Computing local quantities: densities and currents ================================================== In the previous tutorials we have mainly concentrated on calculating *global* properties such as conductance and band structures. Often, however, insight can be gained from calculating *locally-defined* quantities, that is, quantities defined over individual sites or hoppings in your system. In the :ref:`closed-systems` tutorial we saw how we could visualize the density associated with the eigenstates of a system using `kwant.plotter.map`. In this tutorial we will see how we can calculate more general quantities than simple densities by studying spin transport in a system with a magnetic texture. .. seealso:: The complete source code of this example can be found in :download:`magnetic_texture.py ` Introduction ------------ Our starting point will be the following spinful tight-binding model on a square lattice: .. math:: H = - \sum_{⟨ij⟩}\sum_{α} |iα⟩⟨jα| + J \sum_{i}\sum_{αβ} \mathbf{m}_i⋅ \mathbf{σ}_{αβ} |iα⟩⟨iβ|, where latin indices run over sites, and greek indices run over spin. We can identify the first term as a nearest-neighbor hopping between like-spins, and the second as a term that couples spins on the same site. The second term acts like a magnetic field of strength :math:`J` that varies from site to site and that, on site :math:`i`, points in the direction of the unit vector :math:`\mathbf{m}_i`. :math:`\mathbf{σ}_{αβ}` is a vector of Pauli matrices. We shall take the following form for :math:`\mathbf{m}_i`: .. math:: \mathbf{m}_i &=\ \left( \frac{x_i}{x_i^2 + y_i^2} \sin θ_i,\ \frac{y_i}{x_i^2 + y_i^2} \sin θ_i,\ \cos θ_i \right)^T, \\ θ_i &=\ \frac{π}{2} (\tanh \frac{r_i - r_0}{δ} - 1), where :math:`x_i` and :math:`y_i` are the :math:`x` and :math:`y` coordinates of site :math:`i`, and :math:`r_i = \sqrt{x_i^2 + y_i^2}`. To define this model in Kwant we start as usual by defining functions that depend on the model parameters: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_model :end-before: #HIDDEN_END_model and define our system as a square shape on a square lattice with two orbitals per site, with leads attached on the left and right: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_syst :end-before: #HIDDEN_END_syst Below is a plot of a projection of :math:`\mathbf{m}_i` onto the x-y plane inside the scattering region. The z component is shown by the color scale: .. image:: /code/figure/mag_field_direction.* We will now be interested in analyzing the form of the scattering states that originate from the left lead: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_wavefunction :end-before: #HIDDEN_END_wavefunction Local densities --------------- If we were simulating a spinless system with only a single degree of freedom, then calculating the density on each site would be as simple as calculating the absolute square of the wavefunction like:: density = np.abs(psi)**2 When there are multiple degrees of freedom per site, however, one has to be more careful. In the present case with two (spin) degrees of freedom per site one could calculate the per-site density like: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_ldos :end-before: #HIDDEN_END_ldos With more than one degree of freedom per site we have more freedom as to what local quantities we can meaningfully compute. For example, we may wish to calculate the local z-projected spin density. We could calculate this in the following way: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_lsdz :end-before: #HIDDEN_END_lsdz If we wanted instead to calculate the local y-projected spin density, we would need to use an even more complicated expression: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_lsdy :end-before: #HIDDEN_END_lsdy The `kwant.operator` module aims to alleviate somewhat this tedious book-keeping by providing a simple interface for defining operators that act on wavefunctions. To calculate the above quantities we would use the `~kwant.operator.Density` operator like so: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_lden :end-before: #HIDDEN_END_lden `~kwant.operator.Density` takes a `~kwant.system.System` as its first parameter as well as (optionally) a square matrix that defines the quantity that you wish to calculate per site. When an instance of a `~kwant.operator.Density` is then evaluated with a wavefunction, the quantity .. math:: ρ_i = \mathbf{ψ}^†_i \mathbf{M} \mathbf{ψ}_i is calculated for each site :math:`i`, where :math:`\mathbf{ψ}_{i}` is a vector consisting of the wavefunction components on that site and :math:`\mathbf{M}` is the square matrix referred to previously. Below we can see colorplots of the above-calculated quantities. The array that is returned by evaluating a `~kwant.operator.Density` can be used directly with `kwant.plotter.map`: .. image:: /code/figure/spin_densities.