Theory of oscillations in the STM conductance resulting from subsurface defects

· A nonlinear conductivity of a tunnel point contact in the presence of a single defect is investigated. It is shown that the conductance of the contact is an oscillatory function of an applied bias eV and a distance between the contact and the defect. A new non-destructive method of a defectoscopy of metals, which allows finding the defect position below metal surface by the STM method, is grounded theoretically [1].

· An influence of a Fermi surface anisotropy on the conductance oscillations in the presence of the single defect is analyzed. It is shown that the amplitude and period of the conductance oscillations are determined by the local geometry of the Fermi surface. Optimal conditions for determination of defects position in metals with closed and open Fermi surfaces are determined [2, 4].


· An influence of the quantized magnetic field on the conductance of point tunnel contact is studied. A new type of magneto-quantum oscillations of the conductance, the amplitude and period of which depend on the distance between the contact and the defect is predicted. It was shown that the non-monotonic dependence of the conductance on the magnetic field is the superposition of long-period and short-period oscillations, which have a different physical nature [3].

· In the approximation of s-wave scattering the dependence of the conductance of the tunnel point contact on the applied voltage and the position of the defect is obtained. The results are illustrated for a model s-wave phase shift describing Kondo resonance scattering. We demonstrate that multiple electron scattering by the magnetic impurity plays a decisive role in the point contact conductance at voltages near the Kondo resonance. We find that the sign and shape of the Kondo anomaly depends on the position of the defect [5].

· We have investigated the conductance of a normal-superconductor point contact in the tunnel limit and analyzed the quantum interference effects originating from the scattering of quasi-particles by point-like defects. It is found that the amplitude of conductance oscillations increases significantly when the applied bias eV approaches the gap energy of the superconductor Δ. The spatial distribution of the order parameter near the surface in the presence of a defect is also obtained [6].

· The influence of a single magnetic cluster in a nonmagnetic host metal on the spin current j_s and the charge current j in the vicinity of a ferromagnetic STM tip is studied. We show that quantum interference between the partial waves injected from the STM tip and those scattered by the cluster results in the appearance of components perpendicular to the initial polarization of the spin current j_s, which obtain a strongly inhomogeneous spatial distribution (see Fig.). We predict a strong magneto-orientational effect; the conductance oscillations may grow, shrink, or even vanish for rotation of the cluster magnetic moment by an external magnetic field [7].

1. Avotina Ye.S., Kolesnichenko Yu.A., Omelyanchouk A.N., Otte A.F.,  van Ruitenbeek J.M. Method to determine defect positions below a metal surface by STM // Phys. Rev. B, 71, No.11, 115430 (2005).
2. Avotina Ye.S., Kolesnichenko Yu.A., Otte A.F.,  van Ruitenbeek J.M. Signature of Fermi surface anisotropy in point contact conductance in the presence of defects // Phys. Rev. B, 74, No.8, 085411 (2006).  
3. Avotina Ye.S., Kolesnichenko Yu.A., Otte A.F., van Ruitenbeek J.M. Magneto-quantum oscillations of the conductance of a tunnel point-contact in the presence of a single defect // Phys. Rev. B, 75, No.12, 125411 (2007).
4. Avotina Ye.S., Kolesnichenko Yu.A., Roobol S.B., and van Ruitenbeek J.M. Conductance of a tunnel point contact of noble metals in the presence of a single defect // Low. Temp. Phys., 34, No.3, 207 (2008).
5. Avotina Ye.S., Kolesnichenko Yu.A., van Ruitenbeek J.M. The signature of subsurface Kondo impurities in the local tunnel current // J.Phys.: Cond. Mat., 20, No. 11, 115208 (2008).
6. Avotina Ye.S., Kolesnichenko Yu.A., van Ruitenbeek J.M. Influence of a single defect on the conductance of a tunnel point contact between a normal metal and a superconductor // Low. Temp. Phys., 34, No. 11, 936 (2008).
7. Avotina Ye.S., Kolesnichenko Yu.A., van Ruitenbeek J.M. Magneto-orientation and quantum size effects in spin-polarized STM conductance in the presence of a subsurface magnetic cluster // Phys. Rev. B, 80, No. 11, 115333 (2009)