On the other hand, kF may be obtained in multiple ways. That is from the above-mentioned periodicity by diving π by the oscillation period. Another approach is from the ring radius in STM picture reciprocal space. Petersen and colleagues have also shown that Fourier-transform scanning tunneling microscopy may be used to study Fermi contour , and therefore to obtain kF values (Fig. ). Some studies also mention Fermi wavelength λF , a parameter that numerically equals two oscillation periods.