Real-space charge/spin current density

In the step 3, real-space charge/spin current density is calculated optionally. The relevant keyword for the calculation is as follows:

NEGF.tran.CurrentDensity   on        #  default on

• NEGF.tran.CurrentDensity

  If NEGF.tran.CurrentDensity is set to on, the real-space current density is calculated.

In the calculation of the current density, openmx makes the following standard output:

Start Calculation of the currentdensity

  Spin #0
    Sum of current in real space [a.u.] 
      Left(ideal) :    -9.10585e-06
      Right(ideal):    -9.10583e-06
      Left(truncated ):    -8.66971e-06
      Right(truncated):    -8.69926e-06
  Spin #1
    Sum of current in real space [a.u.] 
      Left(ideal) :    -4.54540e-08
      Right(ideal):    -4.54544e-08
      Left(truncated ):    -4.19469e-08
      Right(truncated):    -4.27460e-08

Output: Currentdensity
  Charge-current density along a-axis: ./negf-8zgnr-0.3.curden1.cube
  Spin-current density along a-axis: ./negf-8zgnr-0.3.scurden1.cube
  Charge-current density: ./negf-8zgnr-0.3.curden.xsf
  Spin-current density: ./negf-8zgnr-0.3.scurden.xsf
  Voronoi Charge-current density: ./negf-8zgnr-0.3.curden_atom.xsf
  Voronoi Spin-current density: ./negf-8zgnr-0.3.scurden_atom.xsf

In this case, 6 files:
negf-8zgnr-0.3.curden.xsf, negf-8zgnr-0.3.scurden.xsf,
negf-8zgnr-0.3.curden1.cube, negf-8zgnr-0.3.scurden1.cube,
negf-8zgnr-0.3.curden_atom.xsf, negf-8zgnr-0.3.scurden_atom.xsf,
are generated. These files contain the following quantities:

• System.Name.curden.xsf, System.Name.scurden.xsf

  These files contain the charge and the spin current densities, respectively, on the real space grid. They can be visualized using 'Display$\to$Forces' in XCrySDen.

• System.Name.curden1.cube, System.Name.scurden1.cube

  These files contain the a-component of the charge and the spin current densities, respectively, on real space grid. They can be visualized using VESTA, XCrySDen, and so on.

• System.Name.curden_atom.xsf, System.Name.scurden_atom.xsf

  These files contain the charge and the spin current density respectively averaged in Voronoi polyhedra of each atom. They can be visualized using 'Display$\to$Forces' in XCrySDen.

(Experimentally) When you set

    NEGF.OffDiagonalCurrent   on        #  default off

in an input file for the calculation with the non-collinear magnetism, the following files are outputted.

• System.Name.odcurden_r.xsf, System.Name.odcurden_i.xsf
• System.Name.odcurden1_r.cube, System.Name.odcurden1_i.cube
• System.Name.odcurden_atom_r.xsf, System.Name.odcurden_atom_i.xsf

These files contain the spin off-diagonal component of the current density. Since this quantity becomes generally a complex number, the real part and the imaginary part of that is output separately. As an example, we show in Fig. 42 the currentdensity in the 8-zigzag graphene nanoribbon with an antiferromagnetic junction under a finite bias voltage of 0.3 V. In the vicinity of boundaries, that shows an unphysical behavior because the basis set is not treated correctly in these regions. If you want to calculate more precisely the current density in these regions, please make a supercell larger.

Figure 42: (a) The current density in the 8-zigzag graphene nanoribbon with an antiferromagnetic junction under a finite bias voltage of 0.3 V. (b) Isosurfaces of the a-compontent of that.