80 research outputs found
Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects
We study effect of the adiabatic electron renormalization on the parameters
of the dynamical defects in the ballistic metallic point contact. The upper
energy states of the ``dressed'' defect are shown to give a smaller
contribution to a resistance of the contact than the lower energy ones. This
holds both for the "classical" renormalization related to defect coupling with
average local electron density and for the "mesoscopic" renormalization caused
by the mesoscopic fluctuations of electronic density the dynamical defects are
coupled with. In the case of mesoscopic renormalization one may treat the
dynamical defect as coupled with Friedel oscillations originated by the other
defects, both static and mobile. Such coupling lifts the energy degeneracy of
the states of the dynamical defects giving different mesoscopic contribution to
resistance, and provides a new model for the fluctuator as for the object
originated by the electronic mesoscopic disorder rather than by the structural
one. The correlation between the defect energy and the defect contribution to
the resistance leads to zero-temperature and zero-bias anomalies of the point
contact resistance.
A comparison of these anomalies with those predicted by the Two Channel Kondo
Model (TCKM) is made. It is shown, that although the proposed model is based on
a completely different from TCKM physical background, it leads to a zero-bias
anomalies of the point contact resistance, which are qualitatively similar to
TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.
Point contact spectroscopy of hopping transport: effects of a magnetic field
The conductance of a point contact between two hopping insulators is expected
to be dominated by the individual localized states in its vicinity. Here we
study the additional effects due to an external magnetic field. Combined with
the measured conductance, the measured magnetoresistance provides detailed
information on these states (e.g. their localization length, the energy
difference and the hopping distance between them). We also calculate the
statistics of this magnetoresistance, which can be collected by changing the
gate voltage in a single device. Since the conductance is dominated by the
quantum interference of particular mesoscopic structures near the point
contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield
information on the geometry of these structures. These oscillations also depend
on local spin accumulation and correlations, which can be modified by the
external field. Finally, we also estimate the mesoscopic Hall voltage due to
these structures.Comment: 7 pages, 5 figur
Slow relaxation of conductance of amorphous hopping insulators
We discuss memory effects in the conductance of hopping insulators due to
slow rearrangements of structural defects leading to formation of polarons
close to the electron hopping states. An abrupt change in the gate voltage and
corresponding shift of the chemical potential change populations of the hopping
sites, which then slowly relax due to rearrangements of structural defects. As
a result, the density of hopping states becomes time dependent on a scale
relevant to rearrangement of the structural defects leading to the excess time
dependent conductivity.Comment: 6 pages, 1 figur
Resistivity and 1/f Noise in Non-Metallic Phase Separated Manganites
A simple model is proposed to calculate resistivity, magnetoresistance, and
noise spectrum in non-metallic phase-separated manganites containing small
metallic droplets (magnetic polarons). The system is taken to be far from the
percolation transition into a metallic state. It is assumed that the charge
transfer occurs due to electron tunneling from one droplet to another through
the insulating medium. As a result of this tunneling, the droplets acquire or
lose extra electrons forming metastable two-electron and empty states. In the
framework of this model, explicit expressions for dc conductivity and noise
power of the system are derived. It is shown that the noise spectrum has 1/f
form in the low-frequency range.Comment: 6 pages, 1 fugure include
Transport properties and point contact spectra of Ni_xNb_{1-x} metallic glasses
Bulk resistivity and point contact spectra of Ni_xNb_{1-x} metallic glasses
have been investigated as functions of temperature (0.3-300K) and magnetic
field (0-12T). Metallic glasses in this family undergo a superconducting phase
transition determined by the Nb concentration. When superconductivity was
suppressed by a strong magnetic field, both the bulk sample R(T) and the point
contact differential resistance curves of Ni_xNb_{1-x} showed logarithmic
behavior at low energies, which is explained by a strong electron - "two level
system" coupling. We studied the temperature, magnetic field and contact
resistance dependence of Ni_{44}Nb_{56} point-contact spectra in the
superconducting state and found telegraph-like fluctuations superimposed on
superconducting characteristics. These R(V) characteristics are extremely
sensitive detectors for slow relaxing "two level system" motion.Comment: 4 pages, 5 figure
Kondo Temperature for the Two-Channel Kondo Models of Tunneling Centers
The possibility for a two-channel Kondo () non Fermi liquid state to
appear in a metal as a result of the interaction between electrons and movable
structural defects is revisited. As usual, the defect is modeled by a heavy
particle moving in an almost symmetric double-well potential (DWP). Taking into
account only the two lowest states in DWP is known to lead to a Kondo-like
Hamiltonian with rather low Kondo temperature, . We prove that, in
contrast to previous believes, the contribution of higher excited states in DWP
does not enhance . On the contrary, is reduced by three orders of
magnitude as compared with the two-level model: the prefactor in is
determined by the spacing between the second and the third levels in DWP rather
than by the electron Fermi energy. Moreover, , turns out to be
parametrically smaller than the splitting between the two lowest levels.
Therefore, there is no microscopic model of movable defects which may justify
non-Fermi liquid phenomenology.Comment: 5 pages, 4 .eps figure
Giant Oscillations of Acoustoelectric Current in a Quantum Channel
A theory of d.c. electric current induced in a quantum channel by a
propagating surface acoustic wave (acoustoelectric current) is worked out. The
first observation of the acoustoelectric current in such a situation was
reported by J. M. Shilton et al., Journ. Phys. C (to be published). The authors
observed a very specific behavior of the acoustoelectric current in a
quasi-one-dimensional channel defined in a GaAs-AlGaAs heterostructure by a
split-gate depletion -- giant oscillations as a function of the gate voltage.
Such a behavior was qualitatively explained by an interplay between the
energy-momentum conservation law for the electrons in the upper transverse mode
with a finite temperature splitting of the Fermi level. In the present paper, a
more detailed theory is developed, and important limiting cases are considered.Comment: 7 pages, 2 Postscript figures, RevTeX 3.
Non-equilibrium electronic transport and interaction in short metallic nanobridges
We have observed interaction effects in the differential conductance of
short, disordered metal bridges in a well-controlled non-equilibrium situation,
where the distribution function has a double Fermi step. A logarithmic scaling
law is found both for the temperature and for the voltage dependence of in
all samples. The absence of magnetic field dependence and the low
dimensionality of our samples allow us to distinguish between several possible
interaction effects, proposed recently in nanoscopic samples. The universal
scaling curve is explained quantitatively by the theory of electron-electron
interaction in diffusive metals, adapted to the present case, where the sample
size is smaller than the thermal diffusion length.Comment: Published version, 6 Pages, 6 postscript figures, 1 tabl
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