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Electronic structure of monolayer graphene
Graphene seminar 25/04/13
Andor Kormányos
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Literature
M. I. Katsnelson: Graphene (Cambridge University Press)
Ed McCann: Graphene Nanoelectronics: Metrology, Synthesis, Properties and Applications, pages 237-275 (Springer-Verlag Berlin Heidelberg 2012) ArXiv:1205.4849
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Crystal lattice of monolayer graphene
Hexagonal lattice ofcarbon atoms
Two atoms in the unitcell:
More realistic view
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Electronic structure from DFT calculations
Phys Rev B 77, 035427 (2008) Phys Rev B 82, 245412 (2010)
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Electronic structure from DFT calculations
Phys Rev B 77, 035427 (2008)
Crossing of π bands at the K point of the Brillouin zone
Dispersion is linear in Wavenumber close to the K point
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Band dispersion from Angle Resolved Photoelectron Spectroscopy (ARPES)
Nature Physics 3, 36 (2007)
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Tight-binding model: general theory
It is assumed that the system has translational invariance => we consideran infinite graphene sheet
In general, there are n atomic orbitals in the unit cell
We can form n Bloch functions
An electronic function is a linear combination of these Bloch functions
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Tight-binding model:general theory
The energy of the jth band:
Substituting the expansion
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Tight-binding model:general theory
Minimizing the energy with respect to the coefficients
For the special case of two orbitals per unit cell:
In short
Eigenenergies:
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Tight-binding model : graphene
There are n=2 orbitals in the unit cell.
Assumption: dominant contribution comes from i=j, others can be neglected
Within the summation this expectation value is the same forevery value of i
Diagonal matrix elements
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Tight-binding model : graphene
Carbon atoms on sublattice B are chemically identical to atoms on sublattice A:
Overlap integrals: assuming again that same site contribution dominate (nearest neighbours are B atoms)
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Off-diagonal matrix elements
Tight-binding model : graphene
Assumption: dominant contribution comes from nearest-neighbors, other contributions can be neglected. Looking at, e.g., A type atoms, there are3 nearest-neighbor B atoms
The index “l” depends on the index “i”
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Off-diagonal matrix elements
Tight-binding model : graphene
Assumption: dominant contribution comes from nearest-neighbors, other contributions can be neglected. Looking at, e.g., A type atoms, there are3 nearest-neighbor B atoms
The index “l” depends on the index “i”
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Tight-binding model : grapheneThe matrix element between nearest-neighbor A and B atoms has the samevalue for each neighboring pair:
Note, at this step we have made use of the fact that the atomic orbitals are actually p_z orbitals, hence have a rotational symmetry
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Footnote: general method to obtain tight-binding Hamiltonians
General method: Slater-Koster parametrization of hopping integrals Physical Review 94, 1498 (1954) For recent introduction see,e.g., Sergej Konschuh PhD thesis
Slater-Koster parametrization is still useful, especially in connection with firstprinciples methods and group theory. Recent examples: Bilayer MoS2, arXiv:1304.4831
Spin-orbit coupling in trilayer graphene: Phys Rev B 87, 045419 (2013)
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Tight-binding model : grapheneThe matrix element between nearest-neighbor A and B atoms has the samevalue for each neighboring pair:
Note, at this step we have made use of the fact that the atomic orbitals are actually p_z orbitals, hence have a rotational symmetry
Therefore
Position of atom B relative to atom A
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Tight-binding model : graphene
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Tight-binding model : graphene
Finally
In a similar fashion:
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Tight-binding model : graphene
In summary:
The corresponding eigenenergies:
From
Choice of the origin of the energy axis
Such parameters can be obtained from DFT or experiments
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Tight-binding model : graphene
TB DFTGood qualitative agreement
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Low-energy physics, Dirac-like Hamiltonian
Fermi energy lies at E=0
Only two K points are inequivalent,the others are connected by reciprocalvector, or see the original Brillouin zone
K_, K+ points often called “valleys”
Gaples band structure
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Low-energy physics, Dirac-like Hamiltonian
Introducing the momentum measured from the K point(s)
and expanding f(k) up to first order in p
and we obtain the famous 2D massles Dirac Hamiltonian of graphene
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Low-energy physics, Dirac-like Hamiltonian
Introducing the momentum measured from the K point(s)
and expanding f(k) up to first order in p
and we obtain the famous 2D massles Dirac Hamiltonian of graphene
The actual value of the velocity v is 106 m/s ≈ c/300
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Low-energy physics, Dirac-like Hamiltonian
If we now consider H1 to be an effective Hamiltonian and solve the corresponding
Schrödinger equation:
What about the overlap matrix S?
Finite overlap contributes with ~p2 terms
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Low-energy physics, Dirac-like Hamiltonian
If we now consider H1 to be an effective Hamiltonian and solve the corresponding
Schrödinger equation:
What about the overlap matrix S?
Finite overlap contributes with ~p2 terms
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Low-energy physics, Dirac-like Hamiltonian
If we now consider H1 to be an effective Hamiltonian and solve the corresponding
Schrödinger equation:
What about the overlap matrix S?
Finite overlap contributes with ~p2 terms
Higher order terms in momentum in E(p) are negligible for energies ≤ 1 eV
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Pseudospin and chirality
The eigenstates have two components,reminiscent of spin ½
Looking back to the original definitions, the two components correspondto the relative amplitude of the Bloch function on the A and B sublattice.
This degree of freedom is called pseudospin.
If the wavefunction was finite only on A sublattice → (1,0)T = | ↑ > on B sublattice → (0,1)T = | ↓ >
In graphene, the density is usually shared equally between A and B sublattice.
Some substrates can break this symmetry, though
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Pseudospin and chirality
The eigenstates have two components,reminiscent of spin ½
Looking back to the original definitions, the two components correspondto the relative amplitude of the Bloch function on the A and B sublattice.
This degree of freedom is called pseudospin.
If the wavefunction was finite only on A sublattice → (1,0)T = | ↑ > on B sublattice → (0,1)T = | ↓ >
In graphene, the density is usually sharer equally between A and B sublattice.
Some substrates can break this symmetry, though
Note the index ξ: in addition to pseudospin, there is another degreeof freedom: valley
The valleys are usually not coupled, except in the case of atomic scale scatterers and certain boundaries => two independent Hamiltonians
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Pseudospin and chirality
The particles described by the Dirac Hamiltonian of monolayer graphene have yet another property: they are chiral
This means that the orientation of pseudospin depends on the direction of theelectronic momentum p
To see this more clearly, let's write the effective Hamiltonian as
and define a pseudospin vector as
and a unit vector as
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Pseudospin and chirality
The particles described by the Dirac Hamiltonian of monolayer graphene have yet another property: they are chiral
This means that the orientation of pseudospin depends on the direction of theelectronic momentum p
To see this more clearly, let's write the effective Hamiltonian as
and define a pseudospin vector as
and a unit vector as
Then
therefore σ is linked to the direction of n1
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Pseudospin and chirality
In other words, the eigenstates of the effective Hamiltonian are also eigenstates of the chiral operator
Another way to express this is to calculatewith respect to
In valley K+
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Pseudospin and chirality
Important consequence of the chirality of particles: the probability to scatterinto a direction characterized by the angle φ ( φ=0 corresponds to forwardScattering) is proportional to
For monolayer graphene
No backscattering!