90 research outputs found
Remarks about the Zeeman splitting in quantum dots and single atom transistors
An empirical formula is proposed to calculate the Kondo temperature in
quantum point contacts from the Zeeman splitting. A similar formula was used in
the point-contact spectroscopy (PCS) method to obtain from the Zeeman splitting
of the Kondo peak in the characteristics of the classical Kondo alloys AuFe and
CuMn [1]. Using this formula, it is possible to find the Kondo temperature
taking into account the temperature of the experiment and Lande factor of the
compounds.Comment: 5 pages, 3 Fig
Point-contact spectroscopy of superconductors in the nonequilibrium state
A phase transition of the region of the superconductor near the point contact
into a new nonequi- librium state at the critical density of
nonequilibriumquasiparticles is observed.Comment: 3 pages, 3 figure
Critical current density in the superconducting ceramic in S-c-N point contacts
Measurement of the current-voltage characteristics of point contacts makes it
possible to study the properties of individual crystallites in a
superconducting ceramic. The critical current density in the superconducting
regions of the ceramic , with a size of the order of
several tens of angstroms, is found to attain values of , which
are of the same order of magnitude as the pair-breaking current density, as
evaluated from the formulas of the standard theory of superconductivity.Comment: 2 pages, 2 figure
Reabsorption of nonequilibrium phonons at superconducting point contacts
When there is a deviation from the inequality
( is the contact diameter, and is the energy
relaxation length of the electrons), structural features are produced on the
current-voltage characteristics of contacts at characteristic phonon
energies because of the decrease in due to the accumulation of
nonequilibrium phonons with low group velocities near the contact.Comment: 4 pages, 3 figure
Spectroscopy of phonons in dirty superconducting contacts
The nonlinearities of current-voltage characteristics (IVC) of
superconducting niobium point contacts for voltages of the order of the
characteristic phonon frequencies are investigated. It is shown that in
limiting by dirty contacts , ( is the momentum
mean-free path of electrons), features are observed on the IVC corresponding to
maxima in the first or second derivatives and are situated at energies close to
the characteristic phonon energies. These features are due to the energy
dependence of the excess current, since the nonlinearity of the IVC in the
normal state is several times smaller in absolute magnitude, and its second
derivative is monotonic. For a contact diameter of the order of several
tens of angstroms (the contact resistance is of the order of 100 ),
the thermal effects can be neglected. It is assumed that in the region of the
constriction, the characteristic lengths of the system satisfy the relation
( is
the diffusion length for energy relaxation of the electrons, is the
coherence length) which makes possible inelastic transitions of electrons
between pair chemical potentials differing in energy by an account ( is
the voltage drop across the contact)Comment: 8 pages, 4 figure
Point-contact spectra of in the superconducting state
A qualitative study of the electron-phonon interaction (EPI) spectra in
is carried out by the method of point-contact spectroscopy in
the superconducting state. Anisotropy of the EPI spectra for contacts oriented
in the principal crystallographic directions (along the axis and parallel
to the basal surface) was observed. Approximate values of the positions of the
main features in the phonon density of states were established for both
directions. The additional low-frequency peaks observed in the spectra for
separate crystallographic orientations are associated with the collective
oscillations of the amplitude and phase of charge density waves.Comment: 9 pages, 5 figure
Relaxation of nonequilibrium quasiparticles in a superconductor normal metal point contact
The point-contact spectra of tantalum in the superconducting state, with
, , and counterelectrodes, have been studied. We discovered some
new distinctive features, whose position on the axis is determined by the
critical power required for the injection of nonequilibriumquasiparticles. At
this level of power the band gap decreases abruptly in the vicinity
of the contact. A correction to the point-contact spectrum, with the sign
opposite to that of the usual correction, arises in the region of phonon
energies. The maxima in the spectrum become sharper and their position on
the energy axis becomes stabilized near the values , 11.3,
15.5, and 18 , which correspond to low phonon group velocities in . This is confirmed by the existence of
corresponding flattenings on the dispersion relations of lattice
vibrations. Slow phonons are created near the interface in quasiparticle
recombination and relaxation processes and cause a decrease in and an
increase in the differential resistance in the vicinity of . An
excess quasiparticle charge is accumulated in the region of the contact,
producing a correction to the resistance, which decreases as , , and
increase. These mechanisms are particularly effective in dirty contacts, thus
permitting phonon spectroscopy in the superconducting state even when the
current flow occurs in a nearly thermal mode.Comment: 13 pages, 16 figure
Investigation of the electron-phonon interaction in with the aid of microcontacts
The method of microcontact spectroscopy in the superconducting state was used
to investigate weak nonlinearities of the current-voltage characteristics of
point contacts made of single crystals. The nature of the
spectrum of the electron-phonon interaction was found to vary considerably from
contact to contact, indicating considerable deviations of the composition of
the surface of from stoichiometry. A correlation was established
between the nature of the spectrum and the magnitude of the gap singularities
of the current-voltage characteristics. In the case of "dirty" high-resistance
contacts with strong gap singularities the microcontact spectra were reasonably
reproducible, which made it possible to relate them sufficiently closely to the
microcontact function of the electron-phonon interaction in the bulk material.
It was found that microcontact spectroscopy of this interaction was possible in
the superconducting state not only in dirty contacts, but also in dirty
contacts.Comment: 5 pages, 3 figure
Enhancement of Kondo Temperature in Nanometer-Size Point Contacts
Point-contact spectroscopy is applied to study the energy dependence of
paramagnetic impurities in noble metals. The samples are in the form of the
so-called "mechanically controllable break-junctions" where the investigated
piece of alloy makes a nanowire connecting two bulk electrodes.The lateral
dimensions of the bridge are of the order of a few nm and can be continuously
changed. Three qualitative dependences manifest the size effect while
decreasing the contact diameter: 1) Intensity of Kondo peak due to Kondo
scattering in point-contact spectra decreases slower than the nonlinearities
due to phonon scatterings, 2) The width of the Kondo peak becomes broder, and
3) Due to the Zeeman energy, the splitting of Kondo peak in external and
internal (for a spin glass) fields, is suppressed. Explanation of the phenomena
is given in terms of the theory of Zarand and Udvardi where the local density
of states (LDOS) of conduction electron fluctuates strongly inside a nanowire
due to interference of electronic states. Since Kondo effect is a local probe
of LDOS, the impurities located close to the maxima of LDOS show the increase
of the Kondo temperature and give the primary contribution to the contact
resistance.Comment: 11 pages, 8 figures presented at NATO ARW "Size Dependent Magnetic
Scattering", Pecs, Hungary, 28 May - i June, 200
Spin dynamics in point contacts to single ferromagnetic films
Excitation of magnons or spin-waves driven by nominally unpolarized transport
currents in point contacts of normal and ferromagnetic metals is probed by
irradiating the contacts with microwaves. Two characteristic dynamic effects
are observed: a rectification of off-resonance microwave current by spin-wave
nonlinearities in the point contact conductance, and a resonant stimulation of
spin-wave modes in the nano-contact core by the microwave field. These
observations provide a direct evidence that the magnetoconductance effects
observed are due to GHz spin dynamics at the ferromagnetic interface driven by
the spin transfer torque effect of the transport current
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