* .. specialnote:: Technical Details Although we refer loosely to "densities" and "operators" above, a `~kwant.operator.Density` actually represents a *collection* of linear operators. This can be made clear by rewriting the above definition of :math:`ρ_i` in the following way: .. math:: ρ_i = \sum_{αβ} ψ^*_{α} \mathcal{M}_{iαβ} ψ_{β} where greek indices run over the degrees of freedom in the Hilbert space of the scattering region and latin indices run over sites. We can this identify :math:`\mathcal{M}_{iαβ}` as the components of a rank-3 tensor and can represent them as a "vector of matrices": .. math:: \mathcal{M} = \left[ \left(\begin{matrix} \mathbf{M} & 0 & … \\ 0 & 0 & … \\ ⋮ & ⋮ & ⋱ \end{matrix}\right) ,\ \left(\begin{matrix} 0 & 0 & … \\ 0 & \mathbf{M} & … \\ ⋮ & ⋮ & ⋱ \end{matrix}\right) , … \right] where :math:`\mathbf{M}` is defined as in the main text, and the :math:`0` are zero matrices of the same shape as :math:`\mathbf{M}`. Local currents -------------- `kwant.operator` also has a class `~kwant.operator.Current` for calculating local currents, analogously to the local "densities" described above. If one has defined a density via a matrix :math:`\mathbf{M}` and the above equation, then one can define a local current flowing from site :math:`b` to site :math:`a`: .. math:: J_{ab} = i \left( \mathbf{ψ}^†_b (\mathbf{H}_{ab})^† \mathbf{M} \mathbf{ψ}_a - \mathbf{ψ}^†_a \mathbf{M} \mathbf{H}_{ab} \mathbf{ψ}_b \right), where :math:`\mathbf{H}_{ab}` is the hopping matrix from site :math:`b` to site :math:`a`. For example, to calculate the local current and spin current: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_current :end-before: #HIDDEN_END_current Evaluating a `~kwant.operator.Current` operator on a wavefunction returns a 1D array of values that can be directly used with `kwant.plotter.current`: .. image:: /code/figure/spin_currents.* .. note:: Evaluating a `~kwant.operator.Current` operator on a wavefunction returns a 1D array of the same length as the number of hoppings in the system, ordered in the same way as the edges in the system's graph. .. specialnote:: Technical Details Similarly to how we saw in the previous section that `~kwant.operator.Density` can be thought of as a collection of operators, `~kwant.operator.Current` can be defined in a similar way. Starting from the definition of a "density": .. math:: ρ_a = \sum_{αβ} ψ^*_{α} \mathcal{M}_{aαβ} ψ_{β}, we can define *currents* :math:`J_{ab}` via the continuity equation: .. math:: \frac{∂ρ_a}{∂t} - \sum_{b} J_{ab} = 0 where the sum runs over sites :math:`b` neigboring site :math:`a`. Plugging in the definition for :math:`ρ_a`, along with the Schrödinger equation and the assumption that :math:`\mathcal{M}` is time independent, gives: .. math:: J_{ab} = \sum_{αβ} ψ^*_α \left(i \sum_{γ} \mathcal{H}^*_{abγα} \mathcal{M}_{aγβ} - \mathcal{M}_{aαγ} \mathcal{H}_{abγβ} \right) ψ_β, where latin indices run over sites and greek indices run over the Hilbert space degrees of freedom, and .. math:: \mathcal{H}_{ab} = \left(\begin{matrix} ⋱ & ⋮ & ⋮ & ⋮ & ⋰ \\ ⋯ & ⋱ & 0 & \mathbf{H}_{ab} & ⋯ \\ ⋯ & 0 & ⋱ & 0 & ⋯ \\ ⋯ & 0 & 0 & ⋱ & ⋯ \\ ⋰ & ⋮ & ⋮ & ⋮ & ⋱ \end{matrix}\right). i.e. :math:`\mathcal{H}_{ab}` is a matrix that is zero everywhere except on elements connecting *from* site :math:`b` *to* site :math:`a`, where it is equal to the hopping matrix :math:`\mathbf{H}_{ab}` between these two sites. This allows us to identify the rank-4 quantity .. math:: \mathcal{J}_{abαβ} = i \sum_{γ} \mathcal{H}^*_{abγα} \mathcal{M}_{aγβ} - \mathcal{M}_{aαγ} \mathcal{H}_{abγβ} as the local current between connected sites. The diagonal part of this quantity, :math:`\mathcal{J}_{aa}`, represents the extent to which the density defined by :math:`\mathcal{M}_a` is not conserved on site :math:`a`. It can be calculated using `~kwant.operator.Source`, rather than `~kwant.operator.Current`, which only computes the off-diagonal part. Spatially varying operators --------------------------- The above examples are reasonably simple in the sense that the book-keeping required to manually calculate the various densities