þ In the case of long-range scatterers, we have J ss k x ð"; " Þ¼ðskjj x js kþ A ð"; " Þ ; ð:7þ ð"; " Þ¼ L Explicitly, we have X G sk ð"þg s kð" Þ: ss k ð:8þ ð"; " Þ¼ Xð"ÞXð" Þ ln Xð"Þ Xð" Þ Xð"Þ ln Xð" Þ ð:9þ ln½xð"þ " c Šþln½Xð" Þ " c Š : When " ¼ " we have ð" þ i;"þ iþ ¼ ; ð:þ ð" þ i;" iþ ¼Sð"Þ; jxð" þ iþj Sð"Þ ¼ X ð" þ iþ X ð" þ iþx ð" þ iþ tan X ð" þ iþ : ð:þ Therefore, the dynamical conductivity is given by Z ð!þ ¼ e d" f ð"þ fð" þ h!þ X ðþ X ðþ h h! ð" þ h! i;" iþð" þ h! i;" iþ; ð:þ ð" þ h! i;" iþ ¼; ð:þ for short-range scatterers and ð" þ h! i;" iþ¼ A ð" þ h! i;" iþ ; ð:þ for long-range scatterers. The above results give the static conductivity calculated previously ) in the limit!!. In fact, in the case of shortrange scatterers, the static conductivity becomes ¼ e Sð"F Þþ ; ð:5þ h at zero temperature. In the oltzmann limit A, we have

5 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. j" F j=x ð" F þ iþ and therefore ¼ given by eq. (:) ¼. In the particular case " ¼, the real part of the self-energy vanishes at " ¼ because of the symmetry between "> and "< and eq. (:) gives immediately XðþiÞ ¼i" c expð A=Þ. The conductivity at " F ¼ is also calculated analytically and becomes ¼ e h : ð:6þ This conductivity is given only by natural constants and independent of the scattering strength and the band parameter. Further, it is much smaller than the oltzmann conductivity in the case of weak scattering A. In the case of long-range scatterers, on the other hand, we have ¼ e Sð" F Þþ h ð þ A Þ½ A Sð" F ÞŠ : ð:7þ Dynamical onductivity (units of e / h) 8 6 (a) A = 5 c/ = 5 F/ In the oltzmann limit, we have Sð" F Þ¼A= and ¼ ¼ =. Further, at " F ¼, we have Sð" F Þ¼ and therefore ¼ e e h A h : ð:8þ This value is slightly larger than e = h and dependent on the scattering parameter. This dependence is extremely small in the case of weak scattering A. 5. Numerical Results The important energy scale in the present problem is the cutoff energy " c which is of the order of the band width. However, in this paper, following ref., we shall introduce an arbitrary energy scale " which is assumed to have the same order of magnitude as other relevant energies such as " and Xð" þ iþ and choose the cutoff energy as " c =" ¼ 5. Figure shows some examples of calculated ð!þ for different values of " F and for A ¼ 5 and in the case of short-range scatterers. The frequency dependence is scaled by h!=" F as long as " F 6¼ for weaker scattering A ¼ 5. When " F is very close to, however, the conductivity at! ¼ becomes small and the discrete jump present in the oltzmann conductivity is removed as is more clear in the case of stronger scattering A ¼. The energy scale causing this crossover behavior becomes smaller A leading to a singular behavior of the dynamical conductivity in the weak scattering limit. Figure 5 shows some examples in the case of long-range scatterers. The behavior is essentially same as in the case of short-range scatterers except that effects of scattering are reduced. This reduction corresponds to the increase of the transport relaxation time due to the absence of backward scattering.,6,) 6. Discussion The anomalous behaviors of the static and dynamic conductivities at " F ¼ are closely related to the zero-mode anomaly due to a topological singularity at k ¼ present in Weyl s equation describing the electronic states near the K and K points. In fact, when k is rotated once in the anticlockwise direction adiabatically as a function of time t for a time interval < t < T kðtþ ¼kðÞ, the Dynamical onductivity (units of e / h) (b) Frequency (units of /h) A = c/ = 5 F/ Frequency (units of /h) Fig.. Some examples of calculated dynamical conductivity in the case of short-range scatterers. (a) A ¼ 5. (b) A ¼. wavefunction F sk is changed into F sk expð i Þ, where is erry s phase given by,) Z T ¼ i dt skðtþ d dt ¼ : skðtþ ð6:þ It should be noted that ¼ when the closed contour encircles the origin k ¼ but ¼ when the contour does not contain k ¼. Further, the wave function at k ¼ depends on the direction of k and is undefined at k ¼. These facts show the presence of a topological singularity at k ¼. This nontrivial erry s phase leads to the unique property of a metallic carbon nanotube that there exists no backward scattering and the tube is a perfect conductor even in the

