Nearest-neighbour tight-binding chain
For a periodic chain of N orthonormal localized orbitals, one per site with spacing a, take H_nm=ε₀δ_nm−t(δ_(n+1,m)+δ_(n−1,m)).
Key knowledge
For a periodic chain of N orthonormal localized orbitals, one per site with spacing a, take H_nm=ε₀δ_nm−t(δ_(n+1,m)+δ_(n−1,m)). The normalized eigenvectors φ_n=e^(ikna)/√N, k=2πj/(Na), have E(k)=ε₀−2t cos(ka). In the infinite-chain limit this single band spans ε₀−2|t| to ε₀+2|t|; zero hopping gives the degenerate atomic limit.
Values and equations
H_nm=ε₀δ_nm−t(δ_(n+1,m)+δ_(n−1,m)); φ_n=e^(ikna)/√N; E(k)=ε₀−2t cos(ka)
- H_nm,ε₀,t,E(k)
- Hamiltonian matrix element, on-site energy, declared hopping parameter and band energy J
- a
- Site spacing m
- k
- Allowed wave number m⁻¹
- n,m,j,N
- Integer site, momentum and size labels
- φ_n
- Dimensionless normalized orbital coefficient
- δ_nm
- Kronecker delta
Conditions and scope
- N≥3 on the ring, n and m interpreted modulo N, j=0,…,N−1, a>0, uniform real ε₀ and t, and the specified hopping matrix element −t; k is defined modulo 2π/a. Finite rings sample the dispersion and need not attain both infinite-band endpoints.
- The dispersion is exact for this translation-invariant nearest-neighbour one-orbital Hamiltonian, which approximates an actual solid. Nonorthogonal orbitals, extra hopping, multiple orbitals, disorder and electron interactions require an enlarged model; the formula alone does not decide whether a material is metallic.
Sources · 1
References
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Evidence and review
Solid State Physics: third-year lecture notes (2012): Sections10.1–10.2, equations10.1–10.6: orthonormal orbitals, periodic boundary conditions, real −t nearest-neighbour matrix elements, normalized plane-wave substitution and cosine energy. The positive phase convention is equivalent to the source's negative phase; the infinite-band endpoints follow by the range of cosine and are not asserted for every finite ring.
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