Transport Calculations with wannier90
¶
By setting transport = TRUE
, wannier90
will calculate
the quantum conductance and density of states of a one-dimensional
system. The results will be written to files seedname_qc.dat
and
seedname_dos.dat
, respectively.
The system for which transport properties are calculated is determined
by the keyword transport_mode
.
transport_mode = bulk
¶
Quantum conductance and density of states are calculated for a perfectly
periodic one-dimensional conductor. If
tran_read_ht = FALSE
the transport properties are
calculated using the Hamiltonian in the Wannier function basis of the
system found by wannier90
. Setting tran_read_ht = TRUE
allows the user to provide an external Hamiltonian matrix file
seedname_htB.dat
, from which the properties are found. See
Section Post-Processing
for more details of the keywords required for such calculations.
transport_mode = lcr
¶
Quantum conductance and density of states are calculated for a system where semi-infinite, left and right leads are connected through a central conductor region. This is known as the lcr system. Details of the method is described in Ref. 1.
In wannier90
two options exist for performing such calculations:
-
If
tran_read_ht = TRUE
the external Hamiltonian filesseedname_htL.dat, seedname_htLC.dat, seedname_htC.dat, seedname_htCR.dat, seedname_htR.dat
are read and used to compute the transport properties. -
If
tran_read_ht = FALSE
, then the transport calculation is performed automatically using the Wannier functions as a basis and the 2c2 geometry described in Section Automated lcr Transport Calculations: The 2c2 Geometry.
Automated lcr Transport Calculations: The 2c2 Geometry¶
Calculations using the 2c2 geometry provide a method to calculate the
transport properties of an lcr system from a single
wannier90
calculation. The Hamiltonian matrices which the five
external files provide in the tran_read_ht = TRUE
case are
instead built from the Wannier function basis directly. As such, strict
rules apply to the system geometry, which is shown in
Figure below. These
rules are as follows:
-
Left and right leads must be identical and periodic.
-
Supercell must contain two principal layers (PLs) of lead on the left, a central conductor region and two principal layers of lead on the right.
-
The conductor region must contain enough lead such that the disorder does not affect the principal layers of lead either side.
-
A single k-point (Gamma) must be used.
In order to build the Hamiltonians, Wannier functions are first sorted according to position and then type if a number of Wannier functions exist with a similar centre (eg. d-orbital type Wannier functions centred on a Cu atom). Next, consistent parities of Wannier function are enforced. To distingiush between different types of Wannier function and assertain relative parities, a signature of each Wannier function is computed. The signature is formed of 20 integrals which have different spatial dependence. They are given by:
where \(V\) is the volume of the cell, \(w(\mathbf{r})\) is the Wannier function and \(g(\mathbf{r})\) are the set of functions:
upto third order in powers of sines. Here, the supercell has dimension \((L_x,L_y,L_z)\) and the Wannier function has centre \(\mathbf{r}_c=(x_c,y_c,z_c)\). Each of these integrals may be written as linear combinations of the following sums:
where \(n\) and \(m\) are the Wannier function and band indexes,
\(\mathbf{G}\) is a G-vector, \(U_{mn}\) is the unitary matrix that
transforms from the Bloch representation of the system to the
maximally-localised Wannier function basis and
\(\tilde{u}_{m\Gamma}^{*}(\mathbf{G})\) are the conjugates of the Fourier
transforms of the periodic parts of the Bloch states at the \(\Gamma\!\)
-point. The complete set of \(\tilde{u}_{m\mathbf{k}}(\mathbf{G})\) are
often outputted by plane-wave DFT codes. However, to calculate the 20
signature integrals, only 32 specific
\(\tilde{u}_{m\mathbf{k}}(\mathbf{G})\) are required. These are found in
an additional file (seedname.unkg
) that should be provided by the
interface between the DFT code and wannier90
. A detailed description
of this file may be found in
Section seedname.unkg
.
Additionally, the following keywords are also required in the input file:
-
tran_num_ll
: The number of Wannier functions in a principal layer. -
tran_num_cell_ll
: The number of unit cells in one principal layer of lead
A further parameter related to these calculations is
tran_group_threshold
.
Tutorial of how 2c2 calculations are preformed
can be found in the wannier90
Tutorial.
-
Marco Buongiorno Nardelli. Electronic transport in extended systems: application to carbon nanotubes. Phys. Rev. B, 60:7828, 1999. ↩