624 research outputs found
The signature of subsurface Kondo impurities in the local tunnel current
The conductance of a tunnel point-contact in an STM-like geometry having a
single defect placed below the surface is investigated theoretically. The
effect of multiple electron scattering by the defect after reflections by the
metal surface is taken into account. In the approximation of s-wave scattering
the dependence of the conductance 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.Comment: 13 pages, 4 figures. To be published in J. Phys.: Cond. Ma
Magneto-quantum oscillations of the conductance of a tunnel point-contact in the presence of a single defect
The influence of a quantizing magnetic field to the conductance of a
tunnel point contact in the presence of the single defect has been considered.
We demonstrate that the conductance exhibits specific magneto-quantum
oscillations, the amplitude and period of which depend on the distance between
the contact and the defect. We show that a non-monotonic dependence of the
point-contact conductance results from a superposition of two types of
oscillations: A short period oscillation arising from electron focusing by the
field and a long period oscillation of Aharonov-Bohm-type originated from
the magnetic flux passing through the closed trajectories of electrons moving
from the contact to the defect and returning back to the contact.Comment: 13 pages, 3 figure
Method to determine defect positions below a metal surface by STM
The oscillatory voltage dependence of the conductance of a quantum point
contact in the presence of a single point-like defect has been analyzed
theoretically. Such signals are detectable and may be exploited to obtain
information on defect positions below a metal surface. Both tunnel junctions
and ballistic contacts of adiabatic shape have been considered. The effect of
quantum interference has been taking into account between the principal wave
that is directly transmitted through the contact and the partial wave that is
scattered by the contact and the defect. This effect leads to oscillations of
the conductance as a function of applied voltage. We obtain the dependence of
the period and amplitude of the conductance oscillations on the position of the
defect inside the metal.Comment: 16 pages, 7 figure
Signature of Fermi surface anisotropy in point contact conductance in the presence of defects
In a previous paper (Avotina et al.,Phys. Rev. B Vol.71, 115430 (2005)) we
have shown that in principle it is possible to image the defect positions below
a metal surface by means of a scanning tunnelling microscope. The principle
relies on the interference of electron waves scattered on the defects, which
give rise to small but measurable conductance fluctuations. Whereas in that
work the band structure was assumed to be free-electron like, here we
investigate the effects of Fermi surface anisotropy. We demonstrate that the
amplitude and period of the conductance oscillations are determined by the
local geometry of the Fermi surface. The signal results from those points for
which the electron velocity is directed along the vector connecting the point
contact to the defect. For a general Fermi surface geometry the position of the
maximum amplitude of the conductance oscillations is not found for the tip
directly above the defect. We have determined optimal conditions for
determination of defect positions in metals with closed and open Fermi
surfaces.Comment: 23 pages, 8 figure
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
Voltage dependent conductance and shot noise in quantum microconstriction with single defects
The influence of the interference of electron waves, which are scattered by
single impurities and by a barrier on nonlinear conductance and shot noise of
metallic microconstriction is studied theoretically. It is shown that the these
characteristics are nonmonotonic functions on the applied bias.Comment: 18 pages,5 figure
Conductance of a tunnel point-contact of noble metals in the presence of a single defect
In paper [1] (Avotina et al. Phys. Rev. B,74, 085411 (2006)) the effect of
Fermi surface anisotropy to the conductance of a tunnel point contact, in the
vicinity of which a single point-like defect is situated, has been investigated
theoretically. The oscillatory dependence of the conductance on the distance
between the contact and the defect has been found for a general Fermi surface
geometry. In this paper we apply the method developed in [1] to the calculation
of the conductance of noble metal contacts. An original algorithm, which
enables the computation of the conductance for any parametrically given Fermi
surface, is proposed. On this basis a pattern of the conductance oscillations,
which can be observed by the method of scanning tunneling microscopy, is
obtained for different orientations of the surface for the noble metals.Comment: 8 pages, 5 figure
Influence of a single defect on the conductance of a tunnel point contact between a normal metal and a superconductor
We have investigated theoretically the conductance of a Normal-Superconductor
point-contact in the tunnel limit and analyzed the quantum interference effects
originating from the scattering of quasiparticles by point-like defects.
Analytical expressions for the oscillatory dependence of the conductance on the
position of the defect are obtained for the defect situated either in the
normal metal, or in the superconductor. It is found that the amplitude of
oscillations significantly increases when the applied bias approaches the gap
energy of the superconductor. The spatial distribution of the order parameter
near the surface in the presence of a defect is also obtained.Comment: 13 pages, 4 figure
Theory of oscillations in the STM conductance resulting from subsurface defects (Review Article)
In this review we present recent theoretical results concerning
investigations of single subsurface defects by means of a scanning tunneling
microscope (STM). These investigations are based on the effect of quantum
interference between the electron partial waves that are directly transmitted
through the contact and the partial waves scattered by the defect. In
particular, we have shown the possibility imaging the defect position below a
metal surface by means of STM. Different types of subsurface defects have been
discussed: point-like magnetic and non-magnetic defects, magnetic clusters in a
nonmagnetic host metal, and non-magnetic defects in a s-wave superconductor.
The effect of Fermi surface anisotropy has been analyzed. Also, results of
investigations of the effect of a strong magnetic field to the STM conductance
of a tunnel point contact in the presence of a single defect has been
presented.Comment: 31 pages, 10 figuers Submitted to Low. Temp. Phy
Growth of beryllium oxide nano-structures during thermal treatment of neutron irradiated beryllium
Beryllium oxide nano-structures grown during high temperature oxidation of neutron irradiated beryllium has been investigated. Oxidation of non - irradiated and neutron irradiated beryllium was performed in differential thermal analyzer in an atmosphere of air and its mixture with helium at temperatures up to 1050°C. Structure of beryllium and its oxide was studied by the means of scanning electron microscopy. The growth of beryllium oxide nano-structures – “labyrinth type” layers and nano-rods on the inner surfaces of neutron irradiated beryllium were observed by means of Scanning Electron Microscopy. The size of the rods was 10-100 nm in diameter and up to few micrometers in length. There were no BeO nano-rods found on the surfaces of non-irradiated pebbles after similar treatment. Therefore, it might be concluded that neutron induced irradiation damages, such as dislocation loops, are responsible for the growth of BeO nano-structures.DOI: http://dx.doi.org/10.5755/j01.ms.21.2.6822</p
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