Notes on topological insulators

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Notes on topological insulators ( notes-topological-insulators )

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R. M. Kaufmann, D. Li & B. Wehefritz-Kaufmann index by the eta invariant where 􏰘 b a μ a ( f ) : = 1 􏰘 b 􏰤n [ L ∗2 Ω − L ∗1 Ω ] + 2 η ̃ ( D ( L 1 , L 2 ) ) | ba 1 􏰤n ( 4 . 7 ) μa := η(D(L1, L2)). (4.5) We stress that this analytical Maslov index is real-valued. For example, consider two typical Lagrangians R and L(θ) = R{eiθ} in (R2,ω), the path connecting them can be taken to be φ(t) = ei(lπ+θ)t, for some l ∈ Z where the integer l is added since the Lagrangian R{eiθ} is invariant by multiplying a minus sign. Since D(R,L(θ))φ=−idφ =(lπ+θ)φ dt the eta function of D(R, L(θ)) is η(D(R, L(θ)), s) = 1 [ζH (s, θ/π) − ζH (s, 1 − θ/π)] πs where ζH (s, x) is the Hurwitz zeta function ζH (s, x) = 􏰫∞n=0(n+x)−s for 0 < x < 1. The Hurwitz zeta function has a regular point at s = 0, indeed ζH (0, x) = 1 − x. 2 Hence the real-valued Maslov index is easily obtained by direct computation μa = η(D(R,L(θ)),0) = 1 − 2θ. (4.6) π In order to be compatible with the integral Lagrangian intersection number, i.e. the geometric Maslov index, the integer-valued analytical Maslov index is defined as follows. Define the canonical 1-form on Λ(1) by pulling back the standard 1-form on S1, 􏰠􏰡 dθ 2π which is the generator of the cohomology group H1(Λ(1), Z) ≃ H1(S1, Z) ≃ Z. Now the integer-valued analytical Maslov index is defined for a pair of continuous and piecewise smooth Lagrangians f(t) = (L1(t),L2(t)), for some interval a ≤ t ≤ b, Ω := (Sq)∗ 􏰘b1 a L∗sΩ=π In (R2,ω), let L1 = R,L2 = R{eiθ}, then it is easy to compute the eta invariant dθsi=π [θsi(b)−θsi(a)],s=1,2. i=1 a i=1 η(D(L1, L2)) = 1 − 2θ for 0 < θ ≤ π π 4 0 for θ = 0 1630003-34 −1−2θ for−π≤θ<0  π 4 Rev. Math. Phys. 2016.28. Downloaded from www.worldscientific.com by PURDUE UNIVERSITY on 08/11/17. For personal use only.

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