and currents is still manageable. Now we shall look at the case where we wish to calculate some projected spin currents, but where the spin projection axis varies from place to place. More specifically, we want to visualize the spin current along the direction of :math:`\mathbf{m}_i`, which changes continuously over the whole scattering region. Doing this is as simple as passing a *function* when instantiating the `~kwant.operator.Current`, instead of a constant matrix: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_following :end-before: #HIDDEN_END_following The function must take a `~kwant.builder.Site` as its first parameter, and may optionally take other parameters (i.e. it must have the same signature as a Hamiltonian onsite function), and must return the square matrix that defines the operator we wish to calculate. .. note:: In the above example we had to pass the extra parameters needed by the ``following_operator`` function via the ``param`` keyword argument. In general you must pass all the parameters needed by the Hamiltonian via ``params`` (as you would when calling `~kwant.solvers.default.smatrix` or `~kwant.solvers.default.wave_function`). In the previous examples, however, we used the fact that the system hoppings do not depend on any parameters (these are the only Hamiltonian elements required to calculate currents) to avoid passing the system parameters for the sake of brevity. Using this we can see that the spin current is essentially oriented along the direction of :math:`m_i` in the present regime where the onsite term in the Hamiltonian is dominant: .. image:: /code/figure/spin_current_comparison.* .. note:: Although this example used exclusively `~kwant.operator.Current`, you can do the same with `~kwant.operator.Density`. Defining operators over parts of a system ----------------------------------------- Another useful feature of `kwant.operator` is the ability to calculate operators over selected parts of a system. For example, we may wish to calculate the total density of states in a certain part of the system, or the current flowing through a cut in the system. We can do this selection when creating the operator by using the keyword parameter ``where``. Density of states in a circle ***************************** To calculate the density of states inside a circle of radius 20 we can simply do: .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_density_cut :end-before: #HIDDEN_END_density_cut .. literalinclude:: /code/figure/circle_dos.txt note that we also provide ``sum=True``, which means that evaluating the operator on a wavefunction will produce a single scalar. This is semantically equivalent to providing ``sum=False`` (the default) and running ``numpy.sum`` on the output. Current flowing through a cut ***************************** Below we calculate the probability current and z-projected spin current near the interfaces with the left and right leads. .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_current_cut :end-before: #HIDDEN_END_current_cut .. literalinclude:: /code/figure/current_cut.txt We see that the probability current is conserved across the scattering region, but the z-projected spin current is not due to the fact that the Hamiltonian does not commute with :math:`σ_z` everywhere in the scattering region. .. note:: ``where`` can also be provided as a sequence of `~kwant.builder.Site` or a sequence of hoppings (i.e. pairs of `~kwant.builder.Site`), rather than a function. Advanced Topics --------------- Using ``bind`` for speed ************************ In most of the above examples we only used each operator *once* after creating it. Often one will want to evaluate an operator with many different wavefunctions, for example with all scattering wavefunctions at a certain energy, but with the *same set of parameters*. In such cases it is best to tell the operator to pre-compute the onsite matrices and any necessary Hamiltonian elements using the given set of parameters, so that this work is not duplicated every time the operator is evaluated. This can be achieved with `~kwant.operator.Current.bind`: .. warning:: Take care that you do not use an operator that was bound to a particular set of parameters with wavefunctions calculated with a *different* set of parameters. This will almost certainly give incorrect results. .. literalinclude:: /code/include/magnetic_texture.py :start-after: #HIDDEN_BEGIN_bind :end-before: #HIDDEN_END_bind .. image:: /code/figure/bound_current.*