where A is the area of the aforementioned surface-state pocket. Fe films on W(001) 102 have also produced an anisotropic surface response. They showed anisotropic FOs that decay slower than r -3 , with a period of approx. 1 nm, in accordance with previously developed 107 theoretical model. The model treats Friedel oscillations by means of the stationary phase approximation in the long-range regime. Therefore, the charge density correction with respect to the position R + r and energy E is equal to 102,107 Five years after the initial STM observations of FOs by Crommie et al. , Peterson et al. have conducted a similar study on Cu(111) and herringbone reconstructed Au(111) surface. At low temperatures and low bias voltage, they have observed an additional pattern in the FT-STM power spectrum around a point defect. In 2009, Schouteden et al. has also made a similar observation. Both studies have attributed the second pattern to the bulk Fermi surface. Almost 20 years later, after the first report by Peterson et al., this statement has been debated by Sessi and coworkers . Similarly to most experimental STM papers discussed in this review, they have focused on Cu(111), Ag(111), Au(111) surfaces, known to exhibit an almost 2D nearly-free electron gas quantum structure at the surface . Here, the authors have assigned the second ring to the surface states , based on photoemission data and ab initio calculations . Notably, the two rings they have observed merge into one at bias energies close to the Fermi level. Ishioka et al. have made a rather interesting observation of triangular FOs on the surface of 1T-TiSe2. Unlike the previously proposed purely geometrical explanation 109 , the authors believe that the answer lies within the collective electronic state of the surface. This study also presents a rather rare example of chiral Friedel oscillations, meaning an uneven charge oscillation in the directions of the FOs' propagation.