Computational Solid State Physics 計算物性学特論 第9回 9. Transport properties I: Diffusive transport
Electron transport properties l e : mean free path of electrons l φ : phase coherence length λ F : Fermi wavelength
Examples of quantum transport l e : mean free path of electrons l φ : phase coherence length λ F : Fermi wavelength single electron charging key quantities
Point contact: ballistic quantum conductance
Aharanov Bohm effect: phase coherent quantum magnetic flux
Quantum dot: single electron charging
Shubnikov-de Haas oscillations and quantum Hall effect
Diffusive transport
Equation of motion for electrons Scattering Rate k: wave vecot of Bloch electron
How to solve equations of motion for electrons with scattering? Relaxation time approximation for scattering Direct numerical solution: Monte Carlo simulation Boltzmann equation for distribution function of electrons
Relaxation time approximation equation of motion current density m: effective mass n: electron concentration E: electric field B: magnetic field
Drude model: B=0 conductivity : drift velocity
Drude model: steady state solution in magnetic field : B is assumed parallel to z. : cyclotron frequency drift velocity
Conductivity tensor in magnetic field
no transverse magneto-resistance Hall effect
Monte Carlo simulation for electron motion Drift Scattering Drift Scattering
: current Drift velocity as a function of time
Boltzmann equation r k Motion of electrons in r-k space during infinitesimal time Interval Δt
Equation of motion for distribution function equation of motion for electron distribution function f k (r,t).
Boltzmann equation Steady state Boltzmann equation
Electron scattering detailed balance condition for transition probability
Scattering term assume: elastic scattering, spherical symmetry
Transport scattering time k k’ Θ kk’ Contribution of forward scattering is not efficient. Contribution of backward scattering is efficient.
Linearized Boltzmann equation Fermi sphere is shifted by electric field.
Current density and conductivity
Electron mobility in GaAs
Energy flux and thermal conductivity thermal conductivity
Problems 9 Calculate both the conductivity and the resistivity tensors in the static magnetic fields, by solving the equation of motion in the relaxation time approximation. Study the temperature dependence of electron mobility in n-type Si. Calculate the electron mobility in n-type silicon for both impurity scattering and acoustic phonon scattering mechanisms, by using the linearized Boltzmann equation.