## Materials

The refractive properties of a homogeneous Material can be specified through two different functions:

• RefractiveMaterial, based on the refractive index,
• MaterialBySLD, based on the scattering length density (SLD).

RefractiveMaterial is equally suitable for X-rays or neutrons. However, it does not account for the wave-length dependence of the refractive index. This leads to incorrect results if there is too much spread in the incoming wavelength, as is regularly the case in neutron time-of-flight experiments.

MaterialBySLD is intended for neutron experiments with significant wavelength spread. Refractive indices as function of wavelength are computed internally from constant SLDs.

### Material by refractive index

A Material can be created through

magnetization = ba.kvector_t(1.0, 0.0, 0.0)
<material> = ba.RefractiveMaterial("name", delta, beta, magnetization)

where name is the arbitrary name of the material associated with its complex refractive index $n = 1 - \delta + i\beta$. The magnetization argument is a 3D vector (in A/m). The return value <material> is later used when referring to this particular material.

magnetization can be omitted in material construction. It is assumed to be zero then:

<material> = ba.RefractiveMaterial("name", delta, beta)

### Material by scattering length density

A Material can also be created through

<material> = ba.MaterialBySLD("name", sld_real, sld_imag, magnetization)

or, omitting magnetization again,

<material> = ba.MaterialBySLD("name", sld_real, sld_imag)

Here sld_real and sld_imag are the real and imaginary parts of the material scattering length density (SLD) according to the following convention:

$$SLD = sld_{real} - i \cdot sld_{imag}$$

SLD values for a wide variety of materials can be calculated with numerous online SLD-calculators, e.g. these ones:

The first of these returns values in inverse square angstroms, which are also the input units for MaterialBySLD. Thus the SLD values found with the calculator can be directly copied and pasted into a BornAgain script.

### Default = Vacuum

Both RefractiveMaterial and MaterialBySLD can be created with empty constructors:

<material> = ba.RefractiveMaterial()
<material2> = ba.MaterialBySLD()

In this case the “default” material is created, i.e. a material with the name vacuum, zero SLD/refractive index and zero magnetization.