6 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. Dynamical onductivity (units of e / h) Dynamical onductivity (units of e / h) (a) (b) Long-Range Scatterers A = 5 c/ = 5 F/ Frequency (units of /h) Long-Range Scatterers A = c/ = 5 F/ Frequency (units of /h) Fig. 5. Some examples of calculated dynamical conductivity in the case of long-range scatterers. (a) A ¼ 5. (b) A ¼. presence of scatterers as long as their potential range is larger than the lattice constant. 6,) This erry s phase is closely related to the helicity of a neutrino and the signature change of a wave function under a spin rotation. In fact, the Schrödinger equation near the K point is written as ð ^kþf K ¼ "F K ; ð6:þ using the spin matrix. This shows clearly that the spin is quantized into the direction of k and the wave function is given by a spinor its spin in the k direction. The spin function changes its signature when the spin is rotated by. This well-known signature change under the spin rotation corresponds to the above erry s phase. A singularity at " ¼ manifests itself in magnetic fields even in classical mechanics. The equation of motion is given by h dk dt ¼ e c v : ð6:þ This gives the cyclotron frequency! c ¼ ev =c", where v is the electron velocity given by v ¼jvj ¼=h. The cyclotron frequency! c diverges and changes its signature at " ¼. ) In quantum mechanics ^k x and ^k y satisfy the commutation relation ½ ^k x ; ^k y p Š¼ i=l ffiffiffiffiffiffiffiffiffiffiffiffi, where l is the magnetic length given by l ¼ can be obtained by the condition I ch =e. Semiclassically, the Landau levels k x dk y ¼ ðn þ Þ; l ð6:þ integer n p and an appropriate small correction. This gives " n ¼ ffiffiffiffiffiffiffiffiffiffi pffiffi n þ ð =lþ. ecause of the uncertainty relation k x k y l, k ¼ is not allowed and there is no Landau level at " ¼. However, a full quantum mechanical treatment of a magnetic field leads immediately pffiffiffi pffiffiffiffiffi to the formation of Landau levels at " n ¼ sgnðnþð =lþ jnj n ¼ ; ; ;. A peculiar and intriguing feature is the presence of a Landau level at " ¼. This is presumably related also to the anomaly present at " ¼. In the oltzmann transport theory the conductivity tensor ¼ x; y and ¼ x; y is given by xx ¼ yy ¼ þð! c Þ ; xy ¼ yx ¼! c ð6:5þ þð! c Þ ; where is defined in eq. (:). Using the explicit expressions for! c and, we have xx ¼ þ ; ð6:6þ xy ¼ þ ; rffiffiffiffiffiffiffiffiffi " F ¼ ; ð6:7þ A " where " is the magnetic energy defined by " ¼ =l. ecause the dependence on the Fermi energy is fully scaled by ", the conductivities exhibit a singular behavior at " F ¼ in the limit of the vanishing magnetic field "!. This singularity at " F ¼ disappears in the quantum theory based on the self-consistent orn approximation. Figure 6 compares classical and quantum xx and xy as a function of for A ¼ 5 in the case of short-range scatterers and Fig. 7 shows corresponding comparison in the case of long-range scatterers. In contrast to the oltzmann result the quantum results depend on " =" and xx depends on the Fermi energy in the limit " ="!, in consistent previous results.,)

7 J. Phys. Soc. Jpn., Vol. 7, No. 5, May, T. Ando et al. onductivity [units of (e / h)(a/ )] xx - xy A=5 c/ =5 oltzmann Fermi Energy [units of (A/ Self-onsistent orn /l = /l =. /l =. ) / /l] Fig. 6. The diagonal xx and Hall xy conductivity as a function of the normalized Fermi energy in the case of short-range scatterers. The thick lines represent oltzmann results and the thin lines those obtained in the self-consistent orn approximation. 7. Summary In summary, we have discussed anomalies appearing in the static and dynamic conductivity associated the topological singularity at k ¼. In the oltzmann transport theory, the frequency dependence of the dynamical conductivity is determined by h!=" F leading to a singular jump at! ¼ and " F ¼. Therefore, the static conductivity has a singularity at ð!; " F Þ¼ð; Þ and depends critically on which of! and " F is set equal to zero first. In a selfconsistent orn approximation in which level broadening effects are properly taken into account, this singular dependence is somewhat smoothed out but the corresponding energy scale is determined by the broadening at " F and is extremely small in the case of weak scattering. Acknowledgments This work was supported in part by Grants-in-Aid for Scientific Research and for OE (E ontrol of Electrons by Quantum Dot Structures and Its Application to Advanced Electronics ) from Ministry of Education, ulture, Sports, Science and Technology Japan. Numerical calculations were performed in part using the facilities of the Supercomputer enter, Institute for Solid State Physics, University of Tokyo. onductivity [units of (e / h)(a/ )] xx - xy A=5 c/ =5 oltzmann Self-onsistent orn /l = /l =. /l =. ) S. Iijima: Nature (London) 5 (99) 56. ) See, for example, T. Ando: Semicond. Sci. Technol. 5 () R and references cited therein. ) N. H. Shon and T. Ando: J. Phys. Soc. Jpn. 67 (998). ) Y. Zheng and T. Ando: submitted to Phys. Rev.. 5) J.. Slonczewski and P. R. Weiss: Phys. Rev. 9 (958) 7. 6) T. Ando and T. Nakanishi: J. Phys. Soc. Jpn. 67 (998) 7. 7) T. Ando and Y. Uemura: J. Phys. Soc. Jpn. 6 (97) ) T. Ando: J. Phys. Soc. Jpn. 6 (97) 5; ibid. 7 (97) 6; ibid. 7 (97). 9) T. Ando, T. Nakanishi and M. Igami: J. Phys. Soc. Jpn. 68 (999) 99. ) H. Matsumura and T. Ando: J. Phys. Soc. Jpn. 7 () 657. ) T. Ando, T. Nakanishi and R. Saito: J. Phys. Soc. Jpn. 67 (998) 857. ) M. V. erry: Proc. R. Soc. London A 9 (98) 5.. Fermi Energy [units of (A/ ) / /l] Fig. 7. The diagonal xx and Hall xy conductivity as a function of the normalized Fermi energy in the case of long-range scatterers. The thick lines represent oltzmann results and the thin lines those obtained in the self-consistent orn approximation.