Landau–Zener formula

The Landau–Zener formula is an analytic solution to the equations of motion governing the transition dynamics of a 2-level quantum mechanical system, with a time-dependent Hamiltonian varying such that the energy separation of the two states is a linear function of time. The formula, giving the probability of a diabatic (not adiabatic) transition between the two energy states, was published separately by Lev Landau and Clarence Zener in 1932.

If the system starts, in the infinite past, in the lower energy eigenstate, we wish to calculate the probability of finding the system in the upper energy eigenstate in the infinite future (a so-called Landau-Zener transition). For infinitely slow variation of the energy difference (that is, a Landau-Zener velocity of zero), the adiabatic theorem tells us that no such transition will take place, as the system will always be in an instantaneous eigenstate of the Hamiltonian at that moment in time. At non-zero velocities, transitions occur with probability as described by the Landau-Zener formula.

Landau-Zener approximation

Such transitions occur between states of the entire system, hence any description of the system must include all external influences, including collisions and external electric and magnetic fields. In order that the equations of motion for the system might be solved analytically, a set of simplifications are made, known collectively as the Landau–Zener approximation. The simplifications are as follows:

1. The perturbation parameter in the Hamiltonian is a known, linear function of time
2. The energy separation of the diabatic states varies linearly with time
3. The coupling in the diabatic Hamiltonian matrix is independent of time

The first simplification makes this a semi-classical treatment. In the case of an atom in a magnetic field, the field strength becomes a classical variable which can be precisely measured during the transition. This requirement is quite restrictive as a linear change will not, in general, be the optimal profile to achieve the desired transition probability.

The second simplification allows us to make the substitution

$\Delta E=E_{2}(t)-E_{1}(t)\equiv \alpha t$,

where $\scriptstyle {E_{1}(t)}$ and $\scriptstyle {E_{2}(t)}$ are the energies of the two states at time $\scriptstyle {t}$, given by the diagonal elements of the Hamiltonian matrix, and $\scriptstyle {\alpha }$ is a constant. For the case of an atom in a magnetic field this corresponds to a linear change in magnetic field. For a linear Zeeman shift this follows directly from point 1.

The final simplification requires that the time–dependent perturbation does not couple the diabatic states; rather, the coupling must be due to a static deviation from a $\scriptstyle {1/r}$ coulomb potential, commonly described by a quantum defect.

The Landau-Zener formula

The details of Zener’s solution are somewhat opaque, relying on a set of substitutions to put the equation of motion into the form of the Weber equation and using the known solution. A more transparent solution is provided by Wittig using contour integration.

The key figure of merit in this approach is the Landau-Zener velocity:

$v_{LZ}={{\frac {\partial }{\partial t}}|E_{2}-E_{1}| \over {\frac {\partial }{\partial q}}|E_{2}-E_{1}|}\approx {\frac {dq}{dt}}$,

where $\scriptstyle {q}$ is the perturbation variable (electric or magnetic field, molecular bond-length, or any other perturbation to the system), and $\scriptstyle {E_{1}}$ and $\scriptstyle {E_{2}}$ are the energies of the two diabatic (crossing) states. A large $\scriptstyle {v_{LZ}}$ results in a large diabatic transition probability and vice versa.

Using the Landau-Zener formula the probability, $\scriptstyle {P_{D}}$, of a diabatic transition is given by

${\begin{aligned}P_{D}&=e^{-2\pi \Gamma }\\\Gamma &={a^{2}/\hbar \over \left|{\frac {\partial }{\partial t}}(E_{2}-E_{1})\right|}={a^{2}/\hbar \over \left|{\frac {dq}{dt}}{\frac {\partial }{\partial q}}(E_{2}-E_{1})\right|}\\&={a^{2} \over \hbar |\alpha |}\\\end{aligned}}$

See also

• Adiabatic